Recently, halide superionic conductors have emerged as promising solid electrolyte (SE) materials for all-solid-state batteries (ASSBs), owing to their inherent properties combining high Li+ conductivity, good chemical and electrochemical oxidation stabilities, and mechanical deformability, compared to sulfide or oxide SEs. In this Review, recent advances in halide Li+- and Na+-conducting SEs are comprehensively summarized. After introducing the ionic diffusion mechanism and related governing factors of the crystal structures, we discuss the design strategies, such as the substitution and synthesis protocols, of the halide materials for further improving their properties. We review theoretical and experimental results on electrochemical stabilities and compatibilities with electrode materials. Moreover, we offer a critical assessment of the challenges and issues associated with the development of practical ASSB applications, such as cost considerations, stabilities in atmospheric air, aqueous solutions, and slurry-processing, and the wet-slurry or dry fabrication of sheet-type electrodes (or SE membranes) for large-format ASSBs. Based on these discussions, we provide a perspective on the future research directions of halide SEs, emphasizing the need for expanding the materials space. 

Emerging Halide Superionic Conductors for All-Solid-State Batteries: Design, Synthesis, and Practical Applications

The safety of lithium-ion batteries has caused notable concerns about their widespread adoption in electric vehicles. A nascent but promising approach to enhancing battery safety is using solid-state electrolytes (SSEs) to develop all-solid-state batteries, which exhibit unrivaled safety and superior energy density. A new family of SSEs based on halogen chemistry has recently gained renewed interest because of their high ionic conductivity, high-voltage stability, good deformability, and cost-effective and scalable synthesis routes. Here, we provide a comprehensive review of halide SSEs concerning their crystal structures, ion transport kinetics, and viability for mass production. Furthermore, their moisture sensitivity and interfacial challenges are summarized with corresponding effective strategies. Last, halide-based all-solid-state Li-ion and Li-S pouch cells with energy density targets of 400 and 500 Wh kg−1 are projected to guide future endeavors. This work serves as a comprehensive guideline for developing halide SSEs from material design to practical application.

Prospects of halide-based all-solid-state batteries: From material design to practical application

All solid-state batteries are safe and potentially energy dense alternatives to conventional lithium ion batteries. However, current solid-state batteries are projected to costs well over $100/kWh. The high cost of solid-state batteries is attributed to both materials processing costs and low throughput manufacturing. Currently there are a range of solid electrolytes being examined and each material requires vastly different working environments and processing conditions. The processing environment (pressure and temperature) and cell operating conditions (pressure and temperature) influence costs. The need for high pressure during manufacturing and/or cell operation will ultimately increase plant footprint, costs, and machine operating times. Long term, for solid state batteries to become economical, conventional manufacturing approaches need to be adapted. In this perspective we discuss how material selection, processing approach, and system architecture will influence lithium-based solid state battery manufacturing. 

Processing and manufacturing of next generation lithium-based all solid-state batteries

Halide solid electrolytes have been considered as the most promising candidates for practical high-voltage all-solid-state lithium-ion batteries (ASSLIBs) due to their moderate ionic conductivity and good interfacial compatibility with oxide cathode materials. Aliovalent ion doping is an effective strategy to increase the ionic conductivity of halide electrolytes. However, the effects of ion doping on the electrochemical stability window of halide electrolytes and carbon additive on electrochemical performance are still unclear by far. Herein, a series of Zr-doped Li3-xEr1-xZrxCl6 halide solid electrolytes (SEs) are synthesized through a mechanochemical method and the effects of Zr substitution on the ionic conductivity and electrochemical stability window are systematically investigated. Zr doping can increase the ionic conductivity, whereas it narrows the electrochemical stability window of the Li3ErCl6 electrolyte simultaneously. The optimized Li2.6Er0.6Zr0.4Cl6 electrolyte exhibits both a high ionic conductivity of 1.13 mS cm-1 and a high oxidation voltage of 4.21 V. Furthermore, carbon additives are demonstrated to be beneficial for achieving high discharge capacity and better cycling stability and rate performance for halide-based ASSLIBs, which are completely different from the case of sulfide electrolytes. ASSLIBs with uncoated LiCoO2 cathode and carbon additives exhibit a high discharge capacity of 147.5 mAh g-1 and superior cycling stability with a capacity retention of 77% after 500 cycles. This work provides an in-depth understanding of the influence of ion doping and carbon additives on halide solid electrolytes and feasible strategies to realize high-energy-density ASSLIBs.

New Insights into the Effects of Zr Substitution and Carbon Additive on Li3-xEr1-xZrxCl6 Halide Solid Electrolytes.

Li-rich channels as the material gene for facile lithium diffusion in halide solid electrolytes

DOI: 10.1002/aenm.202101915 in energy storage technology with excellent safety conditions.[1] As the main components of ASSLBs, the solid-state electrolytes (SSEs) play an essential role in ASSLBs.[2] Over the past few years, researches on inorganic solid electrolytes have made significant progress.[3,4] After decades of research effort, various lithium solid electrolytes with high ionic conductivity were developed, including oxide, sulfide, polymer, halide, etc. Among these materials, oxide solid electrolytes such as perovskite-type Li3/8Sr7/16Ta3/4Zr1/4O3, sodium super ionic conductor type Li1.5Al0.5Ti1.5(PO4)3, and garnet-type Li6.4La3Zr1.4Ta0.6 O12 could reach the level of ≈10−4–10−3 S cm−1 at room temperature. However, the poor physical contact due to the inherent mechanical rigidity of oxide SSEs cause hindered ion transport at the interface.[8] At the same time, a high reaction temperature is required in the preparation process, which makes the manufacturing cost high.[9] These problems can be avoided for the ductile sulfide SSEs due to intimate good ionic contacts. For example, thio-lithium super ionic conductor type Li3.25Ge0.25P0.75S4, glass type Li2S-B2S3-SiO2-LiI, glass-ceramic type Li7P3S11, Argyrodite type Li6PS5Cl, Li10GeP2S12, and Li9.54Si1.74P1.44S11.7Cl0.3 exhibit superionic The halide solid-state electrolytes (SSEs) have received significant attention due to their high ionic conductivity and desirable compatibility with cathode materials. However, the reduction potential of the halide is still >0.6 V (versus Li/Li+). Reduction stability is still one of the challenges that need to be addressed. The fluorides have a wide electrochemical stability window due to the large electronegativity of F–. In contrast, Li3YBr6 (LYB) bromides have a narrower electrochemical window, although they have high lithium ion conductivity (>10–3 S cm–1). Herein, a fluorine doping strategy is employed. The interfacial stability between fluoride-doped bromides and lithium metal is researched by cycling of lithium symmetric cells. Li plating/stripping can maintain over 1000 h at 0.75 mA cm–2. Interfacial protection mechanisms investigated by X-ray photoelectron spectroscopy. A fluoride-rich interfacial layer is formed in situ during the cycle, which achieves inhibition of the reduction. The Li metal treated fluorine doping of LYB exhibits significant potential in full cells. In fact, the induction of a stable in situ interfacial layer by fluorine doping can effectively improve the interfacial stability of bromides to lithium metal. Fluorinedoped modification offers a new attempt to realize lithium metal applications in all-solid-state lithium batteries.

Limin Wang

Papers

Superionic Fluorinated Halide Solid Electrolytes for Highly Stable Li-Metal in All-Solid-State Li Batteries

Origin of high electrochemical stability of multi-metal chloride solid electrolytes for high energy all-solid-state lithium-ion batteries