There are no downsides 'per se', that is not really the question. The question is more 'what is the upside'. Until these batteries are in mass production, unle

There are no downsides 'per se', that is not really the question. The question is more 'what is the upside'. Until these batteries are in mass production, unless Toyota is much, much further along then anybody think, it is questionable if 'traditional' batteries might not just be simply better. 'Traditional' lithium ion is also evolving very fast and is produced in numbers that will be incredibly hard to reach, and thus achieving cost parity will be difficult. What Tesla interceded for example is just a further evolution of li-ion but as you add more silicon to the anode, the performance is not actually that different from lithium metal anode (ie what everybody is working on for solid state). Confusingly pure lithium metal anode does not actually have less volume then a silicon anode that is full of lithium-ions. That is very everybody is going, current batteries have 10% silicon anode, Tesla is likely taking it to 20-30% or more in the next couple years (other working on the same probably). There are already companies selling pure silicon anode products that are on the same kind of timeline as solid state companies. They however generally have the advantage of being usable in a standard battery factory. One of the claims to fame for solid-state is safety. That can be true and probably will be. However there are also developments in the traditional space that might make them just as safe, by introducing new seperators. So its kind of race, between different paths of design and maybe even more importantly on how you can scale the technology. You have some startups QuantumScape (VW invested), SolidPower (Ford, BMW invested) and large companies like Toyota really pushing solid state as the future. You have some startups like Sila Nanotechnology (Daimler invested) and some large companies like Tesla push silicon. One question is, really how much better does your car need to be then a current Tesla 3 or comparable. Practically speaking we are not so far away from where you will likely stop just boosting the specs. So it will generally come down to how cheap can you produce the batteries and how many can you actually produce. This is why my bet is that by 2030 the waste majority of EV and even the EV sold will not be solid state.