Fire-extinguishing current collector coated with melamine polyphosphate for lithium-ion batteries with high safety

Abstract

Safety concerns, particularly related to fire and explosions, remain a critical limitation of lithium-ion batteries (LIBs). In this study, we propose an effective approach for enhancing the safety of LIBs while minimizing the degradation of their electrochemical performance by employing an aluminum (Al) current collector coated with a melamine polyphosphate (MPP) layer exhibiting fire-extinguishing properties. The MPP layer, composed of MPP, polyvinylidene fluoride (PVdF), and carbon black, generates phosphorous radicals (PO and HPO) derived from the thermal decomposition of MPP above 400 °C, which enables fire extinguishment by scavenging combustible OH and H radicals. The cathode prepared with an MPP layer exhibits improved self-extinguishing time and comparable electrochemical performance in LIBs compared to those of the cathode prepared with pristine Al in optimized conditions based on the physicochemical properties of the MPP safety layer.

1 Introduction

Due to their high energy density and excellent cycling stability, lithium-ion batteries (LIBs) have become one of the most commonly used energy storage systems in applications such as portable electronics, power tools, and electric vehicles (Kim T. et al., 2019). However, LIBs have serious safety risks caused by the use of flammable organic electrolytes and polyolefin separators (Fan et al., 2018). The temperature of LIBs can increase due to factors including short-circuiting, overcharging, exposure to thermal abuse, and elevated temperatures (Liu et al., 2018). Once the temperature of LIBs exceeds 150 °C, a series of exothermic reactions—including separator contraction, decomposition of the solid-electrolyte interphase (SEI), and structural degradation of cathode materials—are triggered sequentially, potentially resulting in fire or explosion. This phenomenon, known as “thermal runaway,” has emerged as a significant safety concern for LIBs which hinders the growth of the LIB market (Wang et al., 2012; Ribière et al., 2012).

To address the fire hazards associated with LIBs, significant research has focused on designing low-flammability electrolytes containing additives like borates, siloxanes, fluorinated carbonates, phosphates, and phosphazenes (Gan et al., 2022; Guo et al., 2024; Ye et al., 2020). However, these additives significantly compromise electrochemical performance in LIBs due to the low lithium-ion conductivity associated with increased electrolyte viscosity and due to reduced operating potential. Modifications of current collectors with positive temperature coefficient (PTC) materials have also been proposed to shut down the LIBs by increasing electrical resistance caused by contact loss between active materials or between active materials and the current collector when temperatures rise (Song et al., 2024). However, degraded electrode properties such as adhesion strength and electrical conductivity in modified current collectors deteriorate electrochemical performance and energy density in LIBs. Although various strategies have been investigated to improve their safety, few studies have been reported that can simultaneously enhance safety while maintaining electrochemical performances in LIBs.

Here, we report a fire-extinguishing current collector coated with melamine polyphosphate (MPP) to improve safety while maintaining electrochemical performance in LIBs. An MPP layer composed of MPP, polyvinylidene fluoride, and carbon black was uniformly coated onto the Al current collector using a bar-coating process. This MPP existed stably without dissolving in the electrolyte and without redox reaction at a cathode voltage range of 2.5–5.0 V (vs. Li/Li⁺). The polyphosphate molecule of MPP decomposes to form PO and HPO radicals, which extinguish fire by scavenging combustible OH and H radicals. We conducted a systematic investigation on the influence of MPP content on self-extinguishing time, electrode properties, and electrochemical performances. The cathode with an MPP layer showed enhanced self-extinguishing time and similar electrochemical performance to those of the cathode with pristine Al in the optimized condition of the MPP layer. These results suggest that the MPP layer can be a highly safe alternative to pristine Al for LIBs.

