High voltage and long cycling cathode materials based on LiCoO2

Abstract

To meet the ever-increasing demands of portable smart devices, electric vehicles, and grid storage, the pursuit of high energy density remains a longstanding objective in rechargeable battery development. LiCoO2 (LCO) has emerged as the predominant commercial cathode material due to its stable charge/discharge voltage plateaus, high tap density, high volumetric energy density, and excellent electrochemical cycling durability. However, to achieve even higher capacity, it is necessary to explore methods that can enhance the performance of the existing cathodes. One such approach involves raising the cut-off voltage to extract more Li+ ions during the charging process. Unfortunately, exceeding a cut-off charge voltage higher than 4.5 V (vs. Li/Li+) leads to rapid capacity decay that limits its cycle life. It is urgent to find an appropriate modification strategy as the primary tool in improving Li-ion batteries (LIBs). Firstly, considering the significantly different diffusivities of dopant ions (as confirmed by density functional theory calculations), we propose a proof-of-concept strategy to forming core-shell structured LiCoO2 (CS-LCO) via a simple two-step co-doping method. In this process, high diffusivity Al3+/Mg2+ ions occupy the core matrix while low diffusivity Ti4+ ions enrich the shell layer. The presence of Al3+/Mg2+ in the core matrix helps optimize physical properties, such as electrical conductivity and Li+ ion diffusivity. On the other hand, the Ti4+-enriched shell layer, with Ti substitution and stronger Ti-O bond, leads to the reduction in the number of oxygen ligand holes and an enhancement of the oxygen stability. Furthermore, CS-LCO exhibits mitigated phase transition from O3 to H1-3, resulting in reduced contraction of c-axis and structural distortion. Secondly, among different surface coating modifications, we find that the utilization of triethylamine template promotes the phase transformation from traditional amorphous AlPO4 to crystalline AlPO4-5 zeolite on the surface of LCO and provides several advantages. (1) The electrochemically and mechanically stable AlPO4-5 zeolite coating acts as a protective layer to reduce the lattice oxygen loss and the side reactions on the LCO surface. (2) The crystalline AlPO4-5 zeolite establishes a stable diffusion pathway for Li+ ion transport, which accelerates the Li+-desolvation process and improves the Li+ kinetics. (3) The full coverage of the elastic AlPO4-5 zeolite coating layer effectively provides mechanical reinforcements to suppress the phase transition from O3 to H1-3. Consequently, a multifunctional AlPO4-5 zeolite coating layer enables stable 4.6 V high voltage operation of LCO cathode. The third work proposes a novel cathode called CeO2 interspersed massage-ball-like LCO (LCO@CeO2). In addition to their role in surface protection and insulation from electrolyte, CeO2 nanoparticles also play a crucial role in facilitating Li+ conduction by establishing easier pathways for Li+ de/intercalation. Moreover, the interspersed CeO2 nanoparticles contribute to maintaining the reversibility of the lattice oxygen. As a result of these design considerations, the optimized LCO@CeO2 cathode, prepared at 850°C, exhibits outstanding electrochemical performance and effectively mitigates the generation of undesirable gases. Overall, this thesis focuses on the coating and doping strategies to achieve high voltage and long cycling cathodes based on LCO. Satisfactory results are obtained, which will provide guidance in the design and development of future generation of LCO.