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Journal of Power Sources 274 (2015) 496e505

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Engineering study on TiSnSb-based composite negative electrode forLi-ion batteries

H.A. Wilhelm a, d, e, C. Marino b, d, A. Darwiche b, d, P. Soudan a, d, M. Morcrette c, d, e,L. Monconduit b, d, e, B. Lestriez a, d, e, *

a Institut des Mat�eriaux Jean Rouxel (IMN), CNRS UMR 6502, Universit�e de Nantes, 44322 Nantes Cedex 3, Franceb Institut Charles Gerhardt e Agr�egats, Interfaces, Mat�eriaux pour l'Energie, CNRS UMR 5253, Universit�e de Montpellier 2,34095 Montpellier Cedex 5, Francec Laboratoire de R�eactivit�e et Chimie des Solides, CNRS UMR 7314, Universit�e de Picardie Jules Verne, 33 rue Saint Leu, 80039 Amiens Cedex, Franced R�eseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS, 3459 Francee ALISTORE-ERI, FR CNRS, 3104 France

h i g h l i g h t s

� TiSnSb-electrodes with various binders, mass loadings and porosities are studied (81).� CMC shows better performance than CMC/PAAH, CMC buffered at pH3 and Amylopectin (79).� Cycle life decreases for electrodes with higher mass loading and lower porosity (79).� Rate performance is determined by the electrode polarization resistance (72).� TiSnSb/Li1Ni1/3Co1/3Mn1/3O2 cell shows good cyclability with FEC electrolyte additive (84).

a r t i c l e i n f o

Article history:Received 24 July 2014Received in revised form11 September 2014Accepted 8 October 2014Available online 17 October 2014

Keywords:Lithium-ion batteryIntermetallic phaseTiSnSbElectrode formulationCyclabilityRate performance

* Corresponding author. Institut des Mat�eriaux Jea6502, Universit�e de Nantes, 44322 Nantes Cedex 3, F

E-mail address: Bernard.lestriez@cnrs-imn.fr (B. L

http://dx.doi.org/10.1016/j.jpowsour.2014.10.0510378-7753/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Micrometric TiSnSb is a promising negative electrode material for Li-ion batteries when formulated withcarboxymethyl cellulose (CMC) binder and a mixture of carbon black and carbon nanofibers, and cycledin a fluoroethylene carbonate (FEC)-containing electrolyte. Here, other binder systems were evaluated,polyacrylic acid (PAAH) mixed with CMC, CMC in buffered solution at pH 3 and amylopectin. HoweverCMC showed the better performance in terms of cycle life of the electrode. Whatever the binder, cycle lifedecreases with increasing the active mass loading, which is attributed to both the precipitation of liquidelectrolyte degradation products and to the loss of electrical contacts within the composite electrode andwith the current collector as a consequence of the active particles volume variations. Furthermore,calendaring the electrode unfortunately decreases the cycle life. The rate performance was studied as afunction of the active mass loading and was shown to be determined by the electrode polarizationresistance. Finally, full cells cycling tests with Li1Ni1/3Co1/3Mn1/3O2 at the positive electrode were done.60% of the capacity is retained after 200 cycles at the surface capacity of 2.7 mAh cm�2.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The demand for energy, power capability and stable cyclingperformance continue to highly stimulate the current research ofnew electrode materials for Li-ion batteries. Despite their veryattractive specific capacities, intermetallic phases still suffer from

n Rouxel (IMN), CNRS UMRrance.estriez).

limited cycle life mainly due to agglomeration of active elements,detrimental reactivity toward the electrolyte, and volume expan-sion which leads to electrode disintegration upon cycling. Thevarious tentative to solve those problems by preparing compositeseither with carbon or with a second metal has shown only limitedsuccess due to volume variations and high irreversible losses [[1]and references therein]. Among intermetallic phases, micrometricTiSnSb is a promising negative electrode material displaying aspecific gravimetric and volumetric capacity of 540 mAh g�1 and4070 mAh cm�3, respectively. The lithium reaction mechanismwasstudied elsewhere [2]. It shows promising performance in terms of

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505 497

cycle life when formulated with CMC binder and a mixture ofcarbon black and carbon nanofibers, and tested in a FEC-containingelectrolyte [3] allowing to achieve more than 400 cycles giving out400 mAh g�1 at 4C rate (the C rate corresponding to 1 Li h�1, and aspecific current of 93 mA g�1), with a lithium metal counterelectrode.

