A solid state battery has no liquid components. So that should mean more longevity, safety, weight reductions, capacity per pound, and more forgiving thermal properties.

It cannot be understated what an overall improvement it would represent if the technology pans out.

What is the liquid component in existing batteries?

The electrolyte - usually some kind of lithium salt dissolved in an organic solvent.

AFAIK, the electrolyte.

Solid state promises to be safer. Less likely to self-ignite when the battery degrades. The quick charging and performance are a bonus.

The promise is higher capacity because of more durability. There are higher-powered materials which we currently can't use because batteries with existing tech that use them degrade far too quickly.

Safety

Also better performance, since solid state batteries are lighter. More flexible car layouts and longer range since they're more compact. Faster charging due to reduced resistance. More stable in extreme cold or hot temperatures. It truly will revolutionize EVs if they can mass produce these.

Can anyone recommend a good source that quantifies these qualities against LFP/NMC benchmarks?

With LFP being supposedly more safe than NMC would it have a weight benefit as well?