Resolving the relationship between capacity/voltage decay and the phase transition by accelerating the layered to spinel transition

Abstract

Lithium-rich cathode materials are some of the most promising choices for lithium-ion batteries due to their excellent energy density (>900 W h kg⁻¹). However, severe voltage/capacity degradation during cycling has seriously hindered the further commercialization of lithium-rich cathode materials. Current research efforts are focused on enhancing their voltage and capacity retention. Here, the coating of FeF₃ on specific crystal planes is utilized to achieve a degradation trend that is very different from that of the as-received material. Using this as an entry point, the relationship between voltage and capacity degradation was studied in depth. The oriented coated material undergoes a more drastic phase transition during cycling, yet its voltage decay remains basically the same as that of the original sample (769.6 mV after 200 cycles, compared to 723.5 mV for the original sample). Notably, the capacity retention rate is significantly improved (97% after 200 cycles vs. 75% for the pristine material). These findings suggest that the capacity degradation and the voltage decay do not interact with each other and that the phase transition during cycling does not seem to negatively affect the voltage. This conclusion can also be extrapolated to other oxygen-reducing oxide systems to help understand the relationship between capacity and voltage decay. The modification is generalized and applicable to other cathode materials.