Addressing electrode degradation issue in high negative to positive electrode capacity ratio lithium-ion batteries using water-in-salt electrolyte

Abstract

In this study, we investigate the underlying causes of degradation in Li4Ti5O12//LiMn2O4 cells with an N/P ratio of 0.9 utilizing 19.44 m LiN(SO2CF3)(2) and 8.33 m LiN(SO2CF2CF3)(2) (Li(TFSI)(0.7)(BETI)(0.3)center dot 2H(2)O), a hydrate-melt electrolyte. We identify the formation of hydrofluoric acid (HF) during electrochemical reactions as a key factor leading to both Mn dissolution from the LiMn2O4 cathode and rapid self-discharge at room temperature. To address these challenges, we apply a calcium fluoride (CaF2) coating to the electrodes, designed to scavenge HF by reacting to form calcium bifluoride (Ca(HF2)(2)). This reaction underscores the unique quasi-non-aqueous nature of Li(TFSI)(0.7)(BETI)(0.3)center dot 2H(2)O, which facilitates chemical processes not possible in traditional aqueous electrolytes. Electrochemical evaluations demonstrate that the CaF2-coated electrode exhibits improved capacity retention and higher coulombic efficiency than their uncoated counterparts. Despite these enhancements, the rapid self-discharge issue remains, indicating that additional factors contribute to this phenomenon and require further investigation. Our findings highlight the potential of water-in-salt systems, particularly the Li(TFSI)(0.7)(BETI)(0.3)center dot 2H(2)O electrolyte, in advancing lithium-ion battery technology by leveraging their distinct chemical environment. This study provides insights into the mechanisms affecting the stability and performance of hydrate-melt electrolyte for exploiting quasi-non-aqueous systems in energy-storage applications.