Civilingenjörsprogrammet i kemiteknik

Uppsal a universitets l ogotyp

UPTEC K 22001

Examensarbete 30 hp

Februari 2022

An Initial Exploration of Transition Metal Nitroprussides as Electrode Materials for Sodium-ion Batteries

Veronica Enblom Civilingenjörsprogrammet i kemiteknik

Teknisk-naturvetenskapliga fakulteten

Uppsala universitet, Utgivningsort Uppsala

Handledare: William Brant Ämnesgranskare: Reza Younesi

Examinator: Peter Broqvist

Uppsal a universitets l ogotyp

An Initial Exploration of Transition Metal Nitroprussides as

Electrode Materials for Sodium-ion Batteries

Veronica Enblom

Abstract

Na-ion batteries (NIBs) are expected to revolutionise the battery sector by promising an affordable

technology while capitalising on sustainable development. To compete with Li-ion batteries,

however, electrode materials with higher capacities need to be developed. Transition metal

nitroprussides (TM-NPs), NaxM[Fe(CN)5NO]1-y ·zH2O, is a material class derived from one of the

most popular positive electrode materials for NIBs, Prussian blue analogues (PBAs), where one of

the cyano ligands have been replaced by an electroactive nitrosyl (NO) ligand. Thus, in theory

TM-NPs should be able to reach higher capacities than PBAs and therefore be attractive candidates

for high-capacity electrodes. However, if the nitrosyl is redox active in NIBs and how the cycling

behaviour may be affected by the M cation is unknown. The focus in this thesis is therefore to

explore the charge-discharge behaviour of four different TM-NPs (M=Fe, Ni, Mn, and Cu) in

Na-ion half-cell batteries to gain an initial understanding of their electrochemical behaviour and

to set up research questions to be pursued in the future. Based on our observations and previous

studies, we propose that the nitrosyl is electrochemically active in all four TM-NPs, and that it

contributes with a considerable amount of capacity, although with a large voltage hysteresis. It is

further concluded that all M cations apart from Ni were redox active, but to varying degrees on

charging and discharging. We argue that both the redox and the voltage hysteresis is caused by

anisotropic charge transfer within the materials, and that it needs to be understood before

commercialisation of TM-NPs can be realised. Though there are challenges to overcome, the many

interesting attributes of TM-NPs, including anionic redox, anisotropic charge transfer and

structural diversity, makes them promising as a new type of cheap and sustainable electrode

material for NIBs. Teknisk-naturvetenskapliga fakulteten, Uppsala universitet . Utgivningsort U ppsal a. H andl edare: William Brant, Äm nesgranskar e: R eza Younesi, Exami nator: Peter Br oqvist

i

Svensk Populärvetenskaplig Sammanfattning

Kommersialisering av natriumjonbatterier förväntas bli nästa revolutionerande utveckling i

batterivärlden genom att förse världen med ett grönare alternativ till litiumjonbatteriet. För att

natriumjonbatterier ska kunna vara konkurrenskraftiga behöver vi dock utveckla material som

kan lagra en större mängd energi än i dagsläget.

2019 gick nobelpriset till Stanley Whittingham, John B Goodenough, och Akira Yoshino för

utvecklingen av den teknik som möjliggjort dagens mobiltelefoner, bärbara datorer,

pacemakers, och elbilar, nämligen litiumjonbatteriet. Det är tack vare dessa energirika och lätta

batterier som vi så snabbt kunnat övergå till ett trådlöst samhälle och på senare tid kunnat

minska vår användning av fossila bränslen. Idag förväntas de spela en nyckelroll i

bekämpningen mot klimatförändringarna genom att bland annat minska utsläppen från

transportsektorn och lagra överskottsenergi från vind- och solkraft.

Dagens litiumjonbatterier är dock starkt beroende av kritiskt begränsade material eller material

som länge förknippats med människorättsfrågor, så som litium, nickel, och kobolt. Forskare

och ingenjörer försöker nu därför utveckla alternativa tekniker som kan tillverkas på ett mer

ekologiskt och etiskt hållbart sätt för att även utvecklingen av energilagringssystemen skall vara

hållbar. Batterier baserade på natriumjonen i stället för litium, det vill säga natriumjonbatterier,

tros bli nästa stora genombrott bland annat på grund av dess likhet till litiumjonbatteriet och

den stora tillgången av natrium i jordskorpan.

Utvecklingen av natriumjonbatteriet har kommit en lång väg och nyligen blivit en kommersiell

realitet av det kinesiska företaget Contemporary Amperex Technology Co Ltd (CATL). Deras

batterier håller i dagsläget en energimängd runt 160 Wh/kg, vilket kan jämföras med Teslas

litiumjonbatterier som håller runt 200 Wh/kg. För att öka natriumjonbatteriets energidensitet

och därmed dess konkurrenskraftighet behöver dock nya batterimaterial utvecklas. I detta arbete

har vi därför tittat på en outforskad materialklass kallad Nitroprussider för att undersöka deras

potential som ett framtida material för natriumjonbatterier.

Elektroden: Från kemisk energi till elektricitet

Batterier består av två elektroder, en positiv och en negativ, separerade av en jonledande vätska

kallad elektrolyt. Det är i elektroderna som energi omvandlas från kemisk energi till elektricitet

genom vad som kallas redoxreaktioner. Platsen där detta sker kallas redoxcentra. Ju fler

redoxcentra ett elektrodmaterial har, ju mer energi kan vi utvinna från det.

ii

Ett miljövänligt och framgångsrikt elektrodmaterial

”Prussian white” är idag det mest populära elektrodmaterialet för natriumjonbatterier då

materialet kan framställas billigt och hållbart samtidigt som det håller en hög prestanda.

Materialet är endast uppbyggt av kol, kväve, järn, och natrium som alla är bland de mest vanligt

förekommande elementen i jordens skorpa, och har stora porer som tillåter snabb omvandling

av energi. Porositet är dock också grunden till dess största kritik, nämligen låg energiförvaring

per volymenhet, vilket innebär att ett större batteri behövs för att lagra samma mängd energi

som ett kommersiellt litiumjonbatteri. För en ökad energidensitet behöver vi därför titta på

material som liknar Prussian white men som har lägre porositet eller fler redoxcentra.

Extra redoxcentra lagrar mer energi

Nitroprussider liknar Prussian white i uppbyggnad, med undantag för en extra redoxaktiv grupp

kallad ”nitrosyl”, och kan därför framställas på samma billiga och miljövänliga sätt. Denna typ

av material har tidigare undersökts för andra applikationer så som sensorer och vätgasförvaring,

men har endast vid ett tidigare tillfälle testats i ett batterisystem och då ett litiumjonbatteri. Vårt

arbete är därför det första där olika typer av Nitroprussider undersökts i natriumjonbatterier,

och är ämnad som en vägledning för framtida batteriforskning inom området. Som förväntat

kunde vi se att nitrosylen är redoxaktiv i samtliga studerade Nitroprussider under upp- och

urladdning av natriumjonbatterier, vilket visar att de har potential som nya energitäta elektroder.

Komplexitet utmanar prestandan

Den extra reaktiviteten från nitrosylgruppen presenterar dock inte bara möjligheter utan också

utmaningar. Vi kunde se att det krävs mer energi för att ladda upp samtliga batterier jämfört

med den mängd energi som utvinns vid urladdning. Detta är såklart problematiskt eftersom det

innebär att vi förlorar massa energi för varje upp- och urladdningscykel, men är inte en ovanlig

syn tidigt i utvecklingen av nya elektrodmaterial. En mängd olika strategier kan tillämpas för

att undvika energiförlusten så länge orsaken är identifierad. Vi föreslår därför att nästa steg i

utvecklingen bör fokusera på att reda ut materialets komplexa natur på molekylär nivå.

Ett spännande material för framtidens batterier

Batterierna i Teslas elbilar tog runt 20 år att utveckla från det att man föreslog materialet till

vad de är idag på grund av en mängd olika utmaningar. Trots att Nitroprussiderna möter flera

utmaningar innan de kan kommersialiseras, ser vi därför en fundamental potential som innebär

att de kan ha en stor betydelse för utvecklingen av framtidens hållbara batterier.

iii

Table of Contents

Svensk Populärvetenskaplig Sammanfattning ............................................................................ i

Table of Contents ...................................................................................................................... iii

List of Figures ........................................................................................................................... iv

List of Tables ............................................................................................................................. iv

Abbreviations ............................................................................................................................. v

1. Introduction ......................................................................................................................... 1

1.1. Broader Context ........................................................................................................... 1

1.2. Principle of Na-ion Batteries ....................................................................................... 1

1.3. Progress in Electrode Materials for NIBs .................................................................... 3

1.3.1. Negative Electrodes .............................................................................................. 3

1.3.2. Layered Transition Metal Oxides ......................................................................... 4

1.3.3. Polyanionic Based Materials ................................................................................ 5

1.3.4. Prussian Blue Analogues ...................................................................................... 6

1.4. Transition Metal Nitroprussides .................................................................................. 8

1.4.1. Synthesis ............................................................................................................... 8

1.4.2. The Various Structures of TM-NPs ..................................................................... 9

1.4.3. Redox Activity of the Nitrosyl ........................................................................... 12

1.5. Project Aims .............................................................................................................. 14

2. Experimental ..................................................................................................................... 15

2.1. Materials for Synthesis .............................................................................................. 15

2.2. Method ....................................................................................................................... 15

2.2.1. Sample Preparation ............................................................................................ 15

2.2.2. Material Characterisation ................................................................................... 16

2.2.3. Electrochemical Evaluation ................................................................................ 19

3. Results and Discussion ..................................................................................................... 21

3.1. Structure and Purity ................................................................................................... 21

3.2. Composition ............................................................................................................... 24

3.3. Oxidation State of the Nitrosyl and TMs ................................................................... 28

3.4. Galvanostatic Cycling Behaviour .............................................................................. 31

4. Conclusion and Future Work ............................................................................................ 38

Acknowledgements .................................................................................................................. 41

References ................................................................................................................................ 42

iv

List of Figures

Figure 1.1: Architecture and working principle of a Na-ion battery…………………....…. 2

Figure 1.2: General structures of common electrode materials.………………………...…. 3

Figure 1.3: Capacity and voltages of various Layered transition metal oxides……….…… 5

Figure 1.4: Structure of cubic Transition metal nitroprussides…………………….…...… 10

Figure 1.5: Crystallographic structure of Manganese nitroprusside……………….……... 11

Figure 1.6: Crystallographic structure of Copper nitroprusside…………………….…..... 12

Figure 2.1: Experimental setup for synthesis of the samples………………………...….... 16

Figure 2.2: Swagelok battery cell design………………………………………………..... 20

Figure 3.1: X-ray diffractograms of hydrated and dehydrated samples………………..… 22

Figure 3.2: Anisotropic broadening and layer shifting in Copper nitroprusside……….….23

Figure 3.3: Thermogravimetric mass loss curves of hydrated samples……………....…... 26

Figure 3.4: Infrared spectroscopy spectra of hydrated and dehydrated samples………….. 29

Figure 3.5: First one and a half galvanostatic cycling curves for synthesised samples..… 33

Figure 3.6: First five galvanostatic cycling curves for the synthesised samples………….. 35

