In situ X-ray photoelectron spectroscopy investigation of lithium-solid electrolyte interphases

Thesis

Lithium batteries are among the most crucial energy storage technologies, and further enhancing their performance, particularly in energy density, is essential to meet the growing demands of modern applications. For example, in electric vehicles, increasing battery energy density—defined as the energy stored per unit mass or volume—is crucial for extending driving range, a primary concern for consumers. Although battery packs are usually priced per kilowatt-hour of capacity, higher cell-level energy density allows the same capacity to be delivered with reduced mass and volume. Reducing battery mass within the vehicle in turn lowers energy consumption for a given driving distance. Together, these effects show higher energy density indirectly yet appreciably extends driving range.

Solid-state electrolytes (SSEs) are considered one of the most promising advancements for next-generation energy storage. By enabling the use of lithium metal anodes, SSEs offer significantly higher energy densities while enhancing safety through their mechanical suppression of dendrites and intrinsic non-flammability. However, the highly reactive nature of lithium metal and the limited electrochemical stability of SSEs present significant challenges, particularly in the formation and evolution of the solid electrolyte interphase (SEI). This interfacial layer critically influences ionic transport and overall battery stability, yet its formation mechanisms and long-term behaviour remain poorly understood.

To address these challenges, this work utilises in situ X-ray photoelectron spectroscopy (XPS) to investigate SEI formation at the interfaces between lithium metal and several promising SSEs. The SEI characteristics of Li₁₀GeP₂S₁₂ (LGPS) and Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) were compared, revealing that LAGP forms a more passivating SEI with fewer conductive phases. Further investigation on Li₆PS₅Cl suggests that its SEI continues to evolve due to the progressive delithiation of phosphorus-containing species, which explains the variation of reported SEI thickness. Lastly, a graded SEI structure was proposed for LiPON, providing insights into its exceptional stability against lithium metal.

These findings demonstrate the power of in situ XPS in capturing the chemical evolution of the SEI, offering valuable insights into its formation and degradation. When combined with complementary characterisation techniques, this approach enables a more comprehensive understanding of SEI behaviour, aiding the development of more stable and high-performance SSE-based lithium batteries.

Authors: Liang, Y

• DOI: 10.5287/ora-2arnon6ox
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English