The huge influence of the formulation of the composite elec-trode on electrochemical behaviour is now well established [[4]and references therein]. Because the composite electrode is a bi-percolating media (both ionic and electronic), its architecture(dispersion of the active material (AM), conductive additive (C) andbinder (B), porosity …) and composition (C and B content, C shapeand B chemistry …) do influence the electrode behaviour andperformance (energy, power, life). However, the literature on thistopic for intermetallic phases or conversion materials is scarce.With respect to intermetallic phases such as FeSn2, NiSb2 andTiSnSb, Sivasankaran et al. [5] demonstrated the superiority of thecarboxymethyl cellulose binder versus poly(vinylidene fluoride)(PVDF), the standard binder of the battery industry, in achievinglonger cycle life. The improvement is due to the better ability ofCMC compared to PVDF (polyvinylidene fluoride) to form moreresilient AM/C interparticle contacts but also in a better formationof solid electrolyte interphase (SEI) layer on AM. Two conductiveadditives were also compared, acetylene black (AB) and vaporgrown carbon nanofibres (VGCF). Better capacity retention wasachieved with VGCF. As it is a flexible fibre, VGCF forms morecontinuous and resilient conductive paths, compared to AB thateasily looses interparticle contacts during the cycling of AMshowing volume variation. The same paper revealed the high ratecapability of intermetallic phases, which were previously consid-ered as sluggishmaterials. Ponrouch and Palacín studied the impactof the slurry mixing procedure (magnetic stirring, magnetic stirringwith additional sonication steps, ball milling) in the electro-chemical performance of composite electrodes prepared with PVdFas B, Super P carbon as C and Co3O4 as AM [6]. The study showedthe influence of the mixing of the slurry on the homogeneity of thetape casted electrode and a beneficial impact of a sonication pro-cedure. Same group performed systematic formulation on Co3O4based composite electrodes [7]. The influence of diverse parame-ters was studied such as: calendaring pressure applied, composi-tion of the composite electrode, nature of C (carbon super P, carbonnanofibers and carbon nanotubes) and the size of the AM particles.The unpressed electrode experienced an important capacity decayduring the first cycles, whereas the pressed electrodesmaintained agood capacity for several cycles. When the amounts of AM, C and Bwere varied, the best capacity retentions were achieved with thelowest amount of AM (65 wt.%) and the highest content of C(27 wt.%) studied; showing that a key issue for the long termcycling of Co3O4 is the electrical contact with the AM, as for FeSn2,NiSb2 and TiSnSb [5]. A PVdF content of 8 wt.% was found as a goodcompromise between the mechanical stability and the electricalconductivity of the electrodes. For this nanosized AM, no significantchange on the capacity retention was achieved by substituting Cspwith VGCF. The results also showed a strong influence of the par-ticle size on the capacity retention upon cycling. Decreasing theparticle size enhanced the electrolyte decomposition; the electro-lyte decomposition products decreasing the electrochemical ac-tivity of the AM by hindering the diffusion of lithium ions throughthe electrode.

The reported improvements gained by playing with the elec-trode formulation and engineering on the electrochemical perfor-mance of intermetallic phases or conversion materials [5,7]indicate that it is a research well worth continuing to study. Pur-suing our previous work, we have (i) tried to improve the electro-chemical performance by playing on the binder and conductive

additive, (ii) investigated the rate performance and the origin oftheir limitations, (iii) looked at the influence of calendaring withthe aim to increase the composite electrode's volumetric capacity.Finally, we made full cells cycling tests with the Li1Ni1/3Co1/3Mn1/

3O2 (NMC) active material at the positive electrode.

2. Experimental

2.1. Composite electrode formulation

The TiSnSb micro particles (BET 4.89 m2 g�1) were prepared byball milling of Ti, Sn and Sb powders from commercial sources(>99.9%) as described elsewhere [2]. See also Supplementaryinformation 1. Two carbon conductive additives were utilized inorder to optimize the electronic percolation within the compositeelectrode: (i) carbon black Super P (Csp, Timcal, 65 m2 g�1) and (ii)some vapour grown carbon fibres S-type (VGCF-S, Showa Denko,35 m2 g�1). Our standard formulation includes sodium carbox-ymethyl cellulose (CMC, Aldrich, M ¼ 250,000, D.S. ¼ 0.7) used as abinder. The benefit of associating CMC and VGCF into the formu-lation for micron-sized particles is described elsewhere [5,8]. Otherbinders were tested such as: (i) a mixture of CMC and polyacrylicacid (PAAH, Sigma Aldrich) in proportion 1/1, (ii) amylopectin frommaize (Sigma), and (iii) CMC in a citric acid (Sigma Aldrich)/KOHbuffered solution at pH ¼ 3 (0.033 M in citric acid). The differentsolid components were premixed in an agate mortar with thefollowing composition, given in wt%: active material: 70/Csp: 9/VGCF-S: 9/polymer: 12. The components were put into a siliconnitride vial and some deionised water (1.5 mL) or buffered solutionat pH ¼ 3 was added together with three 9.5 mm diameter siliconnitride balls. The mixing was performed with a Fritsch Pulverisette7 mixer at 500 rpm for 60 min. The resulting water-based slurrywas tape-cast onto a 25 mm thick copper foil and dried for 12 h atroom temperature and then another 2 h at 100 �C under vacuum,(and 2 h at 150 �C under vacuum in the case of the CMC þ PAAHmixture). Let's note that the citric acid/KOH mixture is not elimi-nated during drying. Thus, the true composition of the electrode isin this case: 64.4/Csp: 8.3/VGCF-S: 8.3/CMC: 11/citric acidþ KOH: 8.

