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