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Next-Generation Na-Ion Secondary Battery Will Be New Age Energy.

January 18,2011
Solving material problem for sustainable society. High capacity and long life.

Associate Professor Shinichi Komaba 
Department of Applied Chemistry, Faculty of Science Division I

For cell phones, laptop PCs, and hybrid electric vehicles, many of batteries currently used in electrical appliances familiar to us are called lithium-ion batteries. Lithium-ion batteries are excellent in storage capacity and achieve high energy, as compared to the conventional batteries such as lead acid, Ni-Cd, and Ni-MH batteries, and are being widely commercialized in various electrical appliances.

With a view toward achieving a lithium-ion battery having an increased energy density and an increased output power, Associate Professor KOMABA has made extensive and intensive studies, e.g., syntheses of electrode materials and studies about the components of electrolytic solution and the electrode/electrolytic solution interface over the past ten years or more. He is conducting both many collaborative researches with Jananese and foreign companies and researche institutes. Further, recently, he is challenging new fields including studies of sodium-ion batteries, biological fuel cells, and electrochemical sensors.

For example, for solving the firing and safety problems of the current lithium-ion batteries, he has made studies on nonflammable electrolytic solutions usable as a substitute for the combustible electrolytic solution which causes the origin of the battery firing. The studies of this issue are conducted as a part of the NEDO project which is one of the national projects, and, in autumn of last year, advanced materials which achieves high capacity in each of the positive and negative electrodes of the battery were successfully synthesized and a new technique succeeded to design high capacity electrode including functional binders. Consequently, not only the safety problems of the lithium-ion battery solved, but also the remarkable improvement of both the energy density and cycle life of the battery was achieved.

However, the lithium-ion batteries have a problem about their raw material. Essentially, there is an enormous amount of lithium on the Earth, but most of the lithium, which is one of alkali elements, is present in the form of saline water in salt lakes, and the high-quality lithium ore mining sites from which lithium can be easily taken are extremely limited. For this reason, countries all over the world are scrambling for lithium likewise rare metals, and, for realizing a sustainable society, a new battery free of lithium is attractive. With respect to the raw material for battery, Associate Prof. KOMABA pays attention to sodium which is placed immediately below lithium in the periodic table, and is starting a new challenge.

Associate Prof. KOMABA talks about this challenge. The idea of replacing lithium by sodium is very simple, but the main reason that this idea has not yet been realized resides in that such a battery is unsatisfactory in the storage capacity on the positive electrode side and cannot be improved in the life of the negative electrode. For example, NaCoO2, which possesses the similar crystal structure to conventional positive electrode material, LiCoO2, for the lithium-ion batteryexcept that Li is changed to Na, achieves a storage capacity only half of the storage capacity obtained using LiCoO2.

In the sodium-ion battery (see the figures below) being currently studied by Associate Prof. KOMABA, NaNi0.5Mn0.5O2 has achieved a charge storage capacity comparable to the capacity of LiCoO2 which is a positive electrode material for the current lithium-ion battery. Surprizingly, Na(Fe, Mn)O2 has achieved a capacity (150 mAh/g or more) far higher than the capacity obtained using LiCoO2 reported by his group very recently.

In addition, the improvement of the life of the negative electrode has been realized with optimizing electrolyte solutions, succeeding 100 cycles or more of the charging operations. His group is succeeding the excellent charge-discharge performance of the 3 vlot sodium-ion battery, which has been never reported, based upon the materials development of both the electrodes and electrolyte.

Associate Prof. KOMABA is further making a challenge for bringing the sodium-ion battery into practical use.