The demand for safer and higher energy density batteries drives the development of all-solid-state batteries (ASSBs). Current lithium-ion batteries, while ubiquitous, suffer from safety concerns and limited energy density. ASSBs offer a solution by employing nonflammable inorganic superionic conductors, enabling the use of alternative electrode materials like lithium metal. Sulfide solid electrolytes (SEs), such as Li6PS5Cl, exhibit high ionic conductivities and deformability, facilitating ASSB fabrication. However, their low electrochemical oxidative limits hinder their use with high-voltage positive electrodes. Oxide SEs, like Li7La3Zr2O12, possess good electrochemical stability but are brittle. Halide SEs offer a compromise, combining the mechanical sinterability of oxides with the good electrochemical stability of sulfides. Previous research has explored compositional tuning and structural disorder in halide SEs to enhance conductivity, but often utilizes scarce and expensive metals. The present work focuses on leveraging interfacial superionic conduction, a phenomenon where ionic conductivity is boosted at material interfaces, to improve halide SE performance. A clear understanding of this mechanism is crucial for effective material design. This study uses cost-effective materials and mechanochemical synthesis to create halide nanocomposite solid electrolytes (HNSES) with enhanced ionic conductivity and compatibility with sulfide SEs.

Significant progress has been made in developing inorganic solid-state electrolytes (SSEs) for all-solid-state batteries (ASSBs). Sulfide-based SSEs, such as Li6PS5Cl, have gained attention for their high ionic conductivity, but their low electrochemical stability limits their application in high-voltage batteries. Oxide-based SSEs, like Li7La3Zr2O12, demonstrate excellent electrochemical stability but suffer from brittleness. Halide SSEs emerged as a promising alternative due to their combination of good mechanical properties and electrochemical stability. Studies have shown that mechanochemical synthesis and aliovalent substitution can improve the ionic conductivity of halide SSEs, exemplified by Li3YCl6, Li2InCl5, Li3ScCl6, Li2Sc2/3Cl4, Li2ZrCl6, and Li3YbCl6. However, many halide SSEs utilize rare and expensive elements. Recent theoretical and experimental research indicated that the central metal cation and halide anion significantly impact the electrochemical stability of halide SSEs. F-substitution in chloride SSEs enhances oxidative stability but reduces ionic conductivity. While halide SSEs possess high electrochemical oxidation stability, they suffer from poor cathodic stability due to central metal cation reduction. A synergistic approach using halide and sulfide SSEs as catholyte and SE layers, respectively, is proposed to overcome this challenge. Previous studies have explored ionic conduction enhancement in heterostructured systems, with the addition of materials like Al2O3 to LiI improving its ionic conductivity. However, a comprehensive understanding of the interfacial conduction mechanism remains elusive, limiting its application in superionic conductors.

Halide nanocomposite solid electrolytes (HNSES) were synthesized using a mechanochemical method. Stoichiometric mixtures of Li2O (or Na2O), LiCl (or NaCl), and ZrCl4 were ball-milled to produce ZrO2(-ACI)-A2ZrCl6 (A = Li or Na). Li2O acted as an oxygen source, reacting with ZrCl4 to form ZrO2 nanoparticles, while the remaining ZrCl4 and LiCl reacted to produce Li2ZrCl6. Density Functional Theory (DFT) calculations confirmed the thermodynamic feasibility of this synthesis. The resulting HNSES were characterized using various techniques including synchrotron X-ray diffraction (XRD), pair distribution function (PDF) analysis, X-ray absorption spectroscopy (XAS), and cryogenic high-resolution transmission electron microscopy (cryo-HRTEM). XRD confirmed the formation of Li2ZrCl6 with embedded ZrO2 nanoparticles. XAS and PDF analysis revealed structural differences between the HNSES and their individual components, suggesting the presence of interfacial phases. Cryo-HRTEM imaging showed that mechanochemically produced ZrO2 formed a percolating network nanostructure, creating extensive interfaces. AC impedance measurements determined ionic conductivity, revealing a significant enhancement in Li+ and Na+ conduction in the HNSES compared to their single-phase counterparts. DFT calculations were employed to investigate the interfacial conduction mechanism in HNSES, modelling anion exchange between Li2ZrCl6 and ZrO2 at the interface. AIMD simulations were conducted to determine Li+ diffusivity, showing that oxygen substitution leads to an enlarged Li+ transport channel and higher Li+ concentration. ⁷Li MAS-NMR spectroscopy was used to probe the local Li environments, confirming the existence of oxygen-substituted interphases at the interface. Electrochemical energy storage performance of ASSB cells using the HNSES with Li-In negative electrodes and LiCoO2 or S-NCM88 positive electrodes was evaluated. The effects of temperature and using monolayer or bilayer electrolytes were studied. Cyclic voltammetry (CV) tests were performed to assess electrochemical stability.

