• Newly developed battery packs use battery cells with specific energy of 439 Wh/kg, more than 1.5 times that of existing products, to achieve 300 Wh/kg for battery packs

SoftBank Corp. (“SoftBank”) announced it developed a battery pack using a next-generation lithium-metal battery cell manufactured by Enpower Japan Corp. (“Enpower Japan”) with a specific energy of 439Wh/kg for High Altitude Platform Station (“HAPS”) stratospheric wireless communication platforms. SoftBank conducted a charge-discharge cycle test of the battery pack in the stratosphere from January 30 to February 2, 2023 in the United States together with its subsidiary, HAPSMobile Inc. (“HAPSMobile”). To realise HAPS-based wireless communication services, it is essential to develop next-generation batteries with a high specific energy that can be operated in the stratosphere. Although the battery pack was previously tested in a test chamber that simulates the temperatures and atmospheric pressure of the stratosphere, the demonstration marked the first time for SoftBank to successfully test the battery pack in the stratosphere. The tested battery pack for HAPS was developed in cooperation with ENAX Inc. (“ENAX”).

Compared to commercialised lithium-ion batteries, lithium-metal batteries have a higher specific energy and are known to improve cycle life by applying high constraint pressure during charging and discharging. Therefore, when long-life lithium-metal battery cells are assembled into a battery pack, an increase in component weight due to a constraint mechanism poses a development challenge. Also, since a battery pack for HAPS needs to operate correctly even in stratospheric conditions at extremely low temperatures of around -60℃, it is necessary to develop a temperature control system that includes heaters and insulation materials to maintain a constant temperature.

SoftBank developed, in collaboration with Enpower Japan, next-generation lithium-metal battery cells with a world-class specific energy of 439 Wh/kg and an electrolyte that is less prone to gas generation in stratospheric environments with low atmospheric pressure. And, in cooperation with ENAX, SoftBank developed battery pack components that included a constraint mechanism, heaters and insulation materials, and succeeded in reducing the weight of each component. This achievement brought the companies closer to building a battery pack with a specific energy of 300 Wh/kg, which exceeds the approximate 190 Wh/kg of battery packs for HAPS that use commercially available high-performance lithium-ion battery cells.

From January 30 to February 2, 2023, SoftBank and HAPSMobile conducted a charge-discharge cycle test in the stratosphere using the developed battery pack for HAPS. As a result, the newly developed battery pack demonstrated the same level of regular operation in the stratosphere at extremely low temperatures of around -60℃ as shown during the ground tests. Also, it was confirmed that there was no significant difference in the heat retention of the battery pack and the power consumption of the heater when compared with the results of a previous stratospheric environment simulation. As a result of conducting this stratospheric test, SoftBank was able to confirm phenomena and issues specific to the environment in the stratosphere, namely, the establishment of control and communication methods between the stratosphere and the ground, and heat conduction differences of the battery pack when heated in the stratosphere compared to operation on the ground. In the near future, SoftBank aims to develop a large battery pack as a power source for HAPS operation. In addition to HAPS, SoftBank will consider using the battery pack for industrial drone applications.

SoftBank promotes research and development on next-generation batteries that are lightweight, safe, and have high specific energy, considering a perspective of their application in existing devices, HAPS, and other next-generation communication systems. SoftBank will also continue to promote research and development with the aim of realizing high-performance batteries from the perspective of contributing to the SDGs (Sustainable Development Goals).

SoftBank will hold an event called “ギジュツノチカラ Advanced Tech Show 2023” on March 22 and 23, 2023 to showcase various initiatives of SoftBank's Research Institute of Advanced Technology, including the content of this press release. For more details about the event, please visit the event website. (* The event will be held in Japanese only)
Furthermore, readers are invited to visit the Research Institute of Advanced Technology's redesigned website to learn more about the various technology areas and initiatives that the institute is pursuing.