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

Extreme fast charging (XFC, i.e., achieving at least 80% state of charge within 15 minutes) and high current rate cycling remain high desirability criteria for next-generation lithium batteries. While anodes, e.g., graphite and lithium, have been historically acknowledged as the critical hurdles for fast- charging batteries, the stability of cathodes under sustained high current rate cycling has not been well studied. In this work, we have investigated the fast cycling (≦ 15 mins charging time) of LiMn2O4 (LMO) under practical cycling conditions (i.e., 90% loading of cathode material with an areal capacity of 1 mAh/cm2). We find that in the presence of a conventional LiPF6-based carbonate electrolyte, the high rate cycling of LMO brings forth a cascade of bulk LiMn2O4→LiMnO2 phase changes, accompanied by oxygen release, which initiates electrolyte degradation. Together, these factors play a compounding role in the capacity loss during cycling, even with high current rate compatible anodes, e.g., Li4Ti5O12 (LTO). The application of a novel lithium salt, Lithium 1,1,1,3,3,3, (tetrakis) hexafluoroisopropoxy borate (LiBHFip), in a mixture with commercial electrolyte overcomes these limitations due to the formation of a facilitative boron-rich cathode electrolyte interface. Cycling of LMO|LTO cells with the novel electrolyte shows 91% capacity retention after 1500 cycles, thereby demonstrating the long-term stability of this type of Li-ion battery under XFC conditions (e.g., 66% charge within 10 mins) for the first time with 1 mAh/cm2 loading of cathode materials.