To understand what happens to any lithium-based (Li-ion) batteries, one needs to understand what happens to the lithium itself and the lithium salts. As we know that the lithium salts used in batteries are stable at room temperatures, it is crucial to ask what happens to them at 50+ C and below zero temperatures.
Most manufacturers avoid discussing this topic. They assume that an electric car rarely stays in the cold, or very high temperatures suitable for Mediterranean countries in summer, Africa and the like.
Unfortunately, chemistry has no mercy - at high temperatures, the lithium salts used as an energy carrier between the electrodes can decompose. It can happen as a result of heating the battery while it is being discharged (efficiency is not 100%), but also even when it is stationary.
What happens later? Some of the lithium ends up as a membrane on the carbon (or alternative) electrode. A certain part likely ceases to perform its function.
Frost can contribute to the further degradation of the battery, provided we want to charge it quickly at that time. Locally, the temperature will lead to the breakdown of lithium salts.
But frost, in turn, can be helpful - by shrinking it disturbs the surface of the carbon electrode, and geometrically brings the electrodes closer together. The latter factor may not be significant, but we can see it in laboratory tests. It certainly increases the capacity of the battery.