2 Experimental section

2.1 Materials

Polyvinylidene fluoride (PVDF; Mw ≈ 180,000), sodium carboxymethyl cellulose (Na-CMC; average Mw ≈ 250,000, degree of substitution: 1.2), and N-methyl-2-pyrrolidone (NMP; purity 99.5%) were purchased from Sigma-Aldrich (United States). To prepare binder solutions, Na-CMC and PVDF were separately dissolved in deionized water and NMP to concentrations of 1 wt% and 8 wt%, respectively. Graphite powder, LiCoO₂ (LCO), carbon black, and a 40 wt% emulsion of styrene–butadiene rubber (SBR) in DI water were provided by Samsung SDI Co., Ltd. (Republic of Korea). Melamine polyphosphate (MPP) was sourced from Shinyoung Rad. Chem., Ltd. (Republic of Korea).

2.2 Fabrication of MPP-coated Al current collector

MPP, PVDF, and carbon black were mixed in a 75:12.5:12.5 weight ratio based on their solid states to prepare a coating solution, and NMP was used as the solvent. In this case, the coating solution was 5% solid. A bar-coating method was employed to coat the prepared solution onto a 10 μm thick Al foil current collector, which was then dried in a 120 °C oven for 20 min. The thickness of the coating layer was 2 μm. The Al current collector with an MPP ratio of 75% in the coating layer is referred to as “MPP 75.” Similarly, Al current collectors with a functional layer consisting of MPP, PVDF, and carbon black at a ratio of 82.5:8.75:8.75 and 90:5:5 were prepared under the same conditions with the same coating layer thickness (2 μm); these are referred to as “MPP 82.5” and “MPP 90,” respectively.

2.3 Electrode fabrication

To fabricate the cathode, a slurry was prepared by dispersing LCO, PVDF, and carbon black in N-methyl-2-pyrrolidone (NMP) at a solid ratio of 98.5:1:0.5. A THINKY mixer was used to uniformly blend the dispersion at 2000 rpm for 10 min. The cathode slurry was coated onto the pristine prepared Al foil using a doctor blade and was dried at 120 °C for 20 min. The coated electrode exhibited a mass loading of 22 mg cm^-2, and densification was achieved through roll calendering, resulting in a final electrode density of 4.1 g cm^-3. For anode preparation, graphite, Na-CMC, and SBR were dispersed in DIW. The solid content ratio of anode, consisting of graphite, Na-CMC, and SBR, was 97.8:1:1.2. This mixture was homogenized using the THINKY mixer at 2000 rpm for 10 min to obtain a uniform slurry. The anode slurry was applied onto a 6 μm thick copper foil current collector using a doctor blade, followed by drying at 120 °C for 20 min. Subsequent roll calendering was performed to achieve densification. The resulting anode exhibited a mass loading of 12 mg cm⁻² and a density of 1.6 g cm⁻³.

2.4 Characterizations

Thermogravimetric analysis (TGA; Q500, TA Instruments, United States) was conducted under a N₂ atmosphere, heating the samples from 50 °C to 800 °C at a rate of 10 °C/min. Fourier transform infrared (FT-IR) spectroscopy was employed to identify the functional groups in the sample. Spectra were acquired using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, United States) at a 0.5 cm⁻¹ resolution over 64 scans. The pyrolysis products of MPP were collected and then analyzed with gas chromatography–mass spectrometry (GC-MS, 5977A, Agilent Technologies, United States) with He carrier gas under 400 °C. The particle size of MPP in electrolyte was measured by dynamic light scattering (DLS) with Nano ZS (Malvern Instrument, Malvern, UK). For flammable testing, the cathodes thus obtained were prepared at a size of 1 cm × 5 cm. After adding 50 μL of the electrolyte, the cathodes were sealed in a 20 mL vial until the electrolyte was completely wet. The fire from the lighter was applied to the prepared cathodes. The self-extinguishing time (SET) values were calculated by dividing the time it took to turn off the fire after lighting the cathodes were wetted with the electrolyte by the weight of the electrolyte. The adhesion strength of the cathode was evaluated through a 180° peel test using a universal testing machine (Model AGS-J, Shimadzu, Japan). Thru-plane electrical resistance was measured with a source measure unit (Model 2400, Keithley, Tektronix, United States).

