Revealing Valence-Dependent Inductive Effects of Titanium on Phase Transition Behavior toward P2/Tunnel...

P2-type layered oxides are acknowledged as highly promising cathode possibilities for sodium-ion batteries (SIBs); nonetheless, they generally experience significant capacity degradation and slow Na⁺ diffusion kinetics. While P2/tunnel composite structure induced by metal ions has proven to be an effective strategy, conventional Ti-doping strategies focusing solely on Ti⁴⁺ overlook the critical role of valence states in phase transition control. Hence, various titanium precursors (TiO, Ti₂O₃, TiO₂) are chosen to form an intergrown P2/tunnel structure in Na_0.67Ti_0.1Mn_0.9O₂ (NTM). In situ X-ray diffraction (XRD) reveals that all samples undergo successive phase transformations (P3→P2→P2/Tunnel) during thermal treatment, with lower Ti valence states prompting earlier phase transitions and resulting in a higher proportion of the tunnel phase in the final products. Theoretical calculations confirm that, in light of ionic radius-structure correlations, lower valence states are intrinsically associated with reduced dissociation energies (DE), promoting earlier bond cleavage and accelerating reaction kinetics. Moreover, the optimized NTM-10%Ti³⁺ delivers 151 mAh g^-1 initial capacity at 0.1 C and remarkable capacity retention of 82% after 100 cycles at 1 C, attributed to the balanced phase composition and enhanced conductivity. This valence engineering strategy establishes a new pathway for designing high-durability cathodes beyond the limitations of conventional doping approaches.