Ternary Halide Solid Electrolytes

• Ternary halide solid electrolytes are multi-component ion conductors with complex crystal structures and tunable defect chemistries critical for all-solid-state battery performance.
• Mechanochemical and thermal synthesis methods control cation disorder and stacking faults, directly affecting ionic transport properties and activation energies.
• Thermal and mechanical challenges such as low thermal conductivity and brittle grain boundaries require innovative design for scalable, robust electrochemical devices.

Ternary halide solid electrolytes are a class of crystalline or amorphous ion-conducting materials composed of three components, typically involving an alkali or alkaline earth metal (e.g., Li, Na, Mg), a transition or main-group metal, and halide anions (Cl⁻, Br⁻, I⁻). These materials are increasingly investigated as both bulk solid electrolytes and catholytes in next-generation all-solid-state batteries (ASSBs) and related electrochemical devices due to their high ionic conductivities, wide electrochemical windows, chemical compatibility with oxide electrodes, and unique processing advantages. Their physical properties, such as low thermal conductivity, distinct defect chemistry, and tunable structures via synthetic control, are determined by their multicomponent chemistry, complex local and extended crystal structures, and the prevalence of intrinsic and extrinsic disorder.

1. Structural Complexity and Synthesis

Ternary halide solid electrolytes are characterized by complex crystal structures with a large number of atoms per primitive cell, leading to many optical phonon modes. Typical compositions include systems such as Li₃YCl₆, Li₃InCl₆, Li₂ZrCl₆, Na₃YCl₆, and the broader Li₃YCl₆₋ₓBrₓ and Na–Y–Zr–Cl families. Structures are often layered or framework-type, built from corner- or edge-sharing polyhedra (e.g., MCl₆, MBr₆ octahedra), with partially occupied sites and intrinsic vacancies. This atomic complexity, combined with the possibility of cation or anion nonstoichiometry, gives rise to a rich defect landscape and multiple polymorphs.

Synthetic protocols exert strong control over both the extended order and the local disorder in these materials. Mechanochemical methods such as high-energy ball milling are particularly effective at introducing cation (e.g., Li/vacancy, Zr) disorder and stacking faults, yielding phases (e.g., ball-milled Li₂ZrCl₆, Li₃YCl₆) with higher ionic conductivity than those produced by equilibrium solid-state synthesis and annealing (Sebti et al., 2022, Zhong et al., 13 Mar 2024). Heat treatment and cooling rate further dictate phase selection (e.g., P2₁/n vs. P̅3m1 polymorphs in Na₂ZrCl₆), degree of transition metal mixing (Y/Zr), and the manifestation of long-range order or defect-dominated structures (Sebti et al., 2022, Rom et al., 22 Aug 2025).

Synthesis Method	Structural/Defect Features	Typical Impact on Conductivity
Ball milling	Cation/disorder, stacking faults, metastable phases	Increased (often ms/cm range)
High-T annealing/slow cool	Ordered polymorphs, reduced defects	Decreased
Quenching	Kinetic phases, partial disorder	Variable, often increased

2. Ionic Transport Mechanisms and Defect Chemistry

Ionic conduction in ternary halide electrolytes is governed by a combination of intrinsic vacancies, cation disorder, stacking faults, and the extended percolation network provided by the crystal lattice. The foundational microscopic theory describes ionic motion as governed by a Langevin equation, with the mobility μ determined by the inverse of a position-dependent dissipation tensor, itself directly related to lattice vibrational properties and the curvature (Hessian) of the local potential energy surface (Rodin et al., 2021).

Cation disorder (on both the “mobile” alkali/vacancy and the “framework” metal sublattice) is a key parameter that determines percolation thresholds and energy barriers for ion hopping. For example, in Li₂ZrCl₆, fast Li-ion conductivity requires both Li/vacancy and Zr disorder; only non-equilibrium synthesis such as ball milling allows Zr disorder to be “frozen in,” resulting in activation energies as low as 0.28–0.31 eV and conductivities up to ∼0.8 mS/cm at room temperature, compared to much higher activation barriers in ordered samples (Zhong et al., 13 Mar 2024). In the Na–Y–Zr–Cl system, increased mixing of transition metals and the appearance of intrinsic vacancies upon polymorph and disorder control enhance three-dimensional conduction channels, which are further assisted by cooperative anion (Cl⁻) rotations (Sebti et al., 2022).

Stacking faults and off-stoichiometry (e.g., Li₍₃₊₃ₓ₎Y₍₁₋ₓ₎Cl₆) in mechanochemically prepared Li₃YCl₆ enhance Li site connectivity and lower migration barriers (NEB: ∼0.12–0.13 eV in defected regions), providing high lithium-ion conductivities (∼0.49 mS/cm) (Sebti et al., 2022). The relationship between defect concentration (e.g., stacking faults) and ionic conductivity obeys Arrhenius scaling,

$\sigma = \sigma_0\,\exp(-E_a / k_B T)$

with reduced $E_a$ upon defect enrichment.

In certain systems (e.g., MgZrCl₆), despite structural analogy to high-conductivity Li/Na analogues, the stable octahedral coordination of Mg²⁺ leads to large migration barriers (1.77 eV intra-, 0.86 eV interlayer), and ionic conductivities below the detection limit (∼1.4 × 10⁻⁸ S/cm), emphasizing the stringent geometric and energetic requirements for multivalent conduction (Rom et al., 22 Aug 2025).

