Study of the Li⁺ intercalation/de-intercalation behavior of SrLi₂Ti₆O₁₄ by in-situ techniques

The lithium ion intercalation/de-intercalation behavior of SrLi₂Ti₆O₁₄ is studied using in-situ X-ray diffraction (in-situ XRD) and in-situ electrochemical impedance spectroscopy (in-situ EIS) techniques. Significant volume expansion and contraction is observed during Li⁺ intercalation and de-intercalation, respectively. At voltages of 2.5-0.5 V, the entire Li⁺ intercalation process can be divided into two stages. In stage I, corresponding to the 2.5-1.3 V range, ∼60.2% lithium intercalation capacity is obtained with only ∼40.3% relative lattice volume expansion. In contrast, only an ∼39.8% lithium intercalation capacity is obtained with 59.7% lattice volume expansion in stage II, which occurs in a voltage range of 1.3-0.5 V. The results from in-situ EIS reveal that a relatively small charge transfer resistance R_ct can be obtained for charging/discharging at voltages of 2.5-1.3 V. Moreover, R_ct increases with increasing lattice volume expansion when lithium is inserted into the SrLi₂Ti₆O₁₄ lattice at voltages lower than 1.3 V. Cyclic testing in different voltage ranges shows that, compared with ranges of 2.5-0.8 V and 1.42-0.8 V, the best cyclic performance is achieved at voltages of 2.5-1.3 V. This may be attributed to reduced changes in the lattice volume and decreased lithium intercalation resistance.