Discs of 1 cm diameter, corresponding to 0.785 cm2 surface area,were punched out of the electrode tapes for electrochemicalevaluation.

LiNi1/3Co1/3Mn1/3O2 powder was supplied by UMICORERechargeable Battery Materials, Belgium. Carbon black Super P(Csp, Timcal, 65 m2 g�1) was used as conductive additive and pol-yvinylidene fluorine (PVdF) from Solvay as a binder. The differentsolid components were premixed in an agate mortar with thefollowing composition, given inwt%: active material: 85.5/Csp: 4.5/PVdF: 6. This mixture then was further mixed in N-methyl-pyrrolidone (NMP) as a solvent in a glass bottle with one magneticstirrer bar for one day. The resulting slurry was tape-cast onto a25 mm thick aluminium foil and dried 60 �C for 12 h followed byvacuum drying at 100 �C for another 2 h.

2.2. Electrochemical evaluation

The TiSnSb-based electrodes were tested as positive electrodeinto a two-electrode Swagelok-type cell (assembled in a glove box)configuration versus metallic lithium (used as negative and refer-ence electrode) and in two different electrolytes: 1 M LiPF6 in EC/PC/3DMC þ 1% VC (home-made Electrolyte 1) and 1 M LiPF6 in EC/DMC þ 10% FEC, 2% VC (home-made Electrolyte 2). Electrochemicalcharge/discharge curves were recorded between 1.5 V and 0.01 Von a multichannel VMP (BioLogic) system under galvanostaticconditions at different rates. The charge and discharge rate of 1Ccorresponds to 1 Li in 1 h (93 mA g�1 specific current). The

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505498

sequence for cycling is: at the beginning 5 cycles at C, 2C, 4C, 8Cfollowed by a long term cycling at 4C-rate. This hybrid settingallowed testing the material behaviour in terms of power and cyclelife as well. Two batteries were cycled for each test and a goodreproducibility both in terms of specific discharge capacity andcycle life was observed.

To evaluate the practical feasibility of a battery, button cellswere assembled with TiSnSb as the negative electrode and NMC asthe positive electrode. Both electrodes are balanced in order to havethe same surface capacity in mAh cm�2. Cells were cycled in gal-vanostatic mode between 4.2 and 2.0 V at the current of77 mA g�1_TiSnSb, the full charge/discharge taking 7 h (based oninitial capacities).

3. Results and discussion

3.1. The electrochemical parameters studied

In a first approach, we present a series of curves allowing us toestablish the different parameters discussed in this study. First,Fig. 1a shows the 1st and 2nd cycles for an electrode cycled versus

Fig. 1. (a) 1st and 2nd cycles for a TiSnSb electrode cycled versus lithiummetal. The 1stcycle losses and 2nd cycle polarization DV are identified. (b) Discharge, charge andcumulated irreversible capacities versus number of cycles for CMC binder basedelectrode tested with 1 M LiPF6 in EC/PC/3DMC þ 1% VC. The determination of thecycle life is identified by the yellow vertical line. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

lithium and the determination of the 1st cycle irreversible lossesand the polarization. The 1st cycle losses correspond to the capacitydifference (in mAh g or in the number of lithium exchanged) be-tween the capacity in the 1st discharge and the capacity in the 1stcharge as indicated by the arrow. The polarization is calculated asthe difference of the potential in charge and of the potential indischarge for the material half delithiated (50% of state of charge),in the 2nd cycle or the following ones, due to the changes of theelectrochemical curve after the 1st cycle (as discussed elsewhere[2]). Fig. 1b shows the cycling behaviour and the so-called “cyclelife”. The latter is determined by the end of the plateau (where thecapacity is stable in cycling) and corresponds to the cycle numberjust before the capacity drops (yellow bar). On this graph the cu-mulative irreversible losses, defined as the cumulated differencebetween the discharge and charge capacities, rises quickly up tomore than 800 mAh g�1 before the end of life. It corresponds to acoulombic efficiency of about 98%.

