Electrochemical Reactions of Niobium Based Oxide Materials as Anodes for Sodium Ion Batteries
According to the rapid spread of eco-friendly vehicles, it is required to develop the next generation batteries with higher capacity and power, better safety performance and lower cost. We focused on the sodium-ion batteries (SIBs), providing the great chance of low cost and high performance batteries because of the infinite Na sources and wide selectivity of the active materials. In fact, we found the mixed polyanion compounds1-3) which operate at high voltage as cathode materials for SIBs. In contrast, quite a few anode materials have been reported so far. It has developed that hard carbon4,5), alloys6), organic materials7) and titanium based oxides8,9) were acceptable anode materials for SIBs. Among these anode materials, hard carbon is the only candidate materials in terms of high capacity and long cycle life. However, the reaction potential with sodium ion in hard carbon system is very close to the sodium metal deposition, so there is a concern in safety. Therefore, it is expected to develop the novel anode materials with relatively high reaction potential. In this work, we investigate the anode materials with the reaction potential around 1.0V vs. Na/Na+ in order to avoid the risk of sodium metal deposition. While looking for the transition-metal based oxides, we focused on the niobium based oxide materials, and investigated the electrochemical reactions of KNb3O8 as anode material for SIBs.
Niobium based oxide material (KNb3O8) was synthesized by traditional solid state reaction. Morphology and crystal structure were analyzed by SEM and XRD, respectively. The electrochemical properties of KNb3O8 as anode were investigated by the coin type cell with Na metal as counter electrode in non-aqueous electrolyte, and working electrodes that were mixtures of synthesized niobate material, conductive carbon and binder.
3. Results and Discussion
From the result of XRD analysis as shown in Fig.1, the crystal structure of KNb3O8 is orthorhombic with space group Amam (Fig.2). The galvanostatic charge discharge profile of KNb3O8 material in Na cell was shown in Fig.3. The reaction potential plateau around 1.0V vs. Na/Na+ was observed. From these experimental results, we have found that niobium based oxide materials operate as effective anode materials with appropriate reaction potential for SIBs. Electrochemical properties of niobium based oxide materials as anode will be presented in detail.
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