Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (5): 1905027.

Special Issue: Sodium Ion Energy Storage Materials and Devices

• Article •

Electrochemical Mechanism of Na0.44MnO2 in Alkaline Aqueous Solution

Hui Li1,Shuangyu Liu1,Tianci Yuan2,Bo Wang1,Peng Sheng1,Li Xu1,Guangyao Zhao1,Huitao Bai1,Xin Chen1,Zhongxue Chen3,Yuliang Cao2,*()

1. 1 State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection Research Institute Co. Ltd., Beijing 102211, P. R. China
2 Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China
3 Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, P. R. China
• Received:2019-05-06 Accepted:2019-06-03 Published:2019-06-10
• Contact: Yuliang Cao E-mail:ylcao@whu.edu.cn
• Supported by:
the Science and Technology Project of State Grid, China(SGRIDGKJ[2017]841);the National Key Research Program of China(2016YFB0901500);the National Natural Science Foundation of China(21875171);the National Natural Science Foundation of China(21673165)

Abstract:

In recent years, aqueous sodium-ion batteries (ASIBs) have experienced rapid development, and a series of cathode materials for ASIBs has been widely reported. Among these, Na0.44MnO2 possesses the most promising prospects due to its low cost, non-toxic nature, simple synthesis, and structural stability. However, the reported capacity of Na0.44MnO2 in aqueous electrolyte was ~40 mAh·g−1 (less than its theoretical capacity of 121 mAh·g−1), which limits its practical applications. Recently, we developed a novel alkaline Zn-Na0.44MnO2 dual-ion battery using Na0.44MnO2 as the cathode, a Zn metal sheet as the anode, and a 6 mol L−1 NaOH aqueous solution as the electrolyte. In this system, the Na0.44MnO2 electrode presented excellent electrochemical performance with high reversible capacity (80.2 mAh·g−1 at 0.5C) and outstanding cycling stability (73% capacity retention over 1000 cycles at 10C) in alkaline aqueous electrolyte. When the negative potential window was extended to 0.3 V, the Na0.44MnO2 electrode delivered an incredibly high capacity of 345.5 mAh·g−1, which far exceeded the theoretical capacity, but the cycling performance was extremely poor. In that study, X-ray diffraction (XRD) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses revealed that de-intercalation of Na+ and formation of Mn(OH)2 occurred during the discharge process, but the detailed electrochemical mechanism and structural evolution of this process remained unclear. In this study, we used ICP-AES to analyze the elemental composition of discharge products at different discharge depths and found that a small amount of Na+ ions extracted from Na0.44MnO2 electrode since Discharge-120 (corresponding to the discharge capacity of 120 mAh·g−1), and the extraction rate increased gradually with increasing discharge depth. Scanning electron microscope (SEM) and XRD analyses were also carried out to characterize the morphology and phase changes of Na0.44MnO2 electrode during discharge. The results show that the discharge of Na0.44MnO2 electrode in the voltage range 1.95–0.3 V could be divided into the three following steps: (1) the potential range above 1.0 V: Na+ ions de-intercalate reversibly into the tunnel structure of Na0.44MnO2; this discharge mechanism is consistent with that in non-aqueous and neutral aqueous sodium ion batteries. (2) The initial platform region at 1.0 V: in this step, protons (H+) began to insert into the Na+-vacancies in NaxMnO2, and the tunnel structure of NaxMnO2 was still maintained. (3) Subsequent slope region: when the Na+-vacancies in the tunnel structure were fully occupied by protons, further intercalation led to intensification of charge repulsion in the crystal structure. Thus, the tunnel structure collapsed to form a new Mn(OH)2 phase, accompanied by the release of Na+ from the structure. H+ has a smaller radius than Na+; therefore, it could insert into the smaller vacancies in Na0.44MnO2, resulting in higher specific capacity. However, the insertion of H+ will also cause structural damage, which seriously worsens the cycling stability of the Na0.44MnO2 electrode.

MSC2000:

• O646