Synergistic Ti/F Co-doping for Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Cathodes via Ti3+/Ti4+ Redox and Structural Stabilization

Abstract

As a promising high-voltage cathode material candidate for next-generation high-energy-density lithium-ion batteries (LIBs), spinel-type LiNi0.5Mn1.5O4 (LNMO) has attracted significant attention due to its high operating voltage (~4.7 V). However, its intrinsic structural instability and severe surfacial side reactions lead to poor cycling stability and rate capability. This study presents an innovative low-temperature calcination strategy to synthesize Ti/F co-doped LNMO (LNMO-TiF3-x) cathode materials, utilizing TiF3 as a dual-functional dopant. Both experimental and theoretical investigations demonstrate that the introduced Ti3+/Ti4+ redox couple activates additional Mn-based capacity within the 4.2 V region, while contributing a minor yet measurable reversible capacity. Simultaneously, Ti/F co-doping strengthens the lattice through covalent bonding, effectively suppressing transition metal dissolution and synergistically enhancing cycling stability. Furthermore, Ti4+ doping expands the (111) lattice spacing, which facilitates Li+ diffusion kinetics and significantly enhances rate capabilities. The optimized LNMO-TiF3-4 exhibits outstanding performance, delivering 131.9 mAh g-1 at 10 C and maintaining 72.4% capacity retention after 1000 cycles at 5 C. This work not only demonstrates the high electrochemical performance of LNMO-TiF3-x but also offers new insights into bulk-interface synergistic engineering for advanced high-voltage cathode materials.