1

DOI: 10.1002/cssc.201((will be completed by the editorial staff))

History effects in Li-O2 batteries – how initial seeding influences

the discharge capacity

Ali Rinaldi[a], Olivia Wijaya[a], Harry Hoster*[a,c], Denis Y.W. Yu[a,b,c]

Li-air batteries attract a lot of attention due to their high theoretical

energy density. These cells generate electrical power from the

transfer of Li+ ions from a low-potential anode towards a cathode

where they react with O2, forming Li2O2. The total electrical energy of

a Li-O2 cell is in principle determined by the amount of solid Li2O2

formed in the voids of the cathode structure during discharge. In

laboratory experiments, however, Li-O2 systems show “sudden

death” at capacities far below the theoretical ones. Recent studies

showed that hindered electron transport through the existing Li2O2

layer is an important factor that can indeed limit how much thicker

this layer can grow. In this paper, we will demonstrate that not only

the thickness but also the growth history of the product layer is

decisive for the discharge capacity. The discharge of a Li-O2 cell

through Li2O2 formation at a plain carbon cathode is performed in

two steps. First, seed layers equivalent to 400 mAh/g are formed

under potentiostatic conditions at increasing overpotentials, yielding

increasing roughness of the product layers. Subsequently, the

capacities of slow discharge at constant current are determined. The

discharge capacity attainable in this second phase is found to vary

by more than a factor of 3 depending on the history, i.e., the seed

layer. These findings are interpreted based on thorough correlation

with electrochemical impedance spectroscopy data and electron

microscopy images of the electrode morphology at different stages.

We discuss in how far our findings can be understood in terms of O2

transport and reactivity.

Introduction

The Li-air system is considered a promising candidate for next

generation batteries with improved energy density.[1–4]. Li-air

batteries generate electrical power from the transfer of Li+ ions

from a low-potential anode towards a cathode where they react

with O2 and take up an electron from the external circuit. It is

well accepted that Li2O2 is the main reaction product after

discharging in oxygen environment and stable electrolytes.[5]

Several groups have reported Li-O2 cells with high capacity

and good cycle stability.[4,6] Likewise, various solvents and

electrolytes have been tested.[7–13] The principle limit of the

discharge capacity of a Li-O2 cell is determined by the amount

of product that fits into the voids of the cathode structure. In

experimental studies, however, Li-O2 systems show “sudden

death” at capacities way below the theoretical expectations. To

some extent, chemical and electrochemical side reactions from

solvent and electrolyte decompositions are known to increase

the impedance of the cathode and thus also the voltage

drop.[9,10,13–15] Apart from such side reactions one has to

consider that the primary discharge product, Li2O2, will at some

point form the surface for the ongoing electrochemical reaction,

including the electron transport. A recent combined

experimental and theoretical study highlighted that the electron

transport through an Li2O2 layer that reached a certain

thickness can become too sluggish and thus give rise to a

significant voltage drop, the main symptom of “sudden

death”.[16]

It is not only the total amount of Li2O2 that determines whether

or not further discharge is possible. “Sudden death” tends to

occur at lower discharge capacity for higher current

densities.[3,17–19] Such poor rate capabilities were explained by

a deficiency of the reactants (Li+ and O2) near the active sites

at higher discharge rate.[5] This argument would be valid for

discharge at constant high rate, but it is too simple to explain

“history effects” such as the one illustrated in Figure 1: a fast

initial discharge leaves traces in the cathode that substantially

limit a further discharge at lower rates (see detailed discussion

more below). Such observations underline that high-rate

discharge induces an irreversible electrode passivation.

In this paper, we will illustrate and systematically quantify such

history effects in the Li-O2 system in a systematic manner.

Figure 1. Galvanostatic discharge of two carbon cathodes, one with a low

current density of 50 mA gc-1

and another with a two-step discharge with a

current density of 1000 mA gc-1

followed by 50 mA gc-1

.

