Cost-effective and safe rechargeable batteries are required for energy storage in sustainable development. Lithium is widely used as a charge carrier of battery because the standard potential is very low. However, lithium ion battery is still too costly to use in large scale energy storages. Thus, sodium has been attracted as a substitute of lithium because the element is abundant and low-cost and its standard potential is closer to lithium.

Recently some layered materials were found out that they work as cathode materials of sodium ion batteries (SIB). The layers are composed of transition metals and oxygen, and sodium is inserted between the layers. Therefore, these materials have 2D channels of sodium. The layered materials show good capability of sodium, but the layered structure is so unstable that all inserted sodium cannot be extracted. On the other hand, some other 3D channel systems, such as phosphate systems, are enough stable to extract all inserted sodium and their effective capacity can achieve the theoretical value. [1]

In this study, we will report a novel cathode material of a sodium ion battery. We have been studying a new phosphate system material. The material shows the property as a cathode material of the SIB; reversible insertions / extractions of sodium are observed (XRD patterns are shown in Fig.1). The material has high stability on insertion and extraction of sodium ions owing to their tetrahedral structure of phosphate. In addition, by optimizing synthetic process of the material, fine particles were obtained as shown in Fig.2. Therefore the very high capacity, approximately 150 mAh/g at first cycle and over 120 mAh/g after 10 cycles, is achieved. The crystal structure of the material was analyzed precisely using atomic resolution TEM and in-situ XRD measurements with an electrochemical cell attachment. The details will be reported on the day of the session.

[1] A. Yamada, S. C. Chung and K. Hinokuma, J. Electrochem. Soc., 2001, 148, 3