Lithium-ion batteries (LiBs), which charge and discharge by moving lithium ions between two electrodes known as the cathode and anode, remain the most widely used rechargeable batteries worldwide. Over the past decades, however, energy engineers have been assessing the potential of alternative rechargeable batteries based on different materials and ions.
These include sodium-ion batteries, which instead move positively charged sodium ions between an anode and cathode. As sodium is more abundant than lithium, these batteries could potentially be more affordable than LiBs.
The positive electrode in many previously developed sodium-ion batteries is made of a layered oxide, a crystalline material consisting of stacked metal–oxygen layers, with sodium ions positioned between them. Redox reactions, the chemical processes that allow batteries to charge and discharge, typically involve electron transfers centered on metal ions in layered oxide cathodes.
Sometimes, oxygen atoms within the cathode's crystal structure also play a role in these reactions. This can boost energy density, the amount of energy that can be stored for each unit of a battery's mass. Past studies showed that oxidized oxygen in layered oxide cathodes often does not return to its original chemical state after it participates in redox reactions. When a battery is repeatedly charged and discharged, the limited reversibility of the oxidized oxygen can damage cathodes, which can in turn reduce battery performance.
Researchers at Nanjing University and other institutes recently introduced a new strategy that could help overcome this limitation of sodium-ion batteries and enhance their durability. Their proposed approach, outlined in a paper published in Nature Energy, entails incorporating iron into the cathode material to facilitate the transfer of electrons to and from lattice oxygen, improving the reversibility of lattice-oxygen redox reactions.
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