Π-antibonding coupling mediated regeneration strategy for efficient direct regeneration of degraded O3-type cathode materials from sodium-ion batteries.

Abstract

With the advancement of the application of sodium ion batteries in large-scale energy storage and low-cost power scenarios, the direct regeneration of degraded layered oxide cathodes is of great significance for promoting the healthy development of the sodium-ion batteries industry chain. However, existing direct regeneration is limited by the dual challenges of orbital repulsion associated with the surface rock-salt phase and persistent bulk lattice distortion, which limit deep structural repair and hinder practical engineering applications. Therefore, this paper proposes a regeneration strategy mediated by π-antibonding coupling to achieve efficient direct regeneration of degraded O3-type NaNi_0.33Fe_0.33Mn_0.33O₂ (NFM) cathode materials, simultaneously achieving the dual goals of bulk lattice stability and surface structure reconstruction. In this work, π-antibonding coupling describes the π-symmetry hybridization between O 2p and transition-metal 3d t_2g orbitals, with emphasis on the resulting π-antibonding states. V⁵⁺ incorporation modulates this interaction and thereby redistributes the electron density within the oxygen layers. Furthermore, the oxygen-layer sliding energy barrier is effectively reduced, which drives the transformation of the surface rock salt phase into the active P2 phase. The ion diffusion and migration channels are opened up during the regeneration process, which promotes the migration of Na⁺, Li⁺, and Sn⁴⁺ into the bulk phase. The final formation is a composite structure of O3 bulk element gradient doping and surface P2 phase hinge. The volume change of the unit cell of the regenerated cathode material during charging and discharging is controlled at 2.57%, and the interface charge transfer resistance and Na⁺ diffusion resistance are significantly reduced. The regenerated cathode delivers an initial discharge specific capacity of 159.8 mAh g^-1 at 0.1C. At 1C, it retains 82.6% (±0.1%) of its initial capacity after 500 cycles. This work provides a feasible strategy for the direct regeneration of degraded cathode materials from spent sodium-ion batteries.