Lithium-ion batteries rely on a chemical reaction that's happiest around room temperature. In the cold, the electrolyte thickens and ions move sluggishly, so the battery can't deliver power fast enough — your phone may even shut off early even though charge remains. In heat, the opposite problem happens: the reactions speed up too much and permanently degrade the electrode material, which is why a phone left on a hot dashboard loses long-term capacity faster than one used in normal conditions.