# Tailoring eg Orbital Occupancy of Fe in Ni-Doped Na4.3Fe3(PO4)2P2O7 Cathode for High-Performance Sodium-Ion Batteries

## TL;DR

This paper shows how doping a sodium-ion battery cathode with nickel improves performance by optimizing electron configurations and orbital overlaps.

## Contribution

The study introduces a novel electron transfer mechanism from Ni²⁺ to Fe³⁺ and proposes the eg orbital occupancy as a descriptor for electrochemical activity.

## Key findings

- Ni-doped Na4.3Fe3(PO4)2P2O7 shows high conductivity and stable cycling.
- The Fe−O covalency and Na⁺ diffusion are optimized by Ni doping.
- A volcano-shaped relationship is observed between Fe eg occupancy and electrochemical activity.

## Abstract

The Na4.3Fe3(PO4)2P2O7-M cathode constructed by various transition metal cations (M = Ni2+, Mn2+, Zn2+, Co2+ and Cu2+) with different electron configurations for sodium-ion batteries.The Na4.3Fe3(PO4)2P2O7-Ni cathode exhibits superior electronic conductivity, high-rate performance and stable cyclability.A quantitative relationship between the eg occupancy of Fe and the electrochemical activity of The Na4.3Fe3(PO4)2P2O7-M is proposed, serving as an activity descriptor.

The Na4.3Fe3(PO4)2P2O7-M cathode constructed by various transition metal cations (M = Ni2+, Mn2+, Zn2+, Co2+ and Cu2+) with different electron configurations for sodium-ion batteries.

The Na4.3Fe3(PO4)2P2O7-Ni cathode exhibits superior electronic conductivity, high-rate performance and stable cyclability.

A quantitative relationship between the eg occupancy of Fe and the electrochemical activity of The Na4.3Fe3(PO4)2P2O7-M is proposed, serving as an activity descriptor.

The online version contains supplementary material available at 10.1007/s40820-026-02073-3.

Na4Fe3(PO4)2P2O7 (NFPP) is regarded as a prospective cathode for sodium-ion batteries (SIBs) because of its high structural stability and cost-effectiveness. However, its practical application is hindered by intrinsically low electronic conductivity. Herein, an unconventional electron transfer mechanism from Ni2+ to Fe3+ ions is unveiled in Ni-doped Na4.3Fe3(PO4)2P2O7 (NFPP-Ni) cathode, which facilitates electronic coupling within the Fe−O−Ni coordination unit and thereby effectively boosts electron transport. Moreover, the redox kinetics and reversibility of NFPP materials are predominantly governed by the degree of Fe−O covalency. The intermediate eg occupancy of Fe2+, modulated by the presence of Ni2+, optimizes the overlap between Fe d and O p orbitals. The adjustment of Ni dopant strikes a balance between accelerating Na+ diffusion kinetics and mitigating lattice strain during cycling. As a result, the NFPP-Ni electrode displays impressive rate capacity (121.0 mAh g−1 at 0.1C / 80.9 mAh g−1 at 10C) and stable cyclability (89.1% capacity retention after 1000 cycles). More importantly, the relationship between Fe eg orbital occupancy and Fe−O covalency in NFPP as modulated by various transition metal cations (Ni2+, Mn2+, Zn2+, Co2+ and Cu2+) with different electron configurations are systematically elucidated, thereby providing insights for the commercial development of sodium-ion batteries (SIBs). Tuning the eg orbital occupancy of Fe in Na4.3Fe3(PO4)2P2O7 cathode can effectively optimize the spatial overlap between Fe d and O p orbitals with excellent rate capability for sodium-ion batteries. The eg could be a significant descriptor for Fe−O covalency that describes a volcano curve as a function of eg.

The online version contains supplementary material available at 10.1007/s40820-026-02073-3.