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The surface chemistry is one of the most important factor influencing the electrochemical performance of LiBs. The modification of the chemical composition of electrode materials is related to formation of a passive layer, named as a Solid Electrolyte Interphase (SEI) layer1, due to decomposition of electrolyte mostly during the first cycle of charge/discharge. The formation of the SEI layer is irreversible, consumes some amount of electrolyte, and leads to irreversible capacity loss, lower rate capability, cyclability, or electrode degradation. Although a multiple research activities in this domain, there are still a lot of ambiguities concerning the SEI layer, so the famous statement of Martin Winter “The Solid Electrolyte Interphase - The Most Important and the Least Understood”2 is still valid. Various spectroscopic techniques, such as XPS, AES, FTIR, IRAS, or Raman spectroscopy, are used for analysis a mechanism of the SEI layer formation and identification of the SEI components. We present here some recent studies on the surface chemistry of Si thin film electrode material (prepared by Plasma-Enhanced Chemical Vapor Deposition - PECVD) as a function of cycling and electrolyte composition by means of the Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Depending on type of electrolytes and/or the electrolyte additives like vinylene carbonate (VC) and monofluoroethylene carbonate (FEC), having polymerizable features,3,4 it is possible to improve the mechanical properties of the SEI layer and the cyclability of the Si electrode undergoing big volume variations. ToF-SIMS presents several advantages: is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi+) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. Although the big volume variations occurring during the 100 cycles of discharge/charge of the Si thin film electrode (100 nm), the higher specific capacity can be observed in PC/LiClO4 1M than in EC:DMC (1:1)/LiPF6. The ToF-SIMS ion depth profiles show no significant modifications (i.e. no significant volume increase) of the Si thin film electrode after 100 cycles in PC/LiPF6 1M when comparing to PC/LiClO4 1M. Exchanging LiPF6 for LiClO4 in EC/DMC electrolyte leads to important increase of Li- and CO3 - ion signal intensities at the surface and in the bulk of Si thin film electrode, which can be attributed to accumulation of Li-like components (lithiated silicon) and products of electrolyte decomposition as already observed in previous studies.5,6 The enhanced lithiation of Si electrode in a case of PC/LiClO4 1M can explain its higher capacity. The additives (VC or FEC) introduced to PC/LiClO4 1M electrolyte lead to better columbic efficiency and better cyclability of Si electrode. The ToF-SIMS ion profiles show more significant volume changes of Si electrode in a case of cycling in PC/LiClO4 1M electrolyte containing the FEC than the VC additive. The more intense ToF-SIMS Li- ion profile observed for Si electrode cycled in PC/LiClO4 1M with FEC additive can explain also the higher capacity than those obtained in electrolyte with VC addition. In a case of EC:DMC (1:1)/LiPF6 1M electrolyte much lower cycling stability can be observed than in PC/LiClO4 1M. The cycling stability is significantly improved after addition of FEC (2%) to EC:DMC (1:1)/LiPF6 1M electrolyte. The volume increase of Si electrode after 100 cycles estimated from ToF-SIMS sputtering time is much less important in EC:DMC (1:1)/LiPF6 1M than in PC/LiClO4 1M. The FC additive into the EC:DMC (1:1)/LiPF6 1M does not significantly influence the morphological modifications (volume increase) of Si thin film electrode. References: 1. E. Peled, J Electrochem Soc 126 (1979) 2047. 2. M. Winter, Z Phys Chem 223 (2009)1395. 3. M. Ulldemolins, F. Le Cras, B. Pecquenard, V. P. Phan, L. Martin, H. Martinez, J. Power Sources 206 (2012) 245. 4. V. Etacheri, O. Haik, Y. Goffer, G. A. Roberts, I. C. Stefan, R. Fasching, D. Aurbach, Langmuir 28 (2012) 965. 5. C. Pereira-Nabais, J. Światowska, A. Chagnes, F. Ozanam, A. Gohier, P. Tran-Van, C.–S. Cojocaru, M. Cassir, P. Marcus, App. Surf. Sci. 266 (2013) 5 6. C. Pereira-Nabais, J. Światowska, M. Rosso, F. Ozanam, A. Seyeux, A. Gohier, P. Tran-Van, M. Cassir, P. Marcus, ACS App. Mat. & Interf., 6, 2014, 13023.

