The relentless pursuit of higher energy density in lithium ion batteries is a defining trend in modern energy storage technology, driven by demands from electric vehicles and grid-scale storage applications. However, this drive intensifies a critical and persistent challenge: safety. Conventional lithium ion batteries rely on large quantities of liquid electrolytes, typically composed of volatile and highly flammable organic carbonate solvents. These electrolytes represent a significant fuel source within the cell. In the event of internal short circuits, mechanical damage, or thermal abuse, the release and ignition of these solvents can lead to catastrophic failure modes, including fire, explosion, and toxic fume emission, severely hindering the broader adoption of high-energy systems. Therefore, developing strategies to fundamentally enhance the intrinsic safety of electrolytes without compromising electrochemical performance is paramount.
Among various solutions, gel polymer electrolytes (GPEs) present a compelling approach. By immobilizing a liquid electrolyte (plasticizer) within a solid or quasi-solid polymer matrix, GPEs can mitigate leakage issues and offer improved mechanical integrity compared to their liquid counterparts. Nevertheless, the safety concern is not fully eliminated. Under thermal stress, the encapsulated liquid plasticizer can still vaporize and decompose, producing flammable gases that may escape and ignite if the polymer matrix decomposes or ruptures. Consequently, imparting flame-retardant properties to the GPE itself is a logical and necessary step toward creating truly safe lithium ion batteries.
Common strategies for introducing flame retardancy often involve blending flame-retardant additives, such as phosphorus-based compounds (e.g., trimethyl phosphate, triphenyl phosphate), directly into the liquid electrolyte before gelation. While effective to a degree, this physical blending approach has drawbacks. High additive loadings are typically required for significant flame inhibition, which often degrades the electrolyte’s ionic conductivity and, more critically, its electrochemical stability against reduction on the anode surface. This decomposition can lead to poor solid electrolyte interphase (SEI) formation and rapid capacity fading. A more elegant strategy is to chemically anchor the flame-retardant moiety into the polymer backbone itself. This “fixed phosphorus” design ensures the flame-retardant element is non-leachable, maintains a homogeneous distribution, and minimizes direct contact with reactive electrodes, thereby preserving better interfacial stability and long-term cycling performance.
In this work, we design and synthesize a novel flame-retardant gel polymer electrolyte (FR-GPE) based on this principle. The polymer network is constructed via in-situ thermal polymerization of a phosphorus-containing monomer, diethyl vinylphosphonate (DEVP), and a cross-linker, pentaerythritol tetraacrylate (PETEA), using a standard commercial carbonate-based electrolyte as the plasticizer. DEVP provides a high phosphorus content (18.9 wt%), which is crucial for gas-phase flame inhibition, while PETEA forms a robust, cross-linked network to effectively trap the electrolyte. This in-situ polymerization method is versatile and suitable for fabricating various battery form factors. We systematically investigate the physicochemical, electrochemical, and critical safety properties of the resulting DEVP-GPE. The ionic conductivity, electrochemical stability window, and compatibility with graphite anodes and LiFePO4 cathodes are evaluated. Most importantly, its flame-retardant efficacy is quantified, and its performance under abusive overheating conditions is tested in practical, high-capacity pouch cells. Furthermore, thermal analysis is employed to elucidate the mechanism behind its enhanced safety. This comprehensive study demonstrates that the DEVP-GPE successfully reconciles high electrochemical performance with superior intrinsic safety, offering a promising path forward for the development of reliable and hazard-resistant lithium ion batteries.
The quest for safer energy storage is fundamentally linked to the materials at the heart of a lithium ion battery. The electrolyte, serving as the ionic transport medium, is often the most vulnerable component. Our approach focuses on transforming this vulnerability into a strength by creating a multifunctional material that conducts ions effectively while resisting ignition and flame propagation.
Experimental Synthesis and Characterization of DEVP-GPE
The synthesis of the flame-retardant gel polymer electrolyte is a straightforward, one-pot process designed for practicality. The precursor solution is prepared by dissolving the monomer diethyl vinylphosphonate (DEVP), the cross-linker pentaerythritol tetraacrylate (PETEA), and the thermal initiator azodiisobutyronitrile (AIBN, 2 wt% relative to monomers) directly into a commercial liquid electrolyte. The liquid electrolyte used is 1 M LiPF6 in a mixture of ethylene carbonate/diethyl carbonate (EC/DEC) with 5% fluoroethylene carbonate (FEC) additive. The mass ratio of DEVP to PETEA is fixed at 2:1, and the total content of the liquid electrolyte (the plasticizer) is set at 90 wt% of the final GPE to ensure high ionic conductivity. This homogeneous precursor solution is then injected into battery cells or cast onto substrates. Gelation is achieved through in-situ thermal polymerization at 80°C for 20 minutes, resulting in a self-standing, opaque white gel that fully encapsulates the liquid component within a cross-linked polymer network.
