Atomistic Simulation and Characterization of Spinel Li\(_{1+x}\)Mn\(_2\)O\(_4\) (0 ≤ x ≤ 1) Nanoparticles

Lithium-ion batteries, comprising nanoparticulate Ni–Mn–Co (NMC) cathodes that have been used to power electric vehicles, can be improved by blending NMC with Li–Mn–O (LMO). However, LMO undergoes a cubic to tetragonal phase change during charge cycling, which cracks and pulverizes the material, resulting in capacity fading. Structural characterization during the phase transition is the first step in mitigating capacity fading and can be challenging experimentally and computationally. Here, we use simulated amorphization and crystallization to generate atom-level models of the LMO nanoparticles. This simulation strategy does not require any structural information to be predetermined. Instead, the structures evolve “naturally” from amorphous precursors. Analysis of the model Li–Mn–O nanoparticles reveals that they comprise domains of defect-rich spinel, Mn\(_3\)O\(_4\), layered Li\(_2\)MnO\(_3\), and lithium-rich spinel Li\(_{1+x}\)Mn\(_{2–x}\)O\(_4\) phases together with complex microstructural features. The discharge process was modeled by inserting surplus lithium atoms into the nanoparticles, resulting in structural changes, accompanied by a variety of constituent polymorphs. A transitional multigrained structure, between the cubic (Li\(_1\)Mn\(_2\)O\(_4\)) and tetragonal (Li\(_2\)Mn\(_2\)O\(_4\)) phases, is observed at Li\(_{1.75}\)Mn\(_2\)O\(_4\). We also find that microstructural features, such as microtwinning and intrinsic dopants, vacancies, etc., result in a network of Li transport pathways, enabling Li mobility in all three spatial directions.