Improved Electrochemical Performances of Methylated Amorphous Si Electrode Studied By Tof-SIMS
Authors: Swiatowska Jolanta , Pereira-Nabais Catarina , Dalla Corte Daniel Alves , Rosso Michel , Ozanam François , Gohier Aurélien , Tran-Van Pierre , Seyeux Antoine , Cassir Michel , Marcus Philippe 
Published: February 2020

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

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