Synergistic W and Ti Dual Doping for P2/Tunnel Intergrowth Formation: A Pathway to High Stability Sodium Ion Battery Cathodes

Abstract

Although P2-type layered oxides offer promising high specific capacity, they are prone to phase transitions during Na⁺ extraction. This leads to rapid capacity decay and poor cycle stability. Tunnel structured Na_0.44MnO₂ , in contrast, exhibits superior stability but low initial Na⁺ content. In this work, high valence W⁶⁺ and Ti⁴⁺ dopants were chosen to initiate minor tunnel intergrowth within P2 (Na content of 2/3). Additionally, enhanced bond strength of W-O and Ti-O relative to Mn-O lessen Jahn-Teller distortion and thereby stabilize P2 structure. Furthermore, a post synthesis heat treatment was applied to promote additional tunnel phase growth. These complementary modifications collectively stabilize the P2 structure upon cycling. Accordingly, the Na_2/3Mn_0.9Ti_0.05W_0.05O₂ sample, which attains a P2/tunnel ratio of nearly 2:1, delivers a high specific capacity of 120.9 mAh.g^-1 at a current density of 0.1C. Moreover, it exhibits outstanding long term cycling durability achieving 435 cycles at 5 C while retaining 80.24% of its initial capacity. The enhanced performance arises through low charge transfer resistance and a reduced Na⁺ diffusion barrier, as confirmed by both experimental measurements and theoretical calculations. This study elucidates the critical parameters that enable the introduction of tunnel intergrowth and the controlled tuning of phase fractions.