Film-forming mechanism of 1,3-propanediolcyclic sulfate as a bifunctional additive for 4.45 V graphite/LiCoO2 batteries

Abstract

Increasing the cutoff voltage is efficient to increase the energy density of LiCoO2-based lithium-ion batteries to meet consumers' demand for electronic devices. However, higher cut-off voltage may lead to continuous decomposition of electrolyte and shorten the battery's cycle life. Herein, 1,3-propanediolcyclic sulfate (PCS) is designed as a film-forming additive to improve the electrochemical performance of high-voltage graphite/LiCoO2 cells. The combined density functional theory (DFT) calculations and experimental results indicate that PCS participates in the film formation on both LiCoO2 cathode and graphite anode side to regulate the interfacial reaction and inhibit the corresponding gas production. Thus, the capacity retention of 4.45 V graphite/LiCoO2 pouch cells cycled at 45 °C after 300 cycles is elevated from 59.5% to 80.1% by incorporating 1.0 wt% PCS additive into the base electrolyte. It is found that PCS follows the C-O bond breaking mechanism on the positive electrode and S-O bond breaking mechanism on the negative electrode, respectively. In addition, PCS may undergo H-transfer reaction at the cathode in the first cycle because the -O-SO2- group appears as oxidation product in ex-situ X-ray photoelectron spectroscopy (XPS) results. This finding provides new insights for understanding the mechanism of PCS acting as a high-voltage additive.