The rapid advancement of electric vehicles, portable electronics, and large-scale energy storage systems has intensified the demand for high-energy-density and safe electrochemical power sources. Solid-state batteries, particularly all-solid-state batteries, have emerged as promising alternatives to conventional lithium-ion batteries due to their potential for enhanced safety and higher energy density. Among various solid electrolytes, sulfide-based solid electrolytes exhibit exceptional room-temperature ionic conductivities, often in the range of 10−3 to 10−2 S/cm, making them attractive for practical applications. However, the poor mechanical strength of sulfide electrolytes poses significant challenges in forming thin, robust electrolyte layers, which are crucial for maximizing the energy density of solid state batteries. Traditional methods, such as slurry casting or hot-pressing, often involve solvents or high temperatures that can degrade electrolyte performance or increase manufacturing complexity. In this work, we introduce a novel room-temperature dry-processing strategy utilizing pre-fibrillated polymer binders to fabricate ultrathin sulfide solid electrolyte membranes and composite electrodes, enabling high-performance all-solid-state batteries with improved energy density and cycle life.
Solid state batteries rely on solid electrolytes to facilitate ion transport while eliminating flammable organic liquids, thereby enhancing safety. Sulfide solid electrolytes, such as Li6PS5Cl, offer high ionic conductivity but suffer from brittleness, requiring thick layers (e.g., ~1000 μm) under high pressure (e.g., 300 MPa) to maintain structural integrity. This not only reduces the volumetric energy density but also increases the amount of electrolyte needed in composite electrodes, further diminishing the practical specific energy of all-solid-state batteries. To address these issues, we developed a solvent-free, room-temperature dry-processing approach that integrates pre-fibrillated polytetrafluoroethylene (PTFE) as a binder with Li6PS5Cl sulfide electrolyte. This method avoids the detrimental effects of solvent-induced side reactions and high-temperature treatments, preserving the intrinsic properties of the sulfide electrolyte while enabling the production of flexible, ultrathin membranes. The resulting solid electrolyte membranes exhibit high ionic conductivity, low electronic conductivity, and excellent mechanical flexibility, which are critical for advancing solid state battery technology.
The preparation of pre-fibrillated PTFE involved demulsifying a PTFE emulsion followed by vacuum drying at 120°C, resulting in a fibrous powder with interconnected networks. This pre-fibrillated PTFE was then mixed with Li6PS5Cl electrolyte powder in an argon-filled glovebox, with PTFE content optimized between 0.0% and 2.0% by weight. The mixture was roller-pressed at room temperature to form ultrathin membranes (~35 μm in thickness). Similarly, composite cathode membranes were fabricated using lithium niobate-coated lithium cobalt oxide (LCO@LNO) as the active material, Li6PS5Cl as the ionic conductor, vapor-grown carbon fiber (VGCF) as the conductive additive, and a minimal amount of pre-fibrillated PTFE binder. The cathode mixture was roller-pressed into films with controlled mass loadings, ensuring uniform distribution of components. All-solid-state thin-film batteries were assembled by stacking a lithium-indium (Li-In) alloy anode, the sulfide solid electrolyte membrane, and the LCO@LNO composite cathode under controlled pressure, demonstrating the feasibility of this dry-process for constructing high-energy solid state batteries.
Characterization of the Li6PS5Cl solid electrolyte membranes revealed a dense, homogeneous structure with well-distributed PTFE fibers, as confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) mapping. The membranes showed no significant cracks or pores, and the fibrous PTFE network effectively bonded the sulfide electrolyte particles, enhancing mechanical robustness. X-ray diffraction (XRD) and Raman spectroscopy indicated that the crystal structure of Li6PS5Cl remained unchanged after composite formation, highlighting the non-destructive nature of the dry-process. X-ray photoelectron spectroscopy (XPS) further confirmed the chemical stability, with distinct peaks corresponding to CF2 groups from PTFE and unchanged sulfur and phosphorus environments in the sulfide electrolyte.
The ionic conductivity of the membranes was evaluated using electrochemical impedance spectroscopy (EIS) across a temperature range. The results followed the Arrhenius equation, expressed as:
$$ \sigma = A \exp\left(-\frac{E_a}{k_B T}\right) $$
where $\sigma$ is the ionic conductivity, $A$ is the pre-exponential factor, $E_a$ is the activation energy, $k_B$ is Boltzmann’s constant, and $T$ is the absolute temperature. For the optimal membrane with 0.2 wt% PTFE, the room-temperature ionic conductivity reached 3.17 mS/cm, compared to 5.62 mS/cm for the pristine Li6PS5Cl powder. The activation energy for ion transport was calculated as 0.234 eV for the membrane and 0.231 eV for the powder, indicating similar ion conduction mechanisms. The electronic conductivity, determined by DC polarization tests, was significantly lower for the membrane (7.48 × 10−10 S/cm) than for the powder (2.5 × 10−8 S/cm), which is beneficial for suppressing lithium dendrite growth in solid state batteries. Cyclic voltammetry (CV) tests in a Li-In/SS cell showed a slightly wider electrochemical stability window for the membrane, with oxidation onset at 2.6 V (vs. Li-In) and reduction onset at 1.55 V, attributed to the inert nature of PTFE.
