Stabilization of Si negative electrode by Li pre-doping technique and the application to a new energy storage system

Abstract

To extend the cyclability of a hybrid capacitor using Li pre-doped Si negative electrodes (NEs) (Si-CAP), we applied a polyimide binder and fluoroethylene carbonate (FEC) as additives in the Li pre-doping step. This binder successfully improved the cyclability by fixing Si nanoparticles as the NE active material on the Cu foil current collector to resist the volume change caused by Li-Si alloying. After Li pre-doping, there were no cracks on the Si NE surfaces, which had been observed in previous work using sodium carboxymethyl cellulose binder. Furthermore, FEC addition into the electrolyte at Li pre-doping improved homogeneity of the Li-Si alloying. As a result, a large amount of Li was pre-doped to the Si NE and a lower open-circuit potential was maintained. A Si-CAP cell of [Li alloyed Si | activated carbon] using polyimide binder exhibited relatively stable charge/discharge behavior and cyclability was maintained up to 800 cycles. Stability of the charge/discharge performance of FEC-containing Si-CAP was improved by formation of a LiF-rich solid-state interphase film on the Si NE. X-ray photoelectron spectroscopy revealed suppression of decomposition of the propylene carbonate solvent by electrochemical reduction.