Solid State Electrolyte: Ionic Conductivity Measurement and Pressure Effects

1. Background: Solid State Electrolytes and Their Ionic Conductivity

Solid state electrolytes are materials capable of ion conduction in the solid phase, playing a central role in all-solid-state lithium-ion battery performance. By replacing conventional PP/PE liquid-electrolyte separators with solid materials, solid-state batteries significantly reduce the risks of lithium dendrite penetration, electrolyte leakage, and thermal runaway — the dominant safety failure modes in conventional liquid-electrolyte cells.

Solid state electrolytes are classified into four main categories by chemistry: sulfides (highest conductivity, best processability, moisture-sensitive — dry room dew point below −60°C required), oxides (hard and brittle, typically require sintering or combination with liquid electrolytes as semi-solid configurations), polymers (lower conductivity and electrochemical window, limited rate capability), and halides (excellent conductivity and high-voltage stability, currently in research stage).^[1]

In the development path of solid state electrolytes, Japan and South Korea mainly pursue the sulfide route. Particularly in Japan, extensive national efforts and years of deep research have positioned the country at the forefront of solid-state battery technology worldwide. Meanwhile, domestic enterprises in China primarily focus on the oxide route, adopting a semi-solid-state technical approach for faster industrialization. However, influenced by the development trends in Japan and South Korea and guided by domestic policies, an increasing number of Chinese companies are now joining the competition in the sulfide all-solid-state battery route. Key players include leading lithium-ion battery companies such as CATL and BYD, established battery firms like EVE Energy and Fengli, as well as startups such as JW Energy, CBAK Energy, Gotion High-Tech, Beijing National Battery Technology, Zhongke Gu’neng, and Zhongke Shenlan Huize, among others.

Figure 1. Structural comparison of liquid-state (left) and solid-state (right) batteries: solid electrolyte replaces both the liquid electrolyte and PP/PE separator, eliminating the flammable liquid phase and dendrite-penetration risk.^[1]

Pressure significantly influences solid electrolyte ionic conductivity. Under applied pressure, solid-solid interfacial contact between electrolyte particles improves, grain boundary resistance decreases, and effective ionic conductivity increases — effects that are particularly pronounced for sulfide electrolytes but much more limited for oxide electrolytes due to their higher intrinsic hardness. The IEST SEMS1100 Solid state Electrolyte Measurement System was developed specifically to quantify this pressure-dependent ionic conductivity behavior. The SEMS1100 system measures solid electrolyte powder ionic conductivity at controlled pressures of 50–350 MPa via EIS (0.1 Hz–1 MHz) in a hermetically sealed ceramic mold, with real-time thickness monitoring for simultaneous compaction density calculation — providing the measurement foundation for solid electrolyte material screening, pressure optimization, and all-solid-state battery stack design.

2. Experimental Section

2.1 Test Equipment

Ionic conductivity measurement system: IEST SEMS1100 Solid Electrolyte Measurement System paired with Donghua DH7001 electrochemical workstation. The SEMS1100 upgrades traditional powder resistivity and compaction density measurement with specially designed hermetic ceramic-sealed molds and temperature control modules — enabling EIS measurement of solid electrolyte powder ionic conductivity under defined pressure and temperature conditions.

Figure 2. IEST SEMS1100 Solid Electrolyte Measurement System — physical appearance and working principle. Ceramic-sealed mold enables EIS ionic conductivity measurement under 50–350 MPa without electrolyte exposure to ambient air.

2.2 Test Conditions

• Pressure range: 50–350 MPa (servo-controlled), pressure holding time 180 min per step
• EIS parameters: frequency range 0.1 Hz – 1 MHz, perturbation amplitude 10 mV
• Sample 1: Sulfide solid electrolyte LPSC (Li₆PS₅Cl), 0.15 g, φ13 mm ceramic-sealed mold
• Sample 2: Oxide solid electrolyte LLZO powder (Li₇La₃Zr₂O₁₂), 0.1 g, φ13 mm ceramic-sealed mold

3. Test Results

3.1 LPSC Sulfide Solid Electrolyte: Ionic Conductivity Under Pressure

As we all know, sulfide LPSC is a solid state electrolyte that currently rivals organic solvent electrolytes in terms of conductivity. The ion conductivity of LPSC is influenced by factors such as raw material particle size, sintering temperature, and sintering time during the preparation process ^[3]. Additionally, during testing, environmental temperature and pressure during powder compression also have a significant impact on the test results of solid state electrolytes ^[4].