2.5 Electrochemical measurements

All cell components, including the electrodes, were subjected to vacuum drying at 120 °C for 4 h before assembly. For cathode half-cell fabrication, CR2032 coin cells were configured with the prepared cathode as the working electrode and 200-μm-thick lithium metal as the counter electrode. For the electrolyte, 1.5 M LiPF₆ was dissolved in a solvent blend of EC, DMC, and DEC (1:1:2 by volume), and 10 wt% FEC (PANAX StarLyte) was incorporated as an additive. A polypropylene (PP) membrane used as the separator. The manufacturing process of coin full-cell was same as for a half-cell, except that a graphite anode was used as the counter electrode instead of lithium metal. The assembled cells were charged using a constant current–constant voltage (CC–CV) protocol and discharged under constant current (CC) conditions, operating within a voltage window of 3.0–4.55 V at an ambient temperature. Cyclic voltammetry (CV) curves were analyzed using a coin half-cell with pristine Al and MPP 75. The CV analysis was conducted using potentiostat (ZIVE BP2A, WonATech, Republic of Korea) in the voltage range of 2.5–5.0 V with a scan rate of 10 mV s^-1. To analyze the electrochemical impedance spectroscopy (EIS) of each cell, a potentiostat was used with a frequency range of 100 kHz to 1 Hz under an amplitude of 20 mV at the open-circuit voltage condition. After undergoing formation cycles at 0.1 °C and 0.2 °C, the cycling stability and discharge rate capability of the full-cells were evaluated. For the rate performance test, charging was conducted at a constant rate of 0.2 °C.

3 Results and discussion

3.1 Material characterization

Figure 1a shows the schematic illustration of the cathode with a coating layer consisting of melamine polyphosphate (MPP) flame retardant, polyvinylidene fluoride (PVDF), and carbon black on the Al current collector (hereafter referred to as the “MPP safety layer”). As shown in Figure 1b, MPP has a molecular structure composed of melamine and polyphosphate.

FIGURE 1

MPP is a halogen-free fire retardant that decomposes at temperatures above 350 °C, releasing NH₃ gas as the melamine breaks down. Additionally, the polyphosphate component decomposes at approximately 400 °C, resulting in the release of phosphate gases. Fire-extinguishing property is achieved through the synergistic effect of fuel dilution, caused by the NH₃ gas released during melamine decomposition, and radical scavenging, resulting from the phosphate radical released during polyphosphate decomposition (Shao et al., 2021; Cheng et al., 2022). The removal reaction of OH and H radicals by the PO and HPO radicals generated are as follows (Lazar et al., 2020; Li et al., 2025):