[a] Harry E. Hoster,

TUM CREATE Centre for Electromobility

#10-02 CREATE Tower, Singapore 138602E-mail:

harry.hoster@tum-create.edu.sg

[b] Denis Y.W. Yu

School of Energy and Environment

City University of Hong Kong

Tat Chee Avenue, Kowloon, Hong Kong SAR

E-mail; denisyu@cityu.edu.hk

[c] Energy Research Institute at Nanyang Technological University

1 CleanTech Loop, #06-04 CleanTech One, Singapore 637141.

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/cssc.20xxxxxxx.((Please delete if not

appropriate))

www.chemsuschem.org

2

This will involve discharge profiles from experiments with

varied combinations of potentiostatic and galvanostatic

protocols and related electrochemical impedance spectroscopy

(EIS) and scanning electron microscopy data. As a cathode we

have selected graphitized acetylene black. We will discuss how

the attainable discharge capacities can be rationalized via the

structure and composition of the seed layers formed during the

initial discharge time intervals, and in how far this affects the

practical operation of Li-O2 and Li-air batteries.

(a)

(b)

Figure 2. (a) Schematic of the two steps discharge profile at constant

overpotential Li2O2 seeding followed by constant current step. (b) Voltage vs.

time plot of initial seeding step to 400 mAh gc-1

at 2.3 V followed by constant

current at 50 mAh gc-1

.

Results and Discussion

Graphitized acetylene black (GAB) with a BET surface

area of 75.4 m2 g-1 was used as the cathode material in the

experiment. The graphitized form of the carbon black was used

due to the chemical stability against superoxide radicals.[20] For

comparison, the capacities obtained in the study were

normalized with the mass of the GAB, as denoted by mAh gc-1.

Thus, the formation of one monolayer of Li2O2 corresponds to

a discharge capacity of about 55 mAh gc-1. As briefly

mentioned in the introduction, Figure 1 demonstrates the

impact of discharging history on the capacity of the electrode.

Two cells with the same carbon loading were discharged with

two different procedures: one at a constant-current discharge

of 50 mA gc-1 until the voltage reaches 2 V; another one at an

initial discharge of 1000 mA gc-1, followed by a rest of 2 h and a

subsequent discharge of 50 mA gc-1. The Li-O2 cell fully-

discharged at 50 mA gc-1 shows a discharge capacity of about

2500 mAh gc-1. Only 50 % of this capacity is reached for the

second cell that underwent the initial discharge at 1000 mA gc-1.

The large polarization during fast discharge is typically attribu-

ted to poor reaction kinetics, O2 diffusion and/or IR drop. How-

ever the low capacity from the second discharge at 50 mA gc-1

indicates that reactants transport may not be the only problem

limiting the capacity after the high current density discharge at

1000 mA gc-1. After the rest time of 2 h, the cell voltage at 50

mA gc-1 is virtually the same as for a cell that was slowly

discharged to the same extend. The electrolyte and the

electrode should thus be sufficiently equilibrated. Nevertheless,

the cell can only be discharged for another 250 mAh gc-1, which

is apparently less than 1/5 of the remaining capacity reached

by the other cell after this point. The fast discharge obviously

deteriorates the electrode and/or its surface in a manner that

hinders further electrochemical reactions. Once the carbon

cathode surface is fully coated with Li2O2, the dominant

electrochemistry, hence the subsequent material deposition

presumably takes place on the Li2O2 surface.[16,21] The

hypothesis in this study is that the way the initial Li2O2 crystal is

grown on the underlying carbon surface determines the final

capacity. We have therefore designed a set of experiments to

understand how the initial layer of Li2O2 on carbon cathode

affects the total capacity. The experimental procedure is

illustrated in Figure 2.