Based on this, IEST used the SEMS1100 to test the ion conductivity of LPSC under different pressures. IEST SEMS1100 measurement confirms: at pressures exceeding 300 MPa, LPSC powder achieves an ionic conductivity of 0.9 mS/cm — sufficient for all-solid-state battery operation at moderate C-rates — even with relatively small particle sizes (customer-reported D50 ≈ 1 µm).

From the EIS spectra of LPSC powder under pressure, it can be observed that as the pressure increases, the EIS gradually decreases. The reduction in powder EIS indicates an enhancement in ion conductivity. The main reason for this phenomenon is that with increasing pressure, the compaction density of the solid state electrolyte increases, leading to a corresponding decrease in the porosity of the solid electrolyte pellets. This results in a tighter contact between particles, reducing the ion transport resistance at grain boundaries, thereby enhancing the ion transport capability and causing a decrease in EIS ^[5,6].

Figure 3. EIS spectra (left) and ionic conductivity vs. pressure (right) of LPSC sulfide solid electrolyte powder. Impedance decreases systematically with pressure as grain boundary resistance reduces. Peak conductivity: 0.9 mS/cm at >300 MPa.

3.2 LLZO Oxide Solid Electrolyte: Ionic Conductivity Under Pressure

LLZO, as a garnet-type structure oxide solid electrolyte, typically exhibits a conductivity of 0.1 to 0.9 mS/cm after sintering, which is lower than that of organic solvent electrolytes. For LLZO powder itself, the diffusion rate of lithium ions at grain boundaries is much lower than that within the crystal bulk phase, and the number of grain boundaries and particle size have a significant impact on ion conductivity ^[7].

IEST conducted tests on the ion conductivity of LLZO powder under different pressures using the SEMS1100. From the EIS spectra of LLZO powder under pressure, it can be observed that at low pressures, the EIS curve of LLZO powder appears more chaotic, especially in the intermediate frequency region, where the EIS curve is disorderly. As the pressure increases, the EIS of LLZO powder decreases significantly. Although the EIS curve remains somewhat chaotic in the intermediate frequency region, it is generally complete. The main reason for this is that with increasing pressure, the compaction density of the solid state electrolyte improves, leading to better contact between particles and reduced ion transport resistance at grain boundaries.

From the figure and the table data, it can be observed that although the ion conductivity of LLZO increases somewhat under higher pressure, it remains relatively low, reaching only 2.14×10⁻⁵ mS/cm, far below the actual requirements for battery usage. Therefore, in practical applications, LLZO solid electrolytes are mainly used in the form of powder coating, applied onto separators or the surfaces of positive and negative electrodes. They often need to be combined with electrolytes or polymers to achieve the required performance for lithium-ion batteries and meet the practical demands of battery usage.

Figure 4. EIS spectra (left) and ionic conductivity vs. pressure (right) of LLZO oxide solid electrolyte powder.

4. Discussion: Why Solid Electrolyte Type Determines Pressure-Conductivity Behavior

The contrast between LPSC and LLZO pressure-conductivity responses reflects a fundamental materials difference: mechanical compliance. Sulfide electrolytes are soft and ductile — under pressure, particles deform plastically to form intimate grain boundaries with low ionic resistance. Oxide ceramics are hard and brittle — particles do not deform under pressure, leaving residual voids and point contacts at grain boundaries that maintain high resistance regardless of applied load.

• Sulfide LPSC: 0.9 mS/cm at >300 MPa — achieves liquid-electrolyte-comparable conductivity without sintering, suitable as pressed powder in all-solid-state battery stack assemblies.
• Oxide LLZO powder: 2.14×10⁻⁵ mS/cm even at maximum pressure — requires sintering into dense pellets (reaching 0.1–0.9 mS/cm) or coating onto electrodes/separators combined with liquid or polymer electrolytes for practical use.
• Polymer electrolytes (PEO-LiTFSI and similar systems) typically reach 0.01–0.1 mS/cm only at elevated temperatures (60–80°C), limiting their rate capability and complicating room-temperature operation. Conductivity measurement of solid polymer electrolytes by EIS follows the same symmetric cell principle but requires temperature-controlled fixtures.

