Asymmetric Lithium Extraction and Insertion in High Voltage Spinel at Fast Rate

Spinel-structured ordered-LiNi_0.5Mn_1.5O₄ (o-LNMO) has experienced a resurgence of interest in the context of reducing scarce elements such as cobalt from the lithium-ion batteries. O-LNMO undergoes two two-phase reactions at slow rates. However, it is not known if such phenomenon also applies at fast rates. Herein, we investigate the rate-dependent phase transition behavior of o-LNMO through in operando time-resolved X-ray diffraction. The results indicate that a narrow region of the solid solution reaction exists for charge and discharge at both slow and fast rates. The overall phase transition is highly asymmetric at fast rates. During fast charge, it is a particle-by-particle mechanism resulting from an asynchronized reaction among the particles. During fast discharge, it is likely a core-shell mechanism involving transition from Li₀₊Ni_0.5Mn_1.5O₄ to Li₁₊Ni_0.5Mn_1.5O₄ in the outer layer of particles. The Li_0.5Ni_0.5Mn_1.5O₄ phase is suppressed during fast discharge and appears only through Li redistribution upon relaxation.