Influence of Synthesis Routes on the Crystallography, Morphology, and Electrochemistry of Li2MnO3

On the disparity in reporting Li-rich layered oxide cathode materials

Lithium-rich layered oxides are considered one of the most promising cathode materials for next generation lithium-ion batteries due to their extraordinary specific capacity of over 280 mA h g^-1 and superior energy density of over 1000 W h kg^-1. Despite the excellent performance, LRLOs still suffer from low Coulombic efficiency, serious capacity/voltage decay upon cycling, voltage hysteresis, short lifespan, and poor rate capability. Driven by the thirst for high-energy-density battery technologies, various strategies have been developed to address these issues with great progress being achieved in the past several years. However, the emerging disparity among the published results severely precludes meaningful comparisons between different LRLOs and material modification strategies, which has become an impediment to the development and commercialization of LRLOs. Although the significance of standardization has been recognized in the battery community, the standardization of LRLOs is worth particular attention due to their complicated compositions and unique electrochemical properties. This perspective analyzes the underlying parameters that can cause varied and even controversial results observed in LRLOs, from the synthesis procedure to the electrochemical evaluation procedure, followed by preliminary suggestions for the standard protocols of chemical compositions, synthesis pathways, calcination conditions, electrode preparation, battery fabrication, and battery testing. Hopefully, this perspective can help build a reliable baseline for LRLO research, thus aligning the huge research effort toward the practical applications of LRLOs.

Analysing the Implications of Charging on Nanostructured Li2MnO3 Cathode Materials for Lithium-Ion Battery Performance

Capacity degradation and voltage fade of Li2MnO3 during cycling are the limiting factors for its practical use as a high-capacity lithium-ion battery cathode. Here, the simulated amorphisation and recrystallisation (A + R) technique is used, for generating nanoporous Li2MnO3 models of different lattice sizes (73 Å and 75 Å), under molecular dynamics (MD) simulations. Charging was carried out by removing oxygen and lithium ions, with oxygen charge compensated for, to restrain the release of oxygen, resulting in Li2-xMnO3-x composites. Detailed analysis of these composites reveals that the models crystallised into multiple grains, with grain boundaries increasing with decreasing Li/O content, and the complex internal microstructures depicted a wealth of defects, leading to the evolution of distorted cubic spinel LiMn2O4, Li2MnO3, and LiMnO2 polymorphs. The X-ray diffraction (XRD) patterns for the simulated systems revealed peak broadening in comparison with calculated XRD, also, the emergence of peak 2Θ ~ 18-25° and peak 2Θ ~ 29° were associated with the spinel phase. Lithium ions diffuse better on the nanoporous 73 Å structures than on the nanoporous 75 Å structures. Particularly, the Li1.00MnO2.00 shows a high diffusion coefficient value, compared to all concentrations. This study shed insights on the structural behaviour of Li2MnO3 cathodes during the charging mechanism, involving the concurrent removal of lithium and oxygen.

Synthesis-structure relationships in Li- and Mn-rich layered oxides: phase evolution, superstructure ordering and stacking faults

Li- and Mn-rich layered oxides are promising positive electrode materials for future Li-ion batteries. The presence of crystallographic features such as cation-mixing and stacking faults in these compounds make them highly susceptible to synthesis-induced structural changes. Consequently, significant variations exist in the reported structure of these compounds that complicate the understanding of how the crystallographic structure influences its properties. This work investigates the synthesis-structure relations for three widely investigated Li- and Mn-rich layered oxides: Li₂MnO₃, Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. For each compound, the average structure is compared between two synthetic routes of differing degrees of precursor mixing and four annealing protocols. Furthermore, thermodynamic and synthesis-specific kinetic factors governing the equilibrium crystallography of each composition are considered. It was found that the structures of these compounds are thermodynamically metastable under the synthesis conditions employed. In addition to a driving force to reduce stacking faults in the structure, these compositions also exhibited a tendency to undergo structural transformations to more stable phases under more intense annealing conditions. Increasing the compositional complexity introduced a kinetic barrier to structural ordering, making Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ generally more faulted relative to Li₂MnO₃. Additionally, domains with different degrees of faulting were found to co-exist in the compounds. This study offers insight into the highly synthesis-dependent subtle structural complexities present in these compounds and complements the substantial efforts that have been undertaken to understand and optimise its electrochemical properties.

Assessing the local structure and quantifying defects in Ca4Fe9O17 combining STEM and FAULTS

Defects in crystalline structures play a vital role in their properties, so their proper characterization is essential to understanding and improving the behaviour of the materials.