关键词: 锂离子电池, 磷酸酯, 石墨电极, 不可燃电解液, 低温性能

Abstract:

Lithium-ion batteries are extensively used in various applications such as electric vehicles and electrochemical energy storage systems. However, safety concerns related to flammability and low flash point of commercial carbonate electrolytes limit their broad application. The incorporation of nonflammable flame-retardant diethyl ethylphosphonate (DEEP) into carbonate electrolytes has been shown to effectively reduce the risk of battery fires and explosions by mitigating electrolyte combustion. Nonetheless, the strong interaction between DEEP and Li⁺ leads to the infiltration of DEEP into the first solvated shell layer of Li⁺, contributing to the formation of solid electrolyte interphase (SEI) on the graphite anode. The SEI formed through the reductive decomposition of DEEP provides inadequate electron shielding, failing to halt the ongoing decomposition at the interface and leading to the failure of graphite anodes in DEEP-modified carbonate electrolytes. To address this issue, this study adopts a synergistic strategy, using ethylene carbonate as a strong ligand solvent and linear carbonate as a weak ligand solvent. This approach aims to diminish the interaction strength between DEEP and Li⁺, decrease the proportion of DEEP in the first solvated shell layer of Li⁺, and reduce the decomposition of DEEP on the anode. In the developed DEEP-modified carbonate electrolyte with a conventional concentration (~1.15 mol/L), the graphite anode shows an impressive capacity retention of 95.6% after 150 cycles. In addition, the electrolyte remains fluid at -60 ℃, and the graphite/LiFePO₄ battery retains 49.3% of its capacity after 50 cycles at -20 ℃.

Key words: lithium-ion battery, phosphate, graphite electrode, non-flammable electrolyte, low temperature performance