Supplementary Figure S1 exhibits the Fourier transform infrared (FT-IR) spectrum of the MPP obtained, which shows typical MPP peaks represented at 3400, 1678, 1515, 1408, 1274, 1067, and 1016 cm⁻¹ (Fu et al., 2011). Figure 1c shows the thermogravimetric analysis (TGA) curve of MPP under N₂ condition, indicating that MPP undergoes two stages of thermal decomposition. The first stage occurred at 350–400 °C, corresponding to the decomposition of the melamine structure (Hadimani et al., 2022; Liu et al., 2019). The second stage of MPP thermal decomposition, occurring at 400 °C, is attributed to the decomposition of phosphate materials (Liu et al., 2017). The phosphorous radicals resulting from the phosphate decomposition can serve as a flame retardant and contribute to extinguishing fire (Lu et al., 2024). After the pyrolysis process of MPP at 400 °C in the N₂ atmosphere, the gas generated was analyzed through gas chromatography-mass spectrometry (GC-MS). As shown in Figure 1d, the pyrolysis GC-MS analysis revealed the presence of NH₃ (m/z: 17) resulting from melamine decomposition, as well as PO (m/z: 46), HPO (m/z: 47), and HPO₃ (m/z: 79) resulting from phosphate decomposition (Sangale et al., 2019). Moreover, a CO₂ peak (m/z: 44) related to the decomposition of both melamine and phosphate was also observed (Bontemps et al., 2014). As described above, the phosphorous radicals generated by the decomposition of melamine polyphosphate extinguish fire, which could improve the safety of LIBs. To ensure the effectiveness of MPP during the operation of the LIBs, MPP must be a stable material that does not dissolve in the electrolyte and does not react electrochemically during charging/discharging. Supplementary Figure S2 displays optical images of the electrolyte containing 2 wt% MPP immediately after stirring and 12 h later. The MPP in the electrolyte was agglomerated and settled after 12 h, which proves that the solubility of MPP in the electrolyte was low. Additionally, particle size analysis using Nano-ZS revealed that the MPP had an average particle size of approximately 500 nm and remained insoluble in the electrolyte (Supplementary Figure S3). Supplementary Figure S4 presents the cyclic voltammetry (CV) curves within the cathode potential window of 2.5–5.0 V (vs. Li/Li⁺) for both the pristine Al current collector and MPP 75. The CV curve of MPP 75 exhibited no observable oxidation or reduction peaks, indicating electrochemical stability. Notably, MPP 75 displayed lower current than pristine Al at 4 V, which could be attributed to the suppressed reaction between the Al current collector and the electrolyte due to the passivation layer of MPP 75 (Myung et al., 2011; Wang et al., 2017; Yang et al., 2022). These results indicate that the MPP safety layer exists stably without dissolving in the electrolyte and without redox reaction at cathode voltage range; this can improve the safety of LIBs associated with the fire-extinguishing property of MPP.

3.2 Electrode properties

Increasing the MPP concentration in the MPP safety layer could improve the fire-extinguishing property. However, as MPP is a polymer with low electrical conductivity, a high concentration of MPP deteriorates the physicochemical and electrochemical properties of the electrode, including electrical conductivity, adhesion strength, and rate capability (Lee et al., 2018). To investigate the relationship between the physicochemical properties and safety performance for the cathode depending on MPP concentration in the MPP safety layer, cathodes with different MPP concentrations of 75, 82.5, and 90 wt% in the MPP safety layer (“MPP 75,” “MPP 82.5,” and “MPP 90” safety layers, respectively) were prepared.

We investigated the effect of MPP content on fire-extinguishing property. As presented in Figure 2a, the pristine Al current collector wetted with electrolyte showed high flammability and continued to burn until the electrolyte was completely consumed. For MPP 75, the flame burned more weakly than that of the pristine Al. The fire-extinguishing property was improved with increased MPP ratio, and in the case of MPP 90, the flame was completely extinguished within 1.3 s. The fire-extinguishing property of the prepared cathodes was evaluated by measuring the self-extinguishing time (SET), which refers to the duration of burning after ignition. The SET values were calculated by considering the weight of the reacting electrolyte and the burning duration of the prepared cathodes (Ye et al., 2020). The SET value of the pristine Al current collector wetted with electrolyte was 53.0 s g⁻¹, while the SET value of MPP 75 decreased to 39.2 s g⁻¹ (Figure 2b). This is attributed to the production of phosphorous radicals resulting from the thermal decomposition of melamine polyphosphate at high temperature, which can effectively reduce the combustible radicals and enhance the self-extinguishing property (Lazar et al., 2020). As the MPP content increased from 75 wt% to 90 wt%, a significant reduction in the SET was observed with a low SET value of 20.8 s g⁻¹, which is 60% lower than that of the pristine Al (53.0 s g⁻¹). The fire-extinguishing property tests confirmed that increasing the MPP content on the Al current collector enhances the self-extinguishing property of the cathodes, which can improve the safety of LIBs. The electrode properties of adhesion strengths and electrical resistances were evaluated for the cathodes prepared with pristine Al, MPP 75, MPP 82.5, and MPP 90, respectively (Figure 2c). As the MPP content on the Al current collector increased, the adhesion strengths and electrical resistance were linearly degraded. The reduction in adhesion strength and increase in electrical resistance associated with higher MPP content should be taken into account, as these factors influence the electrochemical performance of the electrodes. Therefore, it is crucial to establish conditions that can enhance the safety of the LIBs while simultaneously minimizing the degradation of the performance of LIBs.

