Synergistic effect of lithium salt and trimethyl phosphate for enhanced interface stability in solid-state lithium metal batteries

Polymer electrolytes hold significant potential in solid-state battery applications due to their high flexibility and scalable manufacturing capabilities. However, it is still necessary to address the safety problems and poor rate performance. Herein, the synergistic effect of trimethyl phosphate (TMP) and lithium difluoro(oxalate)borate (LiDFOB) in the in-situ fabricated poly-butyl acrylate (PBA) quasi-solid electrolyte (QSE) is investigated. The non-flammable additive, TMP, has proven to combine with LiDFOB in the electrolytes as TMP-DFOB⁻ pair. The TMP-DFOB⁻ pair can promote the formation of a B–O, B–F, LiF, and Li₃P containing solid electrolyte interphase (SEI) on the lithium anode, enabling stable lithium striping-plating processes. Besides, it enhances the formation of a LiF-rich cathode electrolyte interphase (CEI) and promotes intimate contact between the cathode active materials and PBA-QSE, thereby enhancing the rate performance. Therefore, the Li/PBA-LiDFOB-0.1TMP/LiFePO₄ (LFP) cell delivers discharge capacity of 132 mAh g⁻¹ with a remarkable capacity retention of 95.68 % after 500 cycles at 1 C. Even at 4 C, the discharge capacity reaches 92.1 mAh g⁻¹. This non-flammable PBA-LiDFOB-TMP QSE exhibits potential application in high energy density systems. Furthermore, this work provides new insights into strategies for improving rate performance from lithium salts and functional additives aspects.