Rational Design of Aqueous Na Ion Batteries Toward High Energy Density and Long Cycle Life.

Prussian blue analogues (PBAs) are promising cathode candidates for aqueous Na ion batteries (ANIBs) considering their low-carbon and cost-effective features. However, it is still a huge challenge to achieve desirable energy density coupled with long cycle life due to inherent Na defects in PBAs and the unstable solid-electrolyte interphase (SEI) layer. Herein, we design Na₂C₄O₄ additives as sodium supplements to compensate for Na defects in PBAs, while utilizing the CO₂ products decomposed from Na₂C₄O₄ to construct a robust SEI layer containing Na₂CO₃ species. As proof of concept, our building of full ANIBs using iron-based PBAs and NaTi₂(PO₄)₃ anode with an appropriate amount of Na₂C₄O₄ enable a reversible capacity of ∼144 mA h g^-1 at 0.2 C and an excellent cycling stability of 15,000 cycles with 85% retention at 10 C. The proposed concept is further extended to the manganese-based PBA ANIBs to deliver an energy density of 92 W h kg^-1 with improved cycling stability.