Surface film formation on LiNi_0.5Mn_1.5O₄ electrode in an ionic liquid solvent at elevated temperature

A comparative study is made on the surface film formation on the high-voltage LiNi_0.5Mn_1.5O₄ positive electrode at elevated temperature (55°C) in two different electrolytes; LiPF ₆/organic carbonate and LiTFSI/ionic liquid (propylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide, PMPyr-TFSI). The surface film derived by a decomposition of the former electrolyte is enriched by inorganic fluorinated species, which becomes thicker with cycling to lead a continued electrode polarization and cell failure. In contrast, organic carbon species are dominant in the film derived from the latter electrolyte. This organic-rich film deposited in the earlier period of cycling seems to effectively passivate the positive electrode presumably due to uniform coverage. As a result, the film does not grow with cycling and electrode polarization is not serious to give a stable cycling behavior.