Studies on Electrochemistry and Factors Affecting Phase Transition Behavior of LiMnO2 Polymorphs

Abstract

Co/Ni-free high energy density materials are necessary as positive electrode materials of lithium-ion batteries for electric vehicles. Therefore, electrode materials made from abundant manganese ions are attractive candidates. LiMnO2 with an orthorhombic zigzag layered structure has been studied as potential positive electrode materials. However, LiMnO2 delivers a large reversible capacity after continuous charge/discharge cycles associated with the spinel-like phase transition. In contrast, a non-layered system, cation-disordered rocksalt-type LiMnO2 prepared by mechanical milling shows slow phase transition kinetics to the spinel-like phase on electrochemical cycles.1 In this study, we have synthesized two different polymorphs of LiMnO2 by heat-treatment of rocksalt LiMnO2 and ion-exchange reaction of NaMnO2 with an α-NaFeO2-type layered structure,2 and tested as electrode materials. These different polymorphs show unique phase transition behavior, and from these results, factors affecting electrode performance of LiMnO2 are discussed. X-ray diffraction patterns of different polymorphs of LiMnO2 are shown in Figure 1. The low crystallinity sample with the rocksalt structure obtained by mechanical milling crystallizes into the sample with well-defined diffraction lines after heat-treatment, but the sample clearly shows different diffraction patterns compared with both orthorhombic and α-NaFeO2-type layered phases. Electrochemical properties of LiMnO2 with different crystal structures are compared in Figure 2. Among the tested LiMnO2 polymorphs, the heat-treated sample of rocksalt LiMnO2 delivers a large reversible capacity of 250 mA h g-1 with the highest average discharge voltage. Different polymorphs shows different behavior related to phase transition kinetics and voltage hysteresis on electrochemical cycles. Phase transition kinetics are clearly influenced by original crystal structures and cation distributions. From the results, we discuss the factors affecting electrode properties and phase transition kinetics during charge/discharge processes for the LiMnO2 polymorphs, and the possibility of the development of Ni/Co-free high capacity electrode materials.References Sato et al., and N. Yabuuchi, J. Mater. Chem. A, 6, 13943 (2018).J. Paterson et al., J. ElectroChem., 151 (10) , A1552-A558 (2004) Figure 1