Silicon is regarded as the most promising negative electrode material for next-generation lithium-ion batteries because of the high theoretical capacity. We have developed a Si powder with a flake shape (Si LeafPower®, Si-LP, OIKE & Co., Ltd.) that is 100 nm thick with a 3-5 μm lateral dimension.^[1-3] The Si-LP electrode demonstrates a superior cycle performances because the unique flake structure resists pulverization during charge and discharge cycling. For further improvement of cycle performances, inhibition of reductive decomposition of electrolytes on the Si negative electrode is necessary. We tried to suppress the electrolyte decomposition by an artificial surface coating on Si thin films and investigated formation of solid electrolyte interphase (SEI) by in-situ atomic force microscopy (AFM) coupled with a potential sweep.

An amorphous Si thin film with the thickness of 100 nm was deposited by RF magnetron sputtering on a mirror polished Cu substrate. After that, LiF layer was sequentially deposited on the Si film electrode by the RF sputtering. AFM observations coupled with cyclic voltammetry were performed in a liquid immersion sample stage equipped with electrochemical connectors. The Si thin film as a working electrode and Li wires as a counter and a reference electrodes were immersed in the electrolyte of 1 M LiTFSI/EC+DEC (1:1 by vol.). AFM scan was conducted with a conventional contact mode at a room temperature in an Ar-filled glovebox. The potential was managed with a potentiostat/galvanostat and the sweep rate was 0.5 mV s^-1.

In the first cycle of the cyclic voltammetry for the Si film electrode without LiF, a small cathodic current began to flow ~2 V, and a current peak arose around 0.9 V. This current peak disappeared in the second cycle, which indicated an irreversible reductive decomposition reaction of the electrolyte and formation of SEI on the Si electrode. In the AFM image coupled with the potential sweep, surface deposits emerged from ~0.9 V corresponding to the potential of the electrolyte decomposition. On the other hand, a reductive current peak and surface deposits caused by the electrolyte decomposition were not observed in the Si film electrode with LiF coating. Furthermore, the Coulombic efficiencies of the Si film electrodes improved by the LiF-coating. These results indicated that the LiF layer on the Si electrode prevented the electrolyte decomposition, and thus LiF is a very important component for SEI layer.

Acknowledgments: This work was supported by ALCA-SPRING (JST, Japan) and KAKENHI (JSPS, Japan) Grant Number 16H04649.

References: [1] M. Saito et al., J. Power Sources, 196, 6637 (2011); [2] M. Haruta et al., Electrochimica Acta, 224, 186 (2017).; [3] M. Haruta et al., Electrochimica Acta, 267, 94 (2018).