The appeal of halide-based solid electrolytes lies in their ability to combine properties that are rarely found together. Conventional oxide electrolytes are generally chemically robust, but they are often brittle and require high-temperature sintering to create dense, low-resistance interfaces. Sulfide electrolytes can offer excellent ionic conductivity and mechanical softness, yet they are sensitive to moisture and may release toxic hydrogen sulfide when exposed to air. Polymer electrolytes are easier to process, but their lithium-ion conductivity commonly falls at room temperature. Halide materials occupy an important middle ground, offering ionic conductivities ranging from approximately 10⁻⁴ to above 10⁻³ S cm⁻¹, broad electrochemical stability windows, and comparatively improved tolerance to air.

This combination is especially important for high-voltage cathodes. In a battery, the electrolyte must transport lithium ions while preventing electrons from passing through it. At the same time, it must remain stable against the chemical potential of both electrodes. Many solid electrolytes degrade when placed next to cathode materials charged to more than 4 volts, creating resistive interphases that slow ion transport and reduce battery life. Halide-based compositions, by contrast, can be engineered for improved oxidative stability, making them promising candidates for direct contact with high-voltage layered oxides such as nickel-rich NCM811 and lithium cobalt oxide.

The review organizes halide-based solid electrolytes into five broad families according to the chemical identity and oxidation state of their central metal: divalent, trivalent, tetravalent, pentavalent, and non-metal-centered frameworks. This classification is more than a cataloging exercise. The central metal influences the crystal structure, the distribution of lithium vacancies, the polarizability of the halide anions, and the energy barriers that lithium ions must overcome as they move through the solid. By adjusting these features, researchers can manipulate the balance between structural stability and rapid ion conduction.

Some of the strongest results have emerged from trivalent compounds, including Li₃InCl₆ and Li₃ScCl₆. These materials can reach ionic conductivities of roughly 1 to 3 mS cm⁻¹, a range that begins to approach the performance required for practical solid-state cells. Their behavior is linked to disordered lithium sublattices and carefully controlled vacancies. In a crystalline solid, lithium ions do not move through an empty space as they would in a liquid; instead, they hop between energetically favorable sites. Disorder and vacancies can create additional pathways, reducing the activation energy required for movement.

The researchers also highlight high-entropy halide electrolytes, which contain several different metal species distributed across similar crystallographic positions. One reported composition, Li₂.₂In₀.₂Sc₀.₂Zr₀.₂Hf₀.₂Ta₀.₂Cl₆, achieved an ionic conductivity of 4.69 mS cm⁻¹ and an oxidation stability limit approaching 5.5 volts. High-entropy design introduces chemical complexity that can disrupt unfavorable ordering and generate a broader network of lithium-ion pathways. The same complexity may also help stabilize the material against structural transformations during repeated charging and discharging.

Another striking direction involves oxyhalides, which incorporate oxygen into halide frameworks. The compound Li₃Ta₃O₄Cl₁₀ has been reported to deliver ionic conductivity as high as 9 mS cm⁻¹ at 30°C. Such performance is significant because conductivity at or near room temperature is crucial for electric vehicles and stationary storage systems. Higher conductivity allows a thinner electrolyte layer or lower internal resistance, both of which can improve power capability and reduce energy lost as heat during fast charging and discharging.

Performance depends not only on chemical composition but also on how the electrolyte is made. The review compares mechanochemical processing, co-melting, and wet-chemical synthesis, showing how each route affects particle size, crystallinity, defects, impurities, and contact with electrode materials. Mechanochemical milling can produce intimate mixtures and enable reactions at relatively low temperatures, while wet-chemical approaches may offer better control over composition and morphology. Co-melting can promote uniformity in some systems, although it may require careful control of volatility and thermal stability. These processing choices directly influence critical current density, area-specific resistance, and long-term cycling behavior.

Interface engineering is emerging as one of the most decisive tools in the field. Even a highly conductive electrolyte can perform poorly if it forms a chemically unstable or mechanically fragile boundary with an electrode. Bilayer and dual-electrolyte designs address this challenge by assigning different materials to different sides of the battery. A halide electrolyte can serve as a catholyte, where it faces the oxidizing environment of a high-voltage cathode, while a sulfide electrolyte provides a softer, more conductive interface near the anode. Fluoride-doped halide compositions have also shown promise in improving compatibility with lithium metal, with some configurations maintaining stable lithium stripping and plating for more than 1,000 hours.

According to the review, halide-based cells paired with high-voltage NCM811 and LiCoO₂ cathodes have demonstrated capacity retention of about 70% over 1,600 cycles at a 4C rate, operation at voltages up to 5.5 volts, and projected energy densities approaching 400–500 Wh kg⁻¹. The materials are also being explored beyond conventional lithium-ion chemistry. Halide electrolytes may help stabilize sulfur cathodes in lithium–sulfur batteries, modify reactive air electrodes in lithium–oxygen systems, and support high-voltage sodium-ion solid-state batteries, where reported cells have retained about 90% of their capacity over 300 cycles. The researchers argue that continued progress will depend on scalable synthesis, standardized testing, improved mechanical contact, and a deeper understanding of interfacial reactions. If those challenges can be resolved, halide electrolytes could become a key component of batteries that combine the safety of solid-state architecture with fast charging, high voltage, and substantially greater energy density.

