A research group in China has developed an iron-mediated strategy to improve the stability and energy density of sodium-ion batteries. The researchers incorporated iron into a sodium-deficient manganese-magnesium-iron layered oxide (Na₂/₃Mn₇/₁₂Mg₁/₄Fe₁/₆O₂, or NMMF) cathode, making its lattice-oxygen reactions more reversible and reducing capacity loss during cycling.

“The iron in the NMMF compound is first found to show redox-mediated (RM) catalysis behavior,” co-author Shiyong Chu told pv magazine. “This is the first report of RM in classic alkali metal-ion batteries. Unlike traditional RMs that facilitate surface reactions, as observed in lithium-oxygen and sodium-oxygen (Li/Na-O₂), in this study, the iron-mediated catalytic reaction takes place within the bulk of the lattice-oxygen-activated cathode material, enabling the efficient utilization of lattice-oxygen redox.”

Chu said poor lattice-oxygen redox reversibility occurs in most anionic-redox cathodes, resulting in poor structural and electrochemical stability. The researchers therefore proposed an iron-mediated catalysis strategy to improve redox reversibility and used quantitative mapping of resonant inelastic X-ray scattering to determine the lattice-oxygen redox reversibility of NMMF.

To produce the NMMF cathode, the researchers used a solid-state reaction with sodium carbonate, manganese dioxide, magnesium oxide, and iron oxide as raw materials. They added 5% excess sodium carbonate to compensate for sodium losses during high-temperature processing, mixed the materials in a high-energy ball mill for five hours at 300 rpm, dried the resulting mixture overnight, and pressed it into a 15 mm-diameter cylinder.

The material was then calcined at 900 C for 12 hours under continuous oxygen flow. The researchers used the same process to produce an iron-free material for comparison.

For testing, the team fabricated electrodes containing 80% active material, 10% acetylene black, and 10% sodium-alginate binder and coated them onto aluminum foil. The electrodes were initially tested in coin-type half-cells using sodium metal as the counter electrode. The researchers then incorporated the NMMF cathode into a 15.8 Ah pouch cell with a hard-carbon anode, a polypropylene separator, and a sodium-based electrolyte.

“The redox reversibility of lattice oxygen is dramatically improved from 75% to 99% owing to the RM. To date, this is the highest lattice oxygen redox reversibility reported,” Chu said. “The NMMF electrode shows a high specific capacity of 220 mAh g⁻¹ owing to the redox of lattice oxygen. When NMMF is paired with an hard carbon (HC) anode, the pouch cell achieves an energy density of 206 Wh kg⁻¹, based on the weight of the entire pouch cell.”

Chu said in-situ X-ray absorption near-edge structure (XANES) spectroscopy, machine-learning-based molecular dynamics (MD) simulations, and ⁵⁷Fe Mössbauer spectroscopy confirmed iron-mediated catalytic reactions in the bulk NMMF. He added the iron ions act as redox mediators, transferring electrons between different iron oxidation states and lattice oxygen during charging and discharging. This process allows oxidized lattice oxygen to return more efficiently to its original state, improving the reversibility of the reaction.

Chu explained the team plans to apply the iron-mediated strategy to other layered oxide cathodes and alkali-ion battery systems in follow-up research. It also aims to improve the discharge voltage and cycle life of NMMF-based cells.

“Future work will focus on partnering with enterprises to scale up synthesis and cell manufacturing,” he added.

The study, “Iron-mediated reversible lattice-oxygen redox enables stable 200 Wh kg⁻¹ sodium-ion batteries”, was published in Nature Energy. Researchers from China’s Nanjing University, Zhejiang University, Soochow University, and HoNa New Energy Technology, as well as the United States’ Lawrence Berkeley National Laboratory, contributed to the research.