Transient Behavior in Phase-Transforming Electrodes Revealed By Operando X-Ray Diffraction

Abstract

Solid state phase transformation triggered by lithium insertion is ubiquitous in many commercially available battery electrodes such as LiXFePO4 (LFP), LiXCoO2 and graphite. Understanding the kinetics of phase transformation, in particular the nucleation and growth of a second phase due to the Li compositional changes is key to achieving high cycling rates in these phase separating materials. LFP is a model phase transforming material with a large miscibility gap that thermodynamically favors phase separation (segregation into Li-rich and Li-poor phases) in bulk crystals of sizes larger than 50 nm.It’s commonly accepted that phase transformation at constant current happens via two major pathways: (1) particle-by-particle transformation with a moving interface and (2) concurrent transformation, mediated by a kinetically stabilized solid solution[1-4]. However, realistic battery operation involves erratic charging and discharging behavior such as brief pulses, rests, etc, wherein our current understanding is still lacking. Primarily the relationship between particle dynamics (i.e. the solid solution fraction, active particle population) in an electrode and applied overpotential remains unclear. We herein combine operando X-ray diffraction and numerical phase field simulation to track and understand the transient response of the battery particles during non-constant current conditions. This work highlights the importance of particle dynamics for phase separating electrodes in operation and inspires general battery operating principles that prolong cycle life.