Figure 3.7: Cycling curves for CuNP over different voltages……………………..……... 36

List of Tables

Table 3.1: Cell-parameters and volume change on dehydration of the samples…………. 24

Table 3.2: Stoichiometry and structure of samples compared with reference structures... 27

Table 3.3: Important infrared active vibrations observed in the samples …………..….... 30

v

Abbreviations

ATR: Attenuated total reflection

CuNP: Copper nitroprusside

DEC: Diethyl carbonate

EC: Ethylene carbonate

FeC: Carbon bound Iron

FeN: Nitrogen bound Iron

FeNP: Iron nitroprusside

ICP-OES: Inductively coupled plasma optical emission spectroscopy

IR: Infrared spectroscopy

LIB: Lithium-ion battery

LTMO: Layered transition metal oxide

MLTC: Metal-to-ligand charge transfer

MnNP: Manganese nitroprusside

NaNP: Sodium nitroprusside

NiNP: Nickel nitroprusside

NP: Nitroprusside

PAM: Polyanionic based material

PBA: Prussian blue analogue

NIB: Sodium-ion battery

TGA: Thermogravimetric analysis

TM: Transition metal

TM-NP: Transition metal nitroprusside

XRD: X-ray diffraction

1

1. Introduction

1.1. Broader Context

Rechargeable batteries are an extraordinary technology that play a key role in transitioning from

fossil fuels to renewable energy sources.1,2 But for the development of the grid to be sustainable,

batteries, too, need to be sustainably sourced and made. However, the current state of the art,

Li-ion batteries (LIBs), are heavily dependent on elements like Li, Ni, and Co that are either

critically constrained, or associated with human rights issues.3–7 A more sustainable, and

potentially cheaper, alternative is batteries based on the highly abundant element sodium, i.e.,

Na-ion batteries (NIBs).8–12 This alternative, however, only gained popularity in the last few

years and is thus facing several challenges before catching up to the performance of LIBs. One

of these challenges is designing materials that can reversibly store a high amount of sodium at

a given voltage. The aim of this work is therefore to explore the potential of Transition metal

nitroprussides (TM-NPs) as a new type of battery material for NIBs. Before covering the details

of TM-NPs, an introduction will be given to the working principle of NIBs and state-of-the art

electrode materials.

1.2. Principle of Na-ion Batteries

NIBs are similar to the well-known LIBs in that they have a similar working principle and

architecture. Both consist of two electrodes, that are separated by an electrolyte and an electrical

insulator, often called separator, and externally connected via a conductive wire (Figure 1.1).

This design forces the charged species, the electrons and working ions (Na+ or Li+), to migrate

through separate paths. The driving force is the potential difference between the two electrodes,

which initiates redox reactions when in a closed circuit. On discharge, the electrode with a low

or negative reduction potential is oxidised and the positive electrode reduced. The electrons

pass from the negative to the positive electrode, through the external wire, to perform work. To

maintain charge neutrality, Na+ is simultaneously extracted from the negative electrode into the

electrolyte and intercalated into the positive electrode. Charging the battery entails applying a

current in the opposite direction and forces the charged species the other way, in other words,

favouring reversed redox reactions.13 It should be stated that the electrode at which oxidation

occurs is traditionally defined as the “anode” in electrochemistry, and the reduced electrode a

“cathode”. Although this nomenclature only applies during discharge and single-use batteries,

2

these terms have been adopted by battery science to refer to the negative and positive electrode

respectively and are growing by convention. To avoid confusion in this thesis, they will

henceforth strictly be referred to as negative and positive electrode.

A principal performance criteria for rechargeable batteries is a high amount of reversible energy

stored per cycle, i.e., energy density, at a given power requirement.13 Energy density is the

product of the cell’s voltage and capacity, and thus depends on the potential difference between

the electrodes (voltage) and the number of ions that can be stored in the battery per gram or

volume (capacity). NIBs commonly have a voltage around 3.4–3.6 V and capacities around

150–190 mAh/g and 300 mAh/g for the positive and negative electrode, respectively.12 This

can be compared with commercial LIBs that frequently have a nominal voltage around 3.7 V

and capacities around 200–300 mAh/g for the positive electrode and 370 mAh/g for the negative

electrode.14 Although, there are negative electrodes that go up to and even beyond 800–900

mAh/g for LIBs.15 Thus, developing electrodes with higher capacities is vital if NIBs are to

ever compete with LIBs.

Figure 1.1: Schematic of the architecture and general working principle of a Na-ion battery.

3

1.3. Progress in Electrode Materials for NIBs

To date, the most promising positive electrode materials for NIBs are various Layered

transition-metal oxides (LTMOs), Polyanionic based materials (PAMs) and Prussian blue

analogues (PBAs) (Figure 1.2).12,16 Among these, PBAs are showing the highest reversible

capacities of around 160 mAh/g, approaching common commercialised materials in LIBs.16 For

the negative electrode there is yet no viable commercial material, however, hard carbon will

likely become the preferred choice in the near future. TM-NPs considered in this work are

structurally derived from the positive electrode class of materials PBAs and thus a larger focus

will be given to positive electrode materials. However, for the sake of context and comparison

a brief introduction to current negative electrode materials will be presented.

Figure 1.2: Example structures of (a) a layered transition metal oxide, NaxCoO2, (b) a polyanionic based

material, NASICON, and (c) rhombohedral Prussian white.

1.3.1. Negative Electrodes

Graphite has long been the dominating material used for negative electrodes in LIBs because

of its low potential, low cost, and highly reversible intercalation of Li+ between the many layers.

Na+ intercalation, on the other hand, has proven to be unfavourable because of unstable

thermodynamics, resulting in extremely low capacities.17 Thus, many materials, including

carbon-based materials, titanium-based oxides, alloy and conversion materials, and 2D

transition-metal dichalcogenides, are being considered as new negative electrode materials for

NIBs.12 Carbon-based materials, dominated by the disordered amorphous carbon structure

“hard carbon”, seem to be the most popular candidate so far, with a reversible capacity around

300 mAh/g depending on the synthesis route and treatment.18 Hard carbon, however, suffers

from a low initial capacity due to irreversible binding of sodium to defects and pores, which

4

leads to low capacities in full battery cells, and is also difficult to produce reproducibly.12,19

Subsequently, pre-cycling of hard carbon in a half cell (against Na metal) prior to assembly in

a full cell is necessary, and makes it less convenient for practical purposes.18,19 For this reason,

other material classes, as mentioned above, are being considered as candidates. Titanium based

oxides have gained popularity due to a generally low cost and low operating voltages, but also

suffer from low reversible capacities and poor cycling stability.12 Large progress is hence

needed before it can compete with hard carbon. Alloy and conversion materials and 2D

transition-metal dichalcogenides can both show extremely high capacities. In the former case,

however, the high capacity is given at the expense of structural stability. As alloy and

conversion materials involve the formation of alloys with the working ion, large volume

expansions are inevitable.20 This subsequently leads to large mechanical stresses and eventually

pulverisation of the material during operation.20 2D transition-metal dichalcogenides, on the

other hand, were only recently considered as a candidate for negative electrodes in NIBs.

Hence, there is a general lack of understanding of the working principle and intercalation

mechanism that need to be understood before it can be commercialised.12

1.3.2. Layered Transition Metal Oxides

LTMOs are a complex class of materials with an immense landscape of combinatorial

possibilities and variations in electrochemistry. It is currently the commercial choice for many

LIBs and have subsequently also gained a large interest for NIBs. The general formula for

LTMOs is AxMO2, where A is the charged species (Li+ or Na+ in this case) and M a fraction of

one or several TMs (Figure 1.2(a)). The structure consists of MO2 layers consisting of face

sharing MO6 octahedra. Between these layers the A ions occupy different interstitial sites,

generally prismatic or octahedral sites if A=Na+.21,22 Since M can be a combination of several

TMs, the large combinatorial variety of NaxMO2 makes the material class complex and

challenging, but also leads to many possibilities to tune the electrochemistry, as demonstrated

in Figure 1.3. High initial capacities up to around 200 mAh/g can be reached23,24, but the

reversible capacity remains around 100–150 mAh/g at the highest.25 This capacity loss has been

attributed to oxygen redox reactions, which cause structural degradation during operation in the

form of transition metal (TM) migration and O2 release.25 Another major challenge with

LTMOs are slow Na+ diffusion because of their dense structure.12 Both of these problems,

however, are mitigated in PAMs and PBAs by large open structures and strong covalent

bonding’s within the framework.

5

Figure 1.3: Diagram of capacity and voltage with energy density curves for different transition metal oxides in

half-cell systems. 26

1.3.3. Polyanionic Based Materials

In addition to having open and structurally durable frameworks, PAMs are distinguished by

high redox potentials because of a unique inductive effect.27,28 Generally, PAMs can be

described as inorganic polymeric networks based on (XO4)n- tetrahedra and their derivatives

(XmO3m+1)n- linked by strongly covalently bonded MOx polyhedra (Figure 1.2(b)). The general

formula can be written as NaxMy(XO4)n where X = S, P, Si, As, Mo, or W, and M is a TM.29

This class of materials has been extensively explored both because of their similarity to LIB

polyanionic frameworks and vast variety of combinatorial elements, but mainly because of their

distinctive high voltage outputs, i.e., high redox potentials.30–32 As mentioned earlier, the latter

is desirable to design batteries with high energy densities. The high redox potential of PAMs,

compared to isostructural compounds with identical formal valance, was first described by

Manthiram and Goodenough27 and attributed to an inductive effect of the anion X on the M–O

bond. The strength of the covalent interaction between the M 3d and O 2sp orbitals determines

the redox potential of the material. In other words, a stronger M–O bond gives a lower redox

potential. However, since the electronegative anion X shares a nearest oxygen with M, stronger

X–O bonds subsequently leads to weaker M–O interactions. Thus, raising the redox potential

6

of PAMs compared to many other materials. Moreover, the strong X–O bond inhibits O2

evolution which also gives it a higher thermal stability compared to LTMOs.28 Despite these

advantages, low electronic conductivities and limited capacities still restricts further

application.32 The exception in terms of low conductivity are those based on vanadium,

particularly the NASICON structured Na3V2(PO4)3 and its fluoroderivatives

Na3(VO1-xPO4)2F1+2x.12,31 However, as vanadium and fluoride are both expensive and toxic,

replacing these elements with more benign ones, without harming the battery performance,

poses an ongoing research challenge.

1.3.4. Prussian Blue Analogues

PBAs are currently one of the most attractive material classes for NIBs with high capacities

while capitalising on the principle of affordable sustainable batteries.12,16,33–39 Their generalised

formula can be written as AxM[M’(CN)6]1-y ·zG, where A and G are mobile guest species in the

highly porous M[M’(CN)6]1-y host framework, M and M’ are TMs with a nitrogen- and carbon-

coordination, respectively and y is the number of hexacyanometallate, [M’(CN)6]n-, vacancies.

