Capacity degradation of the LiMn 2 O 4 cathode during high-temperature storage was investigated for various lithium contents. The capacity recovery rate was determined after the storage test, and the phases and valence state of manganese ions were investigated by X-ray diffraction and X-ray absorption fine structure. The storage characteristics depend on the state of charge (SOC); a significantly low recovery rate was found for the shallow SOC region of LiMn 2 O 4 . The phase changes determined after the deterioration were as follows. (i) The lattice parameters decrease with an increase in manganese valence, caused by the formation of defects at the 16d site due to manganese dissolution. (ii) A new phase forms as a second phase. This phase has higher manganese valence and a larger amount of manganese vacancies. The same type of phase was produced by a high-temperature treatment of the shallow SOC phase at 150 • C, which indicates that the phase was stable at high temperatures. (iii) A lower manganese valence state was observed at the spinel surface, which is related to manganese dissolution. Deterioration in the shallow SOC region during high-temperature storage is related to the phase stability of the de-intercalated spinel phases.In recent years, lithium ion batteries have come into wider use for not only portable devices but also as large-scale batteries for pure electric vehicles and hybrid electric vehicles because of their high energy density. Among the cathode materials proposed to date, LiMn 2 O 4 has the advantages of safety, low cost, environmental friendliness and high voltage for use in practical batteries and is the most promising material for large-scale batteries. 1,2 However, it is well known that cell performance gradually fades with dissolution of manganese ions at high temperature. 3,4 The dissolved manganese ions deposit on the anode, particularly in the case of cells with a graphite anode, enhance the formation of a solid electrolyte interface (SEI) layer on the anode, and then cause capacity fade and/or an increase in resistance. [5][6][7][8][9] In order to avoid these problems, there have been several reports that have proposed suppressing manganese dissolution from LiMn 2 O 4 and a partial substitution of other metal elements or lithium for the manganese site. 10,11 Modifications of the spinel have also been proposed such as decreasing the surface area, 12 surface modification, 13 and the addition of borate. 14 Optimization of electrolyte salts 11 and electrolyte additives are other methods that have been proposed to solve the dissolution problem.It has also been reported that storage characteristics and manganese dissolution depend on the state of charge (SOC) of the cells. The dissolution of the manganese spinel was found to increase due to instability of the phase at x = 0.75 to 0.84 in Li x Mn 2 O 4 . 15-18 However, the details of the deterioration mechanism and the manganese compositions have yet to be clarified.Previously, Li et al. reported that the deterioration of storage characte...