UC Berkeley

Rechargeable batteries are a foundational technology for modern energy storage, enabling applications ranging from portable electronics to electric vehicles and grid-scale storage. The accelerating push toward electrification and global decarbonization has intensified the demand for battery systems with higher energy density, improved safety, and long-term reliability. These requirements have motivated growing interest in battery technologies that move beyond conventional liquid-electrolyte lithium-ion systems.All-solid-state batteries have emerged as a leading candidate to address these challenges by replacing flammable liquid electrolytes with inorganic solid-state ion conductors. This architecture offers the potential for enhanced safety and compatibility with high-energy-density electrodes, including high-voltage cathodes and metal anodes. However, the performance and viability of all-solid-state batteries critically depend on the properties of the solid-state electrolyte, particularly its ionic conductivity, electrochemical stability, mechanical ductility, and interfacial stability. Within this landscape, halide-based solid-state electrolytes have recently attracted significant attention as a promising materials class. Compared with conventional oxide and sulfide conductors, halide electrolytes combine high ionic conductivity with wide electrochemical stability windows and favorable mechanical deformability, making them well suited for solid-state battery applications. Understanding and controlling ion transport in halide frameworks is therefore a key scientific challenge for enabling advanced solid-state batteries across both lithium and multivalent chemistries. In lithium-based systems, halide electrolytes are particularly attractive due to their high oxidative stability, enabling compatibility with high-voltage cathodes. Nevertheless, practical deployment remains constrained by the reliance on rare or expensive elements such as Y, Sc, and In, as well as by an incomplete understanding of how disorder and local structural environments govern lithium ion transport. In parallel, rechargeable batteries based on multivalent ions such as Mg2+ offer the potential to surpass the cost and volumetric energy density limitations of Li-ion batteries. Yet, strong electrostatic interactions between multivalent ions and the host lattice severely impede ionic mobility, making the development of fast solid-state Mg2+ inorganic conductors a major scientific and technological challenge. This thesis focuses on elucidating solid-state ion transport mechanisms in halide electrolytes for both Li+ and Mg2+ batteries, with particular emphasis on how metastable structural features, cation ordering, and anion chemistry govern ionic conductivity. By integrating mechanochemical synthesis, advanced structural characterization, electrochemical measurements, and first-principles modeling, this work establishes design principles for enabling fast ion conduction in halide lattices across diverse structural frameworks, including both amorphous and crystalline systems. For Mg-ion transport, this thesis reports the discovery of mechanically soft magnesium gallium halide electrolytes exhibiting room-temperature Mg-ion conductivities of 0.47 mS cm⁻¹, surpassing most inorganic Mg-ion solid conductors. Synthesized via high-energy ball milling, these materials display clay-like deformability that promotes intimate interfacial contact during electrochemical cycling. Detailed analysis reveals that partial anion exchange induced by mechanochemical processing creates undercoordinated magnesium environments within chlorine-rich frameworks, substantially lowering migration barriers for Mg-ion transport. This work demonstrates that deliberate control of local Mg-ion coordination and lattice softness through partial anion exchange in amorphous halide systems can overcome the intrinsic transport limitations of divalent Mg-ions. For Li-ion conduction, this thesis investigates earth-abundant halide electrolytes based on inverse spinel structures. Using Li2MgCl4 as a model system, molecular dynamics simulations reveal that lithium disordering from tetrahedral 8a to octahedral 16c sites significantly lowers the activation energy for Li-ion migration. Guided by these insights, aliovalent zirconium substitution is employed to stabilize cation disorder at room temperature, leading to marked enhancements in ionic conductivity. By decoupling the effects of lithium deficiency level and cation disorder, this work demonstrates that site disorder within the halide framework plays a dominant role in enabling fast lithium transport in inverse spinel electrolytes. This thesis further explores fluorination as a chemical lever to simultaneously enhance ionic conductivity and electrochemical stability in lithium halide electrolytes. In a crystalline LiAlCl4 system, partial substitution of chlorine with fluorine induces lithium deficiency and aluminum excess while preserving the overall monoclinic framework. Multimodal structural characterization shows that fluorination increases the diversity and distortion of local lithium coordination environments and weakens Li–F interactions through preferential Al–F bonding. The optimized fluorinated halide electrolyte exhibits improved Li-ion conductivity and stability against a lithium metal anode. Moreover, fluorination enables access to metastable orthorhombic phases that are inaccessible through conventional synthesis routes, highlighting the critical role of metastability in tuning ion transport. Overall, this thesis establishes halide electrolytes as a versatile and chemically tunable platform for solid-state ion transport in both lithium and multivalent battery systems. By elucidating how cation disorder, anion chemistry, and complex local coordination environments regulate ionic mobility, this work connects ion transport behavior across amorphous and crystalline halide frameworks and identifies common underlying design principles. These insights provide actionable guidance for the rational design of next-generation solid-state electrolytes that combine fast ion conduction, interfacial robustness, and compositional sustainability.