$Li_2SO_4$ coating of Co-free, Li-rich layered oxide cathode for improving electrochemical performance

Abstract

which upon lithiation, presents a Li_2SO_4 coated CF-LLC (S-CF-LLC). The Li_2SO_4 coating prevents the agglomeration of primary particles during lithiation leading to a smaller average primary particle size. Hence, the surface area is increased and the Li diffusion pathways are shortened, leading to increased Li diffusivity. The coating also prevents transition metal (TM) dissolution by acting as a chemically protective barrier when exposed to the electrolyte. The S-CF-LLC was successfully synthesized using a hydrothermal synthesis method with a wet chemical coating step and the synthesis was confirmed using XRD, SEM, XPS, and HR-TEM techniques. Galvanostatic cycling showed that the first cycle capacity increased from 205.1mAhg^-1 to 259.0mAhg^-1 after coating, owing to the improved kinetics. GITT analysis of the increased diffusion coefficient further confirmed the overall improved kinetics. Moreover, the efficacy of the coating was shown through cycle data after 100 cycles, which showed good coulombic efficiency with a high average of 99.5%, good average specific discharge capacity of 231.0 mAhg^-1, and a decrease in voltage fade per cycle by 12.9%. XPS and Raman Spectroscopy were used to confirm that there was less transition metal dissolution for the coated sample, and thus less structural degradation. Overall, the successfully synthesized S-CF-LLC opens up the possibility of practical cobalt-free high-performance batteries.

In the continued evolution toward high-performance lithium-ion batteries, cobalt has presented itself as a major obstacle due to its price, toxicity and supply. Thus, Co-Free, Li-rich Layered Oxide Cathodes (CF-LLC) have become a promising cathode candidate for their exclusion of cobalt and high theoretical capacity. Nevertheless, CF-LLC suffers from issues such as sluggish kinetics, lowered early capacity and voltage fade due to structural degradation. This is due to the increase in cation mixing resulting from the absence of cation-ordering cobalt. To mitigate this, a pre-lithiation sulfate coating of the cathode carbonate precursor was implemented