The fresh carbon cathodes are potentiostatically

discharged at four different potentials (1.7 V, 1.9 V, 2.3 V or

2.6 V) down to a discharge capacity of 400 mAh gc-1 to form a

seed layer, before subsequently discharged at 50 mA gc-1

down to a cut off voltage of 2 V. The 1st discharge step is used

to control the rate of deposition of the seed layer: cell voltages

of 2.6 V and 2.3 V mean low overpotentials and thus slow

deposition, whereas 1.9 V and 1.7 V are further from equi-

librium and thus correspond to fast deposition. The deposition

charge of 400 mAh gc-1 is equivalent to 7 monolayers of Li2O2,

which is only 16% of the maximum capacity of 2500 mAh gc-1

from the carbon electrode (Figure 1). Thus the seed layer will

only occupy a small space and volume on the cathode and can

thus not substantially block any pores in the electrode. The

discharge profile from the low current density step will be used

to monitor the effect of the Li2O2 seed layer. Before each step

the cell is allowed to rest at open circuit voltage for 2 h in

constant O2 flow (Figure 2b).

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Figure 3. Discharge curves of different cells subjected to different discharge

procedures. Potentiostatic until 400 mAh gc-1

, then galvanostatic at 50 mA

gc-1

.

Results of the Li-O2 discharge experiments are shown in

Figure 3. The capacity reached during the galvanostatic dis-

charge at 50 mA gc-1 decreases when the cathode is initially

covered with seed layers formed at higher overpotentials. The

carbon cathode with Li2O2 seeded at 2.6 V (low overpotential)

demonstrates the highest capacity of ~2300 mAh gc-1 while the

cathode with Li2O2 seeded at 1.7 V yields the lowest overall

capacity of ~500 mAh gc-1. The capacities continuously

decrease with decreasing cathode potential (=increasing

overpotential) applied during the seed layer formation. This

steady correlation indicates that the properties of the seed

layer, which acts as electrode for the subsequent complete

discharge, change gradually.

Field emission scanning electron microscopy (FESEM)

images of the cathode were taken to observe the morphology

of Li2O2 deposited after the formation of the seed layer at 400

mAh gc-1 at various overpotentials (Figure 4). Figure 4a is the

fresh carbon cathode before discharging. When the cathode is

discharged at 1.7 V (with the highest overpotential) the

deposition products are scarcely visible and the electrode

surface morphology resembles that of the original carbon black

(see Figure 4b). This is consistent with a recent report on high

current discharge of a carbon membrane.[18] After discharging

at 2.3 V, the deposition products become more visible on the

carbon particles, occupying the inter-particle spaces and fusing

the carbon particles together (Figure 4c). At the lowest

overpotential discharge of 2.6 V, a smooth layer of deposit is

observed on the electrode (Figure 4d). The FESEM images

show that the morphology of the electrodeposition products on

the carbon cathode changes with discharge overpotential, and

thus with the deposition rate. At low overpotentials, i.e. lower

rate, the nucleation and crystal growth of Li2O2 is within the

kinetic or mixed kinetic-mass transport regime, forming a

smooth surface on the cathode. On the other hand, the rough

surface formed at higher overpotentials not only indicates a

high nucleation rate but also a certain degree of mass

transport limitation [22]: pores have a lower probability to

become filled.

Figure 4. FESEM images of (a) fresh carbon cathode; (b, c, d) after Li2O2

seeded to 400 mAh gc-1

under constant voltage discharge at 1.7 V, 2.3 V,

and 2.6 V respectively.

Figure 5. FESEM images after complete discharge at 50 mA gc-1

for two

different seed layers, formed at (a) 1.7 V and (b) 2.6 V, both with thickness

equivalent to 400 mAh gc-1

.

According to Figure 3, the slowly grown smooth layers

allow for more subsequent Li2O2 growth than the rougher ones,

despite the higher surface area of the latter. Furthermore, the

smooth or rough morphology initiated during the seed layer

formation is qualitatively maintained until the end of discharge.

This is illustrated in Figure 5, which shows the electrode

morphology after sudden death during discharge at 50 mA gc-1.

For the following discussions, it is important to note that XRD

analyses after extended discharge at high and low

overpotentials only detected Li2O2 as the dominant discharge

product (Figure S1).

