Ionic liquid electrolytes for high-voltage rechargeable Li/LiNi _0.5Mn_1.5O₄ cells

A high-voltage LiNi_0.5Mn_1.5O₄ cathode material with a cubic spinel structure is synthesized using a citric-acid-assisted sol-gel process. Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI)-based ionic liquids (ILs) with various kinds of Li salts, namely LiTFSI, LiPF₆, and their mixtures, are used as electrolytes for Li/LiNi_0.5Mn_1.5O₄ cells. The IL electrolytes show high thermal stability (>400 °C) and non-flammability, and are thus ideal for high-safety applications. At 25 °C, LiTFSI is more suitable than LiPF₆ as an IL electrolyte in terms of cell capacity, rate capability, and cyclic stability. The IL electrolytes clearly outperform the conventional organic electrolytes at 50 °C, since the latter decomposes at high voltage and corrodes both the Al current collector and LiNi_0.5Mn_1.5O₄, degrading the electrode performance. At such an elevated temperature, using LiPF₆ to partially substitute LiTFSI in the IL electrolyte can effectively suppress Al pitting corrosion and thus improves the cell performance. In the 0.4 M LiTFSI/0.6 M LiPF₆ mixed-salt IL electrolyte, an LiNi _0.5Mn_1.5O₄ discharge capacity of 115 mA h g^-1 (at 0.1 C) is obtained at 50 °C with a high cell voltage of ∼4.7 V.