Delithiation

Delithiation refers to the removal of lithium ions from a lithium-containing host material, typically through electrochemical oxidation during charging in lithium-ion batteries. The process leaves vacancies in the host lattice and increases the oxidation state of transition metal ions, releasing electrons to the external circuit.

In practice, delithiation occurs in materials such as layered LiMO2 (M = Co, Ni, Mn) and olivine LiFePO4. In layered oxides, the formula is often written Li1−xMO2, with x increasing as lithium is extracted; in olivine LiFePO4, Li can be removed to form FePO4. The mechanism can proceed as a solid solution (continuous change in composition and lattice parameters) or via a two-phase reaction (coexistence of Li-rich and Li-poor phases), depending on composition and temperature. Diffusion of Li+ through the lattice governs rate.

Delithiation affects crystal structure and properties. It may cause lattice expansion or contraction, phase transitions, and in some high-delithiation regimes, oxygen loss or structural instability, contributing to voltage fade and capacity loss during cycling. The voltage profile reflects the redox potential of the transition metal and, in some materials, the onset of structural changes.

In energy storage, delithiation is essential for charging a Li-ion cell; the extent of delithiation determines capacity and energy density. Characterization employs electrochemical testing, X-ray diffraction, X-ray absorption spectroscopy, and operando techniques to monitor lattice parameters, oxidation states, and phase evolution.