Two-phase transition of Li-intercalation compounds in Li-ion batteries

Among all electrode materials, olivine LiFePO4 and spinel Li4Ti5O12 are well-known for their two-phase structure, characterized by a flat voltage plateau. The phase transition in olivine LiFePO4 may be modeled in single particle and many-particle systems at room temperature, based on the thermodynamic phase diagram which is easily affected by coherency strain and the size effect. Some metastable and transient phases in the phase diagram can also be detected during non-equilibrium electrochemical processes. In comparison to olivine LiFePO4, spinel Li4Ti5O12 possesses a 'zero strain' property and performs Li-site switching during the phase transition, which lead to a different phase structure. Here, the phase transitions of olivine LiFePO4 and spinel Li4Ti5O12 are systematically reviewed, and the concepts discussed may be extended to other two-phase Li-intercalation compounds in Li-ion batteries.