Further, G is a neutral guest species, most often H2O, and A is a charged species that when

removed coincides with a redox process occurring on the TM centre.37 Thus, in a battery

application, A is considered to be the working ion. PBAs crystallise in what can be called a

molecular perovskite-type structure where cyanide groups octahedrally coordinate metal

centres. These octahedra are subsequently corner linked to form three-dimensional open

frameworks with large interstitial voids for cations. For the majority of compositions and

temperatures, PBAs adopt the face-centred cubic structure, with the Fm-3m space group.40

Although, distortion to a monoclinic, rhombohedral (Figure 1.2(c)), or orthorhombic structure

can occur depending on the type of A ions, their oxidation states, and the material composition,

specifically the A, G and vacancy content.37 It should be stated that PBAs also may consist of

critical elements like cobalt and vanadium, however, unlike many LTMOs and PAMs, they are

not reliant on them to achieve high energy density. Most PBAs in NIBs therefore consist of Fe,

Mn, Ni or Cu. Hexacyanoferrates (M’=Fe) are considered the superior candidates for

commercial scale applications because of their low cost, competitive electrochemical capacity,

thermal and structural stability, and the abundance of Fe resources.16 Among these, the highest

reversible capacity yet reached is by Prussian white, NaxFe[Fe(CN)6]1-y ·zH2O, with a current

practical capacity around 160 mAh/g (93% of theoretical).12,36

7

The largest issues with PBAs are their moisture sensitivity, limited reversibility, and low

volumetric energy densities.41,42 The moisture sensitivity impacts PBAs in two subtle but

distinct ways. Firstly, as PBAs are porous compounds, they readily absorb moisture from the

air into the open structure. Water, when within the structure, increase the redox potential of the

carbon bound iron (FeC) to above the oxidative limit of water (3.94 V vs Na/Na+), subsequently

leading to irreversible gas formation, capacity degradation, and poor cycling lifespan.43,44

Secondly, in presence of oxygen, e.g., in preparation prior to incorporation in a battery cell,

water adsorbed to the surface of the material leaches sodium from the bulk to form NaOH which

subsequently reacts with the framework producing Na4[Fe(CN)6] and Fe(OH)3.42 Na4[Fe(CN)6]

is electrochemically active at a potential around 3.4 V vs Na/Na+ and can thus act as an

electroactive passivating layer.42,45 The effect of Fe(OH)3 on the cell is less known but is

predicted to be detrimental.42 On the other hand, the limited reversibility originates from the

combined effects of two inherent features of the PBA compound: low electronic conductivity

at high sodium contents and a structural transition between a cubic and rhombohedral phase

occurring between 1.2–2 Na+ per formula unit during cycling.37,46 The phase transition itself is

not necessarily detrimental, but comes with a large change in volume (up to 18%) that overtime

leads to particle cracking, loss of contact, and subsequently gradual capacity fading and

polarization increase.37,47 These issues related to moisture sensitivity and limited reversibility

can be mitigated by selective control of composition and drying conditions, but the low

volumetric energy density is inherent to the structure and so cannot be solved without going to

a new structure type.37,42 Therefore, for this type of electrode material to remain competitive,

we must look at materials that are similar to PBAs but fundamentally different to reach higher

volumetric capacities.

8

1.4. Transition Metal Nitroprussides

Transition metal nitroprussides (TM-NPs) are a material class structurally similar to the metal

hexacyanoferrates, only with the exchange of an axial cyano ligand to a nitrosyl (NO) ligand.

Subsequently, their generalised formula is AxM[Fe(CN)5NO]1-y ·zG, where again M is a TM,

and A and G are guest species. TM-NPs have previously been studied for applications such as

a negative thermal expansion material48, hydrogen storage49–51, catalysis52, and sensors53, but

were recently suggested as a new type of positive electrode material for batteries by Mullaliu

et al.54. This group has focused on copper nitroprusside (CuNP), Cu[Fe(CN)5NO], for

applications in LIBs with the aim of increasing the capacity compared to copper

hexacyanoferrate, Cu[Fe(CN)6]. Their results showed that Cu and the nitrosyl group were redox

active during cycling, suggesting that a higher energy density can be obtained.54,55 This

prompted the question of whether this phenomenon is limited to CuNP or if it extends to other

TM-NPs, and encouraged the current investigation. Perhaps by exploring the overall redox

activity in TM-NPs, a deeper understanding can be obtained on the redox processes, their

reversibility, and evolution over multiple cycles.

1.4.1. Synthesis

TM-NPs are primarily synthesised by aqueous co-precipitation between sodium nitroprusside

(NaNP) and a soluble salt of the target TM. The reaction can be described by Reaction 1, where

X is frequently (Cl−)2, (NO3−)2, or SO4

2−.56–60 For many TM-NPs, this reaction is spontaneous

and occurs instantaneously when the two solutions are mixed, but similar to PBAs, size and

stoichiometry of the crystals can be influenced by selectively controlling several parameters

during synthesis. Single crystals can be synthesised by liquid/liquid interdiffusion of 0.1 M

solutions at room temperature through a tetramethoxysilane (TMS) gel, or in an H-shaped

vessel.56,61 The gel method, sometimes called the “slow-diffusion-tube” method, is the slowest

of the two and takes about three to four months to achieve crystals of a size suitable for

diffraction studies.56,57,62 Powders, on the other hand, are more difficult to control but can be

influenced by varying concentrations (frequently around 0.01 M), precursor ratios, mixing

rates, and ageing.59,63,64

MX · nH2O + Na2[Fe(CN)5(NO)] · 2H2O → M[Fe(CN)5(NO)]1-y · zH2O + 2NaX (1)

9

Crystallisation of TM-NPs occur by bridging the cyano ligands in the nitroprusside (NP) anion,

(Fe(CN)5NO)2-, to the TM cations, forming a three-dimensional polymeric network. However,

the NO-ligands remain unbridged, causing structural pores that vary in size and geometry

depending on the composition and structure. Subsequently, TM-NPs, like PBAs, are moisture

sensitive and absorb water into the bulk.65 It is therefore reasonable to assume that a similar

handling, e.g., dehydration, is needed prior to any application in non-aqueous NIBs, and that

varying stoichiometry, water content and guest species can produce a range of polymorphs.

1.4.2. The Various Structures of TM-NPs

The anisotropic nature of the NP ion allows TM-NPs to adopt a variety of crystalline structures.

Generally, these can be described by different relationships between two rigid building blocks:

an octahedrally coordinated iron (NP) and a TM in an octahedral or square pyramidal

coordination environment.57,58,62,66–71 As mentioned, the iron atom is coordinated by five cyano

ligands, four equatorial and one axial, and a nitrosyl (NO+) ligand. However, because the

interatomic bond distance between Fe–NNO is considerably shorter than Fe–C, the octahedra

that is formed is distorted.57,58,62,66–71 Additional distortion is imparted by the larger

electronegativity of the nitrosyl compared to the cyano ligand, which causes a deformation of

the CNeq ligands away from the nitrosyl end such that the angle between Ceq–Fe–NNO is larger

than the angle between Ceq–Fe–Cax.57,62,66,71 Likewise, the TM in its hydrated form generally

adopts an octahedral environment by five cyano ligands and one water molecule. Copper,

however, is an exception as it forms octahedra’s of four cyano ligands and two water

molecules.58 Since water has a lower electronegativity than cyanide, the CN ligands deform

towards the water molecule.57,62,66,71 Moreover, additional water can hydrogen bond to the

coordinated water molecule in all TM-NPs to form crystalline water structures within the

porous network. The degree of hydration depends on the preparative method and the TMs

involved. All crystalline water leaves the pores upon heating around 60–100 °C, generally

leaving an anhydrous phase with a similar framework as the original hydrate.68,70 When the

water is removed, the coordination sphere of the TM becomes incomplete, and results in the

adoption of a square pyramidal coordination.68 Together, the NP and TM polyhedra connect via

the CN- ligands to form the three-dimensional network, where the final structure is determined

by the bond strength between metals and ligands and how the ligands are arranges relative to

each other between metal centres. The degree of octahedral irregularity become larger as the

metal to ligand interactions become larger, and thus depends on the cation occupying the M

10

position and reaches a maximum when the material is dehydrated.70 Similar to PBAs, many

different TMs occupy the M position, including Mo, Cd, Co, etc. Although, for sustainable

battery applications the most interesting are Fe2+, Ni2+, Mn2+, and Cu2+. The structures of these

TM-NPs will therefore be the focus hereafter.

FeNP and NiNP both adopt a cubic phase in the Fm-3m space group when synthesised into a

powder by precipitation.59,65,70 This structure is highly disordered, with relatively large pore

sizes related to vacancies of both NP octahedra and TM atoms, and can therefore not be

described accurately by a single unit cell. However, key local structures of cubic TM-NP exist

and are shown in Figure 1.4. As the TMs are coordinated by a water molecule during synthesis,

two types of cavities tend to form within the structure.65,70 Firstly, the nitrosyl ligands orient

themselves toward each other to form hydrophobic pockets (Figure 1.4(b)). Secondly, large

hydrophilic cavities (Figure 1.4(c)) are formed by the water coordinated end of the TMs where

there is NP vacancies. Instead of an NP octahedra, four more water molecules fill the cavity in

the form of a tetrahedra through hydrogen bonding interactions, giving a total of five water

molecules per formula unit.70 When the water is removed on heating, both the M–N and Fe–C

interatomic distances become shorter, resulting in a shorter unit cell length and a volume

reduction around 2%. The cell volume of NiNP, however, is always smaller than that of FeNP

due to stronger metal to ligand interactions.70 MnNP can also adopt a cubic structure when

rapidly precipitated.59

Figure 1.4: Pores in cubic transition metal nitroprussides (TM-NPs): (a) regular channels due to their inherently

porous structure, (b) NO ligand-cage, and (c) hydrophilic cavity with a nitroprusside (NP) vacancy.

If FeNP is instead slowly grown into a single crystal, it adopts the formation of a monoclinic

trihydrate with space group P21/n.62 This is also the preferred structure for MnNP (Figure

1.5(a)).66 Compared to the cubic structure, the distortions of the octahedra herein are larger,

resulting in a more thermally stable and compact wave-like structure with smaller pores, which

presents an opportunity to overcome the volumetric density issues in PBAs.62,64,66

11

Consequently, only two crystalline water molecules, compared to four in the cubic compounds,

can interact with the coordinated water. Additionally, one of the two zeolitic water molecules

have a shorter bond distance to the coordinated water than the other (2.740 Å vs 2.855 Å for

FeNP and 2.759 Å vs 2.854 Å for MnNP).62,66 According to Brown72, strong hydrogen bonding

has a bond length of approximately 2.73 Å, and becomes progressively weaker when the

interatomic distance increase. Subsequently, the weakly bound water is rapidly lost already at

room temperature, while the remaining two leave the structure at elevated temperatures (≤100

°C).62 The monoclinic trihydrates thus transform into the more symmetric, and slightly more

compact, orthorhombic symmetry with space group Pnma unless kept in solution.59,67 CuNP

can also adopt this structure if prepared as a single crystal.71 MnNP, however, also obtains this

structure through precipitation (Figure 1.5(b)).59 In contrast to the cubic structure, dehydration

of the orthorhombic structure only leads to a very small volume change (<1%).68

Figure 1.5: Crystallographic structure of Manganese nitroprusside as (a) a monoclinic trihydrate

(space group P21/n), and (b) an anhydrous orthorhombic (space group Pnma).

