We investigated the impact of electrolyte difference on lithiation and delithiation properties of a Li1.00Si electrode to improve the Coulombic efficiency (CE) of Si-based electrodes. The results of X-ray diffraction, Raman spectroscopy, and soft X-ray emission spectroscopy demonstrated that a portion of the Li in Li1.00Si desorbed by simply immersing the electrode in an ionic-liquid electrolyte, that is, the phase transition of Li1.00Si to Si occurred. In contrast, this phenomenon was not confirmed in an organic-liquid electrolyte. Instead, the desorbed Li was consumed for the formation of a surface film; thus, the Li in Li1.00Si did not elute into the electrolyte. The addition of vinylene carbonate (VC) to the ionic-liquid electrolyte suppressed the phase transition of Li1.00Si to Si. Although the Li1.00Si electrode showed a low initial CE and poor cycling performance in a VC-free electrolyte, the electrode exhibited a high CE and a remarkable cycle life in the VC-added electrolyte. It was considered that no desorption of the mechanically added Li in Li1.00Si contributed to the superior cycle life; thus, the characteristic ductility, malleability, and high electrical conductivity of lithium silicide should improve the electrochemical performance.
Keywords: film-forming additive; ionic-liquid; lithium silicide; lithium-ion batteries; soft X-ray emission spectroscopy.