Experimental and Theoretical Studies on LiFePO4 Particle Shape Effect in Batteries

LiFePO₄ (LFP) based lithium-ion batteries possess high energy capacity and safety characteristics that are fundamentally superior to those made with other cathode materials. However, the undesired battery discharge capacity due to sluggish electronic and ionic transport in the orthorhombic olivine structure, especially at high rates, limits its wide-spread application. In addition, nonequilibrium phase transformation, arising due to the fact that slow Li diffusion across the two-phase boundaries cannot keep up with the required Li flux at high discharge rate, further diminishes battery capacity. In this study, LFP particle shape has been studied both experimentally and theoretically as a key aspect to achieving a battery capacity close to LFP’s theoretical storage capacity at high charge/discharge rates. Particularly, the effect of particle shape on high-rate discharge and nonequilibrium phase transformation were investigated. Experimentally, we used hydrothermal and solid-state methods to synthesize our particles, and varied time and temperature in order to make plate-shaped and spherical LFP particles. We also used the “mushy-zone” simulation approach to correlate our testing data to the theoretical results.  The spherical particles were found to facilitate Li ions to intercalate/deintercalate during low-rate discharge in comparison to the plate-shaped particles; however, the latter yielded much higher discharge performance owing to shortened Li diffusion length and constant Li flux.