3.2. Playing on the binder

Extensive research was performed especially on silicon in thefield of binder when realizing that the latter played a major role inthe electrode performance [9e20]. Several pioneer papers reportedthe superiority of Na-CMC over polyvinylidene difluoride (PVDF) interms of cycle life and capacity retention with silicon [9e12]. CMCfavours: (i) a much more homogeneous distribution of the carbonblack conductive additive particles and (ii) an efficient networkingprocess of the carbon black and Si particles in the composite elec-trode slurry, due to its extended conformation in solution thatenables the formation of bridges between particles (bridgingmodel). Moreover, CMC shows almost no swelling in carbonatesand has a high elastic modulus. Conversely, PVdF is significantlyswollen by carbonates and becomes an easily deformable materialwith small resistance to both elastic and plastic deformations.Furthermore, CMC could graft to the surface of silicon particlesthrough condensation of its carboxylic groups and silanol groupspresent at the surface of the silicon particles. This way, the slurrypreparation based on CMC alone [12] can be changed by modifyingthe pH of the aqueous solvent. Mazouzi et al. [13] showed that theuse of a citric acid/KOH buffer solution at pH 3 for the preparationof the slurry leads to a higher discharge capacity and a better ca-pacity retention for nanometric silicon. The buffering of the elec-trode slurry at pH 3 is mandatory to achieve extensive grafting ofthe CMC binder on the Si surface. Binders based on other poly-saccharides were also tested: Kovalenko et al. [15] reported thatalginate, a high modulus natural polysaccharide extracted frombrown algae yields remarkable performance even better than CMC.Other polysaccharides such as amylase, amylopectin and glycogenwere investigated by Murase et al. [16]. The best results in terms ofdischarge capacity were obtained with amylopectin. Thanks to itsmoderately branched structure, amylopectin is expected to givegoodmechanical properties to the composite electrode and form anartificial SEI that would stabilize the cumulated irreversible losses.Polyacrylic acid (PAAH) was tested by Magasinski [17] and PAAHcombined with CMC by Koo et al. [18] who proposed a 3D-ther-mally cured polymer prepared by a condensation reaction (dehy-dration) of PAAH and CMC. The reaction takes place between thecarboxylic acid groups of PAAH and the hydroxy moieties of CMC at150 �C under vacuum. Moreover andmore recently, Wang et al. [19]used PAAH with a conversion material, cobalt oxide and observedthat PAAH can effectively improve electrochemical characteristicsof the electrode compared to PVdF. Earlier, Komaba et al. [20] drewthe same conclusion for SiO negative electrode material.

In this context and to our knowledge, none of these promisingbinders or slurry preparation conditions was tested with TiSnSb. In

Table 11st cycle irreversible losses and average reversible discharge capacities (in mAh g�1) at 4C-rate for electrodes containing CMC, CMCþ PAAH, CMC prepared in buffer at pH ¼ 3,amylopectin.

Binder (active mass loading) CMC(1.2 mg)

CMC(4.7 mg)

CMC þ PAAH(1.75 mg)

CMC þ PAAH(4.5 mg)

CMC pH ¼ 3 (2.5 mg) Amylopectin(3.2 mg)

Average irreversible losses (mAh g�1) 100 ± 6 89 ± 2 225.5 ± 19.5 175.5 ± 5.5 262 ± 16 114.5 ± 3.5Average reversible capacity (mAh g�1) 430 ± 5 353 ± 3 477.5 ± 7.5 402.5 ± 12.5 407.5 ± 2.5 387.5 ± 2.5% of irreversible loss 19% 20% 32% 30% 39% 23%

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505 499

the following, we have tested this material with the different for-mulations that were previously tried with silicon: i) CMC alone, ii)PAAH mixed with CMC iii) CMC in buffered solution at pH 3 and iv)amylopectin. The use of PAAH aimed to reticulate the CMC as wellas the use of the buffered solution at pH 3 aimed to graft the CMC atthe TiSnSb surface, both via some ester formation [13,19]. For eachsystem, we have studied the effect of the binder on the irreversiblelosses, on the discharge capacities and on the electrode cycle lives.Table 1 gives the average 1st cycle irreversible losses, expressed inmAh g�1, for the electrodes tested with 1 M LiPF6 in EC/PC/3DMC þ 1% VC as electrolyte. Two “twin” electrodes are measuredand the average value is given as well as the spreading of the values.The TiSnSb loading (in mg) is given to allow the comparison be-tween electrodes with same mass loading. To know the TiSnSbloading per surface area (in mg per cm2) one has to divide thevalues given in the Tables or on the Figures by the factor 0.785.

First of all, we observe that, for a rather low loading (1.2 mgequivalent to 1.5 mg cm�2), the standard system (containing CMC)is characterized by 100 mAh g�1 of first cycle irreversible lossescorresponding to an irreversible loss of 19%. When the active massloading is increased, the average irreversible capacity decreased to90 mAh g�1. The discharge capacity is also decreased due to kineticlimitations (see later) leading to an irreversible loss of 20% com-parable to the low mass loading electrode. The acidification of thesystem, whether via the use of PAAH or via the use of the citric acidbuffer solution with CMC, tends to make the irreversible lossesincrease considerably to 30% whatever the active mass loading.Such result is attributed to the consumption of Li for neutralizationof COOH moieties at the first cycle [21]. The system comprisingamylopectin behaves quite the same as the CMC system in terms of1st cycle irreversible losses with a value of 23%. Table 1 also allowsto compare the reversible capacity (measured for a 4C-rate current)

Fig. 2. Cycle life of electrode made with the various binders as a function of activemass loading when tested with 1 M LiPF6 in EC/PC/3DMC þ 1% VC.

of the electrodes with the various binders. The CMC-based elec-trode displays a reversible capacity slightly above 420 mAh g�1,with a decrease to ~350 mAh g�1 when the active mass loadingincreases. The acidification of the binder systemwith PAAH tends toimprove significantly the reversible capacity as it reaches anaverage value of 470 mAh g�1 for a low active mass loading and400 mAh g�1 for a loading of 4.5 mg. The electrodes realized withthe buffer solution display a capacity which is in line with the CMC-system (extrapolating the capacity for intermediate loadings). Theamylopectin-based electrode reaches almost 400 mAh g�1 with arather high loading, close to 3.2 mg.