FIGURE 2

3.3 Electrochemical performances

The electrochemical properties of the cathodes with pristine Al, MPP 75, MPP 82.5, and MPP 90 MPP safety layers were evaluated via a coin half-cell test. Figure 3a shows the voltage profiles of the cathode half-cells at 0.1  C in the voltage window of 3.0–4.55 V vs. Li/Li⁺. All cells exhibited similar results, with a discharge capacity of ∼194 mAh g⁻¹ and a Coulombic efficiency of ∼93.5% at the low current density of 0.1 °C (Lee et al., 2020). Under 0.2 and 0.5 C operations, the cathodes with MPP 82.5 and MPP 90 safety layers showed slightly reduced discharge capacities relative to those with pristine Al and MPP 75 safety layers, respectively (Figure 3b; Supplementary Figure S5). Electrochemical impedance spectra (EIS) of the cathode coin-type half-cells, obtained at a 50% state of charge (SOC) after formation cycles at 0.1 °C and 0.2 °C, are presented in Figure 3c. Each resistance of the cathode half-cells was analyzed based on the equivalent circuit and is listed in Supplementary Figure S6 and Table 1. All cathode half-cells showed similar values for resistance of the solution (R_s), resistance of the solid-electrolyte interphase layer (R_SEI), and charge transfer resistance in the anode (R_ct,anode). The similar R_s values for all cathode half-cells indicate that the MPP safety layer does not affect the microstructure of the cathode because R_s is closely related to the electrolyte wettability in the electrode (Kim DS. et al., 2019). Additionally, the comparable values for R_SEI and R_ct,anode could be attributed to the identical lithium metal used as anode in all cells. On the other hand, the cells exhibit a noticeable difference in charge transfer resistance (R_ct,cathode) depending on the composition of the MPP safety layer. The R_ct,cathode for cathodes with pristine Al, MPP 75, MPP 82.5, and MPP 90 safety layers was 9.11, 9.96, 12.51, and 14.76 Ω, respectively. The comparable R_ct,cathode for cells employing pristine Al and MPP 75 safety layers, as well as the deterioration of R_ct,cathode in MPP compositions above 82.5 wt%, suggests that the MPP 75 safety layer is a good composition for enhancing the safety of LIBs, which is consistent with the coin half-cell results.

FIGURE 3

The coin full-cells were fabricated with graphite anode and prepared cathodes to evaluate the electrochemical properties. The discharging capacity of the MPP 75 was 195.31 mAh g⁻¹ with a Coulombic efficiency of 93.60%, which indicated the similar property of pristine Al showing 194.89 mAh g⁻¹ with Coulombic efficiency of 93.73% at 0.1 °C in a voltage window of 3.0–4.55 V vs. Li/Li⁺ (Figure 4a). However, contrary to the results from coin half-cell tests, the degradation in electrochemical performances was observed in MPP composition above 82.5 wt% even at a low current density of 0.1 °C. The difference in charge/discharge behavior was more noticeable at current densities of 0.2 and 1 °C, and MPP compositions above 82.5 wt% displayed significantly larger overpotentials during the charge/discharge process than pristine Al and MPP 75 at 1 °C (Supplementary Figure S7; Figure 4b). After undergoing formation at 0.1 °C and 0.2 °C, coin full-cells were cycled at 1 °C under CC–CV charging and CC discharging modes over a voltage range of 3.0–4.55 V to evaluate cycling performance.

