Structural Evolution and Transition Dynamics in Lithium Ion Battery under Fast Charging: An Operando Neutron Diffraction Investigation

Abstract

Fast charging (+. To have a mechanistic understanding of Li⁺ diffusion within LIBs, in this study, structural evolutions of electrodes for a Ni-rich LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) || graphite cylindrical cell with high areal loading (2.78 mAh cm⁻²) are developed for operando neutron powder diffraction study at different charging rates. Via sequential Rietveld refinements, changes in structures of NMC622 and Li_xC₆ are obtained during moderate and fast charging (from 0.27 C to 4.4 C). NMC622 exhibits the same structural evolution regardless of C-rates. For phase transitions of Li_xC₆, the stage I (LiC₆) phase emerges earlier during the stepwise intercalation at a lower state of charge when charging rate is increased. It is also found that the stage II (LiC₁₂) → stage I (LiC₆) transition is the rate-limiting step during fast charging. The LiC₁₂ → LiC₆ transition mechanism is further analyzed using the Johnson–Mehl–Avrami–Kolmogorov model. It is concluded as a diffusion-controlled, 1D phase transition with decreasing nucleation kinetics under increasing chargingrates.