The FESEM data in combination with the discharge curves

confirm that the capacity limitation at higher discharge rate

does not result from clogged pores and concomitant O2 trans-

port limitations. Otherwise, the smooth Li2O2 surface generated

at 2.6 V, where the initial pore structure is virtually filled or

covered, should show the lowest discharge capacity – in con-

trast to our findings. This confirms previous conclusions based

on Li-O2 discharge experiments at flat electrodes.[23]

www.chemsuschem.org

4

The limitation of the discharge capacity due to a period of

rapid Li2O2 growth at 1.7 V also depends on when this rapid

growth occurs. This is illustrated in Figure 6, which shows that

the attainable discharge capacity increases by more than 50%

if the potential of 1.7 V is not applied on the fresh carbon

electrode (as in Figure 4a) but on an electrode pre-covered by

a smoother seed layer equivalent to 400 mAh gC-1 formed at

2.6 V (as in Figure 4d). Taking into account the contribution of

the 2.6 V seed layer, the overall discharge capacity even

increases to 2000 mAh gC-1. In simple words, a sequence

“slow-fast-slow” yields a two times higher discharge capacity

than a sequence “fast-slow”. This suggests that the interface

layer formed between the carbon and the Li2O2 film in the initial

stage of the growth is more important for the overall discharge

capacity than product layers formed later. Further two-step

seeding experiments showing similar trends are summarized in

Table S1.

The data presented so far provide no direct evidence that

the electrochemical discharge reaction is happening on top of

the product layer and not at the interface between the product

and the carbon.

Figure 6. Potentiostatic followed by galvanostatic discharge at 1.7 V and

50 mA gc-1

, respectively. Solid line: fresh carbon cathode (see Figure 3 and

Figure 4a). Dottedline: pre-covered by 400 mAh gC-1

seed layer formed at

2.6 V (see Figure 4d). The pre-applied 2.6 V period is not shown in this plot.

However, our supplementary information (Figure S2) show

results of a test experiment where a Li2O2 cathode similar to

the one in Figure 4d is intentionally poisoned by soaking it in

propylene carbonate (PC), followed by re-introducing the

original fresh electrolyte. Details about this poisoning

experiment will be discussed more below in the context of the

other impedance data. As of now, one should keep in mind

this: since Li2O2 is known to react with PC [24] whereas a

carbon electrode surface should not be affected, we conclude

that the Li2O2 surface is the actual electrode of the ongoing

discharge reaction.

Previous works demonstrated the important role of electron

transport through the Li2O2 layer for the discharge rate and the

discharge capacity of Li-O2 cells.[16] We thus used EIS to

quantify the electrochemically relevant interface properties of

the cathode in more detail. The impedance spectra were

measured under conditions of slow discharge at a DC bias of

2.6 V. A 10 mV AC oscillation between 100 kHz and 10 mHz

was applied. A typical Nyquist plot for the Li-O2 system and the

equivalent circuit are shown in Figure 7a. The first semicircle at

high-frequency (~ 1000 Hz) is attributed to the Li/anode

interface.[25] The second semicircle at low-frequency (~ 0.4 Hz)

strongly depends on the presence of dissolved O2 in the

electrolyte and the overpotential of the cathode (Figure S3-4).

Hence, the second semicircle is attributed to the formation of

Li2O2 on the cathode side. The impedance spectra were further

analysed by fitting the low-frequency semicircle with an

electrical equivalent circuit. The circuit shown in the inset of

Figure 7a consists of a resistance element connected in series

with an RC element. The circuit elements are as follows: R is

the sum of solution resistance and the charge transfer

resistance of the Li anode, Rct is the charge transfer resistance

for the Li2O2 electrodeposition reaction on the cathode and Q is

the constant phase element representing the double layer

capacitance (Cdl). Due to the non-ideal nature of the porous

carbon electrode, the impedance spectra cannot be fitted with

a simple capacitor but require a constant phase element.