Fourier-transform infrared (FTIR) spectroscopy confirms the success of the polymerization. The characteristic C=C double bond stretching vibration peak at approximately 1630 cm-1, present in both DEVP and PETEA monomers, completely disappears in the spectrum of the polymerized DEVP-GPE. Meanwhile, peaks associated with other functional groups, such as the C=O stretch from the acrylate units at ~1726 cm-1, remain intact. This verifies the high conversion of vinyl groups and the formation of the polymer network. Scanning electron microscopy (SEM) reveals the morphology of the GPE supported on a polyolefin separator. The originally porous structure of the separator is uniformly filled and coated by the polymer gel, creating a dense and continuous electrolyte membrane with no visible pores, which is essential for preventing internal short circuits and holding the electrolyte.
The ionic conductivity (σ) of an electrolyte is a fundamental parameter determining its rate capability. The conductivity of the DEVP-GPE is measured by electrochemical impedance spectroscopy (EIS) using a stainless steel (SS) | electrolyte | SS blocking cell across a temperature range. The bulk resistance (Rb) is obtained from the high-frequency intercept of the Nyquist plot. The ionic conductivity is then calculated using the formula:
$$ \sigma = \frac{l}{S \times R_b} $$
where \( l \) is the thickness of the electrolyte membrane and \( S \) is the contact area with the stainless steel electrode. The temperature dependence of ionic conductivity typically follows the Arrhenius relationship:
$$ \sigma = A \exp\left(-\frac{E_a}{RT}\right) $$
where \( A \) is the pre-exponential factor, \( E_a \) is the activation energy for ion transport, \( R \) is the gas constant, and \( T \) is the absolute temperature. A linear fit of \(\ln(\sigma)\) versus \(1000/T\) yields the activation energy.
| Temperature (°C) | Ionic Conductivity (mS cm-1) |
|---|---|
| 25 | 0.54 |
| 35 | 0.57 |
| 45 | 0.63 |
| 55 | 0.66 |
| 65 | 0.70 |
The data shows that the DEVP-GPE exhibits a respectable room-temperature ionic conductivity of 0.54 mS cm-1, which is comparable to that of the pure commercial liquid electrolyte (0.64 mS cm-1). This indicates that the cross-linked polymer network does not severely hinder Li+ ion mobility, as the conduction primarily occurs through the liquid electrolyte phase trapped within the gel. The conductivity increases steadily with temperature, and the calculated activation energy \(E_a\) from the Arrhenius plot is 6.77 kJ mol-1, suggesting a relatively low energy barrier for ion hopping and supporting good low-temperature performance potential. The electrochemical stability window of the DEVP-GPE was assessed by linear sweep voltammetry (LSV) using a Li | electrolyte | SS cell. The DEVP-GPE demonstrates anodic stability up to approximately 4.3 V vs. Li/Li+, which is superior to the liquid electrolyte that begins to decompose just above 4.0 V. This enhanced stability is likely due to the restricted mobility of species within the gel and the stable nature of the polymer matrix itself, making it suitable for use with common high-voltage cathode materials in a lithium ion battery.
Electrochemical Performance in Half-Cell and Full-Cell Configurations
The practical utility of any new electrolyte hinges on its compatibility with standard electrode materials. We first evaluated the long-term cycling performance of the DEVP-GPE against a graphite anode. Graphite | DEVP-GPE | Li half-cells were cycled at a rate of 0.5C between 0.05 V and 2.0 V. For comparison, identical cells using the commercial liquid electrolyte (LE) were tested under the same conditions.
