In recent decades, lithium-ion batteries have been extensively researched and applied, but the growing secondary battery market demands higher energy density and safety. Traditional liquid lithium-ion batteries pose risks of leakage and combustion, making non-flammable all-solid-state batteries a promising solution. Among solid-state electrolyte systems, sulfide-based solid electrolytes stand out due to their excellent processability and high ionic conductivity (10−3–10−2 S/cm). Sulfide electrolytes can be classified into glassy, glass-ceramic, and crystalline types, with crystalline variants like Li10GeP2S12 and Li9.54Si1.74P1.44S11.7Cl0.3 achieving ionic conductivities comparable to liquid electrolytes. However, poor electrochemical stability and high costs limit their widespread use. Argyrodite-type sulfide solid electrolytes, such as Li6PS5X (X = Cl, Br, I), have gained attention for their balanced conductivity and stability. Specifically, chlorine substitution in Li7−xPS6−xClx electrolytes influences ionic conductivity and interfacial stability, yet a comprehensive understanding of chlorine content effects on battery performance remains incomplete. This study systematically investigates the role of chlorine content (x = 1.0–1.5, corresponding to 25–37.5 mol% Cl) in Li7−xPS6−xClx electrolytes for all-solid-state batteries, focusing on ionic transport, interfacial reactions, and overall cell performance. We demonstrate that optimizing chlorine content can balance ionic conductivity and stability, leading to enhanced rate capability and cycling durability in solid state batteries.
We synthesized Li7−xPS6−xClx (x = 1.0–1.5) electrolytes via solid-state sintering. Starting materials, including Li2S, P2S5, and LiCl, were mixed in stoichiometric ratios, ball-milled at 400 rpm for 4 hours, and sintered at 500°C for 8 hours under argon atmosphere. The resulting electrolytes were labeled Cl 1.0 to Cl 1.5 based on the x value. For battery assembly, composite cathodes were prepared by mixing LiNi0.8Co0.1Mn0.1O2 (NCM), Li7−xPS6−xClx, and vapor-grown carbon fiber (VGCF) in a 70:27:3 mass ratio. All-solid-state batteries were constructed with a composite cathode, electrolyte layer, and LiIn anode, cold-pressed at 600 MPa. Structural characterization involved X-ray diffraction (XRD) and Raman spectroscopy, while electrochemical tests included electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic cycling. Distribution of relaxation times (DRT) analysis and X-ray photoelectron spectroscopy (XPS) were employed to probe interfacial changes post-cycling.
The XRD patterns of Li7−xPS6−xClx electrolytes confirmed the argyrodite phase, with peak shifts to higher angles as x increased, indicating lattice contraction due to Cl− substitution for S2−. No impurity phases were detected, suggesting high purity. Crystallinity peaked at x = 1.3, as evidenced by sharp diffraction peaks, while higher chlorine content (x = 1.4–1.5) reduced crystallinity. Raman spectra showed a red shift in the PS43− peak at ~423 cm−1 with increasing x, signifying lattice softening that enhances ion mobility but may compromise stability. The ionic conductivity, measured via EIS, increased monotonically with chlorine content, reaching a maximum of 9.5 mS/cm for Cl 1.5. However, electronic conductivity, determined by DC polarization, remained below 1 × 10−9 mS/cm for all samples. The activation energy for ion conduction, derived from Arrhenius plots, decreased initially with x, achieving a minimum of 0.314 eV for Cl 1.3, before rising for higher x values. This trend highlights the trade-off between anion disorder and crystallinity. CV tests revealed that all electrolytes began oxidizing at 2.6 V vs. Li/Li+, with oxidation current densities increasing with chlorine content, indicating reduced electrochemical stability. These findings underscore the dual role of chlorine in enhancing conductivity while diminishing stability in solid state battery applications.
