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

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 Na0.44MnO2 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 Na0.44MnO2 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 Na0.44MnO2, 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 Na0.44MnO2 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.