Tuning V Redox Couple By Metal-Ion Substitution in Na3V2(PO4)3 Cathode Material for High Energy Density Na-Ion Batteries

Abstract

Recently, the demand for renewable energy has raised with the increase of environmental issues and exhaustion of crude oil. To acquire efficient utilization of the renewable energy, large-scale energy storage systems (ESSs) are necessary that balance the discrepancy between production of and the time required to consume renewable energy. Li-ion batteries have been considered as candidates for large-scale ESSs because they provide high energy density and a long cycle life. However, due to the lack of Li reserves in the Earth and the corresponding growing cost problems, Na-ion batteries have been received great attention as alternative candidates for Li-ion batteries. The Na-ion batteries can replace the Li-ion batteries in the aspects of abundant Na resources, low cost, and easily accessible reserves. Furthermore, the similarity of the electrochemical reaction mechanisms of Na-ion batteries to that of Li-ion batteries has accelerated the development of the Na-ion batteries; thus, various research studies have been conducted to find proper electrode materials for Na-ion batteries. Among various cathode materials of sodium-ion batteries for large-scale applications, NASICON-Na3V2(PO4)3 is an attractive candidate for the cathode material of Na-ion batteries due to its thermal and structural stabilities. Also, Na3V2(PO4)3 shows 117.6 mAh·g-1 theoretical capacity at 3.4 V vs. Na/Na+ using the V3+/4+ redox couple. However, the operating voltage of Na3V2(PO4)3 should be improved to replace the conventional Li-ion battery cathode materials. If V4+/5+ redox reaction, which utilizes higher operating voltage, can be employed in this material, higher operating voltage could be achieved. Herein, we suggest a simple method to enhance the operating voltage of Na3V2(PO4)3. In this work, non-transition metal ions are substituted for V-ions in Na3V2(PO4)3 to utilize V4+/5+ redox reaction. During the (de)intercalation of Na-ions, the oxidation state of the non-transition metal ions is not changed. Hence, some V3+ ions in Na3V2(PO4)3 should be oxidized until V5+ instead of non-transition metal ions while two Na-ions are extracted. Therefore, some part of capacity reacts at 3.4 V using V3+/4+ redox couple and the other part reacts at voltage higher than 3.4 V using V4+/5+ redox couple. Through the tuning of V redox couple, not only the enhancement of energy density of the cathode material but the cost reduction of expensive V source also is possible. The figure shows the initial charge profiles of Na3V2(PO4)3/C and metal-ion substituted Na3V1.5M0.5(PO4)3/C. In case of Na3V1.5M0.5(PO4)3/C, a high voltage V4+/5+ redox reation was clearly observed as the cell was charged above 3.5 V vs. Na/Na+. In the presentation, the detailed structural analyses and electrochemical performance of the materials will be shown. Figure 1