Innovative Approaches and Optimization Pathways for Ionic Conductivity Testing of Oxide-Based Solid-state Electrolytes

1. Background

This article outlines IEST’s solutions for performance evaluation of oxide-based solid-state electrolytes, leveraging our technical expertise.

With the rapid development of the new energy vehicle industry, market demands for batteries have escalated, with high energy density and safety emerging as critical priorities. Compared to liquid-state batteries, solid-state batteries not only address safety concerns inherent to lithium-ion batteries but also offer superior energy density and extended cycle life, positioning them as one of the most dynamic research frontiers in secondary battery technologies.

Current lithium-ion batteries are classified based on liquid electrolyte content. Semi-solid-state batteries (5-10 wt% liquid electrolyte) and all-solid-state batteries (0 wt% liquid electrolyte) are collectively termed solid-state batteries. Semi-solid-state batteries reduce liquid electrolyte usage by incorporating composite electrolytes of oxides and polymers. Oxides are primarily applied as coatings on separators or electrode surfaces, while polymers form framework networks. Additionally, anodes have evolved from graphite to pre-lithiated silicon-based or lithium metal anodes, cathodes transition from high-nickel to high-voltage platforms (e.g., high-nickel, lithium-rich manganese-based cathodes), and separators retain solid electrolyte coatings. Lithium salts such as LiPF₆ are being replaced by LiTFSI, achieving energy densities exceeding 350 Wh/kg. However, flammability risks persist in semi-solid-state systems due to residual liquid electrolytes. In contrast, all-solid-state batteries eliminate liquid electrolytes entirely, employing oxide-, sulfide-, halide-, or polymer-based solid electrolytes as thin-film separators. Oxides currently lead in commercialization progress, while sulfides show the greatest future potential. Anodes and cathodes follow similar upgrade paths as semi-solid systems, with energy densities surpassing 500 Wh/kg. As summarized in the 2022 roadmap “Toward Better Batteries: Solid-State Battery Roadmap 2035+“, numerous component combinations (e.g., anode/cathode active materials, anode/cathode electrolytes, and SE separators) have been explored. Recent breakthroughs highlight oxide-based solid-state electrolytes as one of the earliest developed and most studied materials.

Figure 1. Composition of solid-state battery components

Oxide-based solid-state electrolytes can be structurally categorized into sodium superionic conductors (NASICON), garnet-type, perovskite-type, and amorphous phases. Industry reports indicate that NASICON-type LATP (Li₁₃Al₀.₃Ti₁.₇(PO₄)₃) and garnet-type LLZO/LLZTO (Li₇La₃Zr₂O₁₂/Li₆.₄La₃Zr₁.₄Ta₀.₆O₁₂) have entered scaled production for solid-state battery applications. China is actively advancing the development of oxide-based solid electrolytes, with novel NASICON-type materials such as LZSP (LiZr₂(PO₄)₃) demonstrating superior comprehensive properties being recently reported.

Ionic conductivity evaluation remains a critical focus for researchers. Oxide samples typically undergo high-temperature crystallization (>900°C) to ensure optimal ion transport performance, requiring a densification rate >95% and thickness >500 μm. Post-sintering processes include polishing, sputtering of Au/Pt/Ag/C layers (>5 μm) to ensure reliable electrical contact, and edge insulation to prevent short circuits. Ionic conductivity is assessed using high-frequency impedance analyzers (>10 MHz) integrated with stable pressure-application systems. To address these requirements, IEST has developed the SEMS1100 series solid electrolyte testing system, which enables stable pellet preparation and precise ionic conductivity evaluation under varying pressures. The system concurrently assesses electronic conductivity, compaction density, interfacial stability, and prototype cell performance. Figure 2 illustrates the IEST SEMS1100 system and its application scenarios.

Figure 2. SEMS1100 solid electrolyte ionic conductivity test system and application scenarios

With the advancement of the industry, the traditional labor-intensive sintering and pellet fabrication processes, coupled with metallization (e.g., Au/Pt sputtering), no longer meet the growing demand for rapid ionic conductivity evaluation of oxide-based solid-state electrolytes. According to industry feedback, researchers and engineers increasingly seek to assess ionic conductivity directly on powder-compacted pellets. To address this, a proposed method involves pre-mixing solid electrolytes with binders prior to pressing to enhance densification. However, this approach typically requires a high-temperature binder removal process during post-testing. Internal validation studies indicate that while this method works for certain samples, the introduction of binders may introduce artifacts into test results.

The industry consensus emphasizes combining direct powder compression with high-temperature-controlled platforms. IEST Instrument in close collaboration with Xiamen University’s School of Materials Science, has leveraged iterative testing across diverse sample categories to develop a high-temperature hot-pressing testing platform for oxide-based solid-state electrolyte powders. Compared to conventional sintering, this platform enables rapid ionic conductivity assessment by applying synchronized high pressure (300–500°C) to achieve fast densification and reliable evaluation. Figure 3 illustrates the hot-pressing system schematic and test results of oxide-based solid-state electrolytes measured via direct hot-pressing.

To mitigate thermal interference with testing instruments, the hot-pressing and evaluation modules currently operate independently. Post-fabrication, hot-pressed pellets are coated with silver paste for electrochemical impedance spectroscopy (EIS) analysis. IEST welcomes collaborative discussions with researchers and engineers to explore customized solutions and project partnerships, further advancing standardized testing methodologies in solid-state electrolyte characterization.

Figure 3. Oxide solid electrolyte hot pressing system and test

3. Summary