The cycle life was also studied with the different binders (Fig. 2,Fig. 3, Fig. 4). First of all, let us note the general trend of the drop of

Fig. 3. Discharge/charge curves for (a) amylopectin and (b) PAAH/CMC binders basedelectrodes tested with 1 M LiPF6 in EC/PC/3DMC þ 1% VC.

Fig. 4. Polarization, DV, and cumulative irreversible losses vs. cycle number until the end of cycle life in 1 M LiPF6 EC/PC/3DMC þ 1% VC for (a) the different loadings of the CMCstandard formulation, (b) the CMC e buffer pH 3 and the standard CMC systems with loadings of 2.5 mg, (c) the amylopectin and the standard CMC systems with loadings of 3.5 mg,(d) the CMC-PAAH and standard CMC systems with loadings of 4.5 mg.

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505500

the cycle life when the loading increases. The standard CMC (pre-pared in deionised water) seems to be the best binder system interms of cycle life. The latter is systematically higher of 20e50cycles compared to the one of acidic binder systems for an equiv-alent loading. Amylopectin also gives a shorter cycle life comparedto the standard CMC binder system.We also noticed a constant lossof capacity with amylopectin upon cycling (Fig. 3a) before the endof life compared to for example CMC (Fig.1b) or CMC/PAAH (Fig. 3b)where the reversible capacity is constant before the end of life. Thisdifference could be ascribed to the difference of distribution of thebinder in the electrode. SEM observations showed (in Fig. 5e) thatthe distribution of amylopectin was poor and very heterogeneous(big bunches of polymer can be seen) whereas in the case of CMCbased systems the dispersion of the polymer is optimum as nobunch of polymer can be observed (Fig. 5aed). Poorer binder dis-tribution and resulting lack of adhesion at the interparticle contactslikely explain the shorter cycle life of amylopectin-based electrode[22]. With respect to the mix PAAH þ CMC or the buffer pH 3processing condition, improved performance compared to stan-dard CMC was expected based on a possible grafting of the bindersat the surface of TiSnSb [13,18]. According to the Raman spectra andzeta potential measurements (Supplementary information 2), weindeed could hypothesize that the pH 3 buffering of the electrodeslurry could yield to the grafting between the CMC binder and theMeOH (M ¼ Ti or Sn or Sb) groups. However, the lack of perfor-mance improvement suggests this grafting does not occur. Thesurface of TiSnSb is far more complex than the one of silicon andneed to be investigated more deeply to better understand itsreactivity toward CMC.

The evolution of the cumulative irreversible losses Qirrev and ofthe polarisation (“DV”) upon cycling of the electrodes with thedifferent formulations is shown in Fig. 4. The influence of the binderkind on the viscosity of the electrode slurry made difficult thepreparation of tapes with same active mass loading. In Fig. 4a, theinfluence of the active mass loading is studied for standard CMC.Then, the other binder systems are compared to standard CMC fortwo electrodes with same active mass loading. Classically, DVgathers the contributions of the SEI resistance, the charge transferresistance, the electronic resistance of the carbon network, theionic resistance of the porous network filled by the liquid electro-lyte, and a mass transport or diffusion resistance [23]. In the case ofconversion/intermetallic materials such as TiSnSb, there is more-over an additional potential hysteresis term DV0 of thermodynamicorigin [24]. The different contributions to the electrode's polar-isation will be detailed in the next part. Following the variation inDV gives interesting clues on the failure mechanisms of conversioncompounds [25]. Fig. 4 clearly shows the concomitant increase ofthe cumulative irreversible losses Qirrev and of the DV with cycling.The increase in Qirrev can be associated with continuous liquidelectrolyte degradation [3,5] as a consequence of unstable SEI. Thisphenomenon should produce an increase of DV as a consequence ofthe precipitation of the electrolyte degradation products within thecomposite electrode porosity, at the surface of the active particlesand at the contact spots with carbon conductive particles. Anothersimultaneous cause of DV increase is the loss of electrical contactswithin the composite electrode and with the current collector as aconsequence of the active particles volume variations and me-chanical disintegration of the electrode. The results are presented

Fig. 5. SEM pictures (a) standard CMC in water, (b), CMC in water without Csp, (c) CMC in buffer pH 3, (d) PAAH/CMC and (e) amylopectin binders based electrodes.

Fig. 6. Discharge capacity vs. active mass loading at C, 2C, 4C and 8C currents for thestandard CMC formulation. C rate means 1Li h�1.

