Air-Stable Na_3.5C₆O₆ as a Sodium Compensation Additive in Cathode of Na-Ion Batteries

Sodium-ion battery (SIB) is a candidate for the stationary energy storage systems because of the low cost and high abundance of sodium. However, the energy density and lifespan of SIBs suffer severely from the irreversible consumption of the Na-ions for the formation of the solid electrolyte interphase (SEI) layer and other side reactions on the electrodes. Here, Na_3.5C₆O₆ is proposed as an air-stable high-efficiency sacrificial additive in the cathode to compensate for the lost sodium. It is characteristic of low desodiation (oxidation) potential (3.4-3.6 V vs. Na⁺/Na) and high irreversible desodiation capacity (theoretically 378 mAh g^-1). The feasibility of using Na_3.5C₆O₆ as a sodium compensation additive is verified with the improved electrochemical performances of a Na_2/3Ni_1/3Mn_1/3Ti_1/3O₂ǀǀhard carbon cells and cells using other cathode materials. In addition, the structure of Na_3.5C₆O₆ and its desodiation path are also clarified on the basis of comprehensive physical characterizations and the density functional theory (DFT) calculations. This additive decomposes completely to supply abundant Na ions during the initial charge without leaving any electrochemically inert species in the cathode. Its decomposition product C₆O₆ enters the carbonate electrolyte without bringing any detectable negative effects. These findings open a new avenue for elevating the energy density and/or prolonging the lifetime of the high-energy-density secondary batteries.