The results reveal a stark contrast in cycling stability. The cell with the liquid electrolyte shows a rapid and continuous capacity fade. In contrast, the DEVP-GPE cell exhibits remarkably stable cycling over 1000 cycles. Although the initial cycles show a slightly lower capacity, which can be attributed to the slightly higher initial interface resistance during the formation of a stable SEI, the capacity gradually increases and stabilizes. Crucially, the DEVP-GPE cell avoids the sudden capacity plunge observed in the LE cell after several hundred cycles. The capacity retention after 1000 cycles is a key metric:
$$ \text{Capacity Retention} = \frac{\text{Capacity at cycle N}}{\text{Capacity at a stable reference cycle}} \times 100\% $$
For the DEVP-GPE cell, the capacity retention at the 1000th cycle is 88.7%, dramatically higher than the 25.8% retention of the LE cell. The Coulombic efficiency of the DEVP-GPE cell quickly rises and remains near 99.9% throughout the test, indicating highly reversible lithium plating/stripping and minimal parasitic side reactions. This exceptional performance underscores the advantage of the fixed-phosphorus design: the flame-retardant component does not interfere with the anode interface, allowing for the formation of a robust and conductive SEI layer.
| Sample | 1st Cycle Coulombic Efficiency | Avg. Capacity (cycles 2-10, mAh g-1) | Discharge Capacity at Cycle 100 (mAh g-1) | Discharge Capacity at Cycle 1000 (mAh g-1) | Retention at Cycle 1000 (%) |
|---|---|---|---|---|---|
| Liquid Electrolyte (LE) | 84.8% | 233.1 | 333.3 | 85.3 | 25.8% |
| DEVP-GPE | 62.9% | 205.7 | 333.2 | 303.4 | 88.7% |
To assess performance in a more realistic and applicable configuration, we fabricated 1 Ah capacity pouch-type full cells using LiFePO4 (LFP) as the cathode and graphite as the anode. The DEVP-GPE precursor was injected and polymerized in-situ within the sealed pouch cell. After formation, the cells were cycled at 0.5C rate within a voltage window of 2.0-3.8 V. The full cell delivered an initial discharge capacity of 959.6 mAh, very close to its designed capacity. The cycling performance was excellent, with a discharge capacity of 814.0 mAh remaining after 100 cycles, corresponding to a capacity retention of 80.2%. The average Coulombic efficiency over 100 cycles was 99.73%. These results confirm that the DEVP-GPE is fully compatible with both electrodes in a functioning lithium ion battery, enabling stable long-term operation without compromising energy content.
Flame Retardancy and Thermal Abuse Safety Evaluation
The defining feature of this work is the intrinsic safety imparted by the DEVP-GPE. A simple but telling flame test was conducted first. A strip of separator soaked with the liquid electrolyte ignited immediately upon exposure to a propane torch and continued to burn vigorously until completely consumed. In stark contrast, a self-standing membrane of DEVP-GPE, when held in the flame for 5 seconds and then removed, self-extinguished within 1.5 seconds, emitting only a small, brief flame. This demonstrates a fundamental shift from a highly flammable material to a flame-retardant one. The mechanism is classic gas-phase flame inhibition: upon heating, phosphorus-containing compounds in DEVP decompose to release radical species such as PO• and HPO•. These radicals scavenge the high-energy H• and OH• radicals that propagate the combustion chain reaction in the flame zone, effectively snuffing out the fire.
While informative, material-level tests do not fully capture the complex safety scenario within a sealed lithium ion battery under abuse. Therefore, we conducted an overheating test on the 1 Ah LFP/graphite pouch cells at 100% state-of-charge (SOC). A cylindrical heating rod (200 W) was placed in direct contact with the cell surface inside a fireproof chamber. The cell voltage and surface temperature were monitored throughout the test.
The test outcomes were drastically different between the LE-based cell and the DEVP-GPE-based cell. The liquid electrolyte cell experienced a violent thermal runaway. After a short heating period, the internal pressure from vaporized electrolyte caused the pouch to rupture at the seams, followed by immediate ejection and ignition of the flammable vapors, resulting in a sustained jet fire. The cell voltage plummeted to zero as an internal short circuit occurred.
In remarkable contrast, the DEVP-GPE based lithium ion battery exhibited a much milder failure mode. The cell swelled significantly but did not rupture violently. Only white smoke (likely a mixture of decomposed electrolyte and polymer vapors) emanated from the cell, with no visible flame or fire observed at any point during the test. Crucially, there was no leakage of liquid electrolyte. The temperature profiles provide further insight. The onset of internal short circuit, indicated by a sudden voltage drop, was delayed for the DEVP-GPE cell. Furthermore, the peak surface temperature recorded for the DEVP-GPE cell was over 140°C lower than that of the LE cell at the same time point. The significantly mitigated thermal response highlights the dual protective role of the GPE: 1) The polymer network physically hinders the rapid volatilization and ejection of the liquid plasticizer, slowing down the rate of fuel supply to any potential fire. 2) The phosphorus species released from the polymer matrix chemically inhibit combustion if any fuel does ignite.