3. Thermal, Mechanical, and Electronic Properties

Ternary halide solid electrolytes display anomalously low, glass-like thermal conductivities (κ ∼0.45–0.70 W·m⁻¹·K⁻¹ for Li₃YCl₆ and Li₃InCl₆), despite being crystalline (Cheng et al., 2021). The temperature dependence of κ is glass-like (increasing with T), indicative of extreme phonon scattering and mean free paths approaching atomic spacings. This is attributed to the complex vibrational spectrum (many optical phonons) and extrinsic atomic-scale disorder induced during synthesis. The minimum thermal conductivity is well-predicted by

$$\kappa_{\text{min}} = \left( \frac{\pi}{6} \right)^{1/3} k_B n^{2/3} (v_l + 2v_t)$$

where $n$ is the atomic number density and $v_l, v_t$ the speeds of sound.

The low κ values, while higher than those of typical liquid electrolytes, introduce intrinsic thermal resistance, necessitating careful management of heat flow in battery architectures to avoid localized hot spots. The presence of low-κ, defect-rich halides suggests a need for composite designs or the addition of high-κ fillers for thermal regulation.

Extended defects, particularly grain boundaries and surfaces, are critical determinants of both mechanical and electronic behavior. Grain boundaries have considerably lower work of adhesion ($W_{ad} = 2\gamma - \sigma$) and fracture toughness than the bulk ($K_{Ic} = \sqrt{E G_c} / \sqrt{1 - \nu^2}$), thereby serving as initiation points for intergranular cracking and lithium/metal filament nucleation under electrochemical or mechanical stress (Xie et al., 2023). Electronic structure calculations reveal that these extended defects introduce localized states into the bandgap, increasing the local electronic conductivity and enhancing the propensity for dendrite growth and degradation processes.

4. Compositional Tuning and Predictive Modeling

Compositional modification, such as Br/Cl mixing in Li₃YCl₆₋ₓBrₓ, enables tuning of both the diffusion landscape and the activation energy for ion conduction. Machine learning interatomic potentials (e.g., fine-tuned CHGNet models) now provide accurate predictions of energy, forces, stress, and diffusion characteristics across a wide range of compositions, capturing both structural equilibria and ionic transport (Böhm et al., 10 Oct 2025).

Enumeration and DFT optimization of ordered configurations derived from nominally disordered experimental models afford reliable input for MD simulations, which, combined with temperature sampling and Arrhenius analysis,

$D(T) = \tilde{D}\,\exp(-E_a / k_B T)$

and the Nernst–Einstein relation,

$\sigma(T) = \frac{Nq^2}{V k_B T} D(T),$

connect atomic-scale structure to macroscopic conductivity. For the Li₃YCl₆₋ₓBrₓ family, the maximum ionic conductivity is typically achieved at intermediate Br compositions (experimentally near $x \simeq 4.5$), where wider diffusion bottlenecks enable more facile Li transport. The computational framework captures the dependency of activation energy and σ on Halide composition, providing predictive capability for targeted material optimization.

5. Role of Anion and Cation Chemistry in Polymer-Based Ternary Electrolytes

In polymer-like ternary solid polymer electrolytes (TSPEs), the incorporation of halide or non-halide anions with strong Li⁺ coordination capability (e.g., TFSAM in place of TFSI) fundamentally shifts the transport mechanism (Hoffknecht et al., 2022). Here, Li⁺—rather than being tightly coupled to slow polymer segmental motion—can be coordinated by fast-moving anions, effectively decoupling its mobility from the host matrix. This results in substantial increases in the Li⁺ transference number (by ∼600%), mean squared displacement, and overall Li⁺ conductivity, even if the total ionic conductivity is lower. The improved electrochemical performance in full cells (e.g., increased capacity retention over 300 cycles, improved rate performance at 2C) is attributed to a higher fraction of the ionic current being carried by the Li cation. The general principle is that careful design of the anion or framework chemistry in ternary solid electrolytes—whether crystalline or polymeric—can significantly influence transport selectivity and efficiency.

6. Challenges and Perspectives

Three central challenges emerge from current research on ternary halide solid electrolytes:

• Stability of Disordered States: Many high-conductivity phases rely on non-equilibrium disorder (e.g., Zr disorder in Li₂ZrCl₆). These states can be metastable and may re-order over time or under thermal cycling, resulting in loss of performance (Zhong et al., 13 Mar 2024).
• Scalability and Processing: Mechanochemical synthesis is effective at producing defect-rich, high-conductivity states but is less easily scalable or controlled than conventional high-temperature synthesis. Achieving uniform defect populations at large scale is nontrivial (Zhong et al., 13 Mar 2024, Rom et al., 22 Aug 2025).
• Mechanical and Interfacial Reliability: Brittle failure, grain boundary-induced dendrite nucleation, and low fracture toughness continue to limit practical application, especially for thick polycrystalline pellets (Xie et al., 2023).

Potential research avenues to address these challenges include the deliberate introduction of cation or anion disorder via aliovalent doping or high-entropy design, grain boundary engineering to improve mechanical integrity and suppress electronic leakage, and advanced ML-driven simulation frameworks to inform compositional and structural optimization.

The integration of these approaches is expected to accelerate the design of robust, high-conductivity, and thermally/electrochemically stable ternary halide solid electrolytes, expanding the toolkit for practical all-solid-state and hybrid battery systems.