Optimization of the forming conditions of the solid-state interface in the Li-ion batteries

We optimized the forming conditions of Li-ion batteries from the standpoint of the stability of the solid-state interface (SEI) on the surface of graphite. The optimization processes were focused on the initial two cycles of the Li/graphite half-cell in terms of the current density and the temperature. We evaluated the stability of the SEI from two aspects: (1) self-delithiation at 60 °C of the lithium intercalation compound, which reflects storage stability of the SEI and (2) cycling performance of the Li/graphite cell, which reflects cycling stability of the SEI. It was observed that the low current density and the low temperature favored increasing effectiveness of the SEI formation, and resulted in formation of a stable SEI. However, the SEI formed under such conditions had relatively high impedance. That is, there was a contradictory relationship between the ionic conductivity and the stability of the SEI. Based on the graphite and electrolyte used in this work, we found that the optimum conditions for the formation of the SEI were at the current densities of 10–20 mA/g and at the temperatures of 20–35 °C.