Abstract: This study addresses the issues of rapid cycling capacity decay and poor rate performance in LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode materials by proposing a synergistic modification strategy combining "defect engineering and interfacial protection." Oxygen vacancies were successfully introduced into the bulk phase via NaBH₄ chemical reduction, while a uniform, dense AlPO₄ coating approximately 10 nm thick was formed on the surface using a liquid-phase method. Structural characterization (EPR signal at g=2.003) confirmed the presence of oxygen vacancies, while TEM and EDS analysis demonstrated uniform coverage of the AlPO₄ coating. Electrochemical testing demonstrated that the modified AlPO₄@VONCM523 material exhibited an initial discharge capacity of 185.9 mAh/g at 0.1 C. After 200 cycles at 1 C, its capacity retention improved to 73.66 % (significantly higher than the 50.84 % of the pristine material), and it maintained a reversible capacity of 99.6 mAh/g even at the high rate of 5 C. CV and EIS tests further revealed low electrode polarization (ΔE = 0.12 V) and reduced charge transfer resistance (Rct = 189.17 Ω), indicating significantly improved reaction kinetics. Research indicates that oxygen vacancies enhance the material's electronic conductivity, while the AlPO₄ coating effectively suppresses interfacial side reactions. The synergistic effect of these two factors collectively improves the material's cycling stability and rate performance. This synergistic strategy offers new insights for developing high-performance, long-life cathode materials for lithium-ion batteries.

Key words: Lithium-ion battery, NCM523, oxygen vacancy, AlPO₄ coating, synergistic modification, electrochemical performance