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Achieving Specific Energy of 350 Wh/kg in All-solid-state Battery: Homogenization Technology of Solid Electrolyte
#HAPS, #Next-Generation Battery
Jul 02, 2024
SoftBank Corp.


Blogs
1.Technological Innovation in Next-generation Battery for HAPS: 350Wh/kg All-solid-state Battery
SoftBank Corp. (“SoftBank”) and Enpower Japan Corp. (“Enpower Japan”) have been conducting research and development of next-generation batteries with high specific energy (Wh/kg). The batteries are lightweight and have large capacity for use in High Altitude Platform Stations (HAPS), which provide telecommunication services from the stratosphere. So far, the two companies successfully verified a liquid-type lithium metal battery cell with specific energy of 520Wh/kg *1, and an all-solid-state lithium-metal battery cell with specific energy of 300Wh/kg*2.
Recently, in the development of solid-state batteries, the companies have achieved technological advancements such as increasing the active material ratio through the homogenization of solid electrolytes and thinning the solid electrolyte layers. This has led to the successful demonstration of an all-solid-state lithium-metal battery cell with a specific energy of around 350Wh/kg. (Fig. 1).
*1: Please refer to the press release dated November 21, 2021, "SoftBank Makes Great Progress in the Next-generation Battery Development, Successfully Verifying Three New Technologies Including Development of Cathode Materials for Solid-state Batteries"
https://www.softbank.jp/corp/news/press/sbkk/2021/20211102_01/
in Japanese only
*2: Please refer to the press release dated August 24, 2023, " SoftBank Corp. and Enpower Japan Corp. Successfully Developed All-Solid-State Battery with High Energy Density. Battery achieving specific energy of 300 Wh/kg”
https://www.softbank.jp/en/corp/news/press/sbkk/2023/20230824_01/

Fig. 1. (a) 350 Wh/kg battery cell, (b) Charge-discharge characteristics.
2. Background and Benefits of All-Solid-State Battery Development

Fig. 2. Benefits of all-solid-state batteries.
All-solid-state batteries are batteries containing all components in the solid state. While conventionally utilized lithium-ion batteries (LIB) contain a liquid electrolyte, all-solid-state batteries contain a solid electrolyte. Compared to the liquid electrolyte, the solid electrolyte has several important advantages, as listed below (see also Fig. 2).
◾️ High thermal safety
Liquid electrolytes primarily use flammable organic solvents. In contrast, solid electrolytes are less flammable and offer higher safety even when overheated. This gives all-solid-state batteries extremely high thermal safety, allowing safe usage under extreme conditions.
◾️ Long cycle life
Solid electrolytes are less prone to side reactions compared to liquid electrolytes. In liquid-type batteries, side products can diffuse, and new electrolyte can continue to react. However, solid electrolytes prevent such diffusion, resulting in less degradation and a longer cycle life.
◾️ High voltage
Because diffusion is minimized in solid electrolytes, both the oxidation resistance of the cathode and the reduction resistance of the anode can be individually optimized. This enables higher voltage, expanding the usable voltage range of the active materials and increasing capacity. Additionally, the ability to withstand high voltage allows the use of new high-voltage materials that were previously unusable, broadening the range of material options.
◾️ Fast charging
Solid electrolytes can achieve a high lithium-ion transport number, close to 1. The transference transport number indicates the ratio of ions moving through the electrolyte. The closer this number is to 1, the more efficiently lithium ions move. This increases lithium-ion conductivity, enabling fast charging.
◾️ Wide temperature range
Solid electrolytes do not freeze and increase resistance at low temperatures like liquid electrolytes, nor do they easily vaporize at high temperatures. This ensures stable performance across a wide temperature range. Stable operation at high temperatures also benefits fast charging, as the battery temperature rises during high-current fast charging, accelerating degradation because of electrolyte decomposition. With all-solid-state batteries, which do not degrade at high temperatures, cooling times during fast charging are unnecessary, significantly reducing charging time.
◾️ Compact and lightweight
Solid electrolytes prevent short circuits between adjacent electrodes, allowing the electrodes to be arranged in series within the cell case. This eliminates the need to separate series-connected cells with cases, reducing components like cases and lead wires, making the entire battery more compact and lightweight. This increases design flexibility in portable devices and electric vehicles, enhancing overall device performance. SoftBank, is particularly focused on the long cycle life of all-solid-state batteries as has been shown by SoftBank at experimental level, the potential for cycle life characteristics exceeding those of liquid electrolytes. Realizing the long cycle life of all-solid-state batteries could reduce battery replacement costs. However, technologies that improve cycle life at low energy density may alter the composition of cathode active materials, surface treatment materials, binders, and electrolytes, potentially changing degradation mechanisms and making them unsuitable for high energy density applications. Furthermore, thinning separator layers and thickening cathodes to achieve high energy density may introduce new issues such as dendrite formation and uneven reactions due to surface roughness. Therefore, SoftBank plans to first improve the specific energy to 400Wh/kg and then conduct extensive research on extending the cycle life (Fig. 3).
Among the many advantages of the solid-state batteries, SoftBank is especially focused on their long cycle life. Namely, as has been shown by SoftBank at experimental level, the potential of the solid-state batteries to have cycle life that exceed that of the batteries with liquid electrolytes. If long cycle life of the all-solid-state batteries can be realized in the future, it is expected that the battery replacement cost will be reduced.
On the other hand, technologies that prolongate the cycle life at low energy density may not be applicable when high density is achieved, because the composition of the cathode active material, surface treatment material, binder and solid electrolyte may change, which may influence the degradation mechanism. Also, to increase energy density by making the separator layer thinner or the positive electrode thicker, new issues may arise, such as short circuits and uneven reactions caused by dendrites growth or surface unevenness of the electrodes. For this reason, the development strategy of SoftBank is to first increase the specific energy to 400Wh/kg, and then actually engage in research to increase cycle life (Fig. 3).
By achieving specific energy of 350Wh/kg, SoftBank have taken an important step towards the next development goal of demonstrating 400Wh/kg.