CuNP diverge from the structural behaviour of the Fe, Ni, and Mn analogues and adopts a

structure unique among the TM-NPs during precipitation. In its hydrated form, CuNP

crystallises as an orthorhombic layered dihydrate with space group Amm2 (Figure 1.6(a)).58 The

iron has its usual octahedral coordination of five cyano ligands and a nitrosyl group, but as

previously mentioned copper take on an octahedral coordination of four equatorial cyanide

groups and two axial water molecules in a trans-configuration. Thus, only the axial ligands are

coordinated to the iron, forming a structure of parallel two-dimensional layers of alternating

corner sharing NP and TM octahedra that are stacked in an off-set way along the c-direction

and held together by van der Waals interactions.58 Upon dehydration, the Cu atom loses its

coordinated water resulting in an incomplete coordination sphere. To increase the coordination

number, every second layer shifts half a unit cell length (a/2) withing the a-b plane in the

a-direction such that the Cu coordinates with the axial nitrogen of the cyanide in the adjacent

12

layer. In this new position, copper adopts a square pyramidal coordination with an elongated

top due to a Jahn-Teller distortion thanks to its +2 oxidation state and d9 electronic

configuration.58,71 The resulting anhydrous phase has a tetragonal structure in space group I4mm

(Figure 1.6(b)).58

Figure 1.6: Crystallographic structure of Copper nitroprusside as (a) a hydrous orthorhombic

(space group Amm2), and (b) an anhydrous tetragonal (space group I4mm).

1.4.3. Redox Activity of the Nitrosyl

The NO ligand is particularly known within biochemistry for its redox activity and ability to be

in three different oxidation states, NO+, NO0, and NO-.73,74 As a stable free radical, nitric oxide

(NO) is a highly reactive molecule that easily oxidises into the nitrosonium ion (NO+) or reduces

into the nitroxide ion (NO-).73 This behaviour is attributed to the unpaired electron which resides

in the molecule’s antibonding π* orbital.73 Consequently, nitric oxide is also redox active in

solution with a reduction potential (NO+ + e- ⇌ NO) around +1.2 V versus a saturated calomel

electrode (SCE), although the potential is strongly dependant on the solvent.75 Most

importantly, however, NO is known for its major influence on the chemistry and

electrochemistry of its TM complexes, e.g., TM-NPs.73 The nitrosyl ligand binds to the TM by

the N end, creating an M–N–O bond with a bent configuration.74 In this formation, the metal d

orbital, the π* orbital of the NO, and the molecular orbital that forms between them all have

similar energies, which allows for electron density transfer.63,73,76 The nature of the bond makes

it difficult to assign formal oxidation states to the metal and NO, respectively, as they more

often change between resonance structure.73 However, a general increase in the N–O bond

length is seen when the nitrosyl ligand is reduced to NO- as the π* orbital is further populated.77

Likewise, oxidation of the ligand to NO+ causes the bond to contract. To summarise, it is well

13

known that the nitrosyl ligand is redox active and that it influences the electrochemistry of its

metal complexes, yet it was not until recently that it was examined within a battery system.

Mullaliu et al.54 were the first to study the nitrosyl redox activity of a TM-NP within a LIB in

2017. The study was performed on CuNP that had been bulk synthesised by co-precipitation of

CuSO4 · 5H2O and NaNP. The exact composition, after dehydration at 130 °C for 6 h in air,

was Cu0.8[Fe1.2(CN)5(NO)] · 0.5H2O. Cyclic voltammetry and galvanostatic cycling were

performed at a potential range between 4.0 and 1.5 V vs. Li+/Li. The oxidation states of the

TMs and the nitrosyl were probed by operando X-ray absorption spectroscopy (XAS) and

infrared spectroscopy (IR), respectively. In the first discharge/charge cycle, three main features

were observed: (i) a plateau around 3.4 V corresponding to Fe(2+δ)+/Fe2+ redox, (ii) two plateaus

at 2.9 and 2.5 V, respectively, both attributed to Cu2+/Cu1+ redox with different local

environments, and (iii) a slope from 1.8 V to the lower cut-off voltage (1.5 V) attributed to the

reduction of nitrosyl.54 During charging, an additional slope was observed from 3.8 V to the

upper cut-off voltage (4.0 V) but was not assigned to any specific redox activity. According to

their operando IR results in the same study, however, this slope likely corresponds to oxidation

of the nitrosyl, as will be further discussed in section 3 Results and Discussion. Subsequently,

Mullaliu et al. showed that the nitrosyl ligand in CuNP is redox active during cycling in a

battery, albeit with a large polarisation. Although polarisation on this scale is unusual for battery

materials, it is not uncommon for other anionic redox phenomena, as proven by the large

polarisation of oxygen anionic redox in other materials.78,79 Overall, the initial capacity reached

for the CuNP was between 40 and 120 mAh/g depending on the discharge rate, but only 40

mAh/g (over 90 cycles) were reversible. The same material was later cycled against sodium

metal in a NIB half-cell, in which it reached an initial capacity around 85 mAh/g, and a

reversible capacity of 20 mAh/g.55

While Mullaliu have investigated CuNP over the past 5 years54,55,80–82 there remain other

TM-NP analogues which show promise. Thus, the most pressing question to be answered now

is whether the nitrosyl ligand is chemically active in TM-NP analogues beyond copper.

Investigating the electrochemistry of these materials is therefore of great importance as it could

pave the way for a new class of high capacity, and potentially higher volumetric capacity,

electrode materials for NIBs.

14

1.5. Project Aims

This work is an initial exploration of the charge-discharge cycling behaviour of TM-NPs as a

new class of electrode materials. Because this project is starting an entirely new research area

the aim is to set up research questions which can be pursued in the future. Focus in this report

is on the nitrosyl group and whether it is redox active during cycling in NIBs in TM-NP

analogues beyond copper, and if so, how they differ in behaviour. These questions will be

answered by electrochemically analysing four TM-NPs, FeNP, NiNP, MnNP, and CuNP, in

Na-ion half cell batteries. These four compounds were chosen for two reasons: (i) because of

the frequent use of these TMs in common electrode materials and (ii) because of their distinct

differences in structure. Consequently, all compounds will be characterised in terms of

structure, stoichiometry, and oxidation state, to elucidate differences in cycling behaviour.

15

2. Experimental

The compounds were synthesised by a controlled aqueous co-precipitation method that

produces the material in their hydrated state as powders. Subsequently, dehydration was

required before incorporation in a battery. All TM-NPs were galvanostatically (constant

current) cycled between charged and discharged states five times to measure their voltage

profile as a function of capacity, and in so doing, also measuring vital information about the

redox activity of the nitrosyl ligand. The electrochemistry was then interpreted with help of

structural, stoichiometric and oxidation state characterisation of both the as synthesised and

dehydrated materials. X-ray diffraction (XRD) was used to determine the structures, inductively

coupled plasma-optical emission spectroscopy (ICP-OES) and thermogravimetric analysis

(TGA) to establish the stoichiometry, and infrared spectroscopy (IR) was used to elucidate the

starting oxidation state of the nitrosyl. This chapter will give detailed information about the

materials and methods used.

2.1. Materials for Synthesis

All starting materials were used as purchased. Sodium nitroprusside dihydrate,

Na2[Fe(CN)5(NO)]·2H2O, copper nitrate trihydrate, Cu(NO3)2·3H2O, manganese nitrate

tetrahydrate, Mn(NO3)2·4H2O, and nickel nitrate hexahydrate, Ni(NO3)2·6H2O, were acquired

from Sigma-Aldrich, while iron chloride tetrahydrate, FeCl2·4H2O, was from Honeywell Fluka.

All materials had a purity ≥99%.

2.2. Method

2.2.1. Sample Preparation

Synthesis of the TM-NPs proceeded by a controlled aqueous co-precipitation method between

sodium nitroprusside (NaNP) and the nitrate or chloride salt of the target TM. Each salt,

including NaNP, was weighed out and dissolved in 50 mL distilled water to give a 50 mM

aqueous solution, with the exception of MnNP that required 100 mM of both salts for

precipitation to occur. It is known from previous studies that the synthesis of Mn has difficulties

because of the high solubility of the product in water.56,83 In the synthesis of FeNP, HCl was

added to each solution until the pH was 3 (around 1 mM HCl) to prevent Fe(OH)2 impurities.

After the salts had been dissolved, NaNP and the respective TM solution were simultaneously

16

added into 100 mL distilled water (giving a total ionic concentration of 12.5 mM in all cases

except for Mn2+ where it was 25 mM) at 4 mL/h, using a programmable syringe pump (NE-4000

Two Channel Syringe Pump). By adding the solutions dropwise, and by keeping the

concentration low, the ions are given more time to crystallise which can reduce nitroprusside

and TM vacancies. The reaction vessel was also kept under an inert (N2) atmosphere and

constant stirring. The mixed solutions were allowed to react and rest overnight.

The resulting precipitates were washed and decanted two times with water and one time with

ethanol, before being dried at 70 °C under vacuum for 12 h. Part of each powder was further

dried at 100 °C in a vacuum oven inside a glove box, for 20 h, to fully dehydrate the structure.

Figure 2.1 shows the setup for the controlled co-precipitation and the dried powders that were

obtained.

Figure 2.1: Pictures of synthesis setup (left) and dried powders (right).

2.2.2. Material Characterisation

2.2.2.1. Powder X-ray Diffraction (XRD)

The crystallographic structure and purity of the powders were determined with XRD, one of

the most powerful and frequently used tools to study a materials average bulk atomic structure.

The technique is based on analysing X-rays that have been irradiated on a sample and elastically

scattered by its electrons. The scattered waves constructively interfere at specific scattering

angles as determined by the unit cell dimensions and its symmetry. These are usually plotted as

a function of the experimentally measured scattering angle, 2θ, that are specified with the Bragg

equation84:

𝑛𝜆 = 2𝑑ℎ𝑘𝑙𝑠𝑖𝑛𝜃

17

Here λ is the wavelength of the incident radiation, n is an integer ≥1 that represents the harmonic

order of the diffraction, θ the scattering angle, and dhkl the interplanar distance of a set of parallel

crystallographic planes. The intensity, however, is determined by the structure, i.e., the atom

identities, fractional occupancies, thermal or positional displacements, and relative positions.

XRD is therefore a useful tool to identify the purity and structure of a sample.

Samples were prepared for laboratory XRD by filling the powders into 0.3 mm glass capillaries

in an argon-filled glovebox. The measurements were performed in transmission mode on a Stoe

& Cie GmbH Stadi X-ray powder diffractometer equipped with a Ge monochromator

(single-wavelength Cu Kα1). The scattered radiation was detected by a Mythen 1 K Si strip

detector in sweeping mode with an angular resolution of 2θ = 0.015°. In this thesis, X-ray

diffraction was used to check the purity and extract unit cell dimensions. Thus, Le-bail fits85

(no structural modelling) were performed in JANA200686 to index the pattern, determine

symmetry, and refine cell parameters.

2.2.2.2. Thermogravimetric Analysis (TGA)

TGA is an analytical technique in which the thermal evolution of a material is determined by

measuring the change in mass upon heating or cooling. In practice, this means that the mass of

a few mg of material is measured from inside a furnace while under a controlled temperature

and atmosphere. With this simple set-up, the instrument can be used to follow the thermal

decomposition of a material or reaction with the environment, or to quantify the loss of volatile

components.87

In this work, TGA was used to quantify the amount of crystalline water in the synthesized

samples and to determine the temperature onset of dehydration and decomposition temperature.

