Contribution of Tof-SIMS Ion Profiling to Understanding the Surface and Bulk Modifications of Si Anode As a Function of Electrolyte Composition and Additives
Authors: Swiatowska Jolanta , Pereira-Nabais Catarina , Seyeux Antoine , Ozanam François , Rosso Michel , Cassir Michel , Marcus Philippe 
Published: February 2020

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:

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