Revealing Valence‐Dependent Inductive Effects of Titanium on Phase Transition Behavior toward P2/Tunnel Heterostructure in Sodium‐Ion Battery

Abstract 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 Ti ⁴⁺ overlook the critical role of valence states in phase transition control. Hence, various titanium precursors (TiO, Ti ₂ O ₃ , TiO ₂ ) are chosen to form an intergrown P2/tunnel structure in Na _0.67 Ti _0.1 Mn _0.9 O ₂ (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%Ti ³⁺ 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.