With the rise of solid-state electrolyte batteries, their application prospects in the field of electric vehicles have attracted much attention compared with traditional liquid electrolyte batteries. However, Nick Flaherty reminds us that while these solid-state batteries have great potential, there are still some challenges in bringing them to market.
First, a major advantage of solid-state batteries is that they replace liquid electrolytes, helping to reduce the growth of lithium dendrites in lithium-ion batteries. Lithium dendrites can cause batteries to short-circuit and, in the worst-case scenario, cause fires. In addition, solid-state batteries can also use other materials that cannot be used in liquid batteries, especially metallic lithium as the negative electrode. This may provide higher energy density, and the laboratory has demonstrated batteries with energy densities as high as 1514 Wh/litre.
This lighter weight and higher energy density performance have also sparked interest in electric aircraft design.
However, the development of solid-state batteries faces several challenges, so many observers believe they are unlikely to be produced at scale before 2030. One challenge is that solid-state electrolytes limit the ability of charges to move within the material, specifically limiting charge transport between different material interfaces. This reduces the battery’s power, or specific energy in Wh/kg, because a higher voltage must be used to drive current through the battery. At the same time, this may also reduce the number of charge cycles of the battery, because the higher voltage may cause the solid electrolyte to degrade faster than the liquid electrolyte
Other challenges include processes for mass-manufacturing solid-state batteries for electric vehicles and improving the safety of liquid water electrolyte batteries. Therefore, many research projects are currently exploring new solid-state battery materials and structures while considering their manufacturability.
In summary, discovering the right combination of materials for high-performance solid-state electrolyte EVs is a key area of research, and there are currently three main solid-state electrolyte contenders, including oxides and sulfides (which can be used as ceramic materials) and polymers, The latter is injected in a ceramic holder to carry the current.
A key difference between the development of solid-state batteries and existing lithium-ion batteries is that they require a balance between specific energy (in Wh/kg) and energy density (in Wh/litre). The adoption of all-solid-state batteries will significantly increase the energy density of batteries, which means that at the same energy level, batteries are smaller and lighter. This in turn will enable the battery pack to provide longer range for the same volume, or a lighter, smaller, more economical battery pack for the same range
However, this often has an impact on the battery’s specific energy, or the battery’s power output. This is a bigger issue for heavy-duty electric platforms like trucks and buses, so there are different trade-offs when choosing solid-state battery (SSB) materials. Of course, electric aircraft need to take into account both aspects.
Using smaller, lighter cells allows the battery pack to be fundamentally redesigned, reducing the need for physical protection and cooling so lighter materials can be used. This is a huge improvement for the electric vehicle platform, and even if the battery itself is already in mass production, it will take time to enter mass production. This is why most battery developers regard 2030 as a key time node for SSB technology. They have been working on this technology for several years, going from sample A to sample B, increasing the capacity from 5Ah to 20Ah for early prototype testing by car manufacturers.
Currently, several SSB battery developers are building pilot production lines for small-scale production in 2022 and 2023, with mass production planned for 2024 and 2025. The cells produced on these production lines could be used in cars produced in series between 2028 and 2030, which is why there is interest in this timetable.
Another factor is the impact solid-state batteries have on form factor. The most popular solid-state electrolytes are made from ceramic materials that do not bend, making it impossible to create cylindrical batteries. This is considered a problem because cylindrical cells are the most cost-effective to produce: they can be mass-produced via an automated roll-to-roll process.
Therefore, finding the most cost-effective way to mass-produce pouch-type solid-state batteries is a key area of development. Higher energy density can save cooling costs in other parts of the car, partially making up for the more complex production process, but this is also why people are exploring other non-traditional technologies such as plasma processing and 3D printing.
How solid-state batteries fit into existing manufacturing processes is also critical. Some companies are retrofitting existing lithium-ion battery production lines to use polymer-based semi-solid electrolytes and electrode materials, but using standard materials for other parts of the battery. This will be enough to produce high-end limited edition cars in the future.