Synthesis of Trisiloxane with the Dioxaborolane Group as a Cathode Film-Forming Electrolyte Additive for High-Temperature LiMn₂O₄/Li₄Ti₅O₁₂ Batteries

LiMn₂O₄ batteries have been widely applied as various portable electronic devices and electric vehicles owing to the merits of low cost, high operating voltage, excellent rate capability, and environmental friendliness. However, the poor performance at elevated temperatures remains a serious technical challenge in terms of commercial application purposes. A borate-containing trisiloxane compound of TSMBO is designed and synthesized as a cathode film-forming electrolyte additive to improve the electrochemical performances of LiMn₂O₄/Li₄Ti₅O₁₂ batteries, especially at high temperatures of 55 °C. Atomic force microscopy measurement confirms that the trisiloxane moiety in TSMBO can construct a cathode electrolyte interface (CEI) with higher mechanical strength and better flatness compared to the disiloxane moiety in the TSMBO analogue with a similar chemical structure. The robust CEI film on the surface of the LiMn₂O₄ cathode and the inhibited hydrolysis of LiPF₆ in the electrolyte significantly suppress the dissolution of Mn from the LiMn₂O₄ cathode and maintain the structural integrity of the LiMn₂O₄ lattice over cycling. Thus, the LiMn₂O₄/Li₄Ti₅O₁₂ coin cell using the TSMBO-containing electrolyte with an optimized addition level of 0.5 wt % exhibits a higher capacity retention of 49.3% compared with 34.3% for the baseline electrolyte after 300 cycles under 1C rate at 55 °C. The LiMn₂O₄/Li₄Ti₅O₁₂ pouch cell tests show excellent high-temperature and cycling performance at 55 °C and a higher capacity retention of 90.4% after 500 cycles at 2C compared to 79.7% for that with the baseline electrolyte after 430 cycles at 2C. This work demonstrates that TSMBO is a promising electrolyte additive for practical use to improve the cycling stability of LiMn₂O₄/Li₄Ti₅O₁₂ batteries at elevated temperatures.