Fast oxygen redox enabled by flexible Al–O bonds in P2-type layered oxides for sodium batteries

Sodium-ion batteries are considered ideal candidates for large-scale energy storage systems due to the abundance of sodium resources, low costs, and high safety. Layered transition metal oxides serve as cathode materials for sodium-ion batteries, offering advantages such as simple synthesis, high specific capacity, and good sodium-ion conductivity. Among these, the P2-type structure can further enhance energy density through lattice oxygen redox (LOR). However, the irreversible phase transitions induced by LOR can lead to severe structural distortion and slow sodium-ion diffusion kinetics, limiting their cycling stability and rate performance.

Recently, Researchers from Shanghai Jiao Tong University, Fudan University and Brookhaven National Laboratory innovatively proposed an aluminum (Al) substitution strategy. By introducing flexible Al-O bonds into the P2-type Na_2/3Li_1/6Al_1/6Mn_2/3O₂ (LAM) positive electrode, they successfully suppressed the formation of O-type stacking, promoted the generation of the Z phase with low energy barriers, and alleviated local structural stress, significantly improving sodium-ion diffusion kinetics.

The researchers synthesized the Al-substituted P2-type layered oxide positive electrode using a solid-state sintering method. X-ray diffraction (XRD) and Rietveld refinement results confirmed that LAM retains the typical P63/mmc symmetry, with Al, Li, and Mn uniformly distributed in the transition metal layer. X-ray absorption spectroscopy (XAS) indicated that during the first charging process, the Mn valence state remained unchanged, with capacity fully contributed by oxygen redox; during discharge, part of Mn(IV) was reduced to Mn(III), forming a mixed valence state of Mn(III)/Mn(IV). In situ XRD monitoring showed that upon charging beyond 4.2 V, the material underwent a transition from the P2 phase to the Z phase, rather than the traditional O-type stacking. The orthorhombic prism structure of the Z phase effectively maintained the stability of sodium-ion diffusion channels.

Electrochemical performance tests demonstrated that the LAM positive electrode achieved a reversible capacity of 86 mAh/g at a high current density of 1 A/g, with a capacity retention rate of 62.5% after 100 cycles. Density functional theory (DFT) calculations and crystal orbital Hamilton population (COHP) analysis revealed that the Al-O bond possesses moderate covalency and flexibility. During the sodium extraction/insertion process, local structural stress is alleviated through bond length contraction and octahedral distortion, reducing the sodium-ion migration energy barrier to 0.47 eV, significantly enhancing diffusion kinetics.

This study achieved the controllable generation of the Z phase in P2-type layered oxides through the Al substitution strategy, revealing the mechanism by which flexible Al-O bonds suppress irreversible phase transitions and stabilize local structures. This provides new insights into addressing the structural degradation issues caused by lattice oxygen redox.