The mechanochemical synthesis of HNSES resulted in a significant enhancement of ionic conductivity compared to the single-phase counterparts. Li2ZrCl6 conductivity improved from 0.40 mS cm−1 to 1.3 mS cm−1 for Li+, and Na2ZrCl6 conductivity increased from 0.011 mS cm−1 to 0.11 mS cm−1 for Na+. This improvement was attributed to interfacial superionic conduction caused by the formation of oxygen-substituted compounds at the interfaces between ZrO2 and Li2ZrCl6 (or Na2ZrCl6). DFT calculations indicated that these oxygen-substituted compounds, specifically Li2.5ZrCl5.5O0.5, enlarge Li+ transport channels and increase Li+ concentration at the interface, leading to faster Li+ diffusion. ⁷Li MAS-NMR spectroscopy provided experimental evidence for these oxygen-substituted interphases. The fluorinated HNSE ZrO2-2Li2ZrCl5F demonstrated improved high-voltage stability (up to 5.0 V vs. Li/Li+) and compatibility with Li9PS5Cl, resolving compatibility issues observed with Li2ZrCl6 at elevated temperatures. All-solid-state Li-In||LiCoO2 and Li-In||S-NCM88 cells using ZrO2-2Li2ZrCl5F exhibited good electrochemical performance with high-voltage stability, excellent rate capability, and long-term cycling performance (82.0% capacity retention after 2000 cycles at 400 mA g−1 and 30 °C). The enhanced performance was attributed to the compatibility of ZrO2-2Li2ZrCl5F with both the sulfide SE and the cathode material. DFT calculations showed that mixtures of LCO and various halide SEs exhibited high reaction energies, indicating poor compatibility, whereas the ZrO2-LZCF system showed better compatibility.

This study successfully demonstrates a novel approach to enhance the ionic conductivity of halide solid electrolytes by exploiting interfacial superionic conduction. The mechanochemical synthesis method, combined with the use of cost-effective materials like ZrO2, proved effective in creating nanostructured networks with abundant interfaces. The enhanced conductivity is not simply due to space charge layer effects, but rather a synergistic effect of expanded Li+ transport channels and increased Li+ concentration at the interfaces due to oxygen substitution. The results obtained are highly relevant to the field of all-solid-state battery development, as the enhanced conductivity and compatibility of the HNSES with sulfide SEs are crucial for constructing high-performance ASSBs. The demonstration of high-voltage stability and long-term cycling performance in lab-scale cells highlights the practical significance of this work. The findings expand the material space for high-performance SSEs beyond conventional approaches, opening up new possibilities for ASSB development.

This research presents a novel strategy for improving halide solid electrolytes by leveraging interfacial superionic conduction. Mechanochemical synthesis of halide nanocomposite solid electrolytes (HNSES) successfully increased ionic conductivity and compatibility with sulfide solid electrolytes. This enhancement is attributed to interfacial oxygen substitution, increasing local Li+ concentration and widening transport channels. The F-substituted HNSE shows further improvements in high-voltage stability and compatibility. The fabricated all-solid-state cells using these HNSES demonstrate superior electrochemical performance. Future research could explore the application of this HNSE strategy to other material systems and investigate the optimization of interfacial properties for even further improvements in ASSB performance.

The study primarily focused on laboratory-scale cells, and further research is needed to scale up the synthesis and fabrication processes for commercial applications. The long-term stability of the HNSES under various operating conditions requires further investigation. The current study focuses on specific cathode materials, and wider compatibility testing with various cathode materials is necessary. The DFT calculations used simplified models, and future studies could incorporate more complex models to gain a deeper understanding of the interfacial phenomena.