Nature Energy: Xueliang Sun’s Team Reveals Solid Electrolyte Electronic Conductivity as the Root Cause of Self-Discharge in ASSBs

1. Electronic and Ionic Conductivity of Solid Electrolytes

Research on solid electrolytes has focused primarily on increasing ionic conductivity, with reported values for widely studied inorganic electrolytes ranging from approximately 10⁻⁴ to 10⁻² S cm⁻¹. Their electronic conductivity is much lower, typically around 10⁻⁹ to 10⁻⁸ S cm⁻¹, but it is not necessarily negligible in thin-electrolyte cells. Electronic leakage through the solid electrolyte can allow electrons to move internally from the lithiated anode toward the cathode. This process consumes stored charge while the cell is at open circuit and is referred to as physical self-discharge. Unlike chemical self-discharge caused by irreversible parasitic reactions, much of the capacity lost through physical self-discharge may be recovered during subsequent charging.

Figure 1. Comparative analysis of ionic and electronic conductivities of sulfide, oxide, and halide solid electrolytes, commercial polymer separators, and LiPON thin-film electrolytes.

The consequence of this electronic leakage is illustrated schematically in Figure 1: electrons spontaneously migrate from the (lithiated) anode through the solid electrolyte to the cathode, forming an internal short-circuit-like loop that continuously drains stored energy even when the battery is idle. This is the mechanism of physical self-discharge — fundamentally distinct from the chemical self-discharge caused by interfacial side reactions.

2. Effect of Electrolyte Thickness on Physical Self-Discharge

Laboratory-scale ASSBs typically use pelletized solid electrolytes approximately 1000 μm thick. Practical cells require much thinner electrolyte layers to reduce inactive mass and volume. Therefore, this study compared a 1000 μm electrolyte configuration with a 20 μm configuration to quantify how thickness and electronic conductivity affect physical self-discharge.

Figure 2. Simulation of the physical self-discharge behavior of ASSBs as a function of solid-electrolyte electronic conductivity and thickness for 1000 and 20 μm electrolyte configurations.

For a fixed cell voltage, electrode area, and electronic conductivity, the electronic leakage current scales inversely with electrolyte thickness. Reducing the thickness from 1000 to 20 μm therefore increases the calculated leakage current by a factor of 50 under otherwise identical conditions. The model predicts that electronic leakage becomes increasingly important as the electrolyte approaches thicknesses relevant to practical cells.

3. Experimental Validation: From 3.9% to 94.5% Capacity Loss

Figure 3. Self-discharge behavior of Li₃InCl₆@LiCoO₂/Li₆PS₅Cl/In-Li all-solid-state cells with electrolyte thicknesses of 1000 and 20 μm.

The researchers prepared Li₃InCl₆@LiCoO₂/Li₆PS₅Cl/In-Li all-solid-state cells using a solvent-free dry-film process and evaluated electrolyte thicknesses of 1000 μm and 20 μm. The measurements were consistent with the thickness-dependent trend predicted by the model:

• 1000 μm system: After 1 month of open-circuit storage, voltage remained stable at 4.2V. Total capacity loss was only 3.9%, decomposed into 0.4% physical self-discharge and 3.5% chemical self-discharge — performance comparable to commercial lithium-ion batteries.
• 20 μm system: The voltage decreased from 4.2 V to 3.8 V over the same period, and the capacity loss reached 94.5%. Subsequent charging recovered 83.2% of the lost capacity, indicating that the physical self-discharge accounted for most of the measured loss.
• ysical self-discharge was also observed in cells using Li₃YBr₆ and the sulfide electrolyte reported in this study. This result suggests that electronic leakage is relevant to more than one inorganic solid-electrolyte chemistry.

The 20 μm cell also exhibited greater chemical self-discharge than the 1000 μm cell, at 11.3% versus 3.5%. This association suggests that greater electronic leakage may promote interfacial side reactions.

4. Multi-Scale Mitigation Strategies

Figure 4. Atomic-, micro-, and cell-scale strategies for reducing electronic leakage and physical self-discharge.

The model also evaluated an electron-blocking interlayer between the electrode and solid electrolyte. For a 20 μm electrolyte, a 0.1 μm interlayer with an electronic conductivity below 10⁻¹² S cm⁻¹ maintained 93% capacity after one month in the simulation. This result indicates that interface design may reduce physical self-discharge even when the electronic conductivity of the bulk electrolyte cannot be lowered sufficiently.

5. IEST SEMS Series for Solid-Electrolyte Characterization

Measuring Solid Electrolyte Electronic Conductivity — The Foundation for Self-Discharge Control

The study shows that electronic conductivity must be measured at very low levels when evaluating solid electrolytes for thin-electrolyte ASSBs. The IEST SEMS Series Multi-Dimensional Solid Electrolyte Testing System integrates controlled pressurization, pressure monitoring, thickness measurement, pellet preparation, and electrochemical characterization. These functions allow researchers to evaluate how pressure, thickness, and material compaction affect measured ionic and electronic conductivity.

Figure 5. IEST SEMS Multi-Dimensional Solid Electrolyte Test System — integrating in-situ pressure control, real-time thickness monitoring, and electronic conductivity measurement.

The SEMS system directly addresses the three measurement challenges highlighted by this research:

• Wide-range electronic conductivity measurement: Supports both solid electrolyte electronic conductivity measurement by DC polarization (via electrochemical workstation) and high-precision DC instrumentation, covering 10⁻⁶ to 10⁻¹² S/cm — the full range required to track progress toward the 10⁻¹² S/cm target identified in the study.
• In-situ pressure-thickness-conductivity correlation: The system integrates in-situ pressure regulation with real-time thickness monitoring, enabling researchers to precisely quantify the in-situ pressure-thickness-conductivity correlation for solid electrolytes. Since this study shows that the 1000 μm → 20 μm transition fundamentally changes self-discharge behavior, SEMS is among the few commercial instruments capable of systematically studying this thickness-dependent transition under controlled conditions.
• Multi-dimensional one-stop evaluation: Extending beyond single-property measurement to synchronized characterization of ionic conductivity, electronic conductivity, mechanical response, and thickness evolution — plus integrated automatic pressure-locking for mold-cell assembly — covering the complete workflow from material screening to cell-level design.

6. Conclusions

This study identifies electronic conduction through inorganic solid electrolytes as an important source of physical self-discharge in ASSBs, particularly when the electrolyte is only tens of micrometres thick. In the reported cell tests, reducing electrolyte thickness from 1000 to 20 μm increased the one-month capacity loss from 3.9% to 94.5%. The recovery of much of this lost capacity during subsequent charging supports the contribution of reversible electronic leakage.

The analysis indicates that electronic conductivity should be considered alongside ionic conductivity, electrolyte thickness, and interfacial design when evaluating solid electrolytes. Reducing bulk electronic conductivity remains the primary materials objective, while grain-boundary control, composite electrolytes, and electron-blocking interlayers may provide complementary routes to limiting physical self-discharge.

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