In recent years, the growing demand for high-energy-density chemical power sources in electric vehicles, portable electronics, and large-scale energy storage systems has driven rapid advancements in solid-state electrolyte technologies. Solid-state batteries, particularly all-solid-state batteries, are considered a promising solution due to their potential for enhanced safety and higher energy density compared to conventional lithium-ion batteries with liquid electrolytes. The replacement of flammable organic electrolytes with solid-state electrolytes can mitigate safety risks such as leakage and thermal runaway. Moreover, solid-state electrolytes can suppress lithium dendrite growth, enabling the practical use of lithium metal anodes, which is crucial for achieving high energy density. Among various inorganic solid electrolytes, sulfide-based solid electrolytes, such as Li₂S-P₂S₅, Li₁₀GeP₂S₁₂, and Li₆PS₅X (argyrodite, X=Cl, Br, I), have garnered significant attention owing to their high room-temperature ionic conductivities, typically in the range of 10⁻³ to 10⁻² S/cm. These properties make sulfide solid electrolytes ideal candidates for developing next-generation all-solid-state batteries.
However, sulfide solid electrolytes face challenges in practical applications due to their poor mechanical strength and low Young’s modulus, approximately 20 GPa. This often necessitates the use of thick electrolyte layers (around 1000 μm) under high pressure (e.g., 300 MPa) to form dense structures, which reduces the overall energy density of the battery. Additionally, to ensure sufficient contact between cathode active materials and the solid electrolyte and to minimize interfacial resistance, cathode composites must incorporate large amounts of sulfide electrolyte. This further diminishes the practical energy density of all-solid-state batteries, as the excessive use of electrolyte does not contribute to energy storage. Therefore, developing thin, mechanically robust sulfide electrolyte membranes with minimal binder content is essential for enhancing the performance and viability of all-solid-state batteries.
To address these issues, I propose a novel room-temperature dry-process strategy using pre-fiberized polymer binders to fabricate ultrathin sulfide solid electrolyte membranes and composite cathodes. This approach avoids the use of solvents, which can cause side reactions and degrade ionic conductivity, and eliminates the need for energy-intensive thermal processing. In this work, I demonstrate the preparation of pre-fiberized polytetrafluoroethylene (PTFE) powder through demulsification and drying of PTFE emulsion. By mixing this with Li₆PS₅Cl electrolyte and employing room-temperature roller pressing, I successfully produce ultrathin (~35 μm) composite membranes with high ionic conductivity (3.17 mS/cm). Furthermore, I fabricate composite cathode films using lithium niobate-coated lithium cobalt oxide (LCO@LNO) as the active material, sulfide electrolyte, and conductive agents via the same dry-process method. The assembly of all-solid-state thin-film batteries by stacking ultrathin lithium-indium alloy anodes, electrolyte membranes, and cathode films results in excellent electrochemical performance, including a reversible specific capacity of 134.1 mAh/g, an energy density of 188 Wh/kg, and stable cycling over 100 cycles. This work highlights the potential of room-temperature dry-process fabrication for advancing all-solid-state battery technology.
The dry-process fabrication of sulfide solid electrolyte membranes begins with the preparation of pre-fiberized PTFE binder. Conventional PTFE powders consist of discrete particles without fibrous structures, making them unsuitable for room-temperature film formation. In contrast, pre-fiberized PTFE is obtained by coating PTFE emulsion on a glass substrate, drying it at 120°C under vacuum to evaporate solvents, and then grinding the dried film into a powder with elongated fibers. This fibrous morphology enhances the binding properties, allowing for the formation of flexible and dense membranes under ambient conditions. The Li₆PS₅Cl electrolyte powder, known for its high ionic conductivity and stability, is mixed with varying mass fractions of pre-fiberized PTFE (0.0% to 2.0%) in an argon-filled glove box. The mixture is thoroughly ground in a mortar and transferred to a smooth stainless-steel plate, where it is repeatedly rolled at room temperature to form membranes with thicknesses of 30–40 μm. These membranes are then punched into 10 mm diameter discs for further characterization and battery assembly.