Subject of Research: Halide-based solid electrolytes for advanced all-solid-state batteries

Article Title: Halide-Based Solid Electrolytes for Advanced All-Solid-State Batteries: Design, Interfaces, and Electrochemical Performance

News Publication Date: 22-Jul-2026

Web References: https://doi.org/10.1007/s40820-026-02251-3

References: Nano-Micro Letters, DOI: 10.1007/s40820-026-02251-3

Image Credits: Shivaraju Guddehalli Chandrappa, Gerardo Morell, and Ram S. Katiyar

Keywords

Halide solid electrolytes, all-solid-state batteries, lithium-ion batteries, high-voltage cathodes, lithium metal batteries, sodium-ion batteries, lithium–sulfur batteries, lithium–oxygen batteries, ionic conductivity, interface engineering, high-entropy materials, electrochemistry

Fast charging has always forced lithium-ion batteries into an uncomfortable trade-off. Increasing the charging current can dramatically shorten charging times, but it also accelerates chemical and structural damage inside the cell. At the anode, lithium ions may not be absorbed quickly enough into the host material. Instead, metallic lithium can deposit on the surface, creating irregular structures that increase the risk of internal short circuits, capacity loss and, in extreme cases, thermal runaway. Repeated rapid charging can also destabilize the solid-electrolyte interphase, a thin layer that forms where the electrode meets the electrolyte and controls how ions and electrons move across the interface.

The research team, led by Associate Professor Dongwook Han of Seoul National University of Science and Technology, focused on lithium titanium phosphate, or LTP. This compound has a NASICON-type crystal structure, a framework known for its thermal and structural stability and for providing channels through which lithium ions can move. LTP operates at a relatively high potential compared with conventional graphite anodes, reducing the likelihood of lithium plating during charging. However, its practical performance has been limited by sluggish interfacial kinetics and insufficient conductivity, problems that become especially serious when the battery is pushed to high charging rates.

Rather than using a perfectly balanced chemical composition, the researchers deliberately shifted the ratio of phosphorus to titanium. This “off-stoichiometric” design created a titanium-deficient version of the material. In the resulting anode, titanium phosphate, or TPO, domains formed near the surfaces of LTP particles. These nanoscale or near-surface regions changed how lithium ions interacted with the electrode, effectively creating kinetic gateways at the anode–electrolyte interface.

The significance of the strategy lies in how these altered domains manage the movement of lithium ions. During charging, ions must leave the electrolyte, cross the interface and enter the active particles. Each stage presents an energy barrier, and the interface can become a bottleneck when current is high. According to the researchers, the TPO-rich regions reduce this barrier and provide faster pathways close to the particle surface. The result is a more efficient transfer of lithium ions into the electrode, helping to prevent the accumulation of lithium at the surface that can trigger harmful plating.

The modified structure also appears to improve the anode’s ability to withstand repeated expansion and contraction. The TPO framework contains relatively unconstrained phosphorus–oxygen–phosphorus linkages, which provide additional flexibility within the surrounding structure. These bonds can accommodate the volume changes associated with lithium insertion and removal, reducing the mechanical stress that often leads to cracking, phase degradation or loss of electrical contact. At the same time, the underlying NASICON framework remains sufficiently robust to resist irreversible structural collapse during rapid cycling.

In tests, the off-stoichiometric LTP combined with carbon, known as OS-LTP/C, preserved 86% of its initial capacity at a 10C charging rate. In practical terms, a 10C rate corresponds to charging at a current that could theoretically fill a battery in roughly one-tenth of an hour, although real-world charging time depends on the complete cell design and operating conditions. The pristine LTP/carbon comparison electrode showed a sharp decline in capacity under the same demanding conditions. The redesigned composite also delivered stable cycling for more than 250 cycles, indicating that its fast-charge behavior was not achieved simply by sacrificing durability.

The researchers further tested the material in full cells rather than relying only on simplified laboratory half-cell configurations. These full-cell demonstrations showed similarly strong rate performance and compatibility with high-voltage cathodes. That result is important because anode materials must operate as part of a balanced electrochemical system; a promising electrode can lose its value if it cannot be paired with cathodes that deliver high energy density. Compatibility with higher-voltage cathodes could therefore broaden the range of battery architectures in which the off-stoichiometric LTP strategy may be used.

The work points to a broader design principle for energy-storage materials: performance may be improved not only by discovering entirely new compounds, but also by intentionally introducing controlled chemical imbalance into familiar structures. By engineering the composition near the particle surface, the researchers created a material that combines rapid ion transport, structural flexibility and thermal robustness. Han says the concept could be extended to other rechargeable-battery systems, including all-solid-state batteries, where contact resistance and interfacial ion transport are major technical challenges. Although further testing will be required to evaluate large-format cells, long-term operation and manufacturing scalability, the results offer a promising route toward batteries that charge faster without compromising safety and service life.

Subject of Research: Lithium-ion battery anodes and fast-charging energy-storage materials

Article Title: Unlocking Ultrafast Charging: Synergizing Embedded Pseudocapacitive Domains and Flexible Lattice Dynamics in Off-Stoichiometric LiTi2(PO4)3 Anodes

News Publication Date: 1 June 2026

Web References: https://doi.org/10.1002/adfm.76250

References: Advanced Functional Materials, “Unlocking Ultrafast Charging: Synergizing Embedded Pseudocapacitive Domains and Flexible Lattice Dynamics in Off-Stoichiometric LiTi2(PO4)3 Anodes,” DOI: 10.1002/adfm.76250

Image Credits: Associate Professor Dongwook Han, Seoul National University of Science and Technology, South Korea

Keywords

Fast-charging batteries, lithium-ion batteries, lithium titanium phosphate, LTP anodes, off-stoichiometric materials, NASICON structure, battery safety, electric vehicles, energy storage, materials science