Lithium-ion batteries (LiBs) still dominate the market, but the search for cheaper, resource-abundant chemistries is intense. Sodium-ion batteries are prime candidates: sodium is far more abundant and cheaper than lithium. The main challenge is achieving high energy density without killing cycle life.
Lattice-oxygen redox: more energy, less stability
Many sodium-ion cells use layered oxide cathodes, where metal–oxygen layers are stacked with sodium ions in between. In classic intercalation chemistry, redox reactions are centered on transition metal ions. However, in some high-energy materials the lattice oxygen also participates in redox.
This oxygen redox boosts energy density, but comes with a catch: oxidized oxygen often fails to fully revert to its original state during discharge. Over many cycles, this limited reversibility leads to structural degradation and capacity fade.
Iron as an electron shuttle
A team from Nanjing University and collaborators, publishing in Nature Energy, propose an elegant fix: use iron ions as redox mediators to assist lattice-oxygen redox.
They designed a new layered oxide cathode with the composition Na2/3Mn7/12Mg1/4Fe1/6O2 (Na–Mn–Mg–Fe–O). In this structure:
- During charging, Fe⁴⁺ ions capture electrons from lattice oxygen;
- During discharging, Fe²⁺ ions donate electrons back to the oxidized oxygen via chemical pathways.
With this iron-mediated shuttle, the reversibility of lattice-oxygen redox jumps from about 75% to 99%.
Performance of the new cathode
When used in a sodium-ion pouch cell, the iron-assisted cathode achieved:
- Energy density: 206 Wh/kg;
- Current density: 50 mA/g;
- Cycle life: stable operation for 100 cycles with 87.8% capacity retention.
While still at the lab stage, this iron-mediated lattice-oxygen redox concept offers a promising pathway to more stable, high-energy sodium-ion batteries. If scaled, it could accelerate the commercialization of sodium-ion technology for grid storage, backup systems and cost-sensitive applications.
Source: Phys.org
