The Study On Sodium-storage Performance Of Na_0.44MnO₂ Cathode In Alkaline Solution

Aqueous sodium ion batteries are considered as one of the ideal choices for large-scale energy storage batteries due to their high safety,environment-friendly and low cost.Among these reported positive electrode materials for aqueous sodium ion batteries,Na_0.44MnO₂ with outstanding structural stability,electrochemical stability and low cost attracts the interest of many researchers.However,the reversibly specific capacity of Na_0.44MnO₂ electrode in sodium sulfate solution is only 40 m A h g^-1 due to the influence of proton insertion in neutral solution,which was far less than the theoretical specific capacity(121 m A h g^-1).Therefore,this work was aimed at broadening the stable electrochemical window and improving the reversible capacity of Na_0.44MnO₂ by adopting alkaline electrolyte,exploring the influence of the concentration of Na OH on the performance of storage sodium,revealing the charge-discharge mechanism and developing the Na_0.44MnO₂ electrode with high capacity and high stability.The main research results of this thesis are as follows:（1）Zn/Na_0.44MnO₂ double-ion battery（AZMDIB）was firstly constructed based on Na_0.44MnO₂ positive electrode,Zn negative electrode and 6 M Na OH electrolyte.The capacity of 80.2 m A h g^-1 could be obtained in the voltage range of 1.95-1.1 V,and the AZMDIB also exhibited extremely high rate performance.The reversible specific capacity of 32 m A h g^-1 can still be maintained at a high current density of 50 C.Moreover,excellent cycling performance was demonstrated with a capacity retention rate of 73%after 1000 cycles at 10 C.In order to avoid the influence of Zn anode,aqueous sodium ion battery capacitor was assembled with Na_0.44MnO₂ and activated carbon（AC）as positive and negative electrodes,respectively.In the voltage range of0-1.65 V,AC/Na_0.44MnO₂ sodium ion capacitor could release specific energy of 18.9Wh kg^-1（based on the mass of both the active material of positive electrode and negative electrode）.Moreover,it showed marvelous rate performance and cycling performance,it can still maintain the energy density of 7.7 Wh kg^-1 at the power density of 4.62 k W kg^-1,and the capacity retention of 69.1%after 3500 cycles at a current density of 10 C.（2）We broadened the potential window of Na_0.44MnO₂ electrode to gain the higher capacity.The results displayed that a wide voltage plateau appeared near 1.0 V and the first discharge specific capacity reached 276.6 m A h g^-1,far exceeding the theoretical capacity(121 m A h g^-1).XRD,SEM,and ICP-AES were employed to investigate the detail discharge mechanism of electrode.The results show that the discharge process could be divided into two stages:the first stage is the intercalation of hydrogen ions into the tunnel structure and the structure remains unchanged,corresponding to the platform area in discharge curve.In the second stage,hydrogen ions intercalate and sodium ions de-intercalate,accompanied by the collapse of tunnel structure and formation of Mn（OH）₂ phase,relating to the slope area in discharge curve.（3）We studied the influence of Na OH concentration on electrochemical performance of Na_0.44MnO₂ electrode.The results showed that the increase of Na OH concentration is beneficial to inhibit the insertion of protons and improve cycle performance and the rate performance of electrode,but it would lead to the premature triggering of oxygen evolution reaction at the same time,and the rate performance would also reduce when the concentration was too high.Na_0.44MnO₂ electrode showed the most outstanding electrochemical performance in 8 M Na OH.At the current density of 0.5 C,the reversible specific capacity reached 79.2 m A h g^-1.Even at a high current density of 50 C,it still could maintain the capacity of 35.3 m A h g^-1.In a voltage window of 0.2-1.2 V,the capacity retention after 500 cycles is 64.3%.In addition,we also found that narrowing voltage window could decrease the side reactions to improve the cyclic performance and the exceptional ability of Na_0.44MnO₂electrode to sustain overcharging in alkaline electrolyte.The above results provide the possible technical directions for the development of low-cost cathode material for aqueous sodium ion batteries.