The impedance spectra for the fresh carbon cathode and

those for the electrodes after 400 mAh gc-1 of Li2O2 seeding at

1.7 V and 2.6 V are shown in Figure 7b. While the spectra for

the electrode after seeding at low overpotential (2.6 V) virtually

coincides with that of the fresh cathode, seeding at 1.7 V yields

much larger impedance values.

(a)

(b)

Figure 7. Nyquist plot for Li-O2 battery with carbon cathode and Li metal

anode. (a) Inset shows the equivalent circuit chosen to fit the impedance

behaviour. (b) Nyquist plots for cells taken for fresh cathode, and after

constant voltage discharge to 400 mAh gc-1

. Inset magnifies the data range

for Z < 1000

5

This suggests that fast deposition of Li2O2 leads to larger

charge transfer resistance of the product layer. To further

clarify how deposition rate affects the impedance of the

electrode, EIS measurements were taken at increasing depths

of discharge as depicted in the experimental scheme in Figure

8. The cell was first discharged at a constant voltage for a

limited amount of capacity. Afterwards, the cell was allowed to

rest at Eeq = 2.9 V before the EIS measurement was taken.

The discharge and the EIS steps were then repeated. Using

the equivalent circuit in Figure 7a, the experimental spectra

were fitted to obtain the Rct and Cdl values. Those values are

plotted in Figure 9 as a function of the depth of discharge.

Figure 8. Experimental scheme for constant potential discharge-EIS step to

monitor the progress of impedance parameters at different depth of

discharge. The resting time at OCV is 10 minutes after each step.

Even at the beginning of discharge, Rct is higher when the

electrode is discharged with a higher overpotential: at Q = 100

mAh gc-1, Rct values are 25, 43 and 55 ohm m2 for electrodes

discharged at Eapp = 2.6 V, 1.9 V, and 1.7 V, respectively.

Again, this underlines the relevance of the initial seed layer

and its interface to the underlying carbon. The change in Rct

with discharge capacity is highly dependent on how the

electrode is discharged. Rct slightly decreases until a charge of

about 200mAh gc-1 is reached and then continuously increases.

This fits to trends observable in current-time curves recorded

after potential steps from 2.8 V to different potentials (Figure

S5). The increase in the reaction rate at the beginning of a

constant potential discharge in Figure S5 and the initial

decrease in Rct in Figure 9 can be understood in terms of the

increased surface roughness that increases the effective

electrode area. For growth at low overpotential (Eapp = 2.6 V),

only a small increase in charge-transfer resistance is observed

throughout the deposition of Li2O2 up to 1300 mAh gc-1. On the

other hand, if the electrode is discharged at Eapp = 1.7 V, Rct

increases 4-fold after a capacity of only 400 mAh gc-1 (about 7

monolayers of Li2O2). If the growth at Eapp = 1.7 V occurs on

top of a smooth seed layer pre-deposited at Eapp = 2.6 V,

however, an increase in Rct happens only at 600 mAh gc-1. This

fits to the observation in Figure 6 that the “sudden death”

during discharge at 1.7 V can be delayed by initially seeding a

smooth layer at 2.6 V.

Figure 9. The development of charge transfer resistance (Rct) and double

layer capacitance (Cdl) with capacity at various constant voltage discharges.

Fresh carbon cathode is at Q = 0 mAh gc-1

. Filled circles: no seed layer;

open circles: after 400 mAh gc-1

seed layer deposition at 2.6 V (see text)

