Effects of carbon composite and metal-ion substation on the electrochemical performance of cathode materials ($Na_3V_2(PO_4)_3$ and $Na_{0.7}MnO_2$) for Na-ion batteries

Abstract

Recently, Na-ion batteries have been considered as alternatives for Li-ion batteries due to the crustal abundance and low cost of Na. Therefore, it is necessary to develop electrode materials with high electrochemical performances to commercialize the Na-ion batteries. Especially, development of high-performance cathode materials is necessary which occupy the most part of the manufacturing cost. Cathode materials which have been extensively studied for Na-ion batteries are classified into layered oxides and polyanionic compounds according to their crystal structure. Among the various layered oxides, sodium manganese oxide $(Na_{0.7}MnO_2)$ has been considered potential cathode material due to its high capacity and cost-effective synthetic process. Among the polyanionic compounds, sodium vanadium phosphate (Na₃ V₂(PO₄)₃) have received attention owing to the excellent thermal and structural stability. However, the promising candidates of cathode materials for Na-ion batteries also have problems as follows: (1) sluggish kinetic properties of $Na_{0.7}MnO_2$, (2) poor electronic conductivity and (3) low energy density of Na₃ V₂(PO₄)₃. Herein, three individual studies are performed to solve these problems. In the first part, $Na_{0.7}MnO_2/C$ composite cathode material is fabricated by simple heat treatment for high-power Na-ion batteries. Compared to only 28.07% retention of pristine $Na_{0.7}MnO_2$, 64.55% of the discharge capacity of $Na_{0.7}MnO_2/C$ at a current density of 20 mA $g^-1$ is retained at $400 mA g^{-1}$. The second part covers fabrication of high-power Na₃ V₂(PO₄)₃/C composite cathode materials by synergetic effects of sucrose coating and cellulose template. The improved rate performance of Na₃ V₂(PO₄)₃/C, $77.58 mAh g^{-1}$ at 20-C rate, is acquired from the cost-effective synthetic process with the relatively low amount of carbon, 5 wt.%. In the third chapter, Mg-substituted Na₃ V₂(PO₄)₃/C is synthesized to improve the energy density of Na₃ V₂(PO₄)₃ by utilization of $V^{4+/5+}$ redox reaction. The high-voltage $V^{4+/5+}$ redox reaction in Mg-substituted Na₃ V₂(PO₄)₃/C electrode leads to 3.5% increase of energy density compared with Na₃ V₂(PO₄)₃/C electrode.