Nanjing University Boosts Sodium-Ion Battery Reversibility from 75% to 99% Using Iron-Mediated Cathode Strategy

Forum topic · QianXun · 2026-08-24

Summary

Researchers at Nanjing University, led by Guo Shaohua and Zhou Haoshen, have published a Nature Energy paper describing an iron-mediated strategy that dramatically improves the reversibility of lattice oxygen redox in sodium-ion battery cathodes. In conventional layered oxide cathodes, lattice oxygen redox delivers extra capacity but suffers from oxygen escape and structural collapse, limiting reversibility to roughly 75%. By incorporating iron into a cathode designated NMMF (Na2/3Mn7/12Mg1/4Fe1/6O2), iron ions absorb excess charge during cycling, stabilizing oxygen in the lattice and raising reversibility to 99%, which remains at 98% after 100 cycles. A practical pouch cell built with this cathode achieved an energy density of 206 Wh/kg, a capacity of 15.8 Ah, and 87.8% capacity retention after 100 cycles—approaching mainstream lithium iron phosphate (LFP) performance. Importantly, the cathode uses only abundant, low-cost elements (sodium, manganese, magnesium, iron), avoiding scarce or supply-constrained materials. The advance addresses sodium-ion batteries' key weakness in cycle life, strengthening their case for grid-scale energy storage where cost, safety, and longevity matter most, and potentially allowing them to compete directly with lithium-ion batteries in storage and entry-level electric mobility.

Key points

Nanjing University's Guo Shaohua and Zhou Haoshen team published a Nature Energy study showing how a simple element—iron—can fix a decade-old problem in sodium-ion batteries: the poor reversibility of lattice oxygen redox.

Why sodium-ion batteries matter

Sodium is abundant (essentially free from seawater), cheap, and safer than lithium (less prone to thermal runaway).
The catch: sodium ions are larger than lithium ions, so energy density is inherently lower—relegating sodium batteries to low-end uses (e-bikes, stationary storage).

The "oxygen defection" problem

Layered oxide cathodes rely on lattice oxygen redox: during charging, oxygen atoms participate in electron exchange to add capacity.
When oxygen over-participates, it escapes the lattice (releasing O2 or collapsing the structure), causing irreversible capacity loss.
In traditional materials, lattice oxygen redox reversibility measured only ~75%—roughly one in four oxygen redox events leaves permanent damage.

The iron-mediated fix

The team doped iron into the cathode as a redox buffer. Iron's multiple oxidation states (Fe²⁺/Fe³⁺/Fe⁴⁺) absorb excess charge before oxygen can destabilize, keeping oxygen locked in the lattice.
Result: reversibility improved from 75% to 99%, holding at 98% after 100 cycles.

The cathode: NMMF

Formula: Na₂/₃Mn₇/₁₂Mg₁/₄Fe₁/₆O₂
All components are abundant, cheap metals: sodium, manganese, magnesium, and iron—no scarce or supply-constrained elements, so the route can scale to mass production.

Real-world pouch cell results

Energy density: 206 Wh/kg
Capacity: 15.8 Ah
Capacity retention after 100 cycles: 87.8%
206 Wh/kg reaches the mainstream range of LFP lithium-ion batteries—sodium batteries finally closing the energy-density gap.

Why it matters

Energy storage is one of the largest growth markets of the coming decade, and it prizes cost, safety, and longevity above all. Sodium batteries already win on cost and safety; this work fixes the cycle-life weakness.

Key terms

Lattice oxygen redox: oxygen atoms contribute capacity during charging, but risk escaping the lattice—the root of the capacity-vs-longevity trade-off.
Reversibility: the fraction of a reaction that fully recovers without permanent damage; 99% means near-perfect cycling.
Pouch cell: a flat, flexible-format cell closer to real industrial products than coin cells, making the data more convincing.

Reference

1. Guo S., Zhou H. et al., *Iron-mediated reversible lattice-oxygen redox enables stable 200 Wh kg-1 sodium-ion batteries*, Nature Energy, 2026. DOI: 10.1038/s41560-026-02112-8. 2. Nanjing University news release on the Guo/Zhou team's lattice oxygen redox work, 2026-08-24. 3. Media coverage: iron-mediated strategy raises reversibility from 75% to 99%; pouch cell at 206 Wh/kg, 2026-08-23.