For the composite cathode, LCO@LNO is synthesized via a sol-gel method to improve interfacial stability. Lithium metal is dissolved in ethanol inside a glove box, followed by the addition of niobium ethoxide under stirring. After 30 minutes of mixing, LiCoO₂ powder is introduced, and the suspension is stirred for an additional 60 minutes. Ethanol is removed using rotary evaporation, and the product is dried and calcined at high temperature in an oxygen atmosphere to obtain LCO@LNO with a 3 wt% LiNbO₃ coating. The composite cathode film is prepared by mixing 60 mg of LCO@LNO, 34 mg of Li₆PS₅Cl, 6 mg of vapor-grown carbon fiber (VGCF), and 0.5 mg of pre-fiberized PTFE powder in a mortar. The mixture is rolled into a film on a stainless-steel plate and punched into 10 mm discs. The lithium-indium alloy anode is fabricated by pressing an 8 mm diameter indium foil (36 mg) and an 8 mm lithium foil (1 mg) together under 10 MPa pressure.
The structural and morphological properties of the membranes are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). SEM images reveal that the pre-fiberized PTFE powder exhibits a fibrous network, whereas conventional PTFE appears as isolated particles. The Li₆PS₅Cl composite membrane shows a dense and uniform surface without cracks or pores, with PTFE fibers intertwining around electrolyte particles to form a cohesive structure. Cross-sectional SEM confirms the ultrathin nature of the membrane, approximately 35 μm thick. Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) indicates homogeneous distribution of phosphorus (P), sulfur (S), and fluorine (F), corresponding to Li₆PS₅Cl and PTFE, respectively. XRD and Raman analyses confirm that the crystal structure of Li₆PS₅Cl remains unchanged after composite formation, with no significant shifts in diffraction peaks or vibrational modes. XPS spectra show characteristic signals for CF₂ in PTFE (e.g., C 1s at 292 eV and F 1s at 688.9 eV) in the composite membrane, while S 2p and P 2p spectra are identical to those of pure Li₆PS₅Cl powder, indicating no chemical degradation.
The ionic conductivity of the membranes is evaluated using electrochemical impedance spectroscopy (EIS) with stainless steel (SS) blocking electrodes in a SS|membrane|SS configuration. The impedance spectra are measured over a frequency range of 1 Hz to 1 MHz, and the ionic conductivity (σ_Li⁺) is calculated using the formula:
$$ \sigma_{Li^+} = \frac{L}{R \times S} $$
where L is the membrane thickness (cm), R is the bulk resistance (Ω) obtained from the Nyquist plot, and S is the electrode area (cm²). The electronic conductivity (σ_e) is determined by DC polarization at 0.5 V using the equation:
$$ \sigma_e = \frac{L \times I}{E \times S} $$
where I is the steady-state current (A), and E is the applied voltage (V). The electrochemical stability window is assessed by cyclic voltammetry (CV) in a LiIn|membrane|SS cell, scanning from -0.6 V to 3.6 V (vs. Li⁺/LiIn) at 0.2 mV/s.
The results demonstrate that the ionic conductivity of the Li₆PS₅Cl membrane is highly dependent on the PTFE content. As the mass fraction of PTFE decreases, the ionic conductivity increases, reaching a maximum of 3.17 mS/cm at 0.2 wt% PTFE. This value is lower than that of pure Li₆PS₅Cl powder (5.62 mS/cm) due to the insulating nature of PTFE, but it remains sufficiently high for practical applications. The electronic conductivity of the composite membrane is 7.48 × 10⁻¹⁰ S/cm, which is two orders of magnitude lower than that of the powder (2.5 × 10⁻⁸ S/cm), indicating effective suppression of electron leakage and potential dendrite growth. The CV curves show that the oxidation and reduction onsets for the membrane occur at 2.6 V and 1.55 V (vs. Li⁺/LiIn), respectively, slightly wider than those for the powder (2.5 V and 1.5 V), suggesting improved electrochemical stability due to PTFE.
Temperature-dependent ionic conductivity measurements follow the Arrhenius equation:
$$ \sigma = A \exp\left(-\frac{E_a}{k_B T}\right) $$
where A is the pre-exponential factor, E_a is the activation energy (eV), k_B is Boltzmann’s constant, and T is the temperature (K). The activation energy for Li⁺ transport in the membrane is 0.234 eV, similar to that of the powder (0.231 eV), confirming that the ion conduction mechanism is unaltered by the PTFE binder.
To evaluate the practical performance, all-solid-state batteries are assembled by stacking the lithium-indium alloy anode, Li₆PS₅Cl membrane (or powder for comparison), and LCO@LNO composite cathode in a polyether ether ketone (PEEK) mold. The components are pressed at 300 MPa for the electrolyte and 200 MPa for the electrodes, and the cell is maintained under 30 MPa during testing. Galvanostatic charge-discharge tests are conducted between 2.0 V and 3.7 V at various C-rates.