Fig. 3. Battery development strategy of SoftBank.
3. Current Status of Technological Development: Homogenization Technology of Solid Electrolyte
Although the surfaces of solid electrolytes and cathode layers may appear smooth to the naked eye, they have microscopic irregularities when viewed up close (Fig. 4). These irregularities reduce the adhesion between the cathode active material and the solid electrolyte, increasing interface resistance that affects ion conductivity. This leads to a decrease in battery capacity, output performance, and cycle life, making it challenging to form effective interfaces between the cathode active material and the solid electrolyte, as well as between solid electrolyte layers.
SoftBank has addressed these challenges through the homogenization of solid electrolytes. The benefits of solid electrolyte homogenization are as follows:

Fig. 4. Schematic diagram of the all-solid-state battery interface.
■ Dispersion of conductive material reduces electrode resistance, increases the cathode utilization rate, and extends the battery cycle life.
■ Homogenization of the separator layer and cathode layer reduces interface resistance and improves charge-discharge efficiency.
■ Reaction uniformity resulted in suppression of dendrites growth and short circuits, and extends the battery cycle life.
■ Reducing the cell weight by making the separator layer thinner.
■ Reducing the cell weight by increasing the area of the electrodes.
As shown in Figure 5, when there are large solid electrolyte particles or variations in particle size distribution, conductive materials do not disperse well. Additionally, pressing to form the solid electrolyte layer results in uneven surfaces. Thicker areas have higher resistance, while thinner areas have lower resistance, causing current to concentrate in the thinner sections. This promotes the formation of lithium dendrites, leading to short circuits, uneven reactions, reduced lifespan, and decreased safety.
To address this issue, the solid electrolyte layer is made thicker. However, a thicker solid electrolyte layer is heavier and has higher resistance, resulting in reduced battery capacity and lower energy density by weight. Therefore, SoftBank reviewed the raw material conditions of the solid electrolyte and improved the grinding process to achieve smaller particle sizes and sharper particle size distribution control, resulting in a more homogeneous solid electrolyte. This improvement allowed for a thinner solid electrolyte layer while suppressing short circuits and uneven reactions. However, making the solid electrolyte layer too thin weakens its strength, increasing the risk of short circuits. Thus, the optimal thickness of the solid electrolyte layer was carefully considered to balance weight reduction and short circuit prevention.

Fig. 5. Close-up diagram of the electrode layer interface (before homogenization)
By combining these technologies, SoftBank has demonstrated a solid-state lithium metal battery cell with an energy density of 350Wh/kg. At the electrode level, we have achieved 392Wh/kg and demonstrated 200 cycles. However, challenges remain with large pouch cells, as they tend to short circuit during cycling. Moving forward, SoftBank will continue to develop further materials and electrode homogenization technologies to prevent short circuits, even in larger and stacked electrodes.
4. Future Challenges and Activities
Battery research begins with small model cells and then progresses to practical large cells such as pouch cells. To improve weight energy density, it is essential to scale up the cell size, which reduces the weight ratio of components like the cell case and leads. However, as cells become larger, the impact of uniformity increases, and there is a trade-off with thinning the solid electrolyte layer. SoftBank continues to research and develop next-generation high-capacity batteries, aiming to further improve uniformity.
In collaboration with Tokyo Institute of Technology, SoftBank is working on enhancing solid electrolytes, developing new process technologies, and creating high-capacity positive electrode materials such as high-voltage cathodes. The goal is to demonstrate a weight energy density of 400Wh/kg by FY2024 and then continue development towards achieving over 1,000 cycles for longer battery life.
Currently, the volumetric energy density (Wh/L) is on par with liquid lithium-ion battery cells. However, by using resin foil as a current collector, the volumetric energy density will also be improved. Simultaneously, SoftBank is developing elemental technologies to reduce restraint pressure. These advancements aim to further enhance the performance and practical application of solid-state batteries, expanding their use in aviation fields such as HAPS and drones, IoT devices, and automotive applications, thereby contributing to solving various societal challenges with next-generation batteries.
*3: Please refer to the SoftBank Research Institute of Advanced Technology blog article dated November 20, 2023, “Improvement of Safety by Next-generations Resin Film"
To read the press release regarding this activity, please visit the following website: https://www.softbank.jp/en/corp/news/press/sbkk/2024/20240702_01/