The "naturally degradable" battery. An electromotive force is generated by saturating the battery with water.
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Nippon Telegraph and Telephone Corp (NTT) developed a "naturally degradable" primary battery that does not do harm to soil or living organisms even when left in the natural world.

The electrode, electrolyte and casing (which usually have a high environmental load) of the battery are made of environmentally-friendly materials such as biological and fertilizer-derived materials. The output voltage of the battery is 1.1V (measured current: 1.91mA/cm2).

NTT expects that the battery will be used for sensor devices that are installed outdoors and cannot be easily retrieved. The company exhibited the battery at its private show, "NTT R&D Forum 2018."

The newly-developed battery is an air battery that uses oxygen in the air as a positive-electrode active material. In the positive (air) electrode of an air battery, there needs to be a three-dimensional conductive porous structure for diffusing oxygen contained in the air into the electrode.

Normally, the porous structure is formed by using fluororesin-based binding agent to solidify carbon powder. However, fluororesin generates toxic gas when burned, causing a high load on the environment.

NTT made it possible to form a porous structure without fluororesin binding agent by applying a certain process (of which the company did not disclose the details) to a biological material and carbonizing it.

The separator and the casing of the battery are also made of biological materials. For the negative electrode and electrolyte, NTT used materials that can work as fertilizers. But it did not disclosed the details of the negative electrode and electrolyte.

In a "plant damage test," in which the used battery is crushed, mixed with soil and given to plants to see its influence on the plants, the company confirmed that the battery's components remaining in the soil do not affect the growth of the plants.

One of the challenges in commercializing the battery is its electric capacity, which is about 1/10 that of a dry cell of an equivalent size.

"The battery can be improved in many ways such as electrode and electrolyte," NTT said. "We want to increase its energy density to an equivalent level."