The effect of electrolyte temperature on the passivity of solid electrolyte interphase formed on a graphite electrode Original Research Article

The effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated in 1 M LiPF6-ethylene carbonate/diethyl carbonate (50:50 vol.%) electrolyte, using galvanostatic charge–discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge–discharge curves at 20 °C evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature from 0 to 40 °C. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to structural damages. From both increase of the relative amount of Li2CO3 to ROCO2Li and decrease of resistance to the lithium transport through the SEI layer with increasing SEI formation temperature, it is reasonable to claim that, due to the enhanced gas evolution reactions during transformation of ROCO2Li to Li2CO3, the rising SEI formation temperature increased the number of defect sites in the SEI layer. From the analysis of HRTEM images, no significant structural destruction in bulk graphite layer was observed after charge–discharge cycles. This means that solvated lithium ions were intercalated through the defect sites in the SEI, at most, into the surface region of the graphite layer.