Finding a new energy storage material is a great challenge and sodium is showing great promise. Being one of the two main ingredients in the salt, it is very abundant, non-toxic and cheap. However, it is very difficult to produce a sodium based battery. The problem is, when exposed to air, the metals in a sodium battery’s cathode can be oxidized, decreasing the performance of the battery or even rendering it completely inactive.

In the last years, research on the development of sodium-ion batteries have been making great progress in terms of performance utilizing layered transition-metal oxides and polyanions. It appears that the sodium compounds can be promising compared with their lithium analogs. Combining, the layered metal oxides with polyanions will offer a good compromise between high energy densities and stable cycle life.

Structure of Na2C6O6 and its electrochemical behaviour in Na cells under different conditions showing inconsistent phase transition with chemical and crystal structures.

Using sodium-ion batteries for grid-scale applications will need an active material that combines a high-energy density with sustainability. Given the highly specific theoretical capacity and Earth abundance of disodium rhodizonate (Na2C6O6), it is one of the most promising cathodes for these new sodium-ion batteries. However, technical difficulties have been encountered due to in part at a lower reversible capacity than expected. To address this difficulty a team from Stanford University the phase transformation of the disodium rhodizonate during cycling. They discovered that the cause of the deteriorating redox activity of the cathode. The active-particle size and electrolyte conditions were identified as key factors to decrease the activation barrier. On the basis of this understanding, they modified the four-sodium storage of the cathode improving the energy efficiency and the cycle retention.

Morphological changes during reversible phase transformation and proposed redox mechanism for sodium storage of disodium rhodizonate (Na2C6O6). A schematic of the proposed de/sodiation mechanism of Na2C6O6.

Thanks to this better understanding of the redox mechanism associated with structural and morphological changes during cycling, the team achieved a full utilization of their new battery. Unexpectedly, spontaneous formation of nanostructures enabled facile phase transformation in the cycles, which provided a mechanism for realizing high-rate capability of electrode materials. The resulting electrochemical performances of the new sodium electrode surpass any reported performances of other cathode candidates for sodium-ion batteries.

“This is already a good design, but we are confident that it can be improved by further optimizing the phosphorus anode.”

Yi Cui, Department of Materials Science and Engineering, Stanford University, California

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