Understanding the electrode processes occurring at the electrode/electrolyte interface and in the bulk electrode material is necessary for developing the electrochemical performances of lithium-ion (LIB), sodium-ion, sulfur or metal-air batteries. The main electrode processes are insertion/extraction reactions that induce changes in the host electrode materials; they are accompanied by decomposition of electrolyte that leads to formation of a solid electrolyte interphase (SEI) layer [1]. Apart from the electrode processes, the electrochemical performance of batteries can also be significantly dependent on ionic transport, thus a good understanding of diffusion processes is also necessary. Two principal surface-sensitive techniques are particularly suitable for analyzing surface reactions at electrodes: X–ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). As XPS provides information on chemical composition on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of thicker surface layers and bulk materials. Depth profiling can be either performed by XPS or ToF–SIMS using Ar+ or Cs+ ions, respectively, for sputtering. ToF-SIMS is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi+) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF-SIMS can also serve as a new and more direct method than the commonly used electrochemical methods (i.e. electrochemical impedance spectroscopy, cyclic voltammetry or galvanostatic methods) for precise measurement of ion diffusion in electrode materials. Our approach is to use model thin-film electrodes (i.e. from few tens to few hundreds of nm directly grown on a current collector) with a good surface finishing and having a roughness suitable for analysis by surface sensitive techniques. The application of model thin-film electrode having enlarged surface-to-volume ratio provides clearer and more comprehensible insight into electrode/electrolyte interface reactions without complications from current percolators or binding agents that are used in bulk composite electrode materials. This allows for studies of intrinsic electrochemical and interfacial processes occurring on the electrode surfaces. In the present work, XPS and ToF-SIMS have been used to investigate the chemical and volume modifications of thin-film negative-electrode materials. As an example, two types of high capacity negative-electrode materials will be presented: a conversion-type electrode consisting of iron oxide (with a theoretical capacity of 1007 mAh/g), and an alloying-type electrode consisting of silicon (with a theoretical capacity of 3579 mAh/g). Both negative-electrode materials are interesting and important candidates for application in LiBs due to their abundance and environmental friendliness. For the first time, ToF-SIMS depth profiling have been applied to measure an apparent diffusion coefficient of lithium ions (DLi+) into these electrodes. The DLi+was calculated from the finite integration of Fick’s second law for one-dimensional diffusion. Regardless of the type of electrode material, XPS analysis shows strong intensity attenuation and binding energy positive shift of the principal core level peak corresponding to the main electrode component (Si2p in the case of Si-electrode and Fe2p in the case of Fe2O3-one), confirming the formation of the SEI layers on the surfaces after cycling in PC-LiClO4. The chemical composition of the SEI formed on the two different electrodes is essentially identical, with a principal Li2CO3 constituent and a minor quantity of alkyl carbonates (ROCO2Li) as evidenced by the C1s, O1s and Li1s core level peaks [2,3]. The SEI layer chemistry principally depends on the electrolyte composition [2]. Both, XPS and ToF-SIMS, evidence a dynamic increase/decrease of the SEI layer thickness upon lithiation/delithiation, superimposed to a continuous increase upon cycling. The data reveal the formation of duplex-like SEI layer structure, with the organic carbonate species in the outer region and inorganic salt decomposition products in the inner part of the SEI layer. The ToF-SIMS depth profiles also show irreversible Li trapping in the bulk electrodes and volume variations with the degree of lithiation and the number of cycles [2,4]. These modifications can be detrimental to electrode cycling and capacity retention. A low DLi+ in the order of 10-15 cm2/s calculated from ToF-SIMS depth profiles for these two types of electrodes, indicates that the rate of lithiation is slower than for intercalation-type materials and can have an important influence on their electrochemical performance. References [1] E.Peled, J.Electrochem.Soc.126(1979)2047. [2] C.Pereira-Nabais, J.Światowska, A.Chagnes, F.Ozanam, A.Gohier, P.Tran-Van, C.–S.Cojocaru, M.Cassir, P.Marcus, App.Surf.Sci.266(2013)5. [3] B.Tian, J.Światowska, V.Maurice, S.Zanna, A.Seyeux, L.H.Klein, P.Marcus, J.Phys.Chem. C117(2013)21651. [4] B.Tian, J.Światowska, V.Maurice, S.Zanna, A.Seyeux, L.H.Klein, P.Marcus, Langmuir (2014), DOI:10.1021/la404525v.

