There are many factors to why batteries degrade over time. Batteries are very picky energy dense units. Batteries do not like to be at 100% or 0% and they naturally degrade over time. As the battery goes through charge cycles and the lithium ions pass through the electrolyte solution over and over, and it slowly breaks down. This is called natural degradation of the batter and besides it the number one factor that damages batteries is heat. The liquid electrolytes are like salts because they are very sensitive to changes in heat. This causes it to crystallize and clog up the anodes and cathodes which will stop the ions from being able to store the lithium ions. This will cause the battery to not be in the optimal state of charge, and it will be less efficient. This also includes external heat. Phones can survive being in hot weather perfectly fine, but doing things like leaving a phone in a car on a hot day would not be practical and ruin the battery. All modern smartphones have many hardware and sensors inside of it to help measure temperature and regulate charging.