Date of Award

Fall 2024

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Professor Kevin Huang

Second Advisor

Professor Xinyu Huang

Abstract

Solid-state batteries (SSBs) are emerging as a promising energy storage technology, surpassing traditional liquid electrolyte (LE) counterparts in performance and safety. A critical component in SSBs is the solid-state electrolyte (SSE), which plays a pivotal role in safety, efficiency and stability. SSEs, including sulfides, halides, oxides, and polymers, present distinct advantages and challenges compared to LEs. Among these, oxide-based SSEs are particularly attractive for their balance in ionic conductivity and chemical stability.

This study addresses key challenges related to interfacial stability and electrochemical performance in SSBs by focusing on hybrid polymer-ceramic electrolyte systems. A major challenge to such a SSB system is the stability of the interface between the solid electrolyte and the lithium metal anode, which impacts battery capacity and lifecycle. The interfacial instability issue is deeply rooted in insufficient contacts, formation of detrimental phases and decomposition of electrolytes, causing reduced ionic conductivity, increased interfacial resistance and battery performance decay.

To address these interfacial issues, we investigate hybrid polymer-ceramic electrolyte systems designed to enhance interfacial stability and improve cell performance. Specifically, we explore a multi-functional structure integrating a flexible solid polymer electrolyte (SPE) with a ceramic perovskite electrolyte, Li₆/₁₆Sr₇/₁₆Ta₃/₄Hf₁/₄O₃ (LSTH). The polymethylmethacrylate (PMMA)-polyethylene glycol diacrylate (PEGDA) SPE, with a room-temperature ionic conductivity of 2.20 × 10⁻³ S cm⁻¹, effectively minimizes interfacial resistance, protects LSTH from lithium reduction, reduces concentration polarization, and suppresses lithium dendrite formation. This configuration enables successful cyclability of Li/Li half-cells for 500 cycles and a specific discharge capacity of 129.8 mAh g⁻¹ at 0.1 C for a full cell with LiFePO₄ cathode and lithium anode.

Furthermore, we investigate the copolymerization of polyethylene glycol methacrylate (PPEGMA) and polymethacrylic acid (PDEPMMA) using Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization technique to develop a hybrid polymer-ceramic composite electrolyte system. The polymer chains are then further functionalized with phosphonic acid groups to enable interactions with gadolinium-doped cerium oxide (GDC) phase. In this design, GDC serves dual function: a separator by holding the gel polymer within its scaffold microstructure and facilitating Li-ion movement in gel polymer phase through interactions between RAFT chains and anions. The ethylene glycol segments in gel polymer chains enhance Li-ion conduction and hydrophobic phosphonic acid groups improve adhesion with GDC.

The hybrid electrolyte system demonstrates significant improvements in Li-ion conductivity and interfacial stability with the lithium anode, allowing Li/Li half-cell operation for over 2000 hours at 0.1 mA cm⁻² and critical current densities up to 0.8 mA cm⁻². The interfacial resistance in this system is reduced by more than 50% compared to systems with non-functionalized RAFT polymer, highlighting the effectiveness of the interaction between the RAFT polymer and GDC on performance improvement.

Rights

© 2025, Ziba Rahmati

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