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

Abstract

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 0.9 Mn 1.6 Ni 0.4 O 4 spinels obtained by co-precipitation by optimizing their cooling rates. Highresolution 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.

Cation disorder dominates the defect chemistry of high-voltage LiMn1.5Ni0.5O4 (LMNO) spinel cathodes

“…For lithium vacancies, while V − Li (removing a Li + ion) generates little disturbance to the system, V 0 Li (removing a Li atom) reveals the delithiation mechanism. 16,86 A localised hole is observed on the Ni in proximity to the defect site, with a magnetisation of −0.9 m B which is nearly half of the original value (−1.7 m B ) for Ni 2+ . This suggests that Ni 2+ (d 8 ) is oxidised to Ni 3+ (d 7 ) low-spin conguration, with the Bader charge of 1.33 compared to 1.21 for bulk Ni 2+ in agreement with the reduction in Ni-O bond lengths from 2.05 Å to 1.86-2.05 Å.…”

“…4 The disordered phase generally exhibits better cycling stability and rate capability than the ordered phase, however the connection between disorder and performance has remained unclear, with increasing attention placed on understanding the role of cation disorder. [14][15][16][17][18][19][20][21][22] While some studies found that Mn/Ni disorder increases with oxygen vacancy and Mn 3+ formation, others have suggested that the "disordered" phase from different studies is likely to be samples with partial cation disordering, a varying degree of oxygen content and impurity phases. 13,19,20,[23][24][25] Regardless, the correlation between cation disorder and oxygen content has made it difficult to distinguish the key driving factor for improved performance, illustrating the need to decouple these effects and study their impacts independently.…”

“…4 The disordered phase generally exhibits better cycling stability and rate capability than the ordered phase, however the connection between disorder and performance has remained unclear, with increasing attention placed on understanding the role of cation disorder. 14–22…”