Entropy-increased LiMn2O4-based positive electrodes for fast-charging lithium metal batteries.

Abstract

Fast-charging, non-aqueous lithium-based batteries are desired for practical applications. In this regard, LiMn₂O₄ is considered an appealing positive electrode active material because of its favourable ionic diffusivity due to the presence of three-dimensional Li-ion diffusion channels. However, LiMn₂O₄ exhibits inadequate rate capabilities and rapid structural degradation at high currents. To circumvent these issues, here we introduce quintuple low-valence cations to increase the entropy of LiMn₂O₄. As a result, the entropy-increased LiMn₂O₄-based material, i.e., LiMn_1.9Cu_0.02Mg_0.02Fe_0.02Zn_0.02Ni_0.02O₄, when tested in non-aqueous lithium metal coin cell configuration, enable 1000 cell cycles at 1.48 A g^-1 (corresponding to a cell charging time of 4 minutes) and 25°C with a discharge capacity retention of about 80%. We demonstrate that the increased entropy in LiMn₂O₄ leads to an increase in the disordering of dopant cations and a contracted local structure, where the enlarged LiO₄ space and enhanced Mn-O covalency improve the Li-ion transport and stabilize the diffusion channels. We also prove that stress caused by cycling at a high cell state of charge is relieved through elastic deformation via a solid-solution transition, thus avoiding structural degradation upon prolonged cycling.