For the measurements, ca. 20 mg of sample was weighed in an alumina crucible and placed in

a top loaded Netzsch STA 409 thermal analyzer. Each sample was then individually heated

from 30 to 500 °C under flowing Ar (60 mL/min), at a ramp rate of 5 °C/min. The amount of

crystalline water was calculated by converting the mass loss percentage to gravimetric units

and dividing it by the molecular mass of water. The errors were estimated to 5% based on

previous measurements.

18

2.2.2.3. Inductively coupled plasma-optical emission spectroscopy (ICP-OES)

The cation ratios and sodium content of the samples were analysed by ICP-OES, which is an

accurate tool for determining metal ion concentrations in a compound. With this technique,

samples in solution are ionised in a radiofrequency-induced argon plasma and promoted to an

excited state. Once the ions relax to their ground states, the elements in the samples are

identified by measuring the characteristic wavelengths of the emitted photons. The total number

of photons is detected either in a radial or axial mode and is directly proportional to the

concentration of the elements in the original sample. Solid samples require extraction or acid

digestion to release the elements into a solution before being measured.88,89

ICP-OES measurements were made in a PerkinElmer ICP-OES Avio 200 system in attenuated

radial mode at a sample flowrate of 1 mL/min. Approximately 5 mg of each powder was

prepared by burning away the organic ligands in an oven at 500 °C for 500 min. The remaining

oxides were then dissolved in 3.75 mL of an HNO3:H2SO4:HCl (1:1:3 v/v) solution (ICP grade)

and diluted until the metal ion concentrations were around 10 µg/mL using a solution of 5 vol%

HNO3 in ultrapure Milli-Q water (blank). Before being measured, the solutions were all filtered

through a nylon membrane (VWR Syringe filter) with a 0.2 µm pore size. The metal ion

concentrations were calculated based on a standard provided by PerkinElmer, Pure Plus

Multi-Element Calibration Standard 3. The errors were estimated to 2% based on previous

measurements.

2.2.2.4. Infrared Spectroscopy (IR)

IR spectroscopy is a technique that measures molecular vibrations in a compound by detecting

their characteristic absorption of infrared light. To record a spectrum, a beam of IR light is

transmitted through the sample at different wavelengths, and the intensity of transmitted light

measured. Examining the frequency and intensity of the transmitted or absorbed light reveals

the vibrational frequency of the bonds and the relative proportion of modes absorbing that

energy. Hence, both qualitative and quantitative information is obtained about the bonds that

are present.90 Stronger bonds, i.e., bonds with a higher electronic density in bonding orbitals,

vibrate at higher frequencies. These frequencies are lowered if electrons begin occupying the

antibonding orbitals, weakening the bonds. Consequently, a higher number of electrons in the

antibonding orbitals results in further weakening of the bonds and thus even lower stretching

frequencies. As was discussed in Section 1.4.3 Redox Activity of the Nitrosyl, the LUMO in the

NP anion has an energy close to, and overlaps with, the antibonding π* orbital in the

19

nitrosyl.63,76 Thus, when the negative charge on the NO-ligand increases, as when the nitrosyl

is reduced, the bond within the NO-ligand weakens and vibrates at a lower frequency.77

Although it is difficult to determine the exact oxidation state on the iron and nitrosyl,

respectively, IR spectroscopy can still be used to give an indication of the oxidation states and

was therefore used on the synthesised TM-NPs.

IR measurements were performed over 5000–350 cm-1 on a Perkin Elmer Frontier instrument

with the attenuated total reflection (ATR) accessory, GladiATR. Powder samples were pressed

against the face of a single crystal (ATR crystal) and irradiated with IR light. 100 spectra were

collected for each sample to improve the data quality.

2.2.3. Electrochemical Evaluation

In battery research, the most common technique used to evaluate a materials electrochemical

performance is galvanostatic cycling. For this method, a constant current, positive when

charging and negative when discharging, is passed through the battery cell while recording the

voltage as a function of the total current passed. Multiplying the total current with the time it

was flowing gives the capacity of the cell. Upper and lower voltage cut-offs are set so that

specific electrochemical processes within that voltage range are probed, and the total current

consumed by them can be measured. The current used in the experiment is typically reported

in mAg-1 or in terms of C-rate. The latter is a material specific value that describes the time

required to reach a defined theoretical capacity during charge or discharge. For example, C/20

describes reaching theoretical capacity during (dis)charge in 20 h.91

There are many different cell designs that all have their specific advantages.91 One very

common cell for testing new materials is the Swagelok design (Figure 2.2). As can be seen in

the Figure, the cell consists of conductive cylinders that are pressing together the cell

components along with a spring and fittings. An insulating layer covers the inside of the outer

cylinder to avoid shortage. This cell design is quick to assemble and only requires a few mg of

material. It is therefore suitable for testing materials from small syntheses or with an unknown

behaviour that might require the building of many cells. Subsequently, this was the chosen

design for electrochemical evaluation of the TM-NPs.

20

Figure 2.2: Swagelok battery design in (a) exploded view, (b) assembled view, and (c) simplified schematic.

In this work, galvanostatic cycling was performed in 12 mm diameter Swagelok cells. The

dehydrated TM-NP powders were mixed with 20 wt% conductive additive (C-NERGY SUPER

P Conductive Carbon Black) using a mortar and pestle in an argon-filled glovebox. During

assembly, ca. 5 mg of powder mixture was transferred into an upright standing Swagelok cell

and placed directly onto the bottom aluminium cylinder. Two layers of glass fibre separator

(13 mm) were then gently placed on top of the powder and soaked with 150 µL of electrolyte.

A standard electrolyte, 1 M NaPF6 (Stella) in ethylene carbonate/diethyl carbonate (EC/DEC),

was used in excess to avoid capacity limitations due to electrolyte decomposition. A 12 mm

metallic sodium disk (Aldrich 99.9% trace-metal basis) was placed on top of the soaked

separator followed by an aluminium disk and a spring. The cylinders were lastly pressed

together and tightened in place with a wrench. Cycling was performed on a NEWARE

BTS4000 galvanostat for five cycles, with a current rate of C/20, between 4.2 and 0.25 V versus

Na/Na+ for all TM-NPs. CuNP was further cycled over 3.8 to 0.6 V and 3.8 to 1.5 V to explore

the reversibility within different voltage windows. The amount of sodium corresponding to a

certain capacity for each compound per formula unit (Na+/FU) was determined by the following

equation. The theoretical capacity and sodium ions are given in section 3.3 Oxidation State of

the Nitrosyl and TMs.

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑎+ =𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑

𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦∙ 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑎+

21

3. Results and Discussion

As the structure and composition of TM-NP analogues vary with preparation method, thorough

characterisation of all synthesised materials needed to be performed before electrochemical

cycling in Na-ion cells. The results from the synthesis, including structure, composition, and

oxidation state will thus be presented first, followed by a detailed discussion on their cycling

behaviour.

3.1. Structure and Purity

Symmetry, unit cell dimensions, and purity of the synthesised powders before (H) and after

dehydration (DH) were determined by fitting a model to each XRD pattern and refining the cell

parameters by the Le-bail method85. The resulting diffractograms and assigned symmetry for

each sample is presented in Figure 3.1. No peaks other than the ones corresponding to the

assigned phases were observed for any of the compounds, suggesting that no crystalline

impurities were present in any of the samples. As expected from the synthesis (see section

1.4.2 The Various Structures of TM-NPs), FeNP and NiNP crystallised in the cubic (Fm-3m)

structure while MnNP crystallised as an orthorhombic dihydrate with Pnma symmetry. CuNP

on the other hand, was expected to crystallise as a two-dimensional orthorhombic dihydrate

with Amm2 symmetry and convert to the tetragonal I4mm structure upon dehydration. However,

the I4mm symmetry structure was observed in both the hydrous and anhydrous sample,

indicating that the material underwent partial dehydration at 70 °C under vacuum.

22

Figure 3.1: X-ray diffractograms of hydrated (H) and dehydrated (DH) (a) Fe[Fe(CN)5NO], (b) Ni[Fe(CN)5NO],

(c) Mn[Fe(CN)5NO] and (d) Cu[Fe(CN)5NO]. Some of the major reflection’s indices are indicated.

As the quality of fit for both CuNP samples were lower than the others, a closer inspection was

made. It was observed that every direction with a contribution from the c-direction was broader

than the other directions, indicating anisotropic broadening. To account for this, anisotropic

strain broadening was included along the ⟨001⟩ direction during the Le-bail fitting process,

which as can be seen in Figure 3.2 improved the model. Broadening can indicate a higher degree

of disorder which implies structural irregularity in the c-direction. As mentioned, in the

hydrated phase CuNP arrange in two-dimensional layers that are stacked in an off-set way along

the c-direction. During drying every second of these layers shift within the a-b plane by half a

unit cell length (a/2) in the a-direction to a new alignment along the c-direction (Figure 3.2(a-c)),

while the relative positions of the atoms within the a-b plane does not change. If this process is

incomplete there will be a higher degree of disorder along the c-direction. Therefore, the

observed anisotropic broadening indicates that the dehydration process was either incomplete

or imperfect. It is not certain what effect this has on the electrochemical performance, but the

degree of disorder in layer stacking has been reported to impact electrochemical performance

in other layered phases.92,93 Thus, it may warrant closer inspection in the future and subsequent

optimisation of the drying conditions.

23

Figure 3.2: Structural changes in Cu[Fe(CN)5NO] during dehydration (a-c) and illustration of the anisotropic

broadening in the XRD pattern by comparison of the model (red line) with the data (black dots) in the Le-bail

fitting process before (a) and after (b) adding anisotropic strain broadening along ⟨001⟩ to the fit.

Table 3.1 presents the resulting cell parameters from the Le-bail fit and corresponding volume

change on dehydration. The cubic compounds, i.e., FeNP and NiNP, exhibit an isotropic

volume change of ~2% as the interatomic distances M–N and Fe–C shortens when water is

evolved.70 MnNP, on the other hand, shows an anisotropic change in cell parameters with a

reduction in the a and b unit cell directions while the c-direction increases. One possible

explanation for this is that the loss of water results in stronger interactions between the

equatorial ligands and the TMs but weaker interactions between the TMs and the axial ligands,

but there may be other explanations as well. What implications this has for battery cycling is

uncertain, although, a similar anisotropic change in cell dimensions could be expected from

sodium insertion. This anisotropic change can lead to a reduced total change in volume (<1%)

compared to if it had been isotropic, presenting an opportunity to avoid large volume changes

during cycling and undesirable particle cracking. Finally, a slight volume change was observed

in CuNP even though both samples were found to have the anhydrous tetragonal (I4mm)

24

symmetry, which agrees with the previous suggestion that some water was still present in the

first CuNP sample. When the remaining water evolved from the structure a decrease was

observed in the c-direction, since completely removing the water allows stronger interactions

between the layers in the a-b plane.

Table 3.1: Cell parameters and volume of synthesised materials obtained from Le-bail fitting with corresponding

goodness of fit (GOF). In all materials α=β=γ=90°. The volume change percentage represents the reduction in cell

volume of the dehydrated (DH) material.

