Advanced Energy Materials 2024: IEST In‑Situ Gas Evolution Solution Quantifies Thermal Stability of High‑Safety Solid Polymer Electrolytes

1. Preface

In 2024, a research team led by Professors Dong-Liang Peng and Qingshui Xie at Xiamen University, in collaboration with Professor Jun Lu’s group from Zhejiang University, published a study in Advanced Energy Materials titled “Deep Eutectic Solvent-Based Solid Polymer Electrolytes for High-Voltage and High-Safety Lithium Metal Batteries.” This study addressed the critical challenges facing high-voltage lithium metal batteries, including electrolyte flammability, unstable electrode/electrolyte interfaces, and limited ion transport, by developing a solid polymer electrolyte system based on deep eutectic solvents (DES). By encapsulating a succinonitrile-based DES in situ within an ETPTA polymer network, the researchers constructed a DES-ETPTA solid polymer electrolyte that combines high ionic conductivity, stable interfacial compatibility with lithium metal, and excellent flame retardancy. Cyanoacrylate copolymerization was also introduced to enhance the electrolyte’s oxidative stability in high-voltage LiCoO₂ cathode systems.

Notably, during the thermal-safety evaluation, the research team employed the IEST GVM2200 in-situ gas and volume monitor to continuously monitor pouch cell volume changes under high-temperature conditions. The resulting volume-time data provided quantitative evidence for evaluating thermally induced gassing and swelling in cells containing the DES-ETPTA electrolyte relative to cells containing a conventional liquid electrolyte.

2. Research Background: Why In-Situ Gas Evolution Monitoring Matters for Solid Polymer Electrolytes

Lithium metal anode has an exceptionally high theoretical specific capacity of approximately 3,860 mAh g⁻¹ and a highly negative electrochemical potential of −3.04 V vs. the standard hydrogen electrode (SHE), making it a leading candidate for next-generation high-energy-density batteries. However, conventional liquid electrolytes can present safety concerns because of their flammability and susceptibility to decomposition at high temperatures. Gas evolution during thermal abuse or extended cycling can cause pouch cell swelling and internal pressure build-up, potentially increasing safety risks.

Solid polymer electrolytes (SPEs) represent a promising pathway to address these safety concerns by reducing the use of flammable liquid electrolytes and providing mechanically and thermally stable electrolyte structures. However, these properties depend on the electrolyte composition and polymer architecture. Determining whether and to what degree a specific SPE reduces thermally induced gas evolution requires more than flammability tests or post-mortem cell disassembly.

Visual inspection can indicate whether pronounced pouch-cell swelling has occurred, but it cannot determine the onset of gassing-related volume change, the rate of volume increase, or the total volume change during a test. Without continuous quantitative data, comparisons of thermal behavior among electrolyte systems remain largely qualitative. In-situ volume monitoring addresses this limitation by providing time-resolved volume-change profiles under controlled test conditions.

3. The DES-ETPTA Solid Polymer Electrolyte: Design and Key Properties

The research team designed the DES-ETPTA solid polymer electrolyte by encapsulating a succinonitrile (SN)-based deep eutectic solvent (DES) in situ within an ethoxylated trimethylolpropane triacrylate (ETPTA) polymer network. This dual-component architecture was designed to address the three major limitations of conventional solid polymer electrolyte systems: low ionic conductivity, unstable electrode/electrolyte interfaces, and limited electrochemical stability.

• High ionic conductivity: The DES phase facilitates ionic transport within the ETPTA polymer matrix, supporting room-temperature operation.
• Lithium-metal interface compatibility: The polymer was designed to support stable interfacial contact with lithium metal during repeated plating and stripping.
• Flame retardancy: Incorporation of the non-flammable DES reduces reliance on flammable carbonate-based liquid electrolytes and can help lower electrolyte combustion risk.

To extend the DES-ETPTA electrolyte’s operational voltage window for compatibility with high-voltage LiCoO₂ cathodes (charging to 4.5 V vs. Li/Li⁺), the team incorporated cyanoacrylate copolymerization modification, improving the electrolyte’s oxidative stability at the cathode interface and enabling stable high-voltage LiCoO₂ solid-state battery cycling.

