Anionic Redox Chemistry in Sodium-Ion Battery Cathodes

Anionic redox chemistry harnesses the reversible oxidation and reduction of oxygen anions in cathode materials, offering a pathway to exceed the capacity limits imposed by transition‐metal redox alone. In sodium‐ion batteries, layered transition‐metal oxides can activate oxygen redox at high voltages, delivering substantial extra charge storage. This approach, however, faces challenges such as voltage hysteresis, structural degradation and oxygen loss, which degrade cycle life. Recent insights have shown that precise control of cation ordering and doping can stabilise oxygen electron holes, suppress irreversible oxygen evolution and maintain lattice integrity. Advanced spectroscopic and diffraction techniques reveal that intermediate phases and ligand‐to‐metal coupling play critical roles in reversibility. By balancing cationic and anionic contributions to charge compensation, researchers are now able to design sodium‐ion cathodes with energy densities approaching those of lithium‐ion systems, while using earth‐abundant elements and maintaining operational safety and cost advantages for grid-scale applications.

Research from Nature Portfolio

Studies on polymorphs of sodium-ruthenium oxides have demonstrated that ordering of sodium and transition-metal layers into honeycomb motifs generates an intermediate phase which reorganises frontier orbitals to trigger stable oxygen redox. This structural motif boosts reversible capacity from around 135 to 180 mAh g−1 without severe lattice collapse. In a complementary strategy, incorporation of light boron dopants into layered sodium–nickel–iron–manganese oxides reinforces the oxygen–metal ligand framework, mitigates over-oxidation at voltages above 4.0 V and enables capacity retention above 80% after 200 cycles. Advanced spectroscopic investigations of P2-Na0.78Co0.50Mn0.33Ni0.17O2 have further revealed that oxygen anion holes formed at high voltage serve as the primary charge compensators beyond 4.2 V, confirming the reversible nature of anionic redox when supported by a robust lattice environment.

Anionic Redox Chemistry in Sodium-Ion Battery Cathodes publication trend

The graph below shows the total number of articles in anionic redox chemistry in sodium-ion battery cathodes across all publications each year (not limited to Nature Index journals).

Technical terms

Anionic redox: Reversible oxidation and reduction of oxygen anions (O2−) in a cathode lattice, contributing to charge compensation beyond transition-metal redox.

Cationic redox: Conventional redox processes involving transition-metal ions (such as Mn, Ni or Fe) cycling between different oxidation states during battery operation.

P2-type structure: A layered oxide crystallographic arrangement in which sodium occupies prismatic sites between transition-metal–oxide slabs, influencing ion mobility and phase stability.

Phase transition: Structural rearrangement of the cathode lattice (for example from P2 to O2 stacking) that can induce volume changes, hysteresis and capacity fade.

Honeycomb ordering: A specific cation ordering pattern in layered oxides that creates alternating sites in the transition-metal layer, stabilising intermediate phases and facilitating reversible oxygen redox.

References

1. Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode. Nature Communications (2016).
2. Boron-doped sodium layered oxide for reversible oxygen redox reaction in Na-ion battery cathodes. Nature Communications (2021).
3. Understanding the redox process upon electrochemical cycling of the P2-Na0.78Co1/2Mn1/3Ni1/6O2 electrode material for sodium-ion batteries. Communications Chemistry (2020).
4. Competing Mechanisms Determine Oxygen Redox in Doped Ni–Mn Based Layered Oxides for Na‐Ion Batteries. Advanced Materials (2024).
5. Scientific challenges faced by Mn‐based layered oxide cathodes with anionic redox for sodium‐ion batteries. Carbon Energy (2024).
6. Earth‐Abundant Na‐Mg‐Fe‐Mn‐O Cathode with Reversible Hybrid Anionic and Cationic Redox. Advanced Energy Materials (2023).