This thesis advances the scalable and practical development of halide-based solid electrolytes for all-solid-state lithium-ion batteries by demonstrating that targeted processing - combining wet-chemical synthesis and controlled crystallization - can reproducibly tune structure, transport, and electrochemical properties to yield robust, high-performance electrolytes for device integration. Chapter 1 frames the challenge, contrasting the limitations of oxide and sulfide systems – e.g., interfacial resistance, moisture sensitivity, and limited oxidative stability – with the opportunities afforded by halide solid electrolyte systems. Chapter 2 synthesizes the literature on halide structures, transport mechanisms, and synthesis windows, identifying the key processing parameters that have a pronounced impact on ionic conductivity and electrochemical performance in solid-state cells. This critical review defines the hypotheses and experimental metrics used throughout the work. Building on this foundation, Chapter 3 develops a scalable wet-chemical route for Li3InCl6 and demonstrates that tuning the solvent chemistry enables excellent microstructural control, producing halide solid electrolyte materials with ionic conductivities that far exceed the literature benchmarks for this system. These materials showcase excellent performance in solid-state cells due to their pronounced interfacial stability. Chapter 4 explores crystallization kinetics as a distinct design variable, demonstrating how evaporative and annealing environments (ambient, inert, and vacuum; temperature/time windows) influence phase purity, defect populations, and hopping dynamics, thereby explaining and predicting the observed conductivity trends. Chapter 5 investigates (electro)chemical pathways of degradation of Li3InCl6│Li6PS5Cl interfaces using time-resolved impedance, DRT, and complementary structural/chemical characterisations toward translating materials-level advances to device-relevant understanding. This analysis links interphase growth and associate impedance growth to the processing fingerprints established earlier. Chapter 6 distils the experimental findings into actionable design rules and an outlook for integrating halide electrolytes into high-voltage, long-life ASSLB architectures. Collectively, the thesis offers a coherent, experimentally validated strategy - co-design of wet-chemical processing, crystallization control, and interfacial understanding - that delivers predictable transport behavior and improved device stability, thereby advancing the field toward safer, higher-energy solid-state batteries.