Figure 1. In-situ polymerization of the DES-ETPTA electrolyte to form stable electrode/electrolyte interfaces for long-term cycling of high-voltage lithium metal battery.

4. How IEST GVM2200 In-Situ Gas Evolution Monitoring Provided the Key Evidence

The thermal stability validation phase of the study posed a challenge that conventional characterization methods could not fully resolve. To evaluate the impact of electrolyte chemistry on high-temperature behavior, the research team assembled pouch cells containing either conventional liquid electrolyte or the DES-ETPTA solid polymer electrolyte, and exposed them to the same high-temperature conditions. Visual inspection of the pouch cell exterior (Figures 2a–d) revealed a clear difference: cells containing the liquid electrolyte exhibited noticeable swelling, while those containing the DES-ETPTA remained almost unchanged. However, visual inspection alone could not answer the quantitative questions:

• At exactly what temperature or time point does gas evolution begin?
• Is gas accumulation gradual or does it occur in discrete events?
• What is the total volume change, and how large is the difference between the two electrolyte systems?
• Can the safety improvement be expressed as a reproducible, instrument-verifiable number?

To answer these questions, the research team employed the IEST GVM2200 in-situ gas evolution monitor. Based on the Archimedes buoyancy principle, the instrument continuously recorded cell-volume changes and generated volume-time profiles. These measurements enabled a time-resolved comparison of the thermal responses of the two electrolyte systems.

The GVM2200 measurement results (Figure 2e) showed clearly that the liquid-electrolyte pouch cell exhibited a continuous increase in volume during high-temperature exposure, indicating ongoing gas generation. In contrast, the DES-ETPTA pouch cell maintained an almost constant volume throughout the test. These results provided quantitative, instrument-validated evidence that the DES-ETPTA electrolyte effectively suppresses thermal-induced gas evolution and cell swelling.

Figure 2. Comparison of high-temperature swelling behavior and volume change in pouch cells containing a liquid electrolyte or the DES-ETPTA electrolyte: (a–d) visual observation; (e) in-situ volume change curves measured using the IEST GVM2200.

5. IEST GVM2200: From Qualitative Swelling Observation to Quantitative Volume Change Data

Figure 3. IEST GVM2200 in-situ gassing volume monitoring instrument based on Archimedes buoyancy principle, for real-time pouch cell volume measurement.

As demonstrated in the Xiamen University-Zhejiang University study, the GVM2200 supplemented visual observations of pouch-cell swelling with continuous quantitative volume measurements. Typical application scenarios include:

• High-temperature storage: Monitoring the onset and rate of gassing-related volume changes during storage under controlled high-temperature conditions.
• Overcharge testing: Real-time tracking of electrolyte decomposition and gas evolution under overcharge conditions.
  
• Next-generation anode systems: Gas evolution characterization for silicon-based anodes (volume expansion + SEI gas), high-nickel cathodes (oxygen release), lithium metal batteries, and all-solid-state cells.
• Electrolyte and additive system comparison: Side-by-side quantitative thermal stability evaluation of liquid electrolytes, solid polymer electrolytes, and semi-solid hybrid systems — as demonstrated in this AEM 2024 study.
• Material-to-cell validation: In-situ evaluation of volume change from electrode material development stage through pouch cell verification, enabling early-stage safety screening.

6. Summary

The AEM 2024 study from Xiamen University and Zhejiang University demonstrated the potential of DES-based solid polymer electrolytes for high-safety, high-voltage lithium metal batteries. The DES-ETPTA electrolyte system delivers simultaneous improvements in several key performance and safety attributes, including suppressed thermal gas evolution, stable lithium metal interfaces, reduced flammability, and compatibility with high-voltage LiCoO₂ cathodes operating up to 4.5 V.

The IEST GVM2200 provided the quantitative evidence for the thermal safety comparison in this study. By continuously monitoring cell-volume changes under high-temperature conditions, the system transformed the qualitative observation that “the DES-ETPTA cell exhibits minimal swelling” into quantitative volume–time data that clearly differentiated the solid-polymer electrolyte from the conventional liquid-electrolyte baseline. As solid polymer electrolyte research advances, performance differences between candidate materials are becoming increasingly subtle. Under these circumstances, quantitative and continuous in-situ volume monitoring becomes a primary characterization method.

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