Abstract:

Renewable energy resources (such as wind and solar) are being increasingly utilized to overcome issues of energy shortage and environmental deterioration. However, the intrinsically fluctuant and intermittent character of renewable energy sources hinders their practical application; therefore, batteries have been developed to act as a link between renewable energy sources and consumers. Lithium-ion batteries have become the most advanced battery technology in the last three decades, and have successfully captured the electric vehicles market; however, many concerns have recently arisen about the vastly expanded demand for lithium resources, which contrasts with their limited reserves. In this context, sodium-ion batteries have emerged as a promising alternative because of their intercalation chemistry similar to that of lithium-ion batteries, and the abundance of Na resources in the Earth's crust. Like lithium-ion batteries, the performance and cost of sodium-ion batteries are determined primarily by their cathodes. Among the various cathode materials that have been reported for sodium-ion batteries, Na_0.44MnO₂ is regarded as one of the most promising because of its opened three-dimensional tunnel structure and good chemical stability; it has also been demonstrated in previous studies to have superior cycling stability at room temperature. In practical terms, commercial batteries are often used at high temperatures (above 40 ℃) in summer. Several Mn-based cathode materials for lithium-ion batteries, such as LiMn₂O₄ and LiNi_0.5Mn_1.5O₄, exhibit severe capacity decay at high temperatures. Therefore, the evaluation of the Na_0.44MnO₂ cathode in sodium-ion batteries at high temperatures is critical for its further commercialization. In this study, a Na_0.44MnO₂ cathode is prepared by a facile solid-state method and its electrochemical performance at a high temperature is measured. The electrochemical tests show that the Na_0.44MnO₂ cathode has a capacity retention of 66.5% over 100 cycles and a low reversible capacity of 12.3 mAh∙g^-1 at 10C (1C = 120 mAh∙g^-1). To improve its performance at a high temperature, Al₂O₃-coated Na_0.44MnO₂ is prepared via a liquid-phase method, and the coating effect is evaluated by electrochemical measurements as well as morphological, structural, and chemical composition analyses. The results show that the electrochemical performance of uncoated Na_0.44MnO₂ at 55 ℃ is significantly improved after coating with Al₂O₃; the capacity retention after 100 cycles increases to 79.2%, and the discharge capacity at 10C is increased to 63.6 mAh∙g^-1. The improved performance is clearly attributed to the Al₂O₃ coating, which effectively prevents direct contact of Na_0.44MnO₂ with the electrolyte and alleviates the dissolution of manganese at a high temperature, thus maintaining a stable electrode/electrolyte interface and reducing charge transfer resistance.

Key words: Sodium-ion battery, Na_0.44MnO₂, Al₂O₃ coating, High temperature performance, Manganese dissolution