Toward Improving the Thermal Stability of Negative Electrode Materials: Differential Scanning Calorimetry and In Situ High-Temperature X‑ray Diffraction/X-ray Absorption Spectroscopy Studies of Li₂ZnTi₃O₈ and Related Compounds

Negative electrode materials with high thermal stability are a key strategy for improving the safety of lithium-ion batteries for electric vehicles without requiring built-in safety devices. To search for crucial clues into increasing the thermal stability of these materials, we performed differential scanning calorimetry (DSC) and in situ high-temperature (HT)-X-ray diffraction (XRD)/X-ray absorption (XAS) up to 450 °C with respect to a solid-solution compound of Li_4/3–2x/3Zn_xTi_5/3–x/3O₄ with 0 ≤ x ≤ 0.5. The DSC profile of fully discharged x = 0.5 (Li₂ZnTi₃O₈) with a LiPF₆-based electrolyte could be divided into three temperature (T) regions: (i) T ≤ 250 °C for ΔH_accumⁱ, (ii) 250 °C < T ≤ 350 °C for ΔH_accumⁱⁱ, and (iii) T > 350 °C for ΔH_accumⁱⁱⁱ, where ΔH_accumⁿ is the accumulated change in enthalpy in region n. The HT-XRD/XAS analyses clarified that ΔH_accumⁱ and ΔH_accumⁱⁱ originated from the decomposition of solid electrolyte interphase (SEI) films and the formation of a LiF phase, respectively. Comparison of the DSC profiles with x = 0 (Li[Li_1/3Ti_5/3]O₄) and graphite revealed the operating voltage, i.e., amount of SEI films, and stability of the crystal lattice play significant roles in the thermal stability of negative electrode materials. Indeed, the highest thermal stability was attained at x = 0.25 using this approach.