Abstract:

As the most advanced battery technology to date, lithium-ion battery has occupied the main battery markets for electric vehicles and grid scale energy storage systems. However, the limited lithium reserves as well as the high price raise concerns about the sustainability of lithium-ion battery. Although sodium-ion battery is proposed as a good supplement to lithium-ion battery, expensive and flammable electrolyte components, harsh assembly environments and potential safety hazards have limited the rapid development to a certain extent. The organic electrolyte was replaced with an aqueous solution to construct a new type of aqueous sodium ion battery capacitor (ASIBC). It is of great significance for next-generation energy storage system owing to its low cost, high power, and inherent safety. However, applicable ASIBC system is rarely reported so far. Here, a rechargeable alkaline sodium ion battery capacitors constructed by using Na_0.44MnO₂ cathode, activated carbon (AC) anode, 6 mol∙L⁻¹ NaOH electrolyte, and cheap stainless-steel current collector. Because of high overcharge tolerance of Na_0.44MnO₂ cathode in alkaline electrolyte, the shortcomings of the half-sodium Na_0.44MnO₂ cathode and low initial Coulombic efficiency of AC anode can be resolved by in situ overcharging pre-activation process during first charging. The available capacity of Na_0.44MnO₂ in half cell largely increased from ~40 mAh∙g⁻¹ (neutral electrolyte) to 77.3 mAh∙g⁻¹ (alkaline electrolyte) due to broadened Na⁺ intercalation potential region. Thus, the AC||Na_0.44MnO₂ ASIBC delivers outstanding electrochemical properties with a high energy density of 26.6 Wh∙kg⁻¹ at a power density of 85 W∙kg⁻¹ and long cycling stability with a capacity retention of 89% after 10, 000 cycles. The advantages of the alkaline electrolyte for the AC||Na_0.44MnO₂ ASIBC can be concluded as follows: (1) through the in situ electrochemical pre-activation process, the overcharging oxygen evolution reaction during first charging process can balance the adverse effects of the half-sodium Na_0.44MnO₂ cathode and low initial Coulombic efficiency of AC anode on the energy density of full cell; (2) the overcharging self-protection function can promote the generated oxygen to be eliminated at anode during overcharging, which improves the system safety; (3) the low-cost materials in alkaline environment can be scaled up to construct AC||Na_0.44MnO₂ ASIBC. In addition, the AC||Na_0.44MnO₂ ASIBC also possesses wide operating temperature range, achieving satisfied electrochemical performance at a high temperature of 50 ℃ and a low temperature of −20 ℃. Considering the merits of low-cost, high safety, no toxicity and environment-friendly, we believe the AC||Na_0.44MnO₂ rechargeable alkaline sodium-ion battery capacitors have the potential to be applied to large-scale energy storage.

Key words: Sodium-ion battery capacitor, Alkaline electrolyte, Overcharging self-protection, Low cost, Wide operating temperature range