Compositional Change of Surface Film Deposited on LiNi0.5Mn1.5O₄Positive Electrode

The evolution of surface film on aspect of chemical composition and thickness is monitored on a nickel-doped manganese spinel positive electrode (LNMO, LiNi 0.5 Mn 1.5 O 4 ) for lithium-ion batteries. In the first charging from 4.2 to 4.9 V (vs. Li/Li + ), large amount of lithium fluoride (LiF) deposits at lower potentials (4.2 V) but it is removed when the electrode potential moves to the charging end (4.9 V). The phenomenon of LiF deposition at lower potentials and removal at higher potentials is repeated in the continuing cycles, but the overall LiF concentration becomes progressively lower for the surface films to be enriched by the carbon-containing organic species in the later cycles. Due to the highly resistive nature of LiF, the film resistance shows a strong correlation with the LiF concentration in the films. From the finding that LiF formation is the most significant at 4.2 V in every cycle, under which circumstances LNMO itself is fully lithiated to provide lithium source for LiF formation, hydrogen fluoride (HF) attack on the LNMO surface has been proposed to be the major route for LiF formation.