Compound Cell Parameters (Å) Volume (Å3)

Volume change

(%) GOF

FeNP_H a=b=c=10.3508(4) 1108.962

2.1 1.26

FeNP_DH a=b=c=10.2774(3) 1085.563 1.50

NiNP_H a=b=c=10.1898(1) 1058.031

2.0 1.69

NiNP_DH a=b=c=10.1210(2) 1036.738 1.85

MnNP_H

a=14.1264(4),

b=7.5261(2),

c=10.5549(3)

1122.155

0.71

1.24

MnNP_DH

a=13.7949(5),

b=7.3874(2),

c=10.9330(3)

1114.168 1.34

CuNP_H

a=b=7.1193(2),

c=10.9188(4) 553.4100

0.096

1.96

CuNP_DH

a=b=7.1199(2),

c=10.9065(4) 552.8812 2.08

3.2. Composition

Understanding the composition in TM-NPs is critical to estimating the initial oxidation states,

which ultimately determines the theoretical capacity. Thus, determining y and z in

M[Fe(CN)5NO]1-y ·zH2O is necessary. Cation ratios are particularly important for TM-NPs as

it determines the number of TM or NP vacancies in the material, which in turn determines the

oxidation states of all the species present and what redox reactions to expect. Furthermore,

using the known cation ratios it is possible to determine the relative crystalline water content

(z) from the mass loss in TGA. Measuring the water content is vital to understand the conditions

under which the material can be fully dehydrated and maintained in the dehydrated state. Thus,

compositions were determined through a combination of ICP-OES for cation ratios and remnant

25

sodium content and TGA for the water content. The results are summarised and compared with

previous studies in Table 3.2.

Elemental analysis of the TM-NPs revealed that the compositions of FeNP, MnNP and CuNP

were Fe[Fe(CN)5NO]NA· zH2O, Mn[Fe(CN)5NO]0.98(2)· zH2O and Cu[Fe(CN)5NO]0.89(2)· zH2O,

respectively. Thus, none of the materials showed any signs of remnant sodium from the

synthesis. The cation ratio in FeNP could not be discerned since the two Iron ions in the

structure cannot be differentiated from each other via ICP-OES. Unfortunately, NiNP was

unable to be dissolved into solution and so was not analysed.

As Figure 3.3 shows, the TGA indicates that all samples slowly begin losing water at 50 °C and

are completely dehydrated around 150 °C, or in the case of NiNP around 180 °C, similar to

previous studies.56,94,95 The water content calculated from the weight loss in the TGA curve

between these temperatures corresponds to 4.0(2) (FeNP), 4.2(2) (NiNP), 1.9(1) (MnNP), and

0.46(2) (CuNP) water molecules per formula unit. In all cases, lower values were obtained than

expected from the reference studies.56,58,68,70 This could potentially be due to differences in

nitroprusside or TM vacancies as higher vacancies lead to increased water content.96

Alternatively, it may indicate that all samples were partially dehydrated in the first drying step,

i.e., at 70 °C under vacuum for 12 h. The largest difference between the synthesised and

reference structures is seen for CuNP that contained approximately a fourth of the expected

water content, which would explain why only the anhydrous space group symmetry was

observed in XRD. Once formed, the tetragonal CuNP remains stable even in ambient conditions

and only restore the initial orthorhombic structure after at least a week immersed in water.95

This behaviour is highly unusual for a TM-NP or PBA, but makes handling of the dehydrated

material much easier. After dehydration when heated to higher temperatures, all samples very

slowly begin to decompose with the evolution of first nitrogen oxide (NO) and then cyanogen

gas (C2N2) up until 260 °C, after which they decompose to M[Fe(CN)4] with further gas

evolution.94,95 MnNP, on the other hand, remains stable up to above 300 °C.56 Thus, the window

where the anhydrous material is stable is much larger for MnNP compared to the other

compounds in this study. The origin of the increased stability remains unknown.

26

Figure 3.3: Mass loss in Fe[Fe(CN)5NO] (FeNP), Ni[Fe(CN)5NO] (NiNP), Mn[Fe(CN)5NO] (MnNP) and

Cu[Fe(CN)5NO] (CuNP) upon heating from 30 to 500 °C when measured by TGA. The samples were heated at a

ramp rate of 5 °C/min under flowing Ar (60 mL/min).

IR (Figure 3.4) can give us insight to the water content and bonding in TM-NPs, thus supporting

and building upon the TGA results above. Water exhibits many different peaks depending on

its coordination with respect to other water molecules or polar groups and the types of

interactions between them. The most characteristic spectral features include a broad band of

stretching frequencies extending between 3600–3000 cm-1 attributed to a variety of hydrogen

interactions between water molecules, several sharp peaks around 3650 cm-1 and 1600 cm-1 due

to symmetrical stretching and bending of the water molecules, respectively, and a shoulder or

peak around 800 cm-1 attributed to the liberational modes of coordinated water, i.e., small

rotations about their preferred orientation. Thus, as observed in Figure 3.4 the cubic structures

(FeNP and NiNP) have much more water present in their structures than MnNP and CuNP,

which agrees with the XRD and TGA results. The water in FeNP and NiNP is also interacting

more with each other as evident by the broad band between 3600–3000 cm-1, which suggests

that hydrophilic pores could be present as described in other studies.65 This band is much

smaller in MnNP as the number of water molecules are reduced from 4–5 to two. For the same

reason, the intensity of the vibration corresponding to the liberation of water is reduced in

MnNP compared to both cubic structures. Overall, there is a clear decrease in water vibrations

for all three compounds after dehydration as expected, without any noticeable change in the

other peaks, indicating that the dehydration in vacuum at 100 °C for 20 h was effective. CuNP,

on the other hand, only showed a hint of OH stretching and bending, and was thus almost fully

dehydrated in both cases, which agrees with the XRD and TGA results.

27

Table 3.2: A summary of stoichiometry, water content and structure of synthesised samples given by ICP, TGA and XRD compared to reference structures. Errors are given

as one standard deviation from the measured value and has been calculated from the relative standard deviation given for each sample by the instrument.

Sample Space

group

Water

content

Sodium

content

Fe:M

ratio Final structural formula

Reference structure

Space group Structural formula Type of sample Ref.

Hydrated

FeNP Fm-3m 4.0(2) 0.00(0) NA Fe[Fe(CN)5NO]NA · 4.0(2) H2O Fm-3m Fe[Fe(CN)5NO] · 4.78 H2O Powder 70

NiNP Fm-3m 4.2(2) NA NA Ni[Fe(CN)5NO]NA · 4.2(2) H2O Fm-3m Ni[Fe(CN)5NO] · 4.92 H2O Powder 70

MnNP Pnma 1.9(1) 0.00(0) 0.98(2):1 Mn[Fe(CN)5NO]0.98(2) · 1.9(1) H2O Pnma Mn[Fe(CN)5NO] · 2 H2O Single crystal 56

CuNP I4mm 0.46(2) 0.00(0) 0.89(2):1 Cu[Fe(CN)5NO]0.89(2) · 0.46(2) H2O Amm2 Cu[Fe(CN)5NO] · 2 H2O Powder 58

Dehydrated

FeNP Fm-3m 0.00(0) NA Fe[Fe(CN)5NO]NA Fm-3m Fe[Fe(CN)5NO] Powder 70

NiNP Fm-3m NA NA Ni[Fe(CN)5NO]NA Fm-3m Ni[Fe(CN)5NO]* Powder 70

MnNP Pnma 0.00(0) 0.98(2):1 Mn[Fe(CN)5NO]0.98(2) Pnma Mn[Fe(CN)5NO] Powder 68

CuNP I4mm 0.00(0) 0.89(2):1 Cu[Fe(CN)5NO]0.89(2) I4mm Cu[Fe(CN)5NO] Powder 58

* No anhydrous reference structure was found; thus, fitting was done after the hydrated structure.

28

3.3. Oxidation State of the Nitrosyl and TMs

IR spectra were collected for all samples to determine the initial oxidation state of the Nitrosyl.

The results, together with the results from ICP-OES, were subsequently used to deduce the

oxidation state of the TMs. Spectra for each compound and a summary of important vibrations

are presented in Figure 3.4 and Table 3.3, respectively. The assignments are based on the work

of Benavente et al.56.

The oxidation state of the nitrosyl was determined by the nitrosyl stretching frequencies,

denoted ν(NO). Generally in TM-NPs, this frequency is around 1940 cm-1 for NO+ but

decreases in frequency, by ca. 100–200 cm-1 for more negative oxidation states, when the

electron density increases.56,80,97 In this work, all samples had a nitrosyl stretching frequency

between 1940 and 1945 cm-1 and subsequently a positively charged nitrosyl ligand. Given the

composition, the known NO oxidation state, and that the cyano ligands always have a negative

(1-) charge,98 the sum of charges on both TMs must add up to 4+. How that charge is distributed

over FeC and the TM, however, depends on the presence of charged guest species and vacancies.

For example, in the CuNP synthesised by Mullaliu et al.54 where the Fe:Cu ratio was 1.2:0.8,

Fe had an oxidation state of (2+δ)+ to charge compensate for the Cu vacancies, while it has a

state of 2+ in similar compounds where the TM vacancies are low.63 Therefore, based on the

determined cation ratios of the TM-NPs synthesised herein, FeC must be 2+ which means that

the TMs are also in a 2+ state unless there are NP vacancies. In the case where the TM can have

an oxidation state lower than 2+, as in CuNP, it is possible for the TM to charge compensate

for missing NP anions. Mn can also adopt a 1+ charge, though it is much less stable than 2+ to

4+.99 This may explain why CuNP has a higher tendency for NP vacancies compared to MnNP

but must be confirmed with other techniques. The initial oxidation states of each material were

hence estimated to Fe2+[Fe2+(CN)5NO+], Ni2+[Fe2+(CN)5NO+], Mn2+[Fe2+(CN)5NO+]0.98(2), and

Cu(2-δ)+[Fe2+(CN)5NO+]0.89(2). This entails a maximum theoretical capacity of approximately

197, 195, 198, and 277 mAh/g for FeNP, NiNP, MnNP and CuNP, respectively, corresponding

to Na2Fe2+[Fe2+(CN)5NO-], Na2Ni2+[Fe2+(CN)5NO-], Na2Mn2+[Fe2+(CN)5NO-], and

Na(2.8-δ)Cu+[Fe2+(CN)5NO-]0.89.

29

Figure 3.4: Full IR spectra, from 5000–350 cm-1, of hydrated (H) and dehydrated (DH) samples of

(a) Fe[Fe(CN)5NO] (FeNP), (b) Ni[Fe(CN)5NO] (NiNP), (c) Mn[Fe(CN)5NO] (MnNP), and (d) Cu[Fe(CN)5NO]

(CuNP). The inset in each spectra shows a zoom in of the Cyano and Nitrosyl stretching vibration region.

30

Table 3.3: Summary of important infrared active vibrations observed in hydrous Fe[Fe(CN)5NO] (FeNP), Ni[Fe(CN)5NO] (NiNP), Mn[Fe(CN)5NO] (MnNP), and

Cu[Fe(CN)5NO] (CuNP) measured in attenuated total reflection mode. Assigned by help of reference 56.

