High potential durability of LiNi0.5Mn1.5O4 electrodes studied by surface sensitive X-ray absorption spectroscopy

Phenomena at electrode/electrolyte interface of LiNi 0.5 Mn 1.5 O 4 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 0.5 Mn 1.5 O 4 prepared by pulsed laser deposition (PLD) are used as model electrodes to facilitate the observation of the interface. The thin-film LiNi 0.5 Mn 1.5 O 4 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 LiClO 4 in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate). This is in sharp contrast to the behavior of a LiCoO 2 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 0.5 Mn 1.5 O 4 , leading to good cycling characteristics of LiNi 0.5 Mn 1.5 O 4 in spite of its high operating potential.