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505 501

in Fig. 4a, for three different loadings of the standard CMC formu-lation. In a first approach, DV curves tend “to follow” their associ-ated Qirrev curves suggesting that the DV increase is a consequenceof the electrolyte degradation. But looking into more details, up to100 cycles, the 3 electrodes present the same irreversible losses (inmAh g�1), but have different DV. Only the electrode with a loadingof 3.5 mg sees its DV increase. There is a faster increase of DV in thecase of thicker electrodes, which shows that the simultaneous (i)precipitation of liquid electrolyte degradation products, to the (ii)modification in the texture of the electrode as a consequence ofTiSnSb particles volume variation, results in a severely accelerateddegradation of the electrical wiring of the active mass, leading topremature end of life [26]. Concerning now the acidic systems(Fig. 4b and d), Qirrev are systematically higher than the standardCMC system with same loading. Electrode prepared with CMC inbuffered pH 3 solution (Fig. 4b) seems to present the same qualityof electrical wiring (same DV) at the beginning than the standardCMC. However, DV values increase faster for the system prepared atpH3 than in the standard CMC system in line with the larger irre-versible losses at pH 3. The situation is different with the amylo-pectin system (Fig. 4c). For amylopectin, Qirrev is rather the samethan for standard CMC systemwith same loading, but the DV valuesare higher than for standard CMC. Let us remind that in the case ofamylopectin, the binder could not be uniformly distributed within

Fig. 7. Voltage versus degree of lithiation of TiSnSb curves as for two current regimesfor a loading of (a) 1.2 and (b) 4.9 mg. C rate means 1Li h�1.

Fig. 8. DV (polarisation resistance) versus the current imposed to the cell for thedifferent formulations.

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505502

the electrode structure, leading to electrical wiring of lower qualityand thus faster increase of DV and decrease of the reversible ca-pacity. The CMCwith PAAH system (Fig. 4d) presents the same typeof profile in Qirrev than CMC at pH 3. There is a faster increase of DVfor the system CMC-PAAH compared to standard CMC in line withlarger Qirrev.

3.3. Playing on the conductive additive

In our initial formulation we selected a high content ofconductive additive of 18% and the electrode was kept highlyporous to evaluate the intrinsic TiSnSb performance without hav-ing influence from possible electronic or ionic wiring limitations[27]. In view of increasing the active material content, we investi-gated the impact of the Csp carbon black component in theformulation by comparing the formulation with Csp and theformulation without Csp. Indeed, looking at SEM pictures of theelectrode morphology (Fig. 5) it appears that most of the electronicpercolation is ensured by the network formed by the carbon fibres.Csp is increasing the areal density of contact points between theactive mass and the 3D percolated network of carbon fibers.Regarding the electrochemical properties, investigations gave thefollowing results: i) the carbon black has no significant impact onthe electrode cycle life; ii) the absence of carbon black results in alower discharge capacity. It is lower by ~50 mAh g�1 for loading of2 mg, which can be attributed to a lower fraction of connectedactive particles in the electrode. However, iii) first cycle losses arelower in the electrode containing no carbon black (65 mAh g�1

compared to 95 mAh g�1, or 17% compared to 20%); and iv) thecoulombic efficiency is slightly increased from 98% to 99% in 1 MLiPF6 in EC/PC/3DMC þ 1% VC electrolyte. Supporting data areshown in Supplementary information 3.

3.4. Study of rate performance and the different contributions topolarization

In this part, all the electrodes were prepared with the standardformulation (CMC-in-deionised water). Fig. 6 shows the dischargecapacity versus the active mass loading at different rates. Thegeneral trend is a linear decrease of the capacity with the activemass loading whatever the cycling rate. Fig. 7a and b display thevoltage versus the degree of lithiation of TiSnSb for two differentloadings. With the increase of the current density, the polarisationincreases, the slope of the voltage composition curves get steeperand hits the 0 V limit at lower insertion degree and even earlierwith high loading mass. This increase of polarisation is the mainfactor that explains the decrease of capacity with the currentdensity and the mass loading.

To go deeper in the interpretation of these results, we havedetermined the total cell resistance by plotting the polarizationresistance as a function of the current, I (in A), imposed to the cell(Fig. 8). Linear relationship is observed for the standard CMCformulation, indicating an ohmic behaviour of the electrodes. Thelinear regression gives a slope of 77.4 U and an ordinate at origin of0.31 V corresponding to the potential hysteresis (DV0) of the ma-terial [24]. The polarization thus obeys the following equation,DV ¼ DV0 þ 2R � I, where R is the electrode resistance (38.7 U). Letus note that the system containing amylopectin and the acidifiedsystems (PAAH þ CMC and CMC in buffer pH ¼ 3) tend to followthe same trend. Electrodes containing no Csp give the sameordinate at the origin. However, they show a large dispersion inthe values and a mean slope of 103.8 U, corresponding to anelectrode resistance of 51.9 U. This higher resistance compared toformulations with Csp suggests that the resistance to the lithiuminsertion is influenced by the conductive additive content and is

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505 503

affected by the density of contact points between the percolatednetwork of VGCF and the surface of TiSnSb. The lower capacity ofthe electrodes containing no Csp thus finds its origin in higherelectrode resistance and polarization due to the less numerouscontact points between TiSnSb active mass and the carbonnetwork. In order to precise the different contributions to theelectrode resistance electrochemical impedance spectroscopy (EIS)measurements were also performed (in three electrodes configu-ration) on two electrodes with the standard formulation (withCMC and the Csp þ VGCF mixture) but with different active massloading (1.2 and 4.5 mg). These results are reported and discussedin Supplementary information 4.

