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Wednesday, August 6, 2014

New Rechargeable Cell Has 7x Higher Energy Density Than Li-ion Cells

A Japanese research group developed a rechargeable battery based on a new principle in cooperation with Nippon Shokubai Co Ltd.

The group is led by Noritaka Mizuno, professor at the School of Engineering, the University of Tokyo. The new battery uses the oxidation-reduction reaction between oxide ions and peroxide ions at the positive electrode. The group proved that peroxides are generated and dispersed due to charge and discharge reactions by using a material made by adding cobalt (Co) to the crystal structure of lithium oxide (Li2O) for the positive electrode, verifying a battery system based on a new principle.

The new technology can realize an energy density seven times higher than that of existing lithium (Li)-ion rechargeable batteries, increase capacity, lower price and enhance safety. It is expected to be used for batteries for electric vehicles (EVs) and next-generation stationary batteries.

The oxidation-reduction reaction between Li2O and Li2O2 (lithium peroxide) and oxidation-reduction reaction of metal Li are used at the positive and negative electrodes, respectively, of the new battery. The battery has a theoretical capacity of 897mAh per 1g of the positive/negative electrode active material, voltage of 2.87V and theoretical energy density of 2,570Wh/kg.

At that time, the energy density is 370Wh per 1kg of the positive/negative electrode active material, which is about seven times higher than that of existing Li-ion rechargeable batteries using LiCoO2 positive electrodes and graphite negative electrodes. The theoretical energy density of the new battery is lower than that of lithium-air batteries (3,460Wh/kg). But it has a sealed structure like conventional Li-ion batteries, realizing a high reliability and safety.

This time, as the positive electrode material, the research group used a material made by using a planetary ball mill to add Co to the crystal structure of LiO2. And the group proved that it is possible to realize a battery system in which the oxidation-reduction reaction between oxides and peroxides reversibly proceeds. And it proved that (1) peroxides are generated in the positive electrode for charge, (2) the peroxides are dispersed for discharge and (3) those reactions are repeated, by quantitatively analyzing the peroxides.

The group also proved that neither O2 nor CO2 is generated in the range where it is possible to reversibly charge/discharge the battery.

The positive electrode used in the demonstration test enables to repeatedly charge/discharge the battery with a capacity of 200mAh/g and to quickly charge/discharge the battery with a large current. The positive electrode has a smaller mass ratio of Co than LiCoO2, which is used for existing Li-ion batteries, and possibly lowers costs.

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