Determination of Pyrolysis Product Release from Solid-State Electrolytes

Determination of pyrolysis product release from solid-state electrolytes is a critical safety and characterization test that quantifies the gases evolved when sulfide, oxide, polymer, and halide electrolytes decompose under controlled thermal stress. This article explains the analytical techniques involved, such as TGA-MS, TG-FTIR, and Py-GC/MS, the applicable standards framework, and how accredited third-party laboratories support solid-state battery development, hazard assessment, and regulatory compliance.

Technical Overview

Solid-state electrolytes (SSEs), encompassing sulfide, oxide, polymer, and halide systems, are central to the development of next-generation lithium batteries with higher energy density and improved intrinsic safety. Although SSEs eliminate the flammable liquid electrolyte of conventional lithium-ion cells, they are not chemically inert under thermal stress. When heated beyond their stability window, or during thermal runaway triggered by internal short circuit or external abuse, they decompose and release gaseous products: sulfide electrolytes can emit highly toxic hydrogen sulfide (H2S) together with SO2, COS, and CS2; polymer electrolytes generate H2, CH4, C2H4, and other hydrocarbon fragments; and fluorinated components such as PVdF binders or residual LiPF6 can yield HF, POF3, and PF5.

Determination of pyrolysis product release quantity is therefore a key safety characterization test. A specimen is heated under a controlled temperature program in a defined atmosphere while the evolved gases are continuously or discretely analyzed. The resulting data establish the decomposition onset temperature, gas composition, evolution rate, and cumulative release amount of each species. These quantitative results underpin material screening, hazard classification, thermal runaway modeling, cell and pack design, and compliance demonstration under battery safety frameworks such as IEC 62660-3, GB 38031, and UL 9540A.

Testing Samples

Sulfide-Based Solid Electrolytes

Argyrodite-type Li6PS5Cl, glass-ceramic xLi2S–(1–x)P2S5 systems, and thio-LISICON compounds such as Li10GeP2S12. These materials are moisture sensitive and can liberate H2S upon contact with humidity even below their decomposition temperature, so samples are handled in inert glovebox atmospheres with oxygen and water content typically below 1 ppm and transferred in hermetically sealed containers.

Oxide-Based Solid Electrolytes

Garnet-type Li7La3Zr2O12 (LLZO), NASICON-type LATP and LAGP, and perovskite LLTO ceramics, supplied as powders, sintered pellets, or thin films. Although intrinsically more stable, oxide electrolytes with residual lithium carbonate, moisture, or binder phases can release CO2, H2O, and fluorinated species during heating.

Polymer and Composite Electrolytes

PEO–LiTFSI systems, PVdF-HFP based membranes, in-situ polymerized gel-ceramic hybrids, and composite polymer electrolytes containing ceramic fillers. These samples decompose through chain scission and depolymerization, releasing hydrogen, light hydrocarbons, carbon oxides, and sulfur-containing fragments from lithium salts.

Halide-Based Solid Electrolytes

Emerging chloride and bromide systems such as Li3InCl6, Li3YCl6, and Li3YBr6, which are hygroscopic and may evolve HCl- or HBr-containing species under combined moisture and thermal stress.

Cell-Level and Composite Samples

Electrolyte–electrode composites, separator membranes, cathode-supported bilayers, and small-format solid-state cells or cell fragments, tested to capture interface reactions and realistic off-gassing behavior under abuse conditions.

Testing Items

Typical quantified parameters include:

• Thermal decomposition onset temperature and mass-loss profile (thermogravimetric curve and derivative)
• Qualitative and quantitative identification of evolved gases, including H2S, SO2, COS, CS2, CO, CO2, H2, CH4, C2H4, C2H6, HF, POF3, PF5, H2O, O2, and volatile organic compounds
• Cumulative release amount of each species, reported in µmol/g, mg/g, or weight percent of the original sample
• Gas evolution rate as a function of temperature and time, including peak-release temperatures
• Total gas volume and total mass loss over the full temperature program
• Toxicity-weighted evaluation of hazardous species, particularly H2S and HF, referenced against occupational exposure limits
• Comparison of behavior under inert, dry air, and humidity-containing atmospheres

Testing Methods

Thermogravimetric Analysis Coupled with Mass Spectrometry (TGA-MS)

A specimen of typically 10–20 mg is heated at 5–10 °C/min from ambient temperature to 800–1000 °C under nitrogen or argon purge, following the general principles of ISO 11358 for thermogravimetry. A quadrupole mass spectrometer monitors characteristic ion signals in real time, such as m/z 34 for H2S, m/z 64 for SO2, m/z 60 for COS, and m/z 44 for CO2. Calibration with certified standard gases converts ion currents into quantitative concentration and release profiles.