The double layer capacitance Cdl is also affected by the

discharge protocol. Cdl does not significantly change for Eapp =

2.6 V. Cdl notably decreases for discharge at higher over-

potentials, showing a faster decline if the same charge is trans-

ferred in a shorter time. A similar pronounced decrease of Cdl

after high overpotential seeding is also observed for Cdl values

obtained from cyclic voltammetry experiments in argon

environment (Figure S6). Only for the film that is grown at 1.7 V

after seeding at 2.6 V, shows an increase in Cdl with thickness

(note that the data points shown here start only after the

seeding phase). The small initial slope of all curves in Figure 9

indicates that the area specific double-layer capacity is similar

for fresh carbon and thin Li2O2 films. The decreasing porosity

observed for growth at Eapp = 2.6 V (Figure 4 and Figure 5)

should thus lead to decreasing Cdl values, in agreement with

the observation. However, the even steeper decrease of Cdl

during growth at Eapp = 1.9 V and 1.7 V, where FESEM data

(Figure 4 and Figure 5) indicate that the porosity (and thus the

surface area) of the electrode is maintained high during growth,

does not fit into this simplified picture. Considering that for

these fast growing films Rct is also increasing much faster, we

conclude that most of the material grown at high overpotentials

must be considered “dead” due to its poor electron conductivity.

In other words, only a fraction of the porous surface visible in

Figure 4b and Figure 5a is actually part of the electrode probed

by the impedance experiments. Only after seed layer formation

at 2.6 V, we find an increase in Cdl with film thickness. This fits

to the observation in Figure 6, where the pre-seeding at 2.6 V

www.chemsuschem.org

6

was also found to partially compensate the deteriorating effect

of a subsequent layer formed at 1.7 V.

Our results show that the potential dependent discharge

behaviour can only partially be interpreted in terms of electrode

and product morphology. The second key parameter seems to

be the intrinsic electron conductivity of the Li2O2 product film.

The conductivity seems to become intrinsically worse for

products formed at higher overpotentials. As already

mentioned, one may speculate that (electro-)chemical side

reactions between Li2O2 and the solvent are generating

passivating layers, e.g., poorly conducting Li2CO3. The solvent

used here is 1,2 dimethoxy ethane (DME) which has been

reported to have low reactivity against reduction and super-

oxide attack.[9,10] We thus tested the sensitivity of the overall

discharge process to carbonate layers by intentionally

exposing the product layer to a more reactive solvent. A

smooth Li2O2 film formed at 2.6 V (to 400 mAh gc-1, see Figure

4d) was soaked in propylene carbonate (PC) solvent for 3 days.

The Li2O2 is expected to react with the carbonate solvent to

form Li carbonates.[24] Afterwards, the same electrode was built

into a new cell for EIS measurement and galvanostatic

discharged at 50 mA gc-1, this time again in the original DME

solvent. The results are presented in the supporting

information (Figure S2). To summarize, after the PC soaking

treatment, Rct increased 4-fold but the Cdl remains similar. Rct

then decreased rapidly with further deposition of Li2O2 at 50

mA gc-1 and eventually reached a similar total capacity of about

2400 mAh gc-1. This intentional poisoning test shows that even

if the seed layers formed at higher overpotentials also generate

deposits from side reactions, the system should be able to

recover from this during subsequent slow discharge. As

already mentioned, the poisoning experiment also confirms

that the ongoing electrochemical reaction is happening on top

of the Li2O2 film and not at the carbon. Otherwise, we would

not be able to observe the temporary poisoning effect.

At this point, neither the growth morphology alone nor a

possible formation of carbonates can explain the earlier

sudden death after fast initial discharge. However, our

impedance data clearly indicate that the charge transfer

resistance drastically increases for the films grown at higher

overpotentials. The “rough” deposits above must thus differ

from the “smooth” ones in their electron conductivity. As

mentioned above, the morphologies of the faster grown

deposits resemble electrochemical deposits formed under

mass transport limitations.[22] In the case of Li-O2 discharge,

the transport of either Li+ or O2 in the electrolyte and through

the pores could in principle be decisive. Looking at the low

solubility of O2, only this second species can be critical.[23,26]

