Exploring the evolution process of high-performance amethyst geode-shaped hollow spherical LiFePO4

Abstract

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 a micro-hollow-spherical carbon shell, delivering a specific capacity of 163 mA h g⁻¹ at 0.1 C and 123 mA h g⁻¹ at 10 C. More importantly, we clarified the spheroidization process of spherical LFP particles in our work through a series of characterization studies. It is found that APP and the carbon source play critical roles in the shape evolution process at respective temperature regions. Among them, the cohesive behavior of the APP chain results in surface tension and drives the spheroidization process at low temperatures. The carbon source around the LFP particles confines mass transfer and maintains the hollow spherical structure at high temperatures. The exploration of the spheroidization mechanism is instructive for the use of long-chain precursors to synthesize materials with porous-hollow-spherical morphology via a simple process.