Thermal Degradation Analysis and Safety Mechanism Elucidation
To quantitatively understand how the DEVP-GPE structure enhances safety, thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere. The weight loss profiles of the pure liquid electrolyte (LE), the DEVP-GPE, and the pure polymer matrix (without electrolyte) were compared.
The TGA and derivative thermogravimetry (DTG) curves reveal a multi-stage degradation process for the DEVP-GPE. The first major weight loss step, occurring between room temperature and ~180°C, corresponds primarily to the evaporation and decomposition of the carbonate solvents from the plasticizer. The key observation is that the weight loss rate of DEVP-GPE in this region is significantly lower, and the temperature of the maximum decomposition rate (Tmax) is shifted higher compared to the pure LE. This directly confirms that the cross-linked polymer network acts as a physical barrier, effectively retarding the release of volatile solvents. By the critical temperature of 150°C (where the LE cell typically fails), the DEVP-GPE has lost considerably less mass.
The subsequent degradation stages involve the decomposition of the polymer network itself and any residual salt/solvent. The pure polymer matrix shows high thermal stability, beginning to decompose only above 260°C. The char residue left at 700°C for the DEVP-GPE is higher than for the LE, indicating the formation of a stable carbonaceous layer that can further insulate the cell contents.
| Sample | T-5wt% (°C) | Tmax,1 (°C) | Weight Loss at 200°C | Char Residue at 700°C |
|---|---|---|---|---|
| Liquid Electrolyte (LE) | 28.0 | 104.8 | 92.0% | 2.1% |
| DEVP-GPE | 39.5 | 153.4 | 77.4% | 5.4% |
| Polymer Matrix | 265.9 | 302.1 | ~1.9% | 37.6% |
The integrated safety mechanism of the DEVP-GPE in a lithium ion battery can therefore be summarized as follows. Under normal operation, the gel provides facile Li+ transport and stable interfaces, ensuring excellent cycling. Under thermal abuse conditions: 1) The robust polymer network physically confines the liquid electrolyte, dramatically slowing its vaporization and preventing explosive ejection and leakage. 2) The heat triggers the decomposition of the phosphonate units (DEVP) in the polymer chains, releasing phosphorus-containing radicals (PO•, etc.) into the gas phase. 3) If any flammable vapors do mix with air and reach ignition conditions, these released radicals actively quench the flame propagation reactions, leading to self-extinguishing behavior and preventing a sustained fire. This combination of physical barrier effect and active chemical flame inhibition creates a synergistic safety enhancement that is far superior to conventional liquid electrolytes or physically blended additive systems.
Conclusion and Perspective
In conclusion, we have successfully developed a high-performance, flame-retardant gel polymer electrolyte (DEVP-GPE) through the in-situ copolymerization of a phosphorus-containing monomer within a commercial liquid electrolyte. This design strategically fixes the flame-retardant element into the polymer backbone, avoiding the pitfalls of additive leaching and interfacial incompatibility. The resulting DEVP-GPE exhibits a well-balanced suite of properties essential for next-generation lithium ion batteries:
• Electrochemical Competence: It possesses competitive ionic conductivity (0.54 mS cm-1 at 25°C) and a wide electrochemical window (>4.3 V). It enables outstanding long-term cycling stability in both graphite half-cells (88.7% capacity retention after 1000 cycles) and practical LFP/graphite pouch full cells (80.2% retention after 100 cycles).
• Superior Intrinsic Safety: The DEVP-GPE is inherently flame-retardant, showing rapid self-extinguishing behavior (<1.5 s). When integrated into a 1 Ah lithium ion battery pouch cell, it completely prevents fire and electrolyte leakage during an overheating abuse test, yielding only smoke and a significantly attenuated thermal runaway response with a peak temperature over 140°C lower than a conventional cell.
• Clear Safety Mechanism: Thermal analysis confirms that the cross-linked network physically retards solvent release, while the phosphorus moiety provides active gas-phase flame inhibition upon decomposition.
This work demonstrates that safety and performance in lithium ion batteries are not mutually exclusive goals. By moving beyond simple additive blending to smart molecular design—where safety functionalities are integrated into the electrolyte’s structural framework—we can create materials that meet the rigorous demands of high-energy-density applications. The DEVP-GPE represents a significant step toward this goal, offering a practical and effective strategy for building safer, more reliable lithium ion batteries for electric transportation and large-scale energy storage. Future work may explore optimizing the polymer/plasticizer ratio for specific temperature ranges, integrating other functional monomers for even broader electrochemical stability, and scaling up the synthesis and cell fabrication processes for commercial evaluation.