Why does the capacity of lithium-ion batteries decrease?

Lithium-ion battery capacity decay refers to the phenomenon that lithium-ion batteries gradually lose their available capacity over time and with battery life. What is the mechanism of capacity decay?

1.Volume Change: During the battery’s charge and discharge process, lithium ions insert and extract, causing the lattices of the positive and negative electrode materials to expand and contract to varying degrees. A. The positive electrode material’s microstructure changes, reducing the amount of lithium inserted. Under overcharge conditions, lithium ions rapidly migrate to the negative electrode, causing the positive electrode lattice to collapse. B. The negative electrode graphite can experience a volume change of up to 10% during lithium ion insertion and extraction, leading to particle delamination.

2.SEI film formation: During the battery formation stage, the initial charge and discharge process involves the chemical reaction of lithium ions with specific components in the electrolyte, resulting in the irreversible formation of a solid electrolyte interface at the interface between the negative electrode and the electrolyte. During the charge and discharge process, SEI continuously breaks down and regenerates, resulting in a decrease in active lithium ions, an increase in SEI film thickness, and an increase in internal resistance.

3.Lithium dendrite growth： Under low temperature, fast charge and overcharge conditions, lithium ions continue to move toward the negative electrode. The rate of lithium ion extraction is greater than the rate of embedding, resulting in the deposition of lithium ions near the negative electrode, which leads to a reduction in active lithium.

4.Electrolyte Decomposition: Electrolyte decomposition primarily occurs through two pathways: electrochemical and chemical. Electrochemical decomposition is divided into oxidative decomposition on the positive electrode side and reductive decomposition on the negative electrode side. Oxidative decomposition on the positive electrode side occurs when the positive electrode potential is >4.5V, causing battery bulging and increased interfacial impedance. Reductive decomposition on the negative electrode side occurs when the graphite negative electrode potential is <0.8V, causing a thickening of the SEI and a reduction in active lithium. Chemical decomposition is divided into trace water-catalyzed reactions and high-temperature decomposition. Trace water-catalyzed reactions cause positive electrode corrosion, while high-temperature decomposition causes electrolyte depletion and a tendency toward thermal runaway.

5.Current Collector Corrosion: Current collector corrosion can be categorized as corrosion of the positive electrode aluminum foil at high potentials and corrosion of the negative electrode copper foil at low potentials. When the positive electrode potential exceeds 3.8V, the positive electrode aluminum foil oxidizes and corrodes. Under overcharge conditions, when the negative electrode potential is less than 3V, the copper foil dissolves, migrates to the positive electrode, and deposits on the positive electrode surface. Other issues include conductive agent failure and separator aging.