High-Performance Li3YCl3Br3Halide Solid Electrolytes for Next-Generation Solid-State Batteries

The development of solid-state batteries has emerged as a transformative solution for achieving high energy density and enhanced safety in energy storage systems. Among critical components, halide-based solid electrolytes have gained prominence due to their unique combination of high ionic conductivity, wide electrochemical stability, and compatibility with oxide cathodes. This work introduces a novel wet-chemistry synthesis route for Li₃YCl₃Br₃ (LYCB), a dual-halide electrolyte, and systematically evaluates its structural, electrochemical, and interfacial properties for solid-state battery applications.

Synthesis and Structural Characterization

The LYCB electrolyte was synthesized via an ammonia-assisted wet-chemistry method using stoichiometric ratios of YCl₃·6H₂O, NH₄Br, and LiBr. The precursor phase (NH₄)₃[YCl₃Br₃] was thermally decomposed at 450°C under argon to yield phase-pure LYCB. Structural refinement confirmed a monoclinic framework (space group C2/m) with lattice parameters:
$$
a = 6.809(3)\ \text{Å},\ b = 11.776(6)\ \text{Å},\ c = 6.7759(4)\ \text{Å},\ \beta = 108.79(5)^\circ
$$
The crystal structure features three distinct Li⁺ sites (4h, 4g, 8j), enabling low-energy migration pathways through octahedral-tetrahedral coordination vacancies. Table 1 summarizes key structural parameters derived from Rietveld refinement.

Table 1. Structural parameters of LYCB from Rietveld refinement
Atom	Site	x	y	z	Occupancy
Li1	4h	0	0.1637	0.5	0.973
Y1	4g	0	0.3414	0	0.261
Cl1/Br1	8j	0.2682	0.1633	0.2391	0.507/0.503

Ionic Transport Properties

LYCB demonstrated superior ionic conductivity (2.08 mS cm⁻¹ at 25°C) compared to Li₃YCl₆ (LYC, 0.84 mS cm⁻¹) and Li₃YBr₆ (LYB, 1.52 mS cm⁻¹). The Arrhenius relationship:
$$
\sigma T = A \exp\left(-\frac{E_a}{k_B T}\right)
$$
yielded an activation energy (E_a) of 0.379 eV for LYCB, significantly lower than LYC (0.498 eV). This enhancement arises from halogen mixing effects, which broaden Li⁺ diffusion channels while maintaining structural stability.

Electrochemical Performance in Solid-State Batteries

All-solid-state lithium batteries (ASSLBs) employing LiNi_0.83Co_0.11Mn_0.06O₂ (NCM811) cathodes and LYCB electrolytes exhibited exceptional cycling stability:
$$
\text{Capacity retention} = 93\%\ \text{after 200 cycles at 0.3 C}\ (1\ \text{C} = 170\ \text{mA g}^{-1})
$$
Rate capability tests revealed 115 mAh g⁻¹ at 2 C, outperforming LYB (58 mAh g⁻¹) and LYC (38 mAh g⁻¹). The enhanced performance stems from LYCB’s wide electrochemical window (3.8 V vs. Li⁺/Li) and stable cathode-electrolyte interface (CEI).

Interfacial Stabilization Mechanism

Operando XPS and TEM analyses identified an in situ-formed Y₂O₃ layer at the NCM811/LYCB interface during cycling. This inert phase suppresses oxygen loss from NCM811 while facilitating Li⁺ transport, as described by:
$$
\text{Li}_3\text{YCl}_3\text{Br}_3 + \text{O}_{\text{lattice}} \rightarrow \text{Y}_2\text{O}_3 + \text{LiCl} + \text{LiBr} + \text{Cl}_2/\text{Br}_2 \uparrow
$$
The Y₂O₃ layer (thickness ≈ 5 nm) effectively passivates the cathode surface, reducing interfacial resistance growth from 48 Ω cm² (pristine) to 62 Ω cm² after 200 cycles.

Comparative Analysis of Synthesis Routes

The wet-chemistry method offers distinct advantages over conventional ball milling (Table 2). Scalability and reduced processing time make it commercially viable for solid-state battery production.

Table 2. Comparison of LYCB synthesis methods
Parameter	Wet-Chemistry	Ball Milling
Processing Time	24 h	10 h + Annealing
Particle Size	200-500 nm	50-200 nm
Ionic Conductivity	2.08 mS cm⁻¹	1.12 mS cm⁻¹
Cost Efficiency	High	Medium

Conclusion

This work establishes Li₃YCl₃Br₃ as a premier halide electrolyte for solid-state batteries, combining scalable synthesis with exceptional electrochemical performance. The in situ interfacial engineering mechanism provides fundamental insights for designing stable high-voltage ASSLBs. Future efforts will focus on optimizing halogen ratios and exploring multi-metal doping to further enhance ionic transport while maintaining cost-effectiveness for industrial adoption.