Lithium Sulfate Coating of Co-Free, Li-Rich Layered Oxide Cathode for Improving Electrochemical Performance

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 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; which upon lithiation, presents a Li2SO4 coated CF-LLC (S-CF-LLC). The Li2SO4 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 during deep charging/discharging. The coating also prevents transition metal (TM) dissolution by acting as a protective barrier against HF attack from 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, ATR-FTIR and HR-TEM techniques. Galvanostatic cycling showed that the first cycle capacity increased from 205.1 mAhg-1 to 259.0 mAhg-1 after coating, owing to the particle size reduction. GITT analysis also revealed and increase in the Li diffusion coefficient during deep charging/discharging. 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 %, and a decrease in voltage fade per cycle by 12.9 %. TOF-SIMS analysis was used to confirm the protective mechanism of the Li2SO4 coating layer, and XPS and Raman Spectroscopy were used to confirm the reduction in transition metal dissolution for the coated sample, which was further confirmed through HAADF STEM analysis. Overall, the successfully synthesized S-CF-LLC paves the future for cobalt-free high-performance batteries.