Electrochemical and Chemical Properties of Na₂NiO₂ as a Cathode Additive for a Rechargeable Sodium Battery

Where the anode of an alkali-metal rechargeable battery has the Fermi energy above the LUMO of the electrolyte, an important fraction of the working ions of a cell become irreversibly trapped in a passivating solid-electrolyte-interphase (SEI) layer formed at the anode surface on the initial charge, thus reducing the reversible capacity of the cathode. To compensate for this irreversible loss in a rechargeable sodium-ion battery, a cathode additive that can supply extra Na + ions should be explored. In this work, we have characterized the chemical and electrochemical properties of orthorhombic Na 2 NiO 2 for this purpose. Through a combined electrochemical and structural characterization, Na 2 NiO 2 is found to be transformed irreversibly to NaNiO 2, and the crystalline NaNiO 2 is operating as a cathode in the following charge and discharge reactions. We have evaluated electrochemical characteristics of Na 2 NiO 2 and confirmed that it is a suitable cathode additive to compensate for the irreversible Na + loss at the anode. A full cell with a Sb/C composite anode was successfully designed to demonstrate and verify this cathode strategy.