5. Conclusion

Pressure-dependent ionic conductivity measurement using SEMS1100 and EIS reveals a decisive performance gap between sulfide and oxide solid electrolyte powders: LPSC sulfide reaches 0.9 mS/cm at >300 MPa — comparable to liquid electrolytes — while LLZO oxide powder reaches only 2.14×10⁻⁵ mS/cm under the same conditions, reflecting the fundamental contrast between the ductile, pressure-responsive microstructure of sulfides and the hard, grain-boundary-dominated behavior of oxides.

Key quantitative results: LPSC sulfide solid electrolyte ionic conductivity under >300 MPa = 0.9 mS/cm (practical for all-solid-state battery assembly), with relatively small particle sizes (according to customer feedback, D50 is 1 μm). LLZO oxide solid electrolytes, due to their lower intrinsic ion conductivity and higher hardness, exhibit poorer contact between particles and grain boundaries, particularly evident under low pressure conditions, as reflected in chaotic EIS spectra. LLZO oxide solid electrolyte powder ionic conductivity under maximum pressure = 2.14×10⁻⁵ mS/cm (insufficient for standalone use; requires sintering or polymer/liquid electrolyte combination), far below the actual requirements for battery usage ^[7,8].

In summary, oxide solid state electrolytes typically exhibit low ion conductivity in practical applications and often require combination with polymers or liquid electrolytes. In contrast, sulfide solid state electrolytes can achieve higher ion conductivity after compression and do not require sintering into pellets or combination with polymers or liquid electrolytes. Therefore, sulfide all-solid-state batteries represent the most competitive route. However, challenges such as poor air stability, weak interface stability at the negative electrode, poor solvent compatibility, and reduced ion conductivity after nanostructuring still exist ^[9,10]. These issues require further efforts from industry professionals to resolve. We firmly believe that, with the relentless efforts of numerous researchers, the various challenges of solid-state batteries will gradually be overcome and eventually become part of our daily lives.

6. References

[1] New Material Series Report (I): Solid State Battery Potential Verified, Focus on New Demand for Power Battery Metals, Guotou Securities, January 24, 2024;

[2] Zhang Fangnan, Hive Energy Solid State Battery Focuses on Sulfide Route, Power Battery Sub-Forum of China Electric Vehicle Hundred, March 17, 2024

[3] Jianming Tao.，Unraveling the performance decay of micro-sized silicon anodes in sulfide-based solid-state batteries, Energy Storage Materials，2024

[4] Chanhee Lee, Stack Pressure Measurements to Probe the Evolution of the Lithium-Solid-State Electrolyte Interface, ACS Energy Letters. 2021,

[5] J. Gu, Z. Liang, J. Shi, Y. Yang, Electrochemo-Mechanical Stresses and Their Measurements in Sulfide-Based All‐Solid‐State Batteries: A Review. Advanced Energy Materials, 2022

[6] A. Hayash, N. Masuzawa1, S. Yubuchi, A sodium-ion sulfide solid electrolyte with unprecedented conductivity at room temperature, Nature Communications，2019

[7] How to Measure a Reliable Ionic Conductivity? The Stack Pressure Dilemma of Microcrystalline Sulfide-Based Solid Electrolytes;

[8] Qi,Liu;etc. Challenges and perspectives of garnet solid electrolytes for all solid-state lithium batteries. Journal of Power Sources,2018.

[9] Oh B. Chae, Brett L. Lucht etc. Interfacial Issues and Modification of Solid Electrolyte Interphase for Li Metal Anode in Liquid and Solid Electrolytes, Advanced Energy Meterials,2023,

[10] Gabin Yoon, Sewon Kim, Ju-Sik Kim, Design Strategies for Anodes and Interfaces Toward Practical Solid-State Li-Metal Batteries, Advanced Science

7. FAQ: Solid State Electrolyte Ionic Conductivity