Revealing the Competitive Intercalation between Na+ and H+ into Na0.44MnO2 in Aqueous Sodium Ion Batteries

Abstract Aqueous rechargeable sodium‐ion batteries have attracted increasing attention for large‐scale energy storage applications due to their intrinsic safety and sufficient sodium reserves. The tunnel‐type Na 0.44 MnO 2 has been widely investigated as a promising cathode because of its low cost and high theoretical capacity (120 mAh g −1 ). However, a capacity higher than 45 mAh g −1 for Na 0.44 MnO 2 in aqueous electrolyte is difficult to obtain. Here, it is found that there is a competitive insertion reaction between H + and Na + , and the diffusion energy barriers of Na + are increased along with high Na content in Na 0.44 MnO 2 , but the opposite is true for protons. By decreasing the diffusion energy barrier of Na + and increasing the diffusion energy barrier of protons, a high reversible capacity of 101 mAh g −1 for Na 0.44 MnO 2 in aqueous electrolytes is achieved for the first time. Coupled with a quinone anode, the full cell delivers a high energy density of 60 Wh kg −1 and retains 85% of its capacity for 1200 cycles at a 1 C rate.