Spectral

Feature

Position (cm-1)

Assignment Group/Structure

FeNP NiNP MnNP CuNP

Peaks 3650 3654 3649 (3649) ν(OH) symmetric Water

Band 3600–3000 3600–3000 – – ν (H–O···H) Water

Peak 2179; 2149; (2080) 2194; 2149 (2194) ; (2188); 2174 2207; 2193; (2165) ν(CN) Cyano ligand

Peaks 1941 1942 1945 1940 ν(NO) Nitrosyl ligand

Peak 1616 1616 1656; 1602 (1604) δ(H–O–H) Water

Shoulder Ca. 800 Ca. 800 770 – L(H–O–H) Water

Peaks >680 >680 >680 >680 ν/δ(M–C), ν/δ(M–N),

ν/δ(M–CN), …

Various Metal-ligand

vibrations

Abbreviations: ν, stretching; δ, bending; L, liberation

31

3.4. Galvanostatic Cycling Behaviour

As the intent of this study was to investigate the cycling behaviour of different TM-NPs in

NIBs, all dehydrated samples were galvanostatically cycled in Na half-cells between 4.2 and

0.25 V versus Na/Na+. The resulting cycling curves exhibited varied and complex behaviour.

Thus, to facilitate comparison and rationalisation for the obtained results, discussion will first

be focused on similarities and differences in the first one and a half cycles. This will be followed

by a discussion of changes which occur in the following 5 cycles. Finally, due to the large

difference in behaviour, this section will end with a deeper discussion of the CuNP behaviour.

The resulting galvanostatic cycling (Figure 3.5 and Figure 3.6) shows four distinct behaviours

for the compounds with the exception of one major common characteristic. That is, all materials

exhibit a sloping plateau below 1.0 V (1.3 V in CuNP) to the lower cut-off voltage during

discharge and another plateau above 3.1 V to the upper cut-off voltage during charging, both

corresponding to a significant amount of capacity. Similar behaviour was seen by Mullaliu et

al.54 when cycling CuNP against lithium metal. Using operando XAS and IR, they

demonstrated that the sloping region below 1.8 V versus Li/Li+ (corresponding to ca 1.4 V

versus Na/Na+) was caused by reduction of the nitrosyl. Unfortunately, in this study the redox

processes occurring in the higher voltage plateau during charging were not addressed in the

text. However, their results from operando IR showed a reappearance of the vibration

corresponding to stretching of the NO+ bond at 3.9 V versus Li/Li+ (corresponding to 3.5 V

versus Na/Na+). Thus, it was also shown that the nitrosyl was oxidised during charging to this

potential. It is likely that this was overlooked because of the unusually large voltage

polarisation. As will be discussed later in this thesis, however, voltage polarisation is not

uncommon for systems with anionic redox. Their results therefore suggest that the slopes

observed herein, below 1.0 V (discharge) and above 3.1 V (charge) versus Na/Na+, can be

attributed to reduction and oxidation of the nitrosyl ligand, respectively.

An additional common feature for all samples was the observed oxidation of FeC. When iron is

carbon bound as in the NP anion, it has a redox potential around 3.0 V versus Na/Na+, which is

lower than that of the predicted nitrosyl oxidation potential.54 Thus, while FeC is not redox

active during the first sodium insertion, the high polarisation on the nitrosyl during charging

leads to the oxidation of FeC from 2+ to 3+, allowing the transition metal to contribute to the

redox activity on subsequent cycles. This phenomenon should be observed in all studied

TM-NPs. While a plateau at 3.0 V is observable in FeNP and CuNP, it is more subtle in NiNP

and MnNP where only a small plateau at 3.4 V and a hint of a plateau at 3.5 V can be observed

32

respectively. Therefore, demonstrating that the M’ cation may also impact the redox behaviour

of the nitroprusside structural fragment. In CuNP, Mullaliu et al.54 confirmed both Cu+/Cu2+

reduction around 2.3–2.9 V versus Li/Li+ (1.9–2.5 V versus Na/Na+) on discharge and Fe2+/

Fe3+ oxidation around 3.3–3.5 versus Li/Li+ (2.9–3.1 V versus Na/Na+) on charging with

operando XAS, which confirms the hypothesis that FeC indeed is redox active. Given the

asymmetric redox behaviour and complexity as a function of composition, the exact redox

process will need to be confirmed by operando XAS and IR in a future study. Specifically, to

answer if FeC is solely responsible for the redox activity within a certain potential window or if

both FeC and the nitrosyl are simultaneously active? Does FeC retain its redox activity on

subsequent cycles or is there a charge transfer process which occurs once all sodium is removed

from the structure? Finally, further work would need to be performed to understand the role of

the M’ cation on the redox activity of the nitroprusside. Although for the latter, some insight

can be gained by comparison with their hexacyanoferrate analogues.

By comparing the first one and a half cycles in Figure 3.5 with the redox potentials for the

equivalent hexacyanoferrate analogues, it can be inferred that all TMs apart from nickel are

redox active. Nickel has lately been generally accepted as a non-active TM in similar materials

and is therefore considered to not partake in any redox activity herein.100,101 Manganese on the

other hand, has a Mn2+/Mn3+ redox potential at 3.8 V and can thus be observed as a plateau at

this potential in the middle of the nitrosyl oxidation slope on charging.102 Nitrogen bound iron

(FeN) has a redox potential around 3.0 V, i.e., close to the same potential range as FeC in this

case.36 It is therefore impossible to know whether the plateau in FeNP at 3.0 V is caused by

oxidation of FeC or FeN or both simultaneously without further measurements. Mössbauer

spectroscopy is a tool which can readily distinguish between two iron centres and can probe the

variance in local environment and oxidation state for two distinct Fe ions, and could therefore

be used to analyse this in a future study.42 Moving on, the Cu+/Cu2+ redox potential can be

assigned to the plateau at 2.1 V during charge and discharge by comparison with results from

Mullaliu et al.54. Interestingly, Mullaliu et al. saw two different potentials that they attributed

to Cu reduction (2.3 V and 2.9 V versus Li/Li+, respectively), which they believed corresponds

to two different Cu sites within the structure. They also observed a partial reduction of the FeC

from Fe(2+δ)+ to Fe2+ in the first discharge. Neither the two copper plateaus nor iron reduction,

however, can be observed in the results presented here, probably because of the difference in

composition between our samples. While they had copper vacancies (cation ratio 0.8:1.2 Cu:Fe)

33

that needed to be charge compensated for by the iron, our sample had NP vacancies (cation

ratio 1.0:0.89 Cu:Fe) that “locked” the iron into its Fe2+ state during the first discharge.

Figure 3.5: First one and a half cycles of galvanostatic cycling of (a) Fe[Fe(CN)5NO] (FeNP) and Cu[Fe(CN)5NO]

(CuNP), and (b) Ni[Fe(CN)5NO] (NiNP), and Mn[Fe(CN)5NO] (MnNP), in Na half-cells between 4.2 and 0.25 V.

To compare how each TM-NP evolves over time during cycling, the first five cycles are plotted

in Figure 3.6. NiNP and MnNP both have a very similar cycling behaviour that is dominated

by the nitrosyl activity above 3.1 V and below 1.0 V. NiNP has an initial discharge capacity of

80 mAh/g, but only 38 mAh/g (corresponding to 0.39 Na+/FU, i.e., less than a fourth of the

theoretical amount) is reversible, indicating that the additional first 42 mAh/g might be a result

of side reactions. After the first cycle however, each new cycle almost traces the others without

any loss of capacity. Since nickel is inert, this shows that the nitrosyl redox process is highly

reversible within this operating voltage. MnNP achieves a lower initial capacity compared to

NiNP but gains a similar reversible capacity around 43 mAh/g, corresponding to 0.43 Na+/FU.

It is interesting to note that the discharge capacity increases for each cycle after the second for

MnNP, although no gains are had on charging. This implies side reactions but could perhaps be

an indication of an increasing participation of the manganese ion, i.e., Mn2+/Mn3+ or Mn+/Mn2+

reduction. However, it is impossible to draw any conclusions without further evidence.

Surprisingly, FeNP shows a much higher reversible capacity than what can be predicted

theoretically. That is, 350 mAh/g compared to 197 mAh/g corresponding to 3.5 Na+/FU instead

of 2. What causes this increase is difficult to postulate. It could be side reactions, but since the

reversibility is so high after the first cycle, another phenomenon caused by the redox of one or

both the iron atoms is more likely. We do, for example, see an increased capacity from the

plateau at 3.0 V in the three last charges compared to the second (i.e., first after discharge),

which implies their involvement. Additionally, new plateaus not visible in any of the other

34

TM-NPs also appear after a few cycles, namely the small charge plateau at 0.8–0.9 V and a

minor plateau at 2.0 V. The plateau at 0.8–0.9 V could be partial oxidation of the nitrosyl but

in that case raises the question of why it is not observed in any of the other compounds. The

latter only appears after three cycles and cannot be explained without further work. However,

it does present an intriguing possibility that the nitrosyl can be oxidised without the inherent

voltage polarisation. Note, that the unstable voltage variation at 3.5–3.8 V in the third charging

curve is likely the result of new material being exposed due to particle cracking. Aside from

this one voltage instability, FeNP shows high reversibility.

The electrochemistry of CuNP initially differs from the rest of the TM-NPs in this study but

becomes similar to the others after a few cycles. As also seen in NiNP and MnNP, the initial

capacity of CuNP is much higher than in the following cycles. As the initial capacity is almost

350 mAh/g, which can be compared to its theoretical capacity of 277 mAh/g, some of this

capacity could be attributed to side reactions. In the following cycles, the capacity decreases by

~100 mAh/g (1.0 Na+/FU) in the second and third cycle, and by further ~40 mAh/g (0.40

Na+/FU) per cycle in the fourth and fifth. This coincides with a reduced redox activity of both

the Cu (2.1 V) and Fe (3.0 V), leading to only nitrosyl and minor Fe redox after two to three

discharges. This implies that the capacity fade could be due to a loss of Cu, perhaps Cu

dissolution into the electrolyte, or material degradation. Simultaneous to the fading activity of

Cu, the reduction potential of the nitrosyl goes from a higher value (1.3 V) to the same potential

as in the other TM-NPs (<1.0 V), which suggests that the nitrosyl is more easily reduced when

the Cu is redox active. What causes this potential difference, however, and what role the

different TMs play in the reduction potential is yet unknown. Another unique and interesting

feature in this compound is the high discharge capacity corresponding to Cu reduction in the

first discharge. In theory, the reduction of all Cu2+ to Cu+ should correspond to 1 Na+/FU, but

instead the intercalation of 1.3 Na+/FU (125 mAh/g) is observed, followed by the same amount

of sodium leaving the structure when Fe is oxidised in the subsequent charging without any Cu

activity. This TM redox hysteresis is already known from the work of Mullaliu et al.54 but is

not what one would expect and has yet not been given an explanation. That is, the question

remains why Cu is not active during charging. There is therefore much more work needed to

figure out what happens in this complex system.