This means that Li2O2 growth occurs at conditions of

decreasing O2 availability with increasing deposition rate at

higher overpotentials. There are two ways how this could

reduce the electron conductivity of the deposit. First, it is

possible that apart from the Li2O2 crystallites themselves also

their surfaces and grain boundaries play an important role in

the electron conductivity.[27,28] Recent theoretical and post-

mortem analyses of Li-O2 systems suggested the presence of

lithium superoxide species (LiO2) on the surface of Li2O2 or

grain boundaries.[16,21,29] This oxygen-rich species may be

responsible for the enhanced electronic conductivity of the

Li2O2 and for the subsequent electrochemical formation and

decomposition of Li2O2.[30,31] Under conditions of limited O2

transport, however, less such superoxide species would be

formed, leading to a poorer conductivity. A second possible

scenario could involve O-O bond breaking at high

overpotentials, forming Li2O instead of Li2O2. Li2O deposits

would have a similarly high bandgap as Li2CO3 and would thus

drastically decrease the overall electron conductivity.[23]

Conclusions

The discharge capacity attainable for a Li-O2 cell strongly

depends on the discharge history. If the first few layers are

grown at too high rate and a concomitantly high overpotential,

the discharge capacity is severely lowered. Pore blocking

effects can be ruled out as a reason for this: higher discharge

capacities correlate to smooth morphologies where most pores

are blocked at a rather early state. Electrochemical Impedance

Spectroscopy underlines that it is the intrinsic electron

conductivity of the Li2O2 deposit rather than its morphology that

limits the further discharge. The rougher deposit formed at

higher discharge rates exhibit characteristics of mass transport

limited growth. Comparing charge transfer resistances, double

layer capacities, and the related surface morphologies, we

conclude that the poor conductivity and the rough morphology

are two observations not caused by one another. Instead, both

are a consequence of O2 transport limitations at high

deposition rates. We tentatively suggest that an O2 deficiency

could lead to the formation of less LiO2, an intermediate

product that is known to increase the conductivity of Li2O2.

Alternatively, the high overpotential required for the high

deposition rates could give rise to O-O bond breaking and Li2O

formation, which would also lower the conductivity of the

deposit.

Furthermore, the initial phase of Li2O2 deposition is decisive for

the discharge capacity in two ways. Apart from the deterio-

rating effect of a too fast initial growth, a conditioning step by

forming a smooth seed layer at low rate makes the system less

sensitive to subsequent fast-discharge periods. This underlines

the importance of the carbon-Li2O2 interface for the overall

electrode characteristics and must be considered in the

practical operation protocol of Li-air batteries in the future.

Experimental Section

The cathodes were fabricated by coating a slurry of graphitized

acetylene Black (Sigma Aldrich) with PVDF (HSV900, Arkema) binder in N-Methyl-2-pyrrolidone (NMP) onto a 2320 Celgard separator. The loading of the carbon is 80wt%. The cathodes

were dried at 90°C for 12 hours inside a vacuum-sealed oven before cell assembly. The weight of the active carbon cathode is about ~1.6 mg, with a carbon loading of 0.97–1.03 mg/cm2

and thickness of 20 μm. The cathodes were assembled in an Argon filled glovebox in an electrochemical test cell (ECC-air cell) purchased from EL-Cell GmbH, with Li metal as the anode

and 0.1M LiClO4 in anhydrous 1,2-dimethoxyethane as the electrolyte. Constant voltage and galvanostatic discharge experiments were performed using Arbin Battery tester

BT2000 to a lower voltage cutoff of 2 V vs Li/Li+ or a fixed capacity cutoff (mAh/gc). Electrochemical impedance spectroscopy (EIS) was performed using a Biologic SP 200

potentiostat with a frequency range from 100kHz to 10mHz and AC amplitude of 10 mV. The EIS measurements were

7

performed at the open circuit voltage (~2.9V) or with a 2.6 V vs Li/Li+ DC bias. The EIS spectra were fitted using EC-Lab®

software (Bio-Logic Science Instruments). During the EIS and discharge experiments, the cells were kept at ~23oC under constant flow of O2 (99.999%) at 0.015 ml/min. The

morphology of the cathodes was investigated by FESEM from JEOL JSM-7600F.

