Hysteretic Phenomena between Charging and Discharging in LiNi 0.5 Mn 1.5 O 4-σ

Abstract

Clarification of the mechanism of hysteretic phenomena between charging and discharging in LiNi0.5Mn1.5O4-σ under a closed-circuit voltage is crucial for achieving a high energy efficiency. We performed in situ X-ray diffraction to understand the relationship between the overpotential and the kinetically preferred state of LiNi0.5Mn1.5O4-σ during charging and discharging. Very different crystal structures were observed for the cathode during charging and discharging, especially in the low state-of-charge single-phase reaction region. We consider that charging can be a lithium extraction process through the large lattice spacing, and lattice shrinkage occurs just after lithium diffusion, which enables a continuous reaction. Furthermore, we consider that discharging corresponds to the continuous insertion of lithium, requiring successive “forcible” lattice expansion for the reaction to occur. A kinetic reaction model for the cathode was proposed based on these results to explain the hysteresis for charging and discharging under a closed-circuit voltage.