3.5. Influence of calendaring

The fairly porous electrodes were also densified by uniaxialpressure in order to increase their volumetric capacity. In Fig. 9a arepresented the porosity as well as the electrode volumetric capacity(calculated by multiplying the bulk capacity of TiSnSb by the vol-umic fraction occupied by the active material) versus the pressureapplied on the electrode. The initial porosity (before pressing) isclose to 60e70% and corresponds to a capacity of about500 mAh cm�3. The porosity can be decreased down to 30% whenapplying high enough pressures and leads to a volumetric capacityof about 1100 mAh cm�3. The decrease of the porosity is not linear

Fig. 9. (a) Global porosity of the electrodes with standard CMC binder vs. the uni-axially applied pressure, (b) Cycle life versus porosity.

with pressure: it shows a strong decrease at the beginning and aplateau at about 45% of porosity in the intermediate range ofpressures.

In Fig. 9b the cycle life is presented versus the electrode porosity.Let us note the linear decrease of the cycle life with the porosity, asobserved for silicon-based electrodes [28]. Such behaviour seems tobe specific to electrodes based on active materials showing volumechanges upon lithium (de)insertion. A high porosity or low volumefraction of active mass is required to allow the reversible breathingof the particles [26,29]. The value of 45% global porosity corre-sponds to a good compromise in terms of cycle life (more than 70cycles as shown in Fig. 9b) and volumetric capacity (around850 mAh cm�3 shown in Fig. 9a). This value can be directlycompared to a graphite electrode which delivers 550 mAh cm�3

(30% of porosity, 350 mAh g�1 and a density of 2.2 g cm�3). Finally,let us note that densification does not induce any changes in therate-capability behaviour (not shown). This result is quite unusualas densification of highly porous electrodes usually improveselectronic wiring and rate performance [27]. It tends to demon-strate that the electronic wiring is already excellent as confirmed bythe very low electronic resistance of these electrodes(Supplementary information 4). Furthermore, the irreversible los-ses are rather unaffected by densification (results not shown).

Fig. 10. Capacity, polarization and cumulative irreversible losses vs. cycle number in1 M LiPF6 EC/PC/3DMC þ 1% VC þ 10% FEC of TiSnSb/NMC full cells. TiSnSb electrodewith CMC standard formulation and two different electrode loadings a) 1.3 mg and b)4.2 mg).

Table 2Theoretical energy density of 18,650 cells for LiCoO2/Graphite and LiCoO2/TiSnSb systems as a function of the surface capacity and porosity of the negative electrode. Theporosity of the positive electrode is 15%. The composition of the electrodes (wt%) are given in the experimental section. The thickness of the copper and aluminium currentcollector is 10 and 15 mm, respectively. The thickness and porosity of the separator is 20 mm and 50%, respectively. The mass of the 18,650 can is 6 g. Its height and diameter are6.5 and 1.8 cm, respectively.

LiCoO2/Gr(3.8 V)

LiCoO2/Gr(3.8 V)

NMC/TiSnSb(3.3 V)

NMC/TiSnSb(3.3 V)

NMC/TiSnSb(3.3 V)

NMC/TiSnSb(3.3 V)

Surface capacity (mAh cm�2) 2.5 0.8 2.5 0.8 2.5 0.8Porosity of the negative electrode 20% 20% 60% 60% 40% 40%Capacity of the cell (Ah) 3.2 Ah 1.9 Ah 3.5 Ah 2.5 Ah 3.95 Ah 2.7 AhWeight of the 18,650 cell (g) 50.6 g 45.5 g 53.4 g 56.1 g 57.6 g 58.6 gGravimetric energy density (Wh kg�1) 240 Wh kg�1 160 Wh kg�1 216 Wh kg�1 147 Wh kg�1 226 Wh kg�1 152 Wh kg�1

Volumetric energy density (Wh l�1) 735 Wh l�1 436 Wh l�1 700 Wh l�1 498 Wh l�1 790 Wh l�1 538 Wh l�1