The electrochemical performance of the LCO@LNO composite cathode is summarized in the following tables. Table 1 compares the initial specific capacity, Coulombic efficiency, and cycling stability for powder-based and film-based electrodes in cells with Li₆PS₅Cl powder electrolyte. Table 2 presents the rate capability data, and Table 3 details the performance of the full all-solid-state thin-film battery with the composite membrane.
| Electrode Type | Initial Discharge Capacity (mAh/g) | Initial Coulombic Efficiency (%) | Capacity Retention after 100 Cycles (%) |
|---|---|---|---|
| Powder Cathode | 139.5 | 87.75 | 85.2 |
| Film Cathode | 139.7 | 90.19 | 85.5 |
| C-Rate | Discharge Capacity (mAh/g) |
|---|---|
| 0.1 C | 138.3 |
| 0.2 C | 130.9 |
| 0.5 C | 127.0 |
| 1 C | 119.2 |
| 2 C | 106.6 |
| 5 C | 74.6 |
| 10 C | 30.0 |
| Parameter | Value |
|---|---|
| Initial Discharge Capacity (mAh/g) | 134.1 |
| Initial Coulombic Efficiency (%) | 90.24 |
| Capacity Retention after 100 Cycles (%) | 92.84 |
| Energy Density (Wh/kg) | 188 |
| Areal Capacity (High Load, mAh/cm²) | 2.3 |
| Capacity Retention at High Load after 100 Cycles (%) | 71.8 |
The data indicate that the film-based cathode outperforms the powder-based counterpart in terms of initial Coulombic efficiency and rate capability. For instance, at 0.1 C, the film cathode delivers a discharge capacity of 139.7 mAh/g with 90.19% efficiency, compared to 139.5 mAh/g and 87.75% for the powder. This improvement is attributed to the uniform distribution of active material, electrolyte, and conductive agent in the fibrous PTFE matrix, which facilitates efficient ion and electron transport. The rate performance test shows that the film cathode maintains high capacities even at elevated C-rates, such as 119.2 mAh/g at 1 C and 74.6 mAh/g at 5 C, whereas the powder cathode suffers from significant capacity loss due to poor contact. Long-term cycling at 1 C reveals excellent stability, with the film cathode retaining 92.58% capacity after 500 cycles, far superior to the powder cathode.
In the full all-solid-state thin-film battery, which incorporates the Li₆PS₅Cl membrane and LCO@LNO film cathode, the initial discharge capacity is 134.1 mAh/g at 0.1 C, with a high Coulombic efficiency of 90.24%. The battery exhibits stable cycling over 100 cycles with 92.84% capacity retention, demonstrating the robustness of the dry-processed components. The energy density reaches 188 Wh/kg, a significant enhancement over powder-based cells (48 Wh/kg), highlighting the impact of reducing electrolyte thickness and optimizing electrode architecture. When the cathode loading is increased to 13 mg of LCO@LNO (areal capacity of 2.3 mAh/cm²), the battery still shows reasonable performance, with 71.8% capacity retention after 100 cycles, though some degradation occurs due to increased interfacial resistance.
Furthermore, I explore the feasibility of flexible all-solid-state battery pouches using the same materials. The pouch cells maintain a voltage of 3.566 V after one week of storage, indicating good voltage retention. They function reliably under various conditions, including bending, folding, and cutting, underscoring the mechanical flexibility and safety of the dry-processed membranes. This adaptability is crucial for applications in wearable electronics and other flexible devices.
In conclusion, the room-temperature dry-process fabrication of sulfide solid electrolyte membranes and composite cathodes using pre-fiberized PTFE binder represents a significant advancement for all-solid-state batteries. This method avoids solvent-induced degradation and energy-intensive steps, preserving the high ionic conductivity of Li₆PS₅Cl while enabling the production of ultrathin, flexible, and dense membranes. The resulting all-solid-state batteries exhibit high specific capacity, excellent cycling stability, and enhanced energy density, making them promising for next-generation energy storage. Future work will focus on optimizing the binder content, scaling up the process, and integrating with high-voltage cathode materials to further improve the performance of all-solid-state batteries. The success of this approach underscores the potential of dry-process techniques in accelerating the commercialization of solid-state batteries.
The development of such all-solid-state batteries is pivotal for meeting the escalating demands for safe and high-energy-density power sources. By leveraging the unique properties of sulfide solid electrolytes and innovative fabrication methods, I believe that all-solid-state batteries can overcome current limitations and play a central role in the future of energy storage. The continuous optimization of materials and processes will undoubtedly lead to more efficient and reliable solid-state batteries, driving progress in various technological domains.