Halide and Sulfide Solid Electrolytes for All Solid-State Batteries: Structure and Interface Engineering

Abstract

All-solid-state batteries (ASSBs) are widely regarded as a promising next-generation energy-storage technology due to their potential to deliver enhanced safety, higher energy density, and improved compatibility with high-voltage electrode materials. Central to the advancement of ASSBs is the development of solid electrolyte (SE) that simultaneously exhibit high ionic conductivity, wide electrochemical stability windows, good processability, and robust interfacial compatibility with both lithium anodes and high-energy cathodes. Among the various classes of SEs, halides and sulfides have emerged as leading candidates owing to their unique advantages. Nevertheless, their electrochemical performance is strongly governed by crystallographic disorder, synthesis-dependent defect chemistry, and complex interfacial reactions, many of which remain incompletely understood. This thesis focuses on the design, structural elucidation, and interfacial engineering of halide and sulfide solid electrolytes for high-voltage ASSBs, with particular emphasis on understanding how crystallographic disorder and chemical modification influence lithium-ion transport and interfacial stability. A comprehensive suite of experimental and computational techniques—including synchrotron and neutron diffraction, total scattering and pair distribution function analysis, electron microscopy, X-ray spectroscopies, time-of-flight secondary ion mass spectrometry (ToF-SIMS), electrochemical characterization, and first-principles calculations—is employed to establish robust structure–property–interface relationships. Chapter 3 examines the nature of structural disorder in layered Li₃InCl₆, revealing the critical role of stacking faults and local structural deviations in governing ionic conduction. Solid-state–synthesized Li₃InCl₆ (SS-LIC) exhibits a stacking-fault fraction of approximately 20%, compared to ~2% in water-mediated Li₃InCl₆ (WM-LIC). Despite this pronounced difference in stacking-fault density, both materials display comparable room-temperature ionic conductivities (1.6 vs. 1.4 mS·cm⁻¹) and nearly identical activation energies (0.37 vs. 0.36 eV). Bond valence site energy analysis confirms that the dominant Li⁺ diffusion pathways remain largely unaffected by stacking faults. These findings underscore the structural complexity of layered halide solid electrolytes and demonstrate that stacking faults exert only a limited influence on Li⁺ transport in Li₃InCl₆. Building on this foundation, Chapter 4 explores lithium metal fluorides as an emerging class of solid electrolytes, demonstrating that mechanochemical synthesis and disorder engineering can unlock unexpectedly high lithium-ion conductivity. By probing both amorphous-like and crystalline forms across multiple length scales, this chapter reveals that the introduction of structural disorder—from short-range to long-range—is essential for facilitating Li⁺ transport. Furthermore, the incorporation of LiF into Li₂TiF₆ induces strong local interactions that disrupt long-range order and enhance ionic mobility. As a result, the Li₂TiF₆–LiF composite achieves an ionic conductivity of 2.5 × 10⁻³ mS·cm⁻¹, representing the highest conductivity reported for fluoride-based solid electrolytes and approaching values typical of LiPON and LiNbO₃. In Chapter 5, a dual-halogen substitution strategy is introduced in Li2HfCl6-xF, illustrating how targeted fluorination can simultaneously tune lattice disorder, electrochemical stability, and interfacial chemistry. This chapter systematically investigates the interplay between ionic and electronic conductivity, voltage stability, and overall battery performance in this new family of dual-halogen solid electrolytes. All-solid-state cells employing Li₂HfCl₅.₅F₀.₅ exhibit markedly enhanced electrochemical performance compared to Li₂HfCl₆. This improvement is primarily attributed to the formation of a kinetically stable, LiF-rich cathode–electrolyte interphase (CEI), which suppresses deleterious interfacial reactions, as revealed by ToF-SIMS analysis. Chapters 6 and 7 address cathode–electrolyte interfacial instability in high-voltage ASSBs through surface-engineering strategies designed to suppress parasitic reactions and enable stable cycling under practical operating conditions. In Chapter 6, guided by density functional theory calculations, a conformal and nanometric coating is developed for nickel-rich NMC85 cathodes, effectively suppressing the oxidative decomposition of Li₆PS₅Cl. Cells employing coated NMC85 achieve 82% capacity retention after 200 cycles (2.8–4.3 V vs. Li⁺/Li), compared to only 56% for cells with uncoated cathodes. The coated systems also exhibit superior rate capability and higher reversible capacity. Chapter 7 presents a simple, cost-effective organic coating strategy for Li6PS5Cl that significantly enhances its air stability while preserving high ionic conductivity. The coated electrolyte (DA-LPSCl) withstands exposure to 39% relative humidity for up to 2 hours with minimal structural degradation and maintains an ionic conductivity exceeding 1 mS·cm⁻¹. All-solid-state cells employing DA-LPSCl as the catholyte and a Li–In anode deliver a discharge capacity of 175 mAh·g⁻¹ with 96% capacity retention over 150 cycles at 0.2 C, whereas cells using bare LPSCl retain only 61% under identical conditions. Symmetric Li|DA-LPSCl|Li cells demonstrate stable cycling for over 1000 hours, in sharp contrast to bare LPSCl cells, which short-circuit after approximately 230 hours. Moreover, full cells pairing DA-LPSCl with a lithium metal anode retain 81% of their capacity after 300 cycles at 0.2 C, highlighting performance metrics competitive with state-of-the-art solid-state batteries. Collectively, the studies presented in this thesis demonstrate that disorder engineering and interfacial protection constitute unifying design principles for advancing both halide and sulfide solid electrolytes. The insights gained provide fundamental guidance for the rational design of chemically stable, high-performance solid electrolytes and outline viable pathways toward durable, high-energy all-solid-state battery systems.