To evaluate practical performance, we assembled all-solid-state batteries with NCM-based composite cathodes and LiIn anodes. Batteries employing Cl 1.0 to Cl 1.5 electrolytes in the cathode showed varying initial Coulombic efficiencies and discharge capacities. The Cl 1.3-based cathode delivered the highest rate performance, with discharge capacities of 121 mA·h/g at 2C and 99 mA·h/g at 3C, outperforming Cl 1.0 (76 mA·h/g at 3C). This improvement is attributed to optimal ionic conductivity and interfacial properties. However, cycling stability decreased with higher chlorine content; after 100 cycles at 0.5C, capacity retention followed Cl 1.0 (90%) > Cl 1.3 (87%) > Cl 1.5 (82%). Post-cycling XPS analysis of cathodes revealed increased proportions of polysulfides and phosphorus-sulfur compounds with rising x, confirming greater electrolyte decomposition. FIB-SEM images showed more severe particle cracking and interfacial degradation in Cl 1.5 cathodes, correlating with impedance growth. DRT analysis identified key relaxation processes: high-frequency responses (τ ~ 10−7–10−6 s) corresponding to grain boundaries, mid-frequency peaks (τ ~ 10−5–10−3 s) associated with solid-electrolyte interphase (SEI) and cathode-electrolyte interface (CEI), and low-frequency signals (τ > 10−1 s) related to charge transfer and diffusion. For Cl 1.3 cathodes, charge transfer resistance decreased during cycling, likely due to decomposition products improving particle contact, whereas Cl 1.5 exhibited rapid interface degradation.
When Li7−xPS6−xClx electrolytes were used as separator layers, batteries with Cl 1.0 layers achieved the best cycling performance, retaining 88.4% capacity after 100 cycles at 0.5C, compared to 78.4% for Cl 1.3. EIS and DRT analyses indicated that Cl 1.3 layers led to significant increases in SEI and CEI resistances post-cycling, attributed to mechanical strain and decomposition. SEM of cycled interfaces revealed more surface particles and cracks in high-chlorine electrolytes, hindering ion transport. In contrast, Cl 1.0 layers maintained stable interfaces, facilitating efficient ion conduction. These results emphasize that electrolyte layers require high stability, whereas cathode mixtures benefit from moderate chlorine content to balance conductivity and decomposition.
We further optimized the battery configuration by combining a Cl 1.3-based cathode with a Cl 1.0 electrolyte layer. This cell delivered a high initial discharge capacity of 179 mA·h/g at 0.5C and retained 132.8 mA·h/g after 300 cycles (78.4% retention) under a high active material loading of 20 mg/cm2. The success of this design stems from leveraging the high ionic conductivity of Cl 1.3 in the cathode while using the stable Cl 1.0 layer to minimize interfacial degradation. This approach highlights the importance of tailoring chlorine content in different battery components to achieve superior performance in solid state batteries.
The ionic conductivity (σi) of Li7−xPS6−xClx electrolytes can be modeled using the Nernst-Einstein equation: $$σ_i = \frac{D z^2 F^2 c}{RT}$$ where D is the diffusion coefficient, z is the charge number, F is Faraday’s constant, c is the carrier concentration, R is the gas constant, and T is temperature. The increase in σi with x is due to enhanced Li+ vacancy concentration and anion disorder. The activation energy (Ea) for ion conduction is derived from the Arrhenius relation: $$σ_i T = A \exp\left(-\frac{E_a}{kT}\right)$$ where A is the pre-exponential factor and k is Boltzmann’s constant. The minimal Ea for Cl 1.3 indicates efficient ion transport pathways. Interfacial stability is crucial for long-term cycling in all-solid-state batteries. The growth of interface resistance (Rinterface) over time can be expressed as: $$R_{\text{interface}} = R_0 + k t^n$$ where R0 is the initial resistance, k is a rate constant, t is time, and n is an exponent related to degradation mechanisms. For high-chlorine electrolytes, n values are larger, reflecting accelerated degradation.