Acknowledgements

This work was financially supported by the Singapore National

Research Foundation under its Campus for Research

Excellence and Technological Enterprise (CREATE)

programme.

Keywords: keyword 1 • keyword 2 • keyword 3 • keyword 4 •

keyword 5

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Received: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

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9

Entry for the Table of Contents

FULL PAPER

Text for Table of Contents

Ali Rinaldi, Olivia Wijaya, Harry Hoster*,

Denis Y.W. Yu*

Page No. – Page No.

History effects in Li-O2 batteries -

how initial seeding influences the

discharge capacity

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10

Electronic Supplementary Information (ESI):

History effects in Li-O2 batteries - how initial seeding influences the

discharge capacity

Ali Rinaldi, Olivia Wijaya, Harry Hoster*, Denis Y.W. Yu

Figure S1. X-ray diffraction of commercial Li2O2, and carbon cathodes after extended discharge to ~2000 mAh gc-

1 at 1.7 and 2.6 V. X-ray diffraction measurements of the discharged cathodes were performed with D8

Advance Bruker with Cu Kα radiation (λ = 1.54 Å).

(a)

11

Figure S2. Electrochemical Impedance Spectroscopy showing: (a) Nyquist plot and (b) Rct values of fresh

cathode, discharged at 2.6 V and after soaking in Propylene Carbonate (PC) solvent.

The Li2O2 is expected to react with the carbonate solvent to form Li carbonates.1 To summarize, after the PC

soaking treatment, Rct increased 4-fold but the Cdl remains similar. Rct however decreased rapidly with further

deposition of Li2O2 at 50 mA gc-1 and eventually reaching a capacity of about 2000 mAh gc

-1 , close to the

capacity without soaking in PC (2300 mAh gc-1 , see Figure 3 in the manuscript). The PC-soaking experiments

indicate that minor amount of side reactions is not the dominant factor in reducing the discharge capacity at

higher overpotentials. In addition the PC-soaking experiment confirms the idea that the reaction takes place on

the Li2O2 surface as the PC would react with the Li2O2 forming Li2CO3 and should have no effect on the

graphitized carbon cathode.

1. R. Younesi, Characterization of Reaction Products in the Li-O2 Battery Using Photoelectron

Spectroscopy, Uppsala University, 2012.

(a) (b)

Figure S3. Nyquist plot of Li-O2 cell with Li anode and carbon as cathode AC amplitude of 10 mV and frequency

range of 100 kHz-10mHz, under (a) Argon and (b) O2 flow.

(b)

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Figure S4. Bode plot Li-O2 cell with Li anode and carbon as cathode AC amplitude of 10 mV and frequency range

of 100 kHz-10mHz, under O2 flow.

13

Figure S5. Current profiles from Potential step (CA=Chronoamperometry) experiments of Li-O2 cell with carbon

as cathode, Lithium as anode under flowing O2 gas at 0.015 mL/min.

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Figure S6. (a) Cyclic Voltammogram of fresh cathode in Argon atmosphere at 20mV/sec scan rate. (b) The Cdl

values are determined from the anodic and cathodic current at the potential without the redox couple.

Voltammograms for fresh carbon cathode and after the Li2O2 seeding to 400 mAh gc-1 at 2.6 and 1.7V at 20 mV

sec-1 in Argon atmosphere. (c) Summary of the Cdl values obtained from Cyclic Voltammetry measurements.

(a)

(b) (c)

15

Table S1. The effect of Li2O2 seeding at low overpotential (2.6 V; Step I) to the total discharge capacity of the

cathode.

Step I (slow) Constant voltage discharge at 2.6 V (Li2O2 seeding)

(mAh gc-1 )

Step II (fast) Constant voltage discharge at 1.7 V

(mAh gc-1 )

Step III (slow) Constant current discharge at 50 mA/gc

(mAh gc-1 )

0 400 500

50 (~0.9 monolayer) 400 1067

100 400 930

400 (~7 monolayer of Li2O2) 400 1000