English

Electrochemical Mechanism of Na_0.44MnO₂ in Alkaline Aqueous Solution

• Li Hui ^1, ,
• Liu Shuangyu ^1, ,
• Yuan Tianci ^2, ,
• Wang Bo ^1, ,
• Sheng Peng ^1, ,
• Xu Li ^1, ,
• Zhao Guangyao ^1, ,
• Bai Huitao ^1, ,
• Chen Xin ^1, ,
• Chen Zhongxue ^3, ,
• Cao Yuliang ^{2, ,}

• 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

Corresponding author: Yuliang Cao, Email: ylcao@whu.edu.cn; Tel.: +86-27-68754526

• Received Date: 06 May 2019
  Accepted Date: 03 June 2019
  Revised Date: 02 June 2019
  Available Online: 15 May 2020
• Fund Project:

  The project was supported by the Science and Technology Project of State Grid, China (SGRIDGKJ[2017]841), the National Key Research Program of China (2016YFB0901500) and the National Natural Science Foundation of China (21875171, 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, Na_0.44MnO₂ possesses the most promising prospects due to its low cost, non-toxic nature, simple synthesis, and structural stability. However, the reported capacity of Na_0.44MnO₂ 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-Na_0.44MnO₂ dual-ion battery using Na_0.44MnO₂ 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 Na_0.44MnO₂ 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 Na_0.44MnO₂ 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 Na_0.44MnO₂ 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 Na_0.44MnO₂ electrode during discharge. The results show that the discharge of Na_0.44MnO₂ 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 Na_0.44MnO₂; 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 Na_xMnO₂, and the tunnel structure of Na_xMnO₂ 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 Na_0.44MnO₂, resulting in higher specific capacity. However, the insertion of H⁺ will also cause structural damage, which seriously worsens the cycling stability of the Na_0.44MnO₂ electrode.

Key words:
• Sodium ion battery
•  /  Na_0.44MnO₂
•  /  Alkaline electrolyte
•  /  Electrochemical mechanism
•  /  Proton insertion

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