The poor cyclability of Si electrodes is attributed to crack formation and pulverization of Si active material due to a large volume expansion and contraction during charging and discharging, respectively. It is thus important to suppress the physical stress induced by the large volume changes to improve the cyclability of Si electrodes. We designed a Si powder with a flake shape (Si LeafPowder^Ò, Si-LP, OIKE & Co., LTD.) with 100 nm in thickness and 3-5 mm in lateral dimension, and have reported that Si-LP electrodes have a high initial discharge capacity (ca. 2,500 mAh g^-1) and good cycleability (Fig. 1) [1].

Another serious problem of Si anode is a swelling of the electrode owing to irreversible microstructural changes of Si nano-flakes upon cycling (Fig. 2). The Si-LP sheets in a composite electrode were substantially deformed during alloying/de-alloying cycling and formed the multi-folded layered structure on repeated cycling. Furthermore, many voids, which could accommodate volume changes of Si, were formed on repeated cycling. The multi-folded layered structure with many voids enabled the stable battery operation of the Si-LP electrodes. On the other hand, the thickness of the electrode remarkably increased with cycle number (Fig. 3), which was caused by the deformation of Si-LP sheets and growth of the SEI layer. The electrode swelling was suppressed by the presence of the additives, VC and FEC. The uniform SEI layer formed from the decomposition products of VC or FEC inhibited further decomposition of the electrolyte and suppressed the deformation of the Si-LP sheets. Unfortunately the suppression of the electrode swelling by the additives is not enough, and further suppression of the electrode swelling and/or a new cell design to manage the large volume changes of Si is necessary for practical applications of the Si-LP as a negative electrode in advanced LIBs.

This research was supported by Japan Science and Technology Agency (JST) ALCA-SPRING and Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 16H04649.

References

[1] M. Haruta et al., Electrochim. Acta, 267, 94-101 (2018). DOI: 10.1016/j.electacta.2018.02.062