Kinetic Control of the Li_0.9Mn_1.6Ni_0.4O₄ Spinel Structure with Enhanced Electrochemical Performance

The development of more sustainable societies has become an urgent goal worldwide. Electrical batteries are currently seen as one of the most important energy storage technologies for the development of decarbonized societies. However, many lithium-ion battery manufacturers currently utilize cobalt, a toxic and hazardous mineral, in their batteries. Lithium-deficient manganese nickel oxide spinels are considered promising candidates owing to their high potential and environmental friendliness. Their electrochemical performance highly depends on their average and local structures, such as phase purities, lattice parameters, and cation sites. Thus, a synthesis protocol should be designed to control these structural parameters to improve their electrochemical performance. In this study, we controlled the average and local structures of Li_0.9Mn_1.6Ni_0.4O₄ spinels obtained by co-precipitation by optimizing their cooling rates. High-resolution techniques, including transmission electron microscopy, synchrotron X-ray diffraction, and Auger-composition analysis combined with density functional theory calculations, X-ray absorption spectroscopy, and electrochemical analysis, were used to understand the average and local structural variations and their effects on the electrochemical properties. As a result, the control of oxygen diffusion at different cooling rates can promote the rearrangement of the structure, resulting in a cation-disordered spinel with minimal variations in lattice parameters and composition. Excellent electrochemical properties were noted in the cation-disordered spinel with high crystallinity and a slightly oxygen-rich surface produced via optimized cooling rates.