Exploring the Evolution Process of High-Performance Amethyst Geode-Shaped Hollow Spherical LiFePO4

Hollow spherical lithium iron phosphate (LiFePO4) materials display outstanding electrochemical performance in general. Previous reports on hollow spherical LiFePO4 (LFP) mostly focus on electrochemical performance, while the shape evolution process of spherical LFP particles is not clearly investigated. Therefore, in our work a simple solid-state method was applied to synthesize porous hollow spherical LFP with ammonium polyphosphate (APP) as a spheroidizing agent. The as-prepared LiFePO4 forms a “micro-nano” structure, which is in the form of nano-LFP particles interspersed in micro-hollow-spherical carbon shell, delivering a specific capacity of 163 mAh g-1 at 0.1 C and 123 mAh g-1 at 10 C. More importantly, we clarified the spheroidization process of spherical LFP particles in our work through a series of characterizations. It is found that APP and carbon source play critical roles in shape evolution process at respective temperature regions. Among them, the cohesive behavior of APP chain results in surface tension and drives the spheroidization process at low temprature. Carbon source around LFP particles confines mass transfer and keeps the hollow spherical structure at high temprature. The exploration of spheroidization mechanism is instructive for the use of long-chain precursors to synthesize materials with porous-hollow-spherical morphology via simple process.