5-V plateau observed for Li-rich Li1+Mn2−O4 stoichiometric spinels containing low-temperature-grown Li2MnO3 as an impurity phase

The redox reaction of tetravalent Mn4 + ions has attracted much interest since it is considered that this reaction is closely related to the generation of high capacity in layered cathodes with excess lithium. We considered that to investigate materials, which include tetravalent Mn4 + ions and have a different crystal structure from Li2MnO3, is valuable for understanding the reaction mechanism. The result will reveal whether Mn4 + ion in the Li2MnO3 structure is essential or only existence of Mn4 + ion is sufficient for appearance of the high capacity. In this work, we investigated Li-rich Li1 + xMn2 − xO4 (x = 0.20, 0.25, 0.30, 0.33) stoichiometric spinels. For a Li content of x = 0.33, a capacity of 50 mAh g− 1 was observed in 5-V region. It is possible that the Li-rich spinel includes a small amount of Li 2MnO 3 impurity, which exhibits the 5-V plateau. No significant changes in lattice constants are observed from x = 0.25 to 0.33, which indicates generation of Li 2MnO 3. We estimated the amount as 9.2 wt.% for x = 0.33 by standard addition analysis, and true Li concentration of the spinel component as 0.23. However, the observed 5-V capacity cannot be attributed to only the impurity phase of Li 2MnO 3, because the specific capacity must exceed 540 mAh g − 1 to explain it. It is considered that the observed 5-V capacity is caused by two components of the Li-rich spinel and the other phase like a composite material of Li 2MnO 3/Li-rich spinel. The 5-V capacity for the pure spinel component estimated by extrapolating the 5-V capacity vs. impurity-concentration plot is 7.9 mAh g − 1.