The electrochemical performance of electrode materials in lithium-ion, sodium-ion, sulfur or metal-air batteries can be significantly influenced by the diffusion properties of the exchanged ions. The application of new types of electrodes or battery components (i.e. separators) thus requires a good understanding of the diffusion mechanisms during the discharge/charge process. For lithium-ion batteries (LiBs), the Li-ion distribution in a host electrode material is difficult to measure due to the lack of sensitive and precise techniques. In common practice, the variation of the Li-ion concentration in the host material is deduced from the potential distribution using the Nernst equation and electrochemical characterization techniques such as electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), potentiostatic intermittent titration (PITT), galvanostatic intermittent titration (GITT) and potential relax techniques (PRT). However, it is necessary to develop new and more direct methods for precise measurement of Li-ion diffusion in bulk electrode materials based on precise depth profiling. In the present work, Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS) has been applied for the first time to the measurement of the apparent diffusion coefficient of lithium ions into a conversion-type thin film electrode material (α-Fe2O3). Hematite (α-Fe2O3) is an interesting and important candidate transition metal oxides for application as anode material in LiBs due to its high theoretical capacity (1007 mAh g-1), abundance and environmental friendliness [i,ii,iii,iv]. However, this material suffers from poor electronic/ionic conductivity, which is the main obstacle for improving the charge/discharge rate capability of the battery. So the future of such a conversion-type material in LiBs applications relies on mastering the kinetics of the electrode lithiation reaction [v,vi ]. The analysis of the diffusion coefficient of Li ions in the bulk of this electrode material was based on the in-depth variation of the Li-ion concentration obtained by ToF-SIMS depth profiling a partially lithiated sample (Figure 1). The apparent diffusion coefficient calculated from the infinite integration of Fick’s second law for one-dimensional diffusion was of the order of 10-15 cm2 s−1. This value is in agreement with values obtained from cyclic voltammetry and impedance data. This low value for Li-ion diffusion evidences that the rate of lithiation in this conversion-type material is slower than in intercalation or alloying-type materials. It should be noted that the direct methodology used here for determining the diffusion coefficient can be applied to any type of electrode material or component of the electrochemical cell. It enlarges the horizon for the study of Li diffusion kinetics into electrode for LiBs. Figure 1. (a) ToF-SIMS negative ion depth profiles of Li- ions on the sample discharge to 0.84 V; (b) Normalized in-depth variation of the Li-ion concentration. Acknowledgements: Region Ile-de-France is acknowledged for partial support for the ToF-SIMS equipment. [[i]] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496-499. [[ii]] P. Poizot, S. Laruelle, S. Grugeon, J.-M. Tarascon, Journal of the Electrochemical Society 149 (2002) A1212-A1217. [[iii]] D. Larcher, C. Masquelier, D. Bonnin, Y. Chabre, V. Mason, J.B. Leriche, J.-M. Tarascon, Journal of the Electrochemical Society 150 (2003) A133-A139. [[iv]] D. Larcher, D. Bonnin, R. Cortes, I. Rivals, L. Personnaz, J.-M. Tarascon, Journal of the Electrochemical Society 150 (2003) A1643- A1650. [[v]] X. Xu, R. Cao, S. Jeong, J. Cho, Nano Letters 12 (2012) 4988-4991. [[vi]] A.S. Aricò, P. Bruce, B. Scrosati, J.-M. Tarascon, W.V. Schalkwijk, Nature Materials 4(2005) 366-377.

