Electrochemical performance of 0.5Li₂MnO₃-0.5Li(Mn_0.375Ni_0.375Co_0.25)O₂ composite cathode in pyrrolidinium-based ionic liquid electrolytes

High-energy-density 0.5Li₂MnO₃-0.5Li(Mn_0.375Ni_0.375Co_0.25)O₂ composite cathodes for lithium rechargeable batteries are synthesized using an auto-combustion method. The electrode charge-discharge properties are studied at 25 and 50 °C in Li⁺-containing N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PMP-TFSI) ionic liquid (IL) electrolytes. The IL electrolytes have a high decomposition temperature (∼400 °C) and thus are ideal for high-safety applications. Compared to Li⁺/BMP-TFSI IL, Li⁺/PMP-TFSI IL exhibits higher ionic conductivity and lower viscosity. As a result, the composite cathode shows superior electrochemical performance in Li⁺/PMP-TFSI IL electrolyte. With the increase in cell temperature from 25 to 50 °C, the maximum capacities and rate capabilities of both IL cells improve significantly. Thus at 50 °C, discharge capacities of 304 mAh g^-1 (@10 mA g^-1) and 223 mAh g^-1 (@100 mA g^-1) are obtained for the Li⁺/PMP-TFSI cell. These capacities are superior to those for a control cell made with the same composite cathode and a conventional organic electrolyte. At elevated temperature, the cyclability of the composite cathode in the IL electrolytes is markedly higher than that obtained in a conventional organic electrolyte.