Effects of SEI Formation Additives on Cycle Performance and Surface Morphology in Si-Flake-Powder Anodes

Abstract

We had reported that an 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 and effects of additives, variations of impedance components and the morphology changes 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 in the electrolyte with and without FEC is shown in Fig. 1(a). Capacity retention for Si-LP electrode was substantially improved by the FEC-addition, although the discharge capacity at the 100th cycle decreased to 50% of the initial discharge capacity for the Si-LP electrode without FEC. To understand effects of the FEC-addition, EIS measurements were conducted and the SEI resistance (R SEI) was derived from the impedance spectra obtained at 0.1 V in the charging process. The SEI resistance for the Si-LP without FEC increased with increase in cycle number as shown in Fig. 1(b). By the FEC-addition, the SEI resistance was decreased and its increment with cycle number was substantially suppressed in comparison with the case of additive-free, indicating that reductive decomposition of the electrolyte on the Si-LP electrode was suppressed with a stable SEI film derived from FEC. Cross-sectional SEM images of the Si-LP electrodes after 10 and 30 cycles in the additive-free electrolyte are shown in Fig 1(c) and (d). Deposits of reductive products of the electrolyte were confirmed on Si-LPs. The amount of the deposition increased with cycle number, indicating a continuous decomposition of the electrolyte and a growth of SEI layer. The growth of the SEI layer lades an increment of the SEI resistance. Moreover, we found that the thickness of the Si-LP composite electrode was increased drastically by the growth of SEI layer and bending of Si-LPs, leading to a loss of the electric contact between the Si-LPs. Although Si-LPs bended after cycling, pulverization of Si-LPs was not observed. On the other hand, thinner SEI layer was observed on Si-LPs with the FEC-addition, and the expansion of the composite electrodes was suppressed. The stable SEI layer suppresses increment of the SEI resistance. Ensuring the electric-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 and 16H04649.