For the composite cathode, the LCO@LNO-based membrane exhibited a uniform morphology with integrated active material, electrolyte, and conductive additive, as seen in SEM and EDS analysis. The cathode film thickness was approximately 62 μm, with a mass loading of 6 mg LCO@LNO per 10 mg total electrode weight. Electrochemical performance was assessed in all-solid-state cells with Li-In alloy anodes. The table below summarizes the key properties of the electrolyte and cathode membranes:
| Component | Property | Value |
|---|---|---|
| Li6PS5Cl Membrane | Thickness | ~35 μm |
| Ionic Conductivity (25°C) | 3.17 mS/cm | |
| Electronic Conductivity | 7.48 × 10−10 S/cm | |
| Activation Energy (Ea) | 0.234 eV | |
| LCO@LNO Cathode Membrane | Thickness | ~62 μm |
| Active Material Loading | 6 mg (per 10 mg electrode) | |
| Initial Discharge Capacity (0.1C) | 139.7 mAh/g |
The all-solid-state battery with the dry-processed membranes demonstrated excellent performance. At 0.1C rate, the full cell delivered an initial discharge capacity of 134.1 mAh/g based on the LCO@LNO mass, with a first-cycle Coulombic efficiency of 90.24%. After 100 cycles, the capacity retention exceeded 92%, underscoring the stability of the solid state battery. Rate capability tests showed capacities of 134.1, 122.1, 103.4, 86.1, 65.1, and 33.1 mAh/g at 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, respectively, highlighting the efficient ion and electron transport within the composite electrodes. For higher mass loadings (13 mg LCO@LNO), the area capacity reached 2.3 mAh/cm², with 71.8% capacity retention after 100 cycles at 0.1C. The specific energy of the full solid state battery was calculated as 188 Wh/kg, a significant improvement over powder-based counterparts (48 Wh/kg), using the formula:
$$ E_{\text{specific}} = \frac{C_{\text{discharge}} \times V_{\text{average}}}{m_{\text{total}}} $$
where $C_{\text{discharge}}$ is the discharge capacity, $V_{\text{average}}$ is the average voltage, and $m_{\text{total}}$ is the total mass of the battery components. This enhancement is attributed to the reduced electrolyte thickness and optimized electrode architecture, which minimize inactive material content while maintaining effective interfaces.
Further analysis of the ionic conductivity dependence on temperature for both powder and membrane samples is presented in the table below, derived from Arrhenius plots:
| Sample | Temperature Range (°C) | Ionic Conductivity at 25°C (mS/cm) | Activation Energy, Ea (eV) |
|---|---|---|---|
| Li6PS5Cl Powder | 25–60 | 5.62 | 0.231 |
| Li6PS5Cl Membrane (0.2% PTFE) | 25–60 | 3.17 | 0.234 |
The mechanical properties of the membranes were qualitatively assessed through folding tests, where the electrolyte film could be folded and unfolded without cracking, demonstrating flexibility imparted by the PTFE fibers. This is crucial for practical applications in flexible solid state batteries. Additionally, the electronic conductivity was calculated from steady-state current measurements under a 0.5 V bias using the equation:
$$ \sigma_e = \frac{L \cdot I}{E \cdot S} $$
where $L$ is the thickness, $I$ is the steady-state current, $E$ is the applied voltage, and $S$ is the electrode area. The low electronic conductivity of the membrane reduces parasitic reactions and enhances the overall safety of solid state batteries.
In terms of interfacial stability, the CV curves indicated that the Li6PS5Cl membrane exhibits reversible behavior within the operational voltage window of all-solid-state batteries. The slight improvement in electrochemical stability compared to the powder may be due to the PTFE matrix acting as a barrier against undesirable side reactions. For the cathode, the integration of Li6PS5Cl electrolyte and VGCF conductive agent within the LCO@LNO matrix ensured percolation pathways for both ions and electrons, as evidenced by the high rate performance. The pre-fibrillated PTFE binder maintained particle cohesion without blocking ionic transport, which is essential for long-term cycling in solid state batteries.
To contextualize the performance, we compare the cycle life and capacity retention of our dry-processed solid state battery with literature data on similar systems. The table below summarizes key metrics:
| Battery System | Initial Capacity (mAh/g) | Cycle Number | Capacity Retention (%) | Specific Energy (Wh/kg) |
|---|---|---|---|---|
| This Work (Membrane-Based) | 134.1 | 100 | 92.84 | 188 |
| Powder-Based Counterpart | 139.5 | 100 | ~85 | 48 |
| Typical Sulfide Solid State Battery | 120–140 | 100 | 80–90 | 100–150 |
The superior performance of our approach is attributed to the room-temperature dry-process, which avoids solvent-induced degradation and preserves the high ionic conductivity of the sulfide electrolyte. Moreover, the ultrathin electrolyte membrane reduces the overall weight and volume, contributing to higher energy density. The use of pre-fibrillated binders represents a scalable and energy-efficient alternative to hot-pressing or solvent-based methods, aligning with the industrial requirements for mass production of all-solid-state batteries.
In conclusion, we have successfully demonstrated a room-temperature dry-processing strategy for fabricating sulfide solid electrolyte membranes and composite electrodes for all-solid-state batteries. The Li6PS5Cl-based membranes exhibit high ionic conductivity, low electronic conductivity, and excellent mechanical flexibility, enabling the assembly of high-performance solid state batteries with remarkable cycle life and energy density. This work underscores the potential of dry-process techniques in overcoming the limitations of sulfide electrolytes, paving the way for next-generation solid state batteries with enhanced safety and efficiency. Future efforts will focus on optimizing the binder content, scaling up the membrane production, and exploring other sulfide compositions to further advance the field of all-solid-state batteries.