35

Figure 3.6: Galvanostatic cycling over the first five charge-discharge cycles of (a) Fe[Fe(CN)5NO] (FeNP),

(b) Ni[Fe(CN)5NO] (NiNP), (c) Mn[Fe(CN)5NO] (MnNP), and (d) Cu[Fe(CN)5NO] (CuNP), in Na half-cells

between 4.2 and 0.25 V. Dashed lines indicate the initial one and a half cycles, and a decreasing colour shade

indicates subsequent cycles.

In order to understand the origin of the irreversibility in CuNP, separate cells were cycled over

three different voltages windows. As seen in Figure 3.7, the resulting cycling profiles were

similar in all three cases but with one major difference, namely the degree of Cu reversibility.

When the voltage window was restricted from 4.2–0.25 V to 3.8–0.6 V or 3.8–1.5 V some of

the copper was oxidised on charging and subsequently became more reversible. As mentioned,

Cu is both reduced and oxidised between 1.9–2.5 V versus Na/Na+ and therefore the oxidation

plateau at 2.2–2.5 V observed herein is attributed to Cu.54 Interestingly, Cu seems to become

more reversible when the lower cut-off voltage is set above the nitrosyl reduction potential

(1.5 V), but for some reason this has no larger effect on the total capacity which fades to ⁓60–

75 mAh/g regardless of the voltage range. Additionally, even though Cu is more reversible in

the smaller voltage window (Figure 3.7(c)), it is mainly during discharge. The charging capacity

is still dominated by Fe and nitrosyl oxidation, as in the other voltage ranges. Observe that the

nitrosyl is here oxidised but not reduced which illustrates the hysteresis in the redox reactions

on charging compared to discharge that has previously been mentioned. This hysteresis

suggests that there must be some type of anisotropic charge transfer phenomena occurring in

36

the material which allows some of the electron density to be transferred between overlapping

orbitals and subsequently favouring oxidation or reduction of the various species depending on

the electronic state.

Figure 3.7: Electrochemical characterisation of CuNP over the voltage range (a) 4.2–0.25 V, (b) 3.8–0.6 V, and

(c) 3.8–1.5 V, given as a function of capacity for five cycles. Dark blue indicates the first cycle, and a decreasing

colour shade indicates subsequent cycles.

It is known that the NP anion itself has a strong anisotropic metal-to-ligand charge transfer

(MLCT) that favours donation of electrons from Fe to the nitrosyl ligand more than to the cyano

ligands.98 This does not necessarily mean that FeC always donates its electron density to the

nitrosyl, because in that case FeC would never be observed with a 2+ charge which it is.

However, it does entail that it is easier to oxidise the iron compared to the nitrosyl and easier to

reduce the nitrosyl compared to the iron. This might explain why FeC is only active on charging,

the redox hysteresis of the TMs on charge/discharge, and the large voltage hysteresis of the

nitrosyl in all investigated TM-NPs. If this is true, however, and how the MLCT is affected by

the M’ cation and the structure is currently unknown. Exploring the mechanism of MLCT in

various TM-NPs could thus provide some insight into the complex electrochemical behaviour

of the TM-NPs and perhaps give ideas on how to tune the behaviour and reduce the voltage

hysteresis.

37

Voltage hysteresis is not necessarily an unfavourable property, depending on the application,

but it is detrimental for an electrode material.103 It entails that more energy is required to charge

the battery than what is gained when discharging it, thus resulting in a reduced energy density.

Voltage hysteresis is observed in many materials, including several LTMOs and PAMs, and

spans a range from a few mV to V.104–106 However, the magnitude can depend on whether the

hysteresis observed is caused by extrinsic or intrinsic factors. Extrinsic polarisation is a kinetic

effect caused by a combination of different sources of resistance and current density, e.g., poor

diffusivity due to a rapid charge rate, large particle size, or thick electrodes.105,106 Consequently,

extrinsic polarisation can be modified and reduced to near zero by lowering the

discharge/charge rate to slow down the reactions or by minimising the particle size and the

electrode thickness. Intrinsic polarisation on the other hand, is a thermodynamic effect where

the reduction of a species is thermodynamically different to the oxidation, meaning that the

electronic energy levels change between the charged and discharged states. Thus, it is affected

by inherent material properties such as phase transitions, elastic stiffness, and stoichiometric

concentration.105,106 Intrinsic polarisation can therefore not be reduced by optimising the

cycling conditions, but needs to be identified and treated at a chemical or structural level for

the specific material. For example, often it needs more creative solutions like doping or

selective control of the composition. In electrodes with oxygen anionic redox for example, a

voltage hysteresis up to ⁓2 V associated with the redox activity of oxygen is common.78,79,93,103

Recently, however, a strong link between the superstructure ordering and the magnitude of the

voltage hysteresis was demonstrated, suggesting that some structures of the same compound

can have less hysteresis.79 Consequently, understanding the origin of the voltage hysteresis for

a specific system is critical to minimise it and to design materials with higher energy densities.

In the case of TM-NPs, the origin of the hysteresis is very likely intrinsic and associated with

the anisotropic MLCT.

38

4. Conclusion and Future Work

For a sustainable development of electrical energy storage systems, electrode materials for

NIBs based on earth abundant elements with high gravimetric and volumetric capacities must

be developed. In this work, four TM-NPs, NaxM’[Fe(CN)5NO]1-y ·zH2O, with M’=Fe, Ni, Mn,

and Cu were synthesised and cycled in Na-ion half-cell batteries to gain an initial understanding

of the cycling behaviour and to set up research questions to be pursued in the future. All four

compounds could be synthesised by a facile and controlled aqueous co-precipitation method at

room temperature, and were characterised in three different crystallographic structures, cubic

Fm-3m (FeNP and NiNP), orthorhombic Pnma (MnNP), and tetragonal I4mm (CuNP). Using

the Le-bail method85, a structural irregularity (anisotropic broadening) in the c-direction was

found in CuNP, which has previously not been documented. Herein we concluded that it was

caused by an incomplete or imperfect dehydration process leading to incomplete stacking of

CuNP layers along the c-direction. This phenomenon should be followed up in future studies

when exploring a potential correlation between structural order and electrochemical properties.

It was also concluded that MnNP has a higher thermal stability than the other TM-NPs under

investigation, which potentially could be caused by a lower vacancy content and higher

structural ordering. Finally, IR showed that the nitrosyl ligand had an initial 1+ state in all

compounds. Consequently, the oxidation states of all species were deduced to (M’)2+, FeC2+,

CN-, and NO+.

The resulting galvanostatic cycling curves exhibited varied and complex behaviour, but all

materials exhibited a plateau below 1.0 V on discharge and above 3.1 V on charging, which

based on previous studies could be attributed to reduction and oxidation of the nitrosyl ligand,

respectively. Subsequently, based on our observations and previous studies we propose that the

nitrosyl is electrochemically active in all TM-NPs studied herein, and that it contributes with a

considerable amount of capacity, although with a large voltage hysteresis. Thus, for the first

time, nitrosyl redox is observed in TM-NP battery systems beyond CuNP. Theoretically this

will allow increased gravimetric capacities compared to their hexacyanoferrate analogues.

Assuming reduction of NO+ to NO-, and (M’)2+ to (M’)+ where likely, the theoretical capacities

for the specific stoichiometries were calculated to 197, 195, 198, and 277 mAh/g for FeNP,

NiNP, MnNP and CuNP, respectively.

It can further be concluded that all TMs apart from Ni were redox active to various degrees and

that FeC was active on charging, but not discharge, in all compounds. Thus, a redox hysteresis

39

was also observed, meaning that different species were active on charging and discharging. As

it is known that the NP anion has a strong anisotropic MLCT, we propose that both the redox

hysteresis of the TMs and the voltage hysteresis of the nitrosyl is caused by anisotropic charge

transfer within the materials. However, even though the mechanism behind the MLCT in the

NP anion is known, it is yet unknown how it is affected by the M’ cation and the structure.

Exploring the mechanism behind the anisotropic charge transfer in different TM-NPs could

therefore provide insight into the complex electrochemical behaviour of these compounds and

how to reduce the voltage hysteresis. This can be addressed by monitoring the oxidation state

of the nitrosyl ligand and the TMs using IR spectroscopy and XAS respectively as a function

of charge to unravel the complex charge compensation mechanisms. Since large voltage

hysteresis in electrode materials is detrimental for the energy efficiency, this problem needs to

be solved before TM-NPs can be applicable in battery systems.

Moreover, an interesting and unexpected electrochemical behaviour was observed for FeNP

and CuNP. FeNP, to begin with, showed a much higher reversible capacity than what was

predicted. That is, 350 mAh/g compared to 197mAh/g corresponding to an intercalation of 3.5

Na+/FU instead of 2. Since an increasing involvement of both iron atoms were observed within

the first five cycles, we hypothesise that the unexpectedly high capacity is partly caused by a

phenomenon connected to the anisotropic charge transfer. Although the role of side reactions

will need to be explored. Additionally, new plateaus that are not observed in any of the other

TM-NPs appeared after a few cycles. This presents an intriguing possibility that the nitrosyl

can be oxidised in FeNP without the inherent voltage polarisation. CuNP, on the other hand,

exhibited a huge capacity loss in the first five cycles, which was unique among the studied

TM-NPs. Furthermore, it also exhibited a higher reduction potential for the nitrosyl (1.3 V)

compared to the other compounds (<1.0 V) and a reduced redox activity of Cu with each cycle.

When the Cu was no longer active, and the capacity had severely faded, the reduction potential

of the nitrosyl was lowered to the same potential as in the other TM-NPs. Thus, the capacity

fade in CuNP could be due to a loss of Cu, perhaps Cu dissolution into the electrolyte, or

material degradation. Whatever the reason, however, it was reduced when the voltage window

was restricted to a potential above the nitrosyl reduction.

Clearly much more work needs to be done to understand this class of materials but its many

interesting attributes such as anionic redox, MLCT, and structural diversity, proves that such

work could be fruitful for several research fields. For a battery application, however, the voltage

hysteresis needs to be understood and reduced before any future commercialisation of TM-NPs

40

can be realised. Even though this class of materials is structurally derived from the positive

electrode class of materials PBAs, perhaps they also should be investigated as negative

electrode materials due to the low reduction potential of the nitrosyl? Overall, though there are

challenges that needs to be overcome, TM-NPs can have an impact on the future development

of cheap and sustainable electrode materials for NIBs.

41

Acknowledgements

I greatly want to thank everyone who has been alongside me on this journey over the past few

months. Without their help and guidance, this work would not have been as enjoyable as it was.

Most of all I want to thank my supervisor, Dr William Brant, for all his advice and support. In

particular for pushing and challenging me to perform my best, and for believing I can do so.

Without him, writing this thesis would have been a lot easier, but the end not nearly as

satisfying.

I also want to thank: my subject reviewer, Dr Reza Younesi, for his general support and patience

while waiting for the final thesis; Dr Dickson Ojwang for teaching me how to run the synthesis

and helping me with some of my TGA measurements; Dr Rebecca Clulow for the rest of my

TGA measurements and for the great company she provided at the DESY synchrotron and in

Prague during IUCr2021 mid project; Dr Mikaela Görlin for showing me how to prepare and

run samples for ICP-OES; and Professor Gunnar Westin and Markus Ek for helping me run the

IR measurements.

Lastly, thank you to everyone in the structural chemistry program and the inorganic chemistry

program at the Ångström laboratory for your company and support.

42

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