1. Introduction of next-generation resin film

For current collector of lithium-ion batteries, metal foil such as Copper (Cu) and Aluminium (Al) is used. In terms of materials science, improving specific energy density(Wh/kg) of battery cell mostly depends on the capacity of positive and negative active materials (mAh/g). However, weight reduction of the battery-cells parts is also an effective method. In the weight ratio of lithium metal batteries (LMB) (Figure 1) , the weight of the current collector occupies 21 % of the total weight of the battery cell. By reducing the weight of the current collector, the weight of the battery cell can be reduced and the specific energy density can be improved.
Therefore, SoftBank has conducted the development of next-generation resin film formed thin conductive layers on both sides of the resin film (Figure 2).

Figure 1. Weight ratio of LMB excluding tab and exterior body

Figure 2. Appearance of next-generation resin film(Cu, negative electrode)(left)
and cross section SEM image（right）

2. Issues with welding for stacking

Stacked battery cell forms current collecting path by welding tab section of each layer (in orange area of Figure 3). However, the next-generation resin film does not have electric conductive paths in the vertical direction of the film due to including isolating resin layers in the structure. Moreover, since physical properties such as boiling and melting point and strength are different in resin and metal, it is different to form conductive paths by welding without rapture of the resin layer and the metal layer (Figure 3). Additionally, since it contains an insulating resin layer and the film itself has a high resistance, even if a stacked battery cell could be made, internal resistance would increase and battery performance would decrease.
Therefore, in order to utilize the next-generation resin film in stacked battery cells, it is necessary to establish a technology to weld the tab section, which has many resin/metal layers with low resistance, and to lower the resistance of the next-generation resin film itself.

Figure 3. Issues with welding laminted battery cells and next-generation resin films

3. Demonstration and cycle characteristics of stacked battery cell

In welding tabs and tab leads stacked with the next-generation resin film, we have suppressed factors of the battery performance decrements, including reduction of welding resistance, ensuring of welding strength and raptures and cracks of the film through consideration of various welding methods such as supersonic, resistance, and laser. In addition, we also have reduced the resistance of the next-generation resin film itself by examining various aspects such as the thickness, formation method of the metal layer of the next-generation resin film, the introduction of minute through holes, and the type of resin. By establishment of these technologies, we have successfully demonstrated the stacked battery cell with 3.5 Ah capacity grade.
In order to compare the battery performance of the next-generation resin film and metal foil, we prototyped 3.5 Ah stacked battery cells (positive electrode: 16 layers, negative electrode: 17 layers) using the next-generation resin film and metal foil(Figure 4), and conducted various evaluation tests.

Figure 4. Appearance and constitutions of stacked cells with the resin film.

We conducted cycling tests with conditions by charge: 0.5C 4.2V-CCCV 0.05C cut / discharge: 0.5C-CC 2.5V cut, temperature 25℃ (Figure 5). All configurations showed equivalent performance in terms of capacity retention and coulombic efficiency. We also confirmed equivalent performance in discharge rate tests of 0.1C to 2.0C.

Figure 5. Cycling characteristics of the stacked battery cells

4. Improvement of safety

One of the characteristics with the next-generation resin film is to improve the safety of battery cells by resin fusing. Similar to lithium-ion batteries, when LMB used metal foils as the current collector is short-circuited, a large current continues to flow locally, causing the temperature of the short-circuited part to rise rapidly and to lead to fire or explosion. On the other hand, in the case of the next-generation resin film including low melting point resin layer, it is expected to improve the safety because the current path is cut before ignition by rapid resin fusing if the short circuit occurs.
To verify that property, we conducted a nail penetration test using four types of stacked battery cells prototyped using the next-generation resin film and metal foil.

Movie of a nail panetration test in the case of Al/Cu cell

Movie of a nail penetration test in the case of the resin film (Al)/Cu

Figure 6. Results of the nail penetration tests

When the resin film (Al) was used as the positive current collector, the voltage dropped at the moment of a short circuit, but quickly recovered, and the temperature of the battery cell did not rise (Figure 6). These results show that using the next-generation resin film as the positive current collector improves the safety of battery cells by resin fusing effect.

The next-generation resin film that SoftBank is developing is the material to be expected to improve specific energy density and safety characteristics by replacing metal foil used generally. However, the improved resistance and cycle characteristics are still not sufficient, so in the future, we will roll out the next-generation resin film to material manufacturers and battery manufacturers, further improve the performance, and aim to use it in actual batteries.
SoftBank will continue to approach the development of high weight energy density battery cells for HAPS (High-Altitude Platform Station) from the material level and contribute to their early commercialization.