FIGURE 4

Capacity retention after 100 cycles at 1 °C was 91.53% for pristine Al and 90.26% for MPP 75, indicating the applicability of MPP 75 (Figure 4c). MPP 82.5 showed continuous capacity fading and capacity retention of 85.99% at 100 cycles. In MPP 90, the capacity fading became more severe, and the capacity retention was 78.30% at 100 cycles. The discharge rate capability was tested up to 2.0 °C discharging rate, maintaining the charging rate at 0.2 °C after the 0.1 and 0.2 °C formation cycles (Figure 4d). The pristine Al showed discharge capacities of 191.1, 186.9, 178.1, and 152.9 mAh g⁻¹, respectively, at discharge rates of 0.2, 0.5, 1.0, and 2.0 °C. For the MPP 75, the discharge capacities were 190.5, 185.5, 176.8, and 150.5 mAh g⁻¹, respectively, at 0.2, 0.5, 1.0, and 2.0 °C discharge rates, which presented electrochemical performance comparable to those of the pristine Al. On the other hand, the discharge capacities of MPP 82.5 and MPP 90 at 0.2, 0.5, 1.0, and 2.0 °C were 186.5, 179.4, 171.8, and 134.3 and 182.5, 176.5, 168.1, and 118.6 mAh g⁻¹, respectively, indicating that the deteriorated electrochemical performance in LIBs was caused by the degradation of adhesion strength and electrical resistance associated with increased MPP content on the Al current collector. Based on the electrochemical properties analysis, MPP 75 shows comparable electrochemical performance to that of the pristine Al while significantly reducing the self-extinguishing time, thereby enhancing safety.

To investigate the effect of the MPP incorporation method on electrode structure, the MPP 75 cathode was compared with a cathode in which an equivalent areal amount of MPP (1.7 wt%) was uniformly dispersed within the active material layer. According to the SET results (Supplementary Figure S8a), the MPP 75 cathode exhibited a SET of 39.2 s g⁻¹, representing a 26% reduction compared to the pristine Al. In contrast, the MPP 1.7 wt% cathode, in which MPP is dispersed within the active material layer, showed a 12% reduction, with a SET of 46.6 s g⁻¹. These results suggest that applying MPP as a surface coating on the current collector is more effective for fire suppression than its uniform dispersion within the cathode active material layer. In addition to its safety aspect, the current collector-coated structure also demonstrates superior electrochemical performance than the configuration with MPP uniformly distributed within the cathode active material layer. As shown in Supplementary Figure S8b, the MPP 1.7 wt% cathode exhibits increased overpotential relative to the pristine Al cathode, resulting in inferior performance compared to the MPP 75 cathode, which delivered a discharge capacity of 194.87 mAh g⁻¹ and a Coulombic efficiency of 93.42% in the first cycle. Due to the increased resistance caused by the incorporation of MPP into the active material layer, the 1C/1C cycling performance also deteriorated, with the cycle retention decreasing to 87.2% after 100 cycles (Supplementary Figure S8c). As shown in Supplementary Table S1, although MPP 1.7 wt% shows better adhesion strength and electrical conductivity than MPP 75, embedding MPP particles within the cathode active material layer hinders lithium-ion transport by obstructing the electrode pores, resulting in a significant increase in ionic resistance through the porous structure (R_ion) and a deterioration in cycling performance (Chen and YJJoPS, 2024). These findings suggest that applying a flame retardant such as MPP on the current collector is a more favorable strategy for simultaneously enhancing battery safety and preserving electrochemical performance than incorporating it into the active material layer.

4 Conclusion

This study demonstrated that coating melamine polyphosphate (MPP) onto the aluminum current collector effectively enhances the safety of lithium-ion batteries through the intrinsic fire-extinguishing behavior of the MPP layer. Compared with embedding MPP within the cathode active material layer, applying the MPP coating layer onto the current collector is more favorable for maintaining electrochemical performance, as it minimizes the increase in ionic tortuosity and preserves efficient Li-ion transport pathways within the electrode. Furthermore, the MPP-coated current collector mitigates undesirable interfacial reactions between the Al substrate and the electrolyte. The optimized MPP coating significantly reduced the self-extinguishing time while exhibiting electrochemical properties comparable to those of the pristine Al-based electrode. These findings confirm that the MPP-coated current collector offers a practical and effective strategy for improving battery safety without compromising electrochemical performance.

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