Exploring the Ni redox activity in polyanionic compounds as conceivable high potential cathodes for Na rechargeable batteries

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Although nickel-based polyanionic compounds are expected to exhibit a high operating voltage for batteries based on the Ni2+/3+ redox couple activity, some rare experimental studies on the electrochemical performance of these materials are reported, resulting from the poor kinetics of the bulk materials in both Li and Na nonaqueous systems. Herein, the electrochemical activity of the Ni2+/3+ redox couple in the mixed-polyanionic framework Na4Ni3(PO4)2(P2O7) is reported for the first time. This novel material exhibits a remarkably high operating voltage when cycled in sodium cells in both carbonate- and ionic liquid-based electrolytes. The application of a carbon coating and the use of an ionic liquid-based electrolyte enable the reversible sodium ion (de-)insertion in the host structure accompanied by the redox activity of Ni2+/3+ at operating voltages as high as 4.8 V vs Na/Na+. These results present the realization of Ni-based mixed polyanionic compounds with improved electrochemical activity and pave the way for the discovery of new Na-based high potential cathode materials. A chemical framework that enables low-cost sodium-ion batteries to reach remarkably high operating voltages has been developed. The approach by Stefano Passerini at Helmholtz Institute Ulm in Germany and colleagues uses interlinked phosphate-based anions to produce a porous electrode that incorporates nickel into its redox cycle. Transition metals such as nickel improve the energy density in sodium-ion batteries, but their sluggish kinetics makes them difficult to implement in conventional electrodes. The team fine-tuned a mixture of phosphate and pyrophosphate anions to generate a compound with large diffusion channels for sodium. After coating the electrode with a thin carbon film to boost conductivity, the researchers found the polyanionic structure could withstand nearly 5 volt operation. Higher operating voltages are potentially possible with improved electrolyte selection. The electrochemical activity of Ni2+/3+ redox couple in the mixed-polyanionic framework Na4Ni3(PO4)2(P2O7) is reported for the first time. The application of a carbon coating and the use of an ionic liquid-based electrolyte enable the reversible Na+ (de-)insertion in the host structure accompanied by the redox activity of Ni2+/3+ at operating voltages as high as 4.8V vs. Na/Na+. These results represent an important step towards the realization of Ni- based mixed polyanionic compounds with improved electrochemical activity and pave ways for the discovery of new Na-based cathode materials and the formulation of Na-ion batteries with very high operating voltages.