Relationships between Mn3+ Content, Structural Ordering, Phase Transformation, and Kinetic Properties in LiNixMn2-xO4 Cathode Materials

Micrometer-sized LiNixMn2-xO4 (0.3 <= x <= 0.5) single crystals with (111) surface facets were synthesized and characterized by Li-6 magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and electrochemical studies. All three techniques were sensitive to cation disorder and the corroborated results showed that structural ordering improves with x. The transition from the ordered to the disordered spinel was triggered by an increase in Mn3+ content, which was accomplished either by a change in chemical composition or postsynthesis thermal treatment. Disordering led to increased solid solution behavior, reduced two-phase transformation domains, and improved transport properties during Li extraction and insertion. Further increasing Mn3+ content in already disordered structure extends the solid solution domain and eliminates the presence of phase II; however, this has limited effect on rate capability. The study demonstrates the dominant role of structural ordering in morphology-controlled LiMn1.5Ni0.5O4, and it reveals that the kinetic significance of Mn3+ lies in its ability in triggering structural disordering. The rate performance of the spinels is not directly proportional to the Mn3+ content or the domain size of solid solution transformation in samples where two-phase transition is also present.