| Chlorine Content (x) | Ionic Conductivity (mS/cm) | Electronic Conductivity (mS/cm) | Activation Energy (eV) | Decomposition Onset Voltage (V vs. Li/Li+) |
|---|---|---|---|---|
| 1.0 | 3.2 | 5.6 × 10−10 | 0.345 | 2.6 |
| 1.1 | 4.8 | 7.2 × 10−10 | 0.331 | 2.6 |
| 1.2 | 6.5 | 8.9 × 10−10 | 0.322 | 2.6 |
| 1.3 | 8.1 | 9.8 × 10−10 | 0.314 | 2.6 |
| 1.4 | 8.9 | 8.5 × 10−10 | 0.328 | 2.6 |
| 1.5 | 9.5 | 7.1 × 10−10 | 0.337 | 2.6 |
In terms of interfacial reactions, the decomposition of Li7−xPS6−xClx at the cathode involves oxidation to LiCl, Li3PS4, and sulfur species: $$\text{Li}_{7-x}\text{PS}_{6-x}\text{Cl}_x \rightarrow \text{LiCl} + \text{Li}_3\text{PS}_4 + \text{S}_x^{0}/\text{S}_{x}^{2-}$$ Further oxidation yields P2S5 and S. The extent of decomposition, quantified by XPS, increased with x, leading to higher interface resistance. For the anode side, the formation of a stable SEI is critical. The Li+ transference number (tLi+) in these electrolytes approaches unity, minimizing polarization. The overall cell performance can be evaluated using the specific capacity (C) as a function of cycle number (N): $$C(N) = C_0 \exp(-kN)$$ where C0 is the initial capacity and k is the degradation rate. For optimized cells, k is minimized through interface engineering.
Our findings demonstrate that chlorine content in Li7−xPS6−xClx electrolytes plays a critical role in balancing ionic conductivity and interfacial stability for all-solid-state batteries. The Cl 1.3 composition offers an optimal compromise, enabling high rate capability in cathodes, while Cl 1.0 layers provide durable interfaces. Future work should explore halogen doping strategies and advanced characterization to further enhance the performance of solid state batteries. By systematically optimizing material properties, we can accelerate the development of safe, high-energy-density all-solid-state batteries for next-generation energy storage.
| Battery Configuration | Initial Discharge Capacity (mA·h/g, 0.5C) | Capacity Retention After 100 Cycles (%) | Rate Performance (3C Capacity, mA·h/g) | Interface Resistance Growth (ΔR, %) |
|---|---|---|---|---|
| NCM@Cl 1.0 | Cl 1.0 | LiIn | 163 | 90 | 76 | 120 |
| NCM@Cl 1.3 | Cl 1.3 | LiIn | 178 | 87 | 99 | 150 |
| NCM@Cl 1.5 | Cl 1.5 | LiIn | 170 | 82 | 85 | 200 |
| NCM@Cl 1.3 | Cl 1.0 | LiIn | 179 | 88.4 | 98 | 110 |
The degradation mechanisms in solid state batteries are influenced by multiple factors, including mechanical stress, chemical reactions, and ion transport limitations. For Li7−xPS6−xClx electrolytes, the relationship between chlorine content and stability can be described using a stability parameter (S): $$S = \frac{\sigma_i}{\sigma_e \cdot R_{\text{growth}}}$$ where σi is ionic conductivity, σe is electronic conductivity, and Rgrowth is the rate of interface resistance growth. Higher S values indicate better overall performance. In our study, Cl 1.3 exhibited the highest S when used in cathodes, whereas Cl 1.0 was superior for electrolyte layers. This nuanced understanding enables the design of robust solid state battery systems with prolonged lifespan and high efficiency.
In conclusion, the optimization of chlorine content in Li7−xPS6−xClx electrolytes is pivotal for advancing all-solid-state batteries. Through tailored material design and interface engineering, we can overcome current limitations and realize the full potential of solid state batteries for sustainable energy storage. Continued research into sulfide-based electrolytes will undoubtedly contribute to the commercialization of high-performance all-solid-state batteries.