Operando X-Ray Diffraction Boosting Understanding of Graphite Phase Evolution in Lithium-Ion Batteries.

The fast charging/discharging performance of lithium-ion batteries is closely related to the properties of electrode materials, especially the phase evolution and Li+ diffusion kinetics. The phase evolution and intrinsic properties of an electrode material under different C-rates can be investigated by applying operando X-ray diffraction (XRD). In this study, a transmission X-ray diffractometer is used in operando monitoring the behaviors of NCM811/Graphite pouch cells during charging/discharging at low rate (0.1C) and high rate (2.5C), especially the structure changes, phase evolution, and relaxation of graphite anode. The variations in XRD patterns, as well as and the inconsistency between the state of charge (SOC) of full cells and the SOC of electrodes, are explained based on genetic algorithm and shrinking annuli model. Furthermore, from the perspectives of monitoring and identification of electrode state, structural design of materials and electrodes, and optimization of charging/discharging protocols, practical suggestions for understanding the state and improving the performance of electrodes are proposed. Operando X-ray diffraction is applied with genetic algorithm and shrinking annuli model to investigate the phase evolution in NCM811/Graphite cells especially the graphite, a bottleneck for fast-charging. The poor kinetics of Li+ insertion and diffusion in stage 1 and 2 of graphite lead to unsatisfactory rate performance, asymmetric phase transitions, and severe inconsistency between battery state of charge (SOC) and electrode SOC at 2.5C.image