We had reported that am amorphous silicon flake powder (Si LeafPowder®, Si-LP), of which the lateral dimension and the thickness were ~4 μm and 100 nm, respectively, demonstrated superior cycle performances.^{[1, 2]} Cyclability of the Si-LP electrodes was successfully improved by an addition of solid electrolyte interface (SEI)-forming additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC). However, the capacity fading mechanism of the Si-LP electrodes has not been clarified. In order to understand the capacity fading of the Si-LP, variations of impedance components and the surface morphology associated with a decomposition of the electrolytes were investigated.

Test electrodes were composed of Si-LP (83.3 wt.%), Ketjen Black (5.6 wt.%) and carboxymethyl cellulose sodium salt (11.1 wt.%), and coated on a Cu foil. The loading of the Si-LP composite was approximately 0.4 mg cm^-2. Electrolytes were 1 M LiPF₆ dissolved in a mixture of ethylene carbonate and diethyl carbonate (EC+DEC, 1:1 by vol.) with and without additive of 10 wt.% VC or FEC. Charge and discharge tests were conducted at C/2 rate in the CC-CV mode between 1.5 and 0.02 V using a two-electrode coin-type cell. The electrochemical impedance spectroscopy (EIS) was performed using a three-electrode cell in which impedance spectra were obtained by applying an AC voltage of 10 mV over the frequency range of 0.03 Hz to 300 kHz. Surface and cross-sectional morphologies of the Si-LP electrodes after cycling were observed by a scanning electron microscope (SEM) without air exposure.

Cyclability of the Si-LP electrodes is shown in Fig. 1(a). Capacity retention for Si-LP electrodes was substantially improved by the VC- and FEC-addition, although the discharge capacity at the 100th cycle decreased to 50% of the initial discharge capacity for the Si-LP electrode in the additive-free electrolyte. To understand effects of VC- and FEC-addition, EIS measurements were conducted. Impedance components of the SEI layer, the electronic contact and the charge transfer were derived from the impedance spectra obtained at 0.1 V in the charging process. The SEI resistance of the Si-LP in the additive-free electrolyte increased with increasing cycle number as shown in Fig. 1(b), and the contact resistance also increased. The increments of the SEI and contact resistances with cycle number were suppressed by the VC- and FEC-addition.

A thick SEI layer, originated from a continuous decomposition of the electrolyte, was observed by SEM on the Si-LP cycled in the additive-free electrolyte. Furthermore, we found that the thickness of the Si-LP composite was increased drastically by the growth of SEI layer, leading to a loss of the electronic contact between the Si-LPs. On the other hand, thinner SEI layers were observed on the Si-LPs cycled in the VC- and FEC-added electrolytes, and the expansion of the electrodes was suppressed. The continuous decomposition of the electrolyte was prevented by the stable SEI layer formed from decomposition products of VC and FEC, and therefore the increments of the SEI and contact resistances with cycle were suppressed. Ensuring the electronic-conducting path between active materials by the stable SEI layer is necessary to achieve a superior cycle performance of Si anodes.

Acknowledgments: This work was supported by JST-ALCA-SPRING and JSPS-KAKENHI Grant Number 25820335.

References: [1] M. Saito et al., J. Power Sources, 196, 6637 (2011).; [2] M. Haruta et al., Electrochemistry, 83, 837 (2015).