The Myth of Solid State Batteries?
The latest craze in energy storage seems to be waning recently. More specifically, the promise over solid electrolytes that can conduct Lithium ions between the anode and cathode does not seem to be panning out. To be fair, a fully solid-state battery would solve the two main problems that plague Li-ion batteries now. First, the elimination of the liquid electrolyte, which serves as the fuel for a battery cell in thermal runaway, would enhance the safety of Li ion batteries. Second, a solid state electrolyte would allow for a Lithium metal anode, rather than another intercalation material like graphite, thereby increasing the energy density of the battery and completely removing range anxiety for electric vehicles.
So yes, in principle, a solid-state battery would be awesome. But, when it comes to EV’s or grid storage, those pesky economics always get in the way. There is pretty a fundamental reason why a solid state battery would be difficult to produce on a large scale, and conversely, why a liquid-based based battery is perfect for mass production. The reason concerns how to get the Lithium ion out of and into the cathode. The cathode is an intercalation material, meaning the Lithium ion enters the solid and diffuses through the molecular structure of the cathode. This diffusive transport is not very fast. Therefore, the cathode must be a fine powder. If the cathode is a fine powder, then there is a lot of surface area for the Lithium ions to enter the cathode material, but not a lot of volume through which the ions need to diffuse. To further complicate the issue, the cathode is not electrically conductive. In order to get the electron out, an electrical conductor (carbon) needs to be in the mix. Incorporating a liquid electrolyte involves simply taking the cathode/carbon powder mixture, and soaking it in the liquid so that all of the holes are filled up, giving the ions superhighways of transport.
By contrast, the perfect solid state cathode must be a good mixture of the electrical conductor, the Li-ion conductor, and the intercalation material. Without a good enough mixture, Li ions cannot access available intercalation sites, and the capacity of the cell drops to a small fraction of what it could be. Such low capacity cells, unfortunately, represent the only data that have been publicly disclosed by companies that specialize in solid state Li batteries like Sakti3 and Seeo –despite all of the hype. Production of a quality cell likely entails molecular assembly, vacuum deposition, or a similar time- and cost-intensive manufacturing process.
Now, something interesting has happened. Seeo was gobbled up by Bosch, and Sakti3 was gobbled up by Dyson – despite early investments by GM. Volkswagon was supposed to have announced last summer whether or not to use Quantumscape’s solid state batteries in their next generation of cars. No announcement was made. The implication is that, although it may be possible to make a very high capacity cell using a solid electrolyte, the cost makes large-format battery applications – like cars and grid storage – unattainable. But there may still hope for smaller-format applications – like vacuum cleaners, appliances, etc. These applications may be less cost sensitive and more driven by battery size and weight. It will be exciting to see what Dyson and Bosch do with the technology.