Look a little closer
A rechargeable battery is not a bucket refilled with electrons. In a lithium-ion cell, ions shuttle between electrodes while materials change chemical state. The trip is never perfectly reversible, so usable lithium and active material gradually decline while resistance rises.
During early charging, electrolyte decomposes on the anode to form a solid-electrolyte interphase. This layer protects against further decomposition, but creating and repairing it locks away some lithium. Continued growth can also make ion transport more difficult.
Electrode particles expand and contract as lithium enters and leaves. Repeated strain can create microscopic cracks and break electrical connections. Newly exposed surfaces form more interphase material, consuming additional lithium and electrolyte.
Heat speeds unwanted side reactions. Extremely fast charging, especially when a cell is cold, can encourage lithium to plate on graphite rather than insert safely. Holding some chemistries at a very high state of charge for long periods can also accelerate aging.
Battery-management systems limit voltage and temperature and may keep displayed zero and 100 percent inside the true chemical limits. Users cannot prevent all aging, but avoiding unnecessary heat and following manufacturer charging guidance can reduce avoidable stress.
Capacity fade is therefore a sum of small changes rather than one failure. Loss of lithium inventory, particle cracking, electrolyte reactions and rising impedance progress according to time, temperature, charge rate and depth of cycling as well as cycle count.