Thermogravimetry Coupled with Fourier-Transform Infrared Spectroscopy (TG-FTIR)

Evolved gases pass through a heated transfer line and gas cell, where infrared spectra are collected continuously. TG-FTIR provides real-time identification of IR-active species and enables quantitative analysis of gases such as CO2, CO, SO2, COS, and HF through chemometric calibration against reference spectra.

Pyrolysis–Gas Chromatography–Mass Spectrometry (Py-GC/MS)

Flash or stepwise pyrolysis at defined temperatures (for example, 300, 500, and 800 °C) is followed by chromatographic separation and mass-spectral identification of individual organic and inorganic products. Py-GC/MS delivers the most complete compositional picture of complex decomposition mixtures and supports semi-quantitative release estimation.

Sealed-Vessel Gas Evolution and Pressure Monitoring

Larger samples, from hundreds of milligrams to several grams, are heated in sealed ampoules or small autoclaves equipped with pressure transducers. The accumulated gas is sampled and analyzed by micro-GC or GC configured with TCD for permanent gases, FID for hydrocarbons, and FPD or SCD detectors for sulfur compounds. For acid gas quantification, evolved streams are passed through absorption traps followed by ion chromatography or spectrophotometric determination.

Thermal Runaway Simulation and Vent Gas Analysis

Adiabatic calorimetry (ARC) or accelerated heating to runaway is combined with gas collection bags, and the vent gas is characterized using the analytical train aligned with the vent gas analysis methodology of UL 9540A. This approach provides release data representative of genuine abuse conditions and supports thermal runaway evaluation concepts referenced in IEC 62660-3 and GB 38031. Because dedicated international standards for solid electrolyte gas release are still maturing, accredited third-party laboratories apply these established evolved-gas-analysis and battery safety frameworks in adapted, documented test protocols.

Applications

Quantitative pyrolysis release data serve multiple purposes across the solid-state battery value chain:

• Research and development screening to down-select electrolyte chemistries with the lowest gas generation and highest thermal stability
• Safety and hazard assessment, including H2S exposure scenarios, ventilation design, and gas sensor placement in battery packs and manufacturing facilities
• Input data for thermal runaway propagation models, including gas generation terms used in thermodynamic and CFD simulations
• Regulatory and certification support for electric vehicle and stationary storage applications under GB 38031, GB/T 36276, IEC 62660-3, IEC 62619, and UL 9540A
• Manufacturing quality control, including batch consistency verification and detection of residual solvents, binders, or salts that off-gas during heating
• Comparative benchmarking of sulfide, oxide, polymer, and halide electrolytes for technical reports, academic publications, and intellectual property filings
• Interface compatibility studies between electrolytes and cathode materials under elevated temperature

FAQ

Why is hydrogen sulfide release from sulfide electrolytes such a major concern?

H2S is acutely toxic, with a distinctive rotten-egg odor at very low concentrations and rapid olfactory fatigue at higher concentrations. Sulfide electrolytes such as Li6PS5Cl and Li2S–P2S5 glasses can generate H2S through hydrolysis with ambient moisture and through thermal decomposition during abuse events. Quantifying the maximum possible release per gram of material is essential for exposure modeling, facility design, and emergency planning.

What sample sizes and forms can be tested?

TGA-MS and TG-FTIR typically require 10–20 mg of powder, pellet fragment, or membrane. Sealed-vessel and vent gas methods accommodate hundreds of milligrams to several grams, including composite electrodes and small cells. Reactive sulfide samples are prepared and loaded under inert atmosphere to prevent pre-test degradation.

What temperature range and heating programs are used?

Standard screening covers ambient to 800–1000 °C at 2–10 °C/min under inert purge, with optional isothermal holds at critical temperatures. Faster ramps, dry air atmospheres, humidity-controlled environments, and adiabatic modes are available to approximate short-circuit or thermal runaway conditions more closely.

How are the results reported?

Deliverables typically include thermogravimetric and heat-flow curves, temperature- and time-resolved concentration profiles for each detected species, cumulative release amounts in µmol/g or weight percent, peak-release temperatures, total gas volume, and an interpretation section comparing results with relevant exposure and safety criteria.

Can this testing fully simulate a real thermal runaway event?

Adiabatic calorimetry combined with vent gas collection provides the closest laboratory representation of genuine runaway chemistry. Linear-ramp screening tests are comparative in nature but remain highly valuable for ranking materials and quantifying intrinsic release potential under well-defined conditions.

Which standards govern this testing?

No single international standard yet addresses solid electrolyte gas release directly. Laboratories therefore apply thermogravimetry principles from ISO 11358, calorimetric practice from ISO 11357 and ASTM E1131, compositional TG analysis from ASTM E1131, and battery safety frameworks including IEC 62660-3, IEC 62619, GB 38031, GB/T 36276, and UL 9540A within validated, documented protocols.