Lithium-ion batteries work by moving lithium ions between an anode and a cathode through a liquid electrolyte. During charging, ions move from cathode to anode; during discharge, they move back and release energy. This cycle can theoretically repeat many times, but in practice each cycle leaves behind small chemical side effects.
Over time, microscopic layers build up on the anode and cathode surfaces, making it harder for ions to move freely. This is what causes a battery's total capacity and maximum output to gradually decline. Manufacturers generally consider dropping to around 80% capacity after 500 full charge cycles to be normal aging.
Temperature directly affects how fast this happens; batteries stored or charged in high heat, especially above 35°C, degrade chemically much faster. Cold environments temporarily reduce performance too, but with a lower risk of permanent damage.
Different lithium-ion chemistries, like NMC (nickel manganese cobalt) and LFP (lithium iron phosphate), strike different balances between energy density and cycle life; LFP tends to last more cycles while NMC packs more energy into the same size, so phone makers usually balance the two.
The most effective thing users can do is avoid extreme heat, avoid keeping the battery at 100% constantly, and use quality, PD-compliant chargers. Charjett's protection-circuit products help reduce the risk of voltage fluctuations damaging the battery over time.