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SoftBank Corp. and Enpower Greentech Successfully Verify 450+ Wh/kg Energy Density Battery and Succeed in Development of Core Technologies for Longer Battery Life

Prototype battery using SoftBank and Empower Greentech core technology
Prototype battery using SoftBank and Empower Greentech core technology

In March 2020, SoftBank Corp. (“SoftBank”) and US-based Enpower Greentech Inc. (“Enpower Greentech”) signed a joint research agreement for the development of lightweight, large capacity, and high specific energy density next generation batteries, with applications for IoT and cellular base stations. Both companies have just successfully verified a high energy density (450+Wh/kg) lithium metal battery, and are also pleased to announce the successful development of core technologies that will extend the service life of lithium metal batteries. The core technologies developed thus far include an ultra-thin coating film (less than 10nm), which prevents dendrites*1 from forming on the surface of lithium metal, and an electrolyte, which achieves both high voltage and high coulombic efficiency (charge-discharge efficiency) *2.

Lithium metal used in the 450Wh/kg cell has long been considered the paragon of anode materials; however, dendrites that subsequently form during the charge/discharge process often diminished battery capacity in a short time span. By coating the surface of the lithium metal with an inorganic substance (an “inorganic lithium metal-coating technology”), SoftBank and Enpower Greentech managed to prevent dendrite formation by blocking direct contact with the liquid electrolyte and creating a stable solid electrolyte interface (SEI) film*3. To verify this, both companies used a lithium metal electrolyte covered with an ultra-thin (less than 10nm) inorganic film. Collecting charge/discharge data on a coin-type lithium symmetric cell (a battery used for lab measurements), we were able to maintain a consistently low overvoltage even after 500 hours of continuous operation. In the future, both companies aim to apply this method to the 450Wh/kg cell to achieve an even longer battery life. For more details on the experiment, please refer to the Appendix.

Enpower Greentech is a US startup company engaged in the research, development, and commercialization of next-generation batteries, including all-solid-state cells. The company also has a research base in Japan (“Enpower Japan”). In 2015, they began developing high-capacity electrolyte materials, including solid electrolytes. Furthermore, since October 2017, Enpower Greentech has been collaborating on developing all-solid-state cell materials with a research team led by Professor John Goodenough of the University of Texas at Austin and a Nobel laureate in chemistry. Professor Goodenough made the following remarks regarding the results of the joint research by SoftBank and Enpower Greentech.

Prof. Goodenough’s remarks:

“I am pleased to see the excellent work of the Enpower and SoftBank team on the development of advanced battery technologies and on applications that contribute to the SDGs (Sustainable Development Goals). My research team is honored to contribute to this worthwhile effort from a fundamental materials science perspective, and I wish the Enpower and SoftBank team every success in this endeavor.”

SoftBank is currently undertaking a variety of initiatives aiming to narrow the digital divide, secure communication systems in the event of disasters, and reduce CO2 emissions. SoftBank believes that high-performance batteries will be essential in supporting the many technologies and devices that are necessary to tackle these challenges over the long term. A 450Wh/kg battery, which is expected to be achieved using the material technologies jointly developed by SoftBank and Enpower Greentech, would have twice the energy density of lithium-ion batteries on the market today. This battery will be applied not only to IoT devices and cellular base stations, but also to “Sunglider,” a solar-powered unmanned aircraft system designed for stratospheric communications (using High Altitude Platform Station, or HAPS, technology). A solution being developed by SoftBank’s subsidiary HAPSMobile Inc., Sunglider will fly in the stratosphere 20 km above ground.

SoftBank and Enpower Greentech will continue to conduct various research activities into high-capacity next generation batteries to solve social issues through its business activities.


  1. *Dendrite: Needle-shaped crystals of lithium metal grow when the battery is repeatedly charged/discharged. If dendrites continue to grow unchecked, it can cause a short circuit between the cathode and anode, leading to combustion and other problems.
  2. *Coulombic efficiency (charge/discharge efficiency): The ratio of the discharge capacity during a discharge to the charge capacity during a charge. The higher the coulombic efficiency, the more charge capacity can be used for discharge without waste it, resulting in a longer battery life.
  3. *Solid electrolytes interface (SEI) film: a film with lithium-ion conductivity formed at the interface between the anode and the electrolyte in a lithium-ion battery (LIB) or secondary lithium metal battery in the event of a charge/discharge.

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