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Sodium-ion batteries could light path to new power cells

An experiment on sodium-ion batteries is conducted at the Hagiwara Laboratory at Kyoto University.

TOKYO -- Research is continuing apace on sodium-ion batteries, cells that could deliver more juice to gadgets much more cheaply than current power cells.

     A group led by Shinichi Komaba, a professor at Tokyo University of Science, has developed technology that increases the charge capacity of the cell by five to seven times per unit of weight, compared with conventional batteries. Although the results are preliminary, the new battery's quick-charge characteristics are comparable to the lithium-ion batteries used to power most portable devices today. The research group is working to further enhance the cell's performance, and it hopes to bring the technology to market in five to 10 years.

Start me up

Batteries work by moving ions between two electrodes, one positive and one negative. With sodium-ion batteries, sodium ions inside the cell move from the negative electrode to the positive electrode through an electrolyte; electrons follow this ion path, which generates the electric current to power whatever is hooked up to the battery. When the battery is charging, ions and the corresponding electrons flow in the opposite direction, returning to the negative electrode.

     Research into sodium-ion batteries has been going on since the 1980s, but as capacity increased, battery life dwindled. From the middle of the last decade, however, this started to change, as the electrolytes and the materials comprising the electrodes were tweaked. Little by little, better materials for charging and discharging have been discovered and other problems overcome.

     The basic makeup of sodium-ion batteries is the same as that of their lithium-ion counterparts. The main drawback of lithium is its rarity. It is most commonly found in South America, but only in small deposits. Sodium, on the other hand, can be readily collected from seawater and other sources, which would make sodium-ion batteries far less expensive if they can be perfected.

     The drawback is that sodium ions are larger and heavier than lithium ions. To keep the power flowing, it is necessary to use different materials for the positive and negative electrodes and for the electrolyte.

     Komaba began his search for these materials by looking at what has been used in lithium-ion batteries. He eventually hit upon a material known as hard carbon for use as the negative electrode, while for the electrolyte, a solution that is not susceptible to electrolysis even with repeated charging and discharging was selected. In 2009, a battery was created that saw only a negligible decline in performance even after 100 charge cycles.

     And improvements have continued. A layered material with manganese and minute amounts of nickel now can serve as the positive electrode. Through joint research with associate professor Kazuyasu Tokiwa, performance has been improved by using black phosphorus in the negative electrode. This change increased the capacity of electricity that could be stored to around five to seven times that of batteries made with hard carbon.

     Black phosphorus is created by applying high pressures to phosphorus in the laboratory. There are issues in industrial production, but with black phosphorus, a sodium-ion battery's capacity is enhanced and is less likely to deteriorate even after repeated charges. This has raised hopes that one lingering problem of sodium-ion batteries -- that the lifespan grows shorter as capacity increases -- can be overcome.

     Komaba's team has tested various materials that could influence the performance of sodium-ion batteries to increase voltage while further extending capacity and lifespan. The team says there is still much room for improvement.

Right connections

Kyoto University professor Rika Hagiwara and associate professor Toshiyuki Nohira are in charge of a team that is researching the use of molten salts as the electrolyte. Molten salts are the liquid form of substances containing sodium. Their low viscosity means that when they are used as the electrolyte, ions can move more quickly.

     In 2013, working with Sumitomo Electric Industries, Hagiwara's team created a prototype of a sodium-ion battery that works even at 20 C. In the past, it was necessary to add heat with a heater to make molten salt, but no longer. The goal is to produce a battery that can operate even at minus 30 C.

     The viscosity of molten salt readily increases as the temperature drops. Hence, it is necessary to find a material with low viscosity but which still contains sodium. On the other hand, performance is better at higher temperatures. At present, the batteries can operate at around 90 C, and progress is being made on development of new materials that could work even at 120 C.

     "We expect applications for emergency power supplies, from mobile phone base stations in around three to five years to household stationary batteries in five to 10 years and batteries for electric vehicles in a decade," Hagiwara said.

     Sodium is the sixth most common element on the Earth's surface. It exists in abundance in sea water and rock salt, making it relatively cheap to collect.

     But to use it in sodium-ion batteries, the element must be processed chemically to refine it and increase its purity. Such costs are thought to put the process on a par with making lithium-ion batteries, so it is believed that material costs will be easy to reduce significantly. It is anticipated that the use of rare metals will eventually be eliminated, even for the electrodes.

     Completely solid batteries, in which even the electrolyte is a solid, and air batteries that use air for charging and discharging, are among the leading candidates for next-generation batteries. Realizing them will require overcoming numerous problems. These include reducing costs, increasing capacity and improving durability and safety, such as for use in electric vehicles.

     According to Komaba, the big issue is finding a way to underscore how a sodium-ion battery differs from other batteries and the applications they could have. Production of sodium-ion batteries can probably make use of know-how from the manufacturing of lithium-ion batteries.

     However, as in lithium-ion batteries, the electrolyte in sodium-ion batteries is a readily flammable organic solvent. Although similar safety measures are used in both types of batteries, the danger of fire remains. 

     Research and development of next-generation batteries is very competitive worldwide. Japan's technology and economy ministries are supporting such R&D, while the U.S. is pursuing such research as a national project centered on institutions under the Department of Energy. South Korea and China have also produced a variety of results.

     Compared to its automakers and materials and parts manufacturers, Japan's battery manufacturers have a weak research presence. This raises the fear that even if the country leads in basic research, it could lag in commercializing the technology.

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