Abstract: Cobalt-free lithium-rich manganese-based cathode materials are widely regarded as promising candidates for lithium-ion batteries due to their high specific capacity and low cost. However, their practical application remains hindered by several challenges, including low initial coulombic efficiency, rapid voltage decay, and poor cycling stability. Compared with polycrystalline materials, single-crystal counterparts offer superior structural stability and cycling durability, however, their relatively high surface area tends to intensify interfacial side reactions, thereby adversely affecting their electrochemical performance. In this work, polyvinylidene fluoride (PVDF) was employed as a fluorine source to modify single-crystal Li_1.2Ni_0.25Mn_0.55O₂. The influence of fluorine incorporation on the crystal structure, morphology, and electrochemical behavior was comprehensively investigated. The results show that the fluorine-doped material, Li_1.2Ni_0.25Mn_0.55O_1.95F_0.05, exhibits enhanced performance, delivering an initial discharge capacity of 219.68 mAh/g at 0.5 C and retaining 95.09% of its capacity after 100 cycles. This study presents a viable approach for optimizing cobalt-free lithium-rich cathode materials, providing valuable insights for the development of next-generation high-energy-density lithium-ion batteries.