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

1. The Overlooked Electronic Conductivity of Solid Electrolytes

The all-solid-state battery (ASSB) research community has invested enormous effort in maximizing the ionic conductivity of solid electrolytes, driving values from 10⁻⁴ to 10⁻² S/cm — competitive with liquid electrolytes. Yet the electronic conductivity of these same materials has been systematically neglected, operating under the tacit assumption that “electronic conductivity of solid electrolytes can be ignored.” This study demonstrates that assumption is fundamentally flawed.

Figure 1. Electronic conductivity of sulfide oxide halide solid electrolytes comparison. Inorganic solid electrolytes (sulfide, oxide, halide) show ionic conductivity of 10⁻⁴–10⁻² S/cm but electronic conductivity of 10⁻⁸–10⁻⁹ S/cm. Commercial polymer separators (10⁻¹⁵–10⁻¹⁸ S/cm) and LiPON thin-film electrolyte (10⁻¹⁴ S/cm) are 6–7 orders of magnitude lower.

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. The Thickness Effect: Why Thin Electrolytes Are Catastrophic

Laboratory-scale ASSBs typically use thick pelletized electrolytes (~1000 μm) for ease of handling. Practical high-energy-density cells, however, require thin-film electrolytes of 20 μm or less to minimize the electrolyte’s volumetric and mass contribution. This study demonstrates that this inevitable downscaling catastrophically amplifies the self-discharge problem.

Figure 2. Electrochemical modeling of the electrolyte thickness effect on all-solid-state battery self-discharge. Thick (1000 μm) vs thin (20 μm) electrolyte configurations and their simulated self-discharge behavior as a function of electronic conductivity.

The physics is intuitive: physical self-discharge current (I_leak) is inversely proportional to electrolyte thickness. Thinning the electrolyte from 1000 μm to 20 μm — a 50× reduction — increases the leakage current by 50× for the same electronic conductivity. This is why the self-discharge penalty becomes catastrophic precisely at the thickness scales required for commercial viability.

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

Figure 3. Experimental validation using Li₃InCl₆@LiCoO₂/Li₆PS₅Cl/In-Li all-solid-state batteries. (a–e) 1000 μm: voltage stable at 4.2V, 3.9% loss (0.4% physical + 3.5% chemical). (f–j) 20 μm: voltage drops to 3.8V, 94.5% loss with 83.2% recoverable — proving physical self-discharge dominance.

The research team fabricated Li₃InCl₆@LiCoO₂/Li₆PS₅Cl/In-Li all-solid-state batteries using a solvent-free dry-film process, testing both 1000 μm and 20 μm electrolyte thicknesses. The experimental results matched the simulations with striking precision:

• 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 decayed continuously from 4.2V to 3.8V over the same period, with a staggering 94.5% capacity loss. Crucially, 83.2% of this loss was recoverable upon recharging — direct evidence that the dominant degradation mechanism was physical self-discharge (reversible electron leakage), not chemical self-discharge (irreversible side reactions).
• Universal phenomenon: The physical self-discharge was also observed in Li₃YBr₆ (halide) and Li₇P₃S₄ (sulfide) electrolyte systems, confirming that the problem is intrinsic to the entire inorganic solid electrolyte family, not a material-specific defect.

A secondary but important finding: the thin electrolyte system showed higher chemical self-discharge (11.3% vs. 3.5% for thick) — indicating that the increased electronic leakage current also accelerates interfacial side reactions, compounding the degradation through a self-reinforcing loop.

4. Multi-Scale Mitigation Strategies

Figure 4. Multi-scale framework for suppressing physical self-discharge, spanning atomic-scale bandgap engineering, micro-scale composite design, and cell-level interface engineering to block electronic leakage in solid electrolytes.

The most practically actionable finding is that interface engineering works even when bulk electronic conductivity cannot be reduced. Simulation confirmed that a 0.1 μm electron-blocking interface layer (σe < 10⁻¹² S/cm) inserted between the electrolyte and electrode sustains >93% capacity retention in 20 μm systems. This decouples the self-discharge problem from the fundamental limits of current solid electrolyte materials.

5. IEST Solution: Precise Measurement Is the First Step

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

This Nature Energy study identifies a clear quantitative target — solid electrolyte electronic conductivity must be reduced from 10⁻⁸ to 10⁻¹² S/cm to solve physical self-discharge. The first and most fundamental step toward this goal is precise, reliable electronic conductivity measurement. This is precisely the capability gap that IEST Instrument’s Multi-Dimensional Solid Electrolyte Testing System (SEMS series) is engineered to close.

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

• Electronic conductivity is not negligible: Mainstream inorganic solid electrolytes show σe of 10⁻⁸–10⁻⁹ S/cm — 6–7 orders higher than commercial polymer separators — forming a significant electronic leakage channel.
• Thickness amplifies the problem: The electrolyte thickness effect on all-solid-state battery self-discharge is severe: 1000 μm → 20 μm raises 1-month capacity loss from 3.9% to 94.5%.
• Clear quantitative target: Matching commercial Li-ion self-discharge (<2.5%/month) requires σe ≤ 10⁻¹² S/cm — a target no current inorganic solid electrolyte meets.
• Multi-scale solutions exist: Bandgap engineering, grain-boundary control, composite electrolytes, and — most practically — interface engineering to block electronic leakage (0.1 μm layer, σe <10⁻¹², >93% retention).
• Measurement is the enabling capability: Precise electronic conductivity measurement — the core capability of IEST SEMS — is the essential first step toward achieving the 10⁻¹² S/cm target.

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