Understanding and Controlling the Degradation Mechanisms at Cathode-Electrolyte Interfaces in All-Solid-State Lithium-Ion Batteries
| Autor: | Kim, Younggyu |
|---|---|
| Rok vydání: | 2022 |
| Druh dokumentu: | Diplomová práce |
| Popis: | All-Solid-State Li-ion Batteries with Li₇La₃Zr₂O₁₂ solid electrolyte enable higher energy density compared to conventional batteries with liquid electrolytes since they are compatible with Lithium metal anode. Despite their promises, stability issues at the interface between cathode and solid electrolyte need to be solved for their implementation. The interface needs to be chemically stable at high temperature during sintering. Electrochemical and chemo-mechanical stabilities at the interface are necessary during the operation of the battery for good cyclability. In order to study interfacial stabilities we developed model system with thin film cathode. The cell design allowed characterization of the interface by interface-sensitive techniques without the needs of destructive techniques. We studied interfacial degradation between LiNi₀.₆Mn₀.₂Co₀.₂O₂ (NMC622) cathode, and Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte. We evaluated thermal stability in controlled gas environments (Air, O₂, N₂, humidified O₂, CO₂) to identify contributors for secondary phase formations and their effect on charge transfer properties. Li₂CO₃, La2Zr2O7, and La(Ni,Co)O₃ formed at the NMC622|LLZO interface when annealed at 700 °C in air, which increased the interfacial resistance by 2 orders of magnitude. Sintering in gas environment without CO₂ and H₂O (g) was necessary to obtain chemically stable interfaces. Sintering in O₂ gave excellent chemical stability and interfacial resistance comparable to lowest values obtained in literature with protective coatings at the interface. Sintering in N₂ caused oxygen loss at high temperature, but secondary phases did not form. NMC622|LLZO interface was electrochemically unstable due to limited oxidation stability of LLZO. Electrochemical degradation at the interface reduced Ni during potentiostatic hold at 4.3 V vs Li/Li⁺, and formed reduced phases with Ni²⁺ and Co²⁺ from cycling at 80 °C. Electrochemical degradation decreased capacities by overpotential increase. Reduction was not observable when cycling temperature was lowered to room temperature, indicating that the reaction could be kinetically inhibited. Stress due to lattice parameter changes of NMC622 during cycling caused intergranular cracks in NMC622 film and delamination at NMC622|LLZO interface. Chemo-mechanical degradation caused abrupt capacity decrease by disconnecting Li-ion conduction pathway, so it should be avoided for better cyclability. Understanding of interfacial degradation offers guidelines for designing All-Solid-State Li-ion batteries with better interfacial stability. Ph.D. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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