Improving cyclic stability of LiNi0.6Co0.2Mn0.2O2-SiOx/graphite full cell using tris(trimethylsilyl)phosphite and fluoroethylene carbonate as combinative electrolyte additive

Abstract

Fluoroethylene carbonate (FEC) has been proven an effective electrolyte additive for silicon-based Li-ion cells. However, studies have shown that it will promote the formation of acid during storage and cycling in LiPF6-based electrolyte, thus deteriorating the performance of cell. Herein, tris(trimethylsilyl)phosphite (TMSPi) and fluoroethylene carbonate (FEC) are used as combinative electrolyte additive for LiNi0.6Co0.2Mn0.2O2-SiOx/graphite (NCM622-SiOx/Gr) coin-type full cell. Electrolyte storage experiments show that TMSPi can eliminate the acid present in FEC-containing electrolyte. Theoretical calculations and electrochemical analysis confirm that TMSPi could be oxidized previously on NCM622 to optimize the cathode electrolyte interphase (CEI) and FEC can also be reduced in advance on SiOx/Gr to modify the solid electrolyte interphase (SEI). Such a full cell with 3% TMSPi + 5% FEC as electrolyte additive has the best electrochemical performance; the specific discharge capacity is maintained at 156.1 mAh g−1 after 200 cycles at rate of 1 C between 2.8 and 4.25 V. X-ray photoelectron spectroscopy results of electrode surface show that CEI has a certain amount of Li2CO3 and LixPOyFz and lower LiF, while SEI has a certain amount of Li2CO3 and LiF and less LixPOyFz, which could be beneficial to improve the cyclic stability of NCM622-SiOx/Gr full cell.