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505504

3.6. Full Li-ion cells and energy density calculation

To evaluate the practical feasibility of a battery, coin cells wereassembled with TiSnSb as the negative electrode and NMC as thepositive electrode. The cycling conditions are the following: the cellvoltage varies between 4.2 V and 2.0 V at the current of77 mA g�1_TiSnSb, the full charge/discharge taking 7 h (based oninitial capacities). In full cell configuration, it was decided to addFEC as an additive in the electrolyte since it has been demonstratedin Ref. [3] that the electrochemical properties of TiSnSb in FECþ VCelectrolyte gave a much better cycling ability. Our objective in thispart is not to detect the mechanisms of electrode failure but moreto demonstrate the potentialities TiSnSb as a new negative elec-trode for Li-ion batteries. In Fig. 10, the results with two differentTiSnSb mass loading (1.3 and 4.2 mg cm�2) corresponding to0.8 mAh cm�2 and 2.7 mAh cm�2 surface capacity, respectively, arepresented. NMC electrodes were balanced in order to match thecapacity of the two different loadings. In good agreement with thetheoretical capacity the first discharge capacity is equal to 0.84 and2.76 mAh cm�2. A first irreversible capacity is observed of respec-tively 27% and 24% for thin and thick electrodes. These values areslightly higher than those measured with electrodes versus lithium(Table 1) and can be due to the addition of 10% of FEC that con-tributes to the formation of a thicker but more protective SEI layers.Upon cycling, we observe for both electrodes a gradual decrease ofcapacity upon cycling with about 60% of capacity retained after 200cycles for the thicker electrode. Contrary to the electrodesmeasured versus lithium, the capacity is not stable versus cyclenumber and gradually fades, while in half-cell a fast decrease of thecapacity at the end of life is observed. The fading of the capacity infull cell can be explained by the progressive delithiation of thepositive electrode as a consequence of the loss of lithium ionswithin the liquid electrolyte degradation products, phenomenonthat is not significant with lithium electrodes in half-cell since ahuge excess of lithium is present in that case. Interestingly, it wasnot observed any increase of the polarization whatever the activemass loading contrary to measurements in half-cells. However,half- and full-cells were not cycled with the same electrolyte. Wemay assume that the constancy of the polarization in full cells isdue to the addition of FEC, which forms a more stable SEI and re-sults in lesser degradation of the liquid electrolyte after the 1stcycle. As a consequence, the electrical contacts are not degradedupon cycling.

Finally, based on these results, the theoretical energy density ofa 18650 cell with TiSnSb electrodes was calculated using a home-made program as a function of active mass loadings and porosityand compared with the state-of-the-art 18650 LiCoO2/Graphite Li-ion cells that has been optimized for more than 20 years. The re-sults are presented in Table 2. Theoretically, 147Wh kg�1 of full cellbatteries can be obtained with thin electrodes whereas216 Wh kg�1 for the thicker electrodes with 60% of porosity in the

negative TiSnSb electrode, values that are below these of LiCoO2/Graphite cells. However, if the porosity is decreased to 40%, thevolumetric energy density can be raised above that of the state-of-the-art. Nevertheless, decreasing the porosity unfortunately de-creases the cyclability of the TiSnSb electrode.

4. Conclusion

Amongst the various binders used to formulate TiSnSb-basednegative electrodes for lithium-ion batteries, CMC in deionisedwater remains the best compromise in terms of 1st cycle irre-versible losses, reversible discharge capacity and cycle life. The rateperformance of the electrodes is limited by the polarization voltageof the electrode, which is the sum of two terms: the voltage hys-teresis of TiSnSb and an ohmic resistance that is dependent on theamount of conductive additive. The cycle life is limited due to thesimultaneous (i) precipitation of liquid electrolyte degradationproducts, to the (ii) variation in the texture of the electrode as aconsequence of TiSnSb particles volume variation, which results ina degradation of the electrical wiring of the active mass. The latterfactor is emphasized in electrode with non-homogeneous binderdistribution. The densification of the electrodes, originally veryporous (60%), down to 45% porosity does not play on the rateperformance but decreases the cycle life, showing that as for sili-con, a high porosity or low volume fraction of active mass isrequired to allow the reversible breathing of the active particlesshowing volume changes upon lithium (de)insertion. Coin cellsassembled with TiSnSb as the negative electrode and NMC as thepositive electrode and cycled in 1 M LiPF6 EC/PC/3DMC þ 1%VC þ 10% FEC electrolyte show a 1st cycle irreversible loss of24e27% and a about 60% of capacity retained after 200 cycles withan initial 2.7 mAh cm�2 surface capacity. Theoretical calculationsshow that a gravimetric energy density nearly equal to and avolumetric energy density superior by about 10% to that of thestate-of-the-art LiCoO2/Graphite system could be reached with theLiNi1/3Co1/3Mn1/3O2/TiSnSb for 2.7 mAh cm�2 surface capacity andporosity of TiSnSb electrode equal to 40%. As prospects, the trendsthat need to be followed are: identifying an electrolyte compositionsuited to TiSnSb to substantially decreases the irreversible losses;finding a composite electrode architecture allowing the reversiblefunctioning of TiSnSb at low electrode porosity.

Acknowledgements

This research was performed in the framework of ALISTORE-ERIand the ANR Program no. ANR-11BS09-009-01.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2014.10.051.

H.A. Wilhelm et al. / Journal of Power Sources 274 (2015) 496e505 505

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