2025 Theses Doctoral

Chemo-Mechanics and Interfacial Stability of High-Energy-Density Li Battery Materials

Sun, Kerry

This thesis investigates the chemo-mechanical dynamics and interfacial degradation mechanisms in advanced lithium battery chemistries, focusing on sulfur and silicon electrodes and enabling solid-state electrolytes. By leveraging characterization techniques like operando acoustic transmission, ex-situ X-ray techniques, and electro-analytical techniques, this work provides novel insights into the limitations and design optimizations necessary for high-energy-density battery systems.

Chapter 2 explores lithium iron phosphate (LFP) in all-solid-state batteries with sulfide argyrodite solid electrolytes. Contrary to expectations, LFP exhibits significant chemical and electrochemical instabilities at the interface with the solid electrolyte, leading to ion transport hindrances and capacity fade. The study reveals that interfacial degradation is not limited to the LFP-electrolyte interface but also at the conductive carbon-electrolyte interface. LiNbO₃ coatings partially mitigate these effects but fail to protect against degradation at the carbon-electrolyte interface, emphasizing the importance of optimizing interfacial stability in solid-state LFP batteries.

Chapter 3 introduces operando acoustic transmission as a non-invasive technique to study high-energy sulfur electrodes in lithium-sulfur (Li-S) batteries, challenged by sulfur’s solid-liquid-solid transition and polysulfide dissolution in liquid electrolytes. By decoupling sulfur’s intrinsic material property changes from electrode dilation, acoustic time-of-flight (ToF) and damping analysis offers real-time insights into sulfur’s state-of-matter transitions and material property dynamics. This method effectively tracks complex chemo-mechanical dynamics; we then correlate acoustic ToF changes to polysulfide dissolution and capacity fade, demonstrating the potential of acoustic transmission for guiding the design of next-generation sulfur cathodes.

Chapter 4 integrates the learnings from Chapters 2 and 3 to study silicon (Si) anodes in all-solid-state batteries using ultrasound transmission. Si anodes have a high specific capacity but experience significant volume expansion during lithiation, leading to stress-induced contact loss and electrolyte cracking. This study reveals a nonlinear relationship between stress evolution and state-of-charge during metastable LixSi phase transitions. Additionally, amplitude signal analysis provides visualization of real-time porosity dynamics of Si electrodes to highlight the morphological dynamics of Si lithiation and delithiation. These findings highlight the necessity of advanced mechanical design strategies to mitigate stress and improve cycling stability in Si-based solid-state batteries.

Chapter 5 examines high-energy molten sulfur Li batteries (T > 120°C), informing design principles for the electronic conductive network within the cathode composite. The morphology of carbon additives is shown to significantly impact capacity utilization and retention. Short-range carbons improve utilization but induce parasitic side reactions, whereas long-range carbons enhance retention but limit utilization. Intermittent current interrupt analysis can map SoC-dependent ohmic and mass transport limitations linked to Li₂S/Li₂S₂ electrodeposition.

Overall, this thesis provides a comprehensive understanding of how high-capacity electrode materials behave and their interactions with solid electrolytes. The findings contribute to the rational design of high-energy-density batteries, highlighting the importance of optimizing interfacial stability, conductive networks, and design around mechanically active electrode materials to enhance cycle life and performance.