Silicon is considered as a promising anode material for Li-ion batteries due to its ability to insert large amounts of lithium, delivering a very high theoretical specific capacity of 3579 mAh/g, which is almost 10 times higher than graphite electrode (372 mAh/g). Nevertheless, a high volume variation (280 % for Li15Si4) during lithiation leading to a morphological damage of electrode materials and a huge capacity loss (of about 30%) observed during the first charge are the major drawbacks for application of Si as anode material. These damages can be considerably decreased or avoided by using nanosized materials, such as Si nanowires (SiNW), allowing better accommodation of volume variation.[i],[ii] Another solution to limit the consequences of these damages is to use electrolyte additives like vinylene carbonate (VC) and monofluoroethylene carbonate (FEC), having polymerizable features and the possibility of forming a SEI layer with improved mechanical properties.[iii],[iv] A new way to improve the Si electrode performances is the modification of the chemical bulk composition of the Si electrode material. In this work, we present the improved electrochemical performance of methylated amorphous silicon (a-Si0.9(CH3)0.1:H), a new type of Si-based material containing methyl (CH3) groups. In order to focus on the material itself, we compare thin-film electrodes of this material[v] to similar electrodes of hydrogenated amorphous Si (a-Si:H),[vi] without using additives or nanosize shaping of the material. The chemical modifications of these two electrodes induced by the electrochemical lithiation process were studied by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) negative-ion depth profiles. Experiments were performed in propylene carbonate (PC, purity > 99.7 %, 30 ppm H2O, Sigma-Aldrich) containing 1 M LiClO4 (purity > 99.99%, battery grade, Sigma-Aldrich). As compared to a-Si:H, the modified chemical composition of a-Si0.9(CH3)0.1:H has an influence on the electrochemical behaviour by shifting the lithiation plateau from 200 mV to 160 mV. The most important difference between the two types of electrodes is the instantaneous formation of a thin, stable and homogenous Solid Electrolyte Interphase (SEI) layer on a-Si0.9(CH3)0.1:H electrode during the first cycle. On the contrary, a thick unstable SEI layer is formed on a-Si:H, with possible dissolution/oxidation and cracking of the layer formed during the first cycle and continuous uptake of electrolyte decomposition products during the following discharge/charge cycles. The ion-depth profiles obtained by ToF-SIMS also evidence significant volume variations and swelling of a-Si:H electrodes which can lead to large morphological modifications, electrode cracking. The a-Si0.9(CH3)0.1:H electrode also shows much lower irreversible capacity during the first 100 discharge/charge cycles, as compared to the a-Si:H electrode. Finally, the diffusion coefficient of Li ions (determined from Li- ToF-SIMS profile) is found to be one order of magnitude higher in a-Si0.9(CH3)0.1:H than in a-Si:H, confirming its improved electrochemical performance. References [i] C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zang, Y. Cui, Nat. Nanotechnol. 3, 31 (2008). [ii] B. Laïk, D. Ung, A. Caillard, C.-S. Cojocaru, D. Pribat, J.-P. Pereira-Ramos, J. Solid State Electrochem. 14, 1835 (2010). [iii] M. Ulldemolins, F. Le Cras, B. Pecquenard, V. P. Phan, L. Martin, H. Martinez, J. Power Sources 206, 245 (2012). [iv] V. Etacheri, O. Haik, Y. Goffer, G. A. Roberts, I. C. Stefan, R. Fasching, D. Aurbach, Langmuir 28, 965 (2012). [v] L. Touahir, A. Cheriet, D. Alves Dalla Corte, J.-N. Chazalviel, C. Henry de Villeneuve, F. Ozanam, I. Solomon, A. Keffous, N. Gabouze, M. Rosso, J. Power Sources 240, 551 (2013). [vi] C. Pereira-Nabais, J. Światowska, A. Chagnes, F. Ozanam, A. Gohier, P. Tran-Van, C.–S. Cojocaru, M. Cassir, P. Marcus, App. Surf. Sci. 266, 5 (2013).

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