Influence of NaOH Concentration on Sodium Storage Performance of Na0.44MnO2

Aqueous sodium ion batteries (ASIBs) have attracted considerable attention for large-scale energy storage because of their prominent advantages of low cost, high safety, and environment-friendliness. Among the reported cathode materials for ASIBs, Na0.44MnO2 exhibits outstanding structural and hydrochemical stability, and hence is of much interest to research scholars. However, the reversible capacity of Na0.44MnO2 in most of the reported ASIBs was only 40 mAh.g(-1) due to the restriction of stable working windows, although the in spite of theoretical capacity is121 mAh.g(-1). Recently, we reported a Zn/Na0.44MnO2 dual-ion battery (AZMDIB) based on a Na0.44MnO2 positive electrode, Zn negative electrode, and 6 molL(-1) NaOH electrolyte. The alkaline solution lowered the proton insertion potential and expanded the stable working window of the Na0.44MnO2 electrode, thus enhancing the reversible capacity to 80 mAh.g(-1). Previous studies have demonstrated that the composition, concentration, and pH of the electrolytes have significant effects on the stable electrochemical window, rate performance, cycling performance, and other electrochemical properties of aqueous batteries. In addition, it has been reported that the co-intercalation of hydrogen ions can be inhibited by increasing the pH of the electrolyte in order to improve the cyclic stability of the electrode. Therefore, exploring the effect of electrolyte concentration and pH on the electrochemical performance of Na0.44MnO2 can provide insight into the design and optimization of high-performance Zn/Na0.44MnO2 aqueous batteries. Hence, in this work, rod-like Na0.44MnO2 was synthesized by ball milling and subsequent high-temperature calcination, and the influence of NaOH concentration on the electrochemical performance of the Na0.44MnO2 electrode was investigated by adopting five different concentrated electrolytes, 1, 3, 6, 8, and 10 mol.L-1 NaOH. The results showed that an increase in NaOH concentration is beneficial for preventing the insertion of protons and improving the cycling performance and the rate performance of the electrode; however, it also leads to premature triggering of the oxygen evolution reaction. Moreover, the rate performance would decrease at high NaOH concentration. The Na0.44MnO2 electrode showed optimal electrochemical performance in 8 mol.L-1 NaOH. At a current density of 0.5C (1C = 121 mA.g(-1)), a reversible specific capacity of 79.2 mAh.g(-1) was obtained, and a capacity of 35.3 mAh.g(-1) was maintained even at a high current density of 50C. In the potential window of 0.2-1.2 V (vs. NHE), the capacity retention after 500 weeks was 64.3%, which increased to 78.2% when the potential window was reduced to 0.25-1.15 V, because of the fewer side reactions. In addition, Na0.44MnO2 showed an exceptional ability to sustain overcharging up to 30% in a concentrated alkaline electrolyte (based on the reversible capacity of 79.2 mAh.g(-1)), and the discharge capacity within 80 cycles was almost steady. The above mentioned results form the basis for possible technical directions toward the development of low-cost cathode materials to be used in ASIBs.