High potential durability of LiNi_0.5Mn_1.5O ₄ electrodes studied by surface sensitive X-ray absorption spectroscopy

Phenomena at electrode/electrolyte interface of LiNi_0.5Mn _1.5O₄ are studied by in situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), ex situ X-ray photoelectron spectroscopy (XPS), and electrochemical tests. Flat and well-defined thin films of LiNi_0.5Mn_1.5O₄ prepared by pulsed laser deposition (PLD) are used as model electrodes to facilitate the observation of the interface. The thin-film LiNi_0.5Mn_1.5O₄ electrode showed good cycling characteristics at around 4.7 V vs. Li/Li ⁺. The TRF-XAS measurements reveal that nickel and manganese species at the surface have almost the same chemical states and local environments as those in the bulk when in contact with organic electrolyte solutions (1 mol dm^-3 LiClO₄ in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate). This is in sharp contrast to the behavior of a LiCoO₂ electrode, in which the surface cobalt species is irreversibly reduced by soaking to the organic electrolyte solutions, leading to gradual material deterioration during the delithiation/lithiation cycling (D. Takamatsu et al., Angew. Chem. Int. Edit., 51 (2012) 11597). It is suggested that the electrolyte decomposition products detected by XPS form a protective layer to restrict the reduction of the surface species of LiNi _0.5Mn_1.5O₄, leading to good cycling characteristics of LiNi_0.5Mn_1.5O₄ in spite of its high operating potential.