Extreme fast charging and stable cycling of LiMn2O4 – Li4Ti5O12 lithium batteries by suppression of cathode phase changes

Extreme fast charging (XFC, i.e., achieving at least 80% state of charge within 15 minutes) remains as a high-desirability criterion for next-generation lithium batteries. While the anodes, e.g., graphite and lithium, have been historically acknowledged as critical hurdles for fast-charging batteries, the stability of the cathodes under sustained high-rate cycling has not been well studied. In this work, we have investigated the fast cycling (10 - 15 minutes charging time) of LiMn2O4 (LMO) under practical conditions (90% loading of cathode material with an areal capacity of 1 mAh/cm2). We find through advanced characterisation that in the presence of LiPF6-based electrolyte, the high rate cycling of LMO causes a cascade of bulk LiMn2O4→LiMnO2 phase changes, accompanied by oxygen release, initiating electrolyte degradation. Together, these factors play a compounding role in the capacity loss during cycling, even with high-rate compatible anodes, e.g., Li4Ti5O12 (LTO). The combination of a commercial, high-capacity, micro-structured LMO cathode and novel electrolyte, prepared by using a novel lithium salt, lithium 1,1,1,3,3,3, (tetrakis) hexafluoroisopropoxy borate, in a mixture with commercial electrolyte, overcomes these limitations. The performance improvement is attributed to the synergy between the unique cathode structures and the formation of a facilitative boron-rich cathode electrolyte interface. Cycling of the LMO|LTO cells show 98 mAh/g initial discharge capacity at 4C-4D cycling with the novel electrolyte with 91% capacity retention after 1500 cycles, while charging to 90% of total capacity in 10 minutes. This work, for the first time, demonstrates the long-term stability of practical high loading LMO-LTO based Li-ion battery under XFC conditions, which has been upscaled and reproduced in a single-layer pouch cell.