Microcapsule Modification Strategy Empowering Separator Multifunctionality to Enhance Safety of Lithium-Metal Batteries.

The uncontrollable growth of lithium dendrites and the flammability of electrolytes are the direct impediments to the commercial application of high-energy-density lithium metal batteries (LMBs). Herein, this study presents a novel approach that combines microencapsulation and electrospinning technologies to develop a multifunctional composite separator (P@AS) for improving the electrochemical performance and safety performance of LMBs. The P@AS separator forms a dense charcoal layer through the condensed-phase flame retardant mechanism causing the internal separator to suffocate from lack of oxygen. Furthermore, it incorporates a triple strategy promoting the uniform flow of lithium ions, facilitating the formation of a highly ion-conducting solid electrolyte interface (SEI), and encouraging flattened lithium deposition with active SiO2 seed points, considerably suppressing lithium dendrites growth. The high Coulombic efficiency of 95.27% is achieved in Li-Cu cells with additive-free carbonate electrolyte. Additionally, stable cycling performance is also maintained with a capacity retention rate of 93.56% after 300 cycles in LFP//Li cells. Importantly, utilizing P@AS separator delays the ignition of pouch batteries under continuous external heating by 138 s, causing a remarkable reduction in peak heat release rate and total heat release by 23.85% and 27.61%, respectively, substantially improving the fire safety of LMBs.

Fully bio-based and intrinsically flame retardant unsaturated polyester cross-linked with isosorbide-based diluents.

Unsaturated polyester resins (UPR) are composed of prepolymers and styrene diluents, while the former are produced by co-polycondensation between diol, unsaturated diacid and saturated diacid. In this work, bio-based UPR prepolymers were synthesized from bio-based oxalic acid, itaconic acid, and ethylene glycol, which were then diluted with bio-based isosorbide methacrylate (MI). Meanwhile, the phenylphosphonate were introduced into the molecular chains of prepolymers to achieve intrinsic flame retardancy of bio-based UPR. The potential of the reactive MI diluents as substitutes of volatile styrene, was also assessed through the volatility test, curing kinetics and gel contents analysis. For UPR materials with styrene diluents, the UPR materials can achieve UL-94 V0 level and the 28% of limiting oxygen index (LOI) with 2.63 wt% of phosphorus contents. By contrast, the UPR materials with MI diluents can reach UL-94 V0 level with only 2.14 wt% of phosphorus contents. As the phosphorus contents were further increased to 2.63 wt%, UPR materials can achieve highest 29%, while the peak of heat release rate (PHRR) and total heat release (THR) were decreased by 68.01% and 48.62%, respectively. The Flame Retardancy Index (FRI) was also used to comprehensively evaluate the flame retardant performance of UPR composites. Compared with neat UPR, the composites with MI diluents and phosphorus containing structures increased from 1.00 to 6.46. The mechanism for improved flame retardancy was analyzed from gaseous and condensed phase. Additionally, the tensile strengths of bio-based UPR materials with styrene and MI diluents were studied. This work provides an effective method to prepared high-performance and fully bio-based UPR materials with improved flame retardant properties and safety application of reactive diluents.

Preparation of BMI monomers containing phosphate and phosphonate structure to enhance the flame retardant and toughness of BMI.

Aiming at enhancing the toughness and fire safety of bismaleimide (BMI), BMI monomers containing phosphate and phosphonate structure (BDTP and BDTDP) were designed and prepared. With incorporation of 5 wt% BDTP and BDTDP, the peak value of heat release rate (PHRR) of BMI/BDTP-5 and BMI/BDTDP-5 decrease by 59.4% and 52.4%, respectively. The total smoke production (TSP) of BMI/BDTP-5 and BMI/BDTDP-5 are of 8.3% and 13.1% reduction, respectively. Meanwhile, BMI/BDTP-5 and BMI/BDTDP-5 possess UL-94V-0 rating, which indicates that BMI is endowed with better flame retardant performance by modification of designed BMI monomers. Besides, the impact strength of BMI/BDTP-5 and BMI/BDTDP-5 increase by 146.3% and 90.2%, respectively. The comprehensive performance of BMI/BDTP-5 is better than that of BMI/BDTDP-5. And the effect of phenyl phosphate structure in BDTP and phenyl phosphonate structure in BDTDP on BMI performance is explored.

Hierarchical Ti3C2Tx@BPA@PCL for flexible polyurethane foam capable of anti-compression, self-extinguishing and flame-retardant.

It is of great importance to fabricate flexible polyurethane foam (FPUF) with superior mechanical properties and flame retardancy for practical applications. Herein, organosilicon and phenyl phosphorus compounds were synthesized and grafted on the surface of Ti3C2Tx (Ti3C2Tx@BPA@PCL) via in-situ polymerization. Then, the FPUF composites were fabricated, combining intrinsic flame retardancy (9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide-diethanolamine: DH-DOPO) (addition amount: 20 wt%) and Ti3C2Tx@BPA@PCL (addition amount: 4 wt%). Attributed to the rigid structure of Ti3C2Tx@BPA@PCL, the tensile strength and compression strength of FPUF showed 24.0% and 253% increase, respectively. In addition, anti-fatigue properties of FPUF composites during the cyclical test were dramatically enhanced. In contrast to pure FPUF, 36.1% and 44.0% reductions in peak heat release rate (pHRR) and total heat release (THR) were achieved for the FPUF containing Ti3C2Tx@BPA@PCL and DH-DOPO, the production rate of carbon dioxide (CO2) and carbon oxide (CO) also decreased by 40.3% and 52.1%, respectively. FPUF4 showed self-extinguishing behavior, and passed the vertical burning test (VBT). This work provides a facile approach to preparing high-performance FPUF with enhanced mechanical property and flame retardancy.

MOF-derived 3D petal-like CoNi-LDH array cooperates with MXene to effectively inhibit fire and toxic smoke hazards of FPUF.

The toxic smoke produced by the combustion of flexible polyurethane foam (FPUF) may not only caused casualties, but also polluted the environment. Here, double metal hydroxide derived from ZIF-67 (MOF-LDH) modified Ti3C2TX (Ti3C2TX@MOF-LDH) was innovatively designed to solve the serious smoke and fire hazards of FPUF. The FPUF nanocomposite containing 6 wt% Ti3C2Tx@MOF-LDH achieved a 16.1% reduction in total smoke production (TSP) along with 22.2% reduction in peak smoke production rate (PSPR), which greatly reduced the hazard of smoke. At the same time, toxic gases, such as carbon monoxide (CO), carbon dioxide (CO2), and aromatic compounds, showed the same reduction pattern. In addition, the heat release of FPUF nanomaterials was also suppressed. In particular, the FPUF/Ti3C2Tx@MOF-LDH 3.0 achieved 110.4% and 76.1% increase in compressive strength and tensile strength, respectively, confirming the effective mechanical enhancement. Therefore, this work provided a new reference for the preparation of high-performance FPUF nanocomposites with low smoke, low fire hazard and excellent mechanical properties.

Construction of bismaleimide resin with enhanced flame retardancy and mechanical properties based on a novel DOPO-derived bismaleimide monomer.

It's known that the application of bismaleimide resins (BMI) is limited due to its brittleness and poor flame retardancy. A novel type of BMI monomer (MADQ) based on the typical phosphorus series flame retardant DOPO is designed to improve the fire safety of BMI. Besides, aliphatic long chain structure is introduced in MADQ, which is supposed to be conducive to reducing the rigidity of the BMI cross-linked network and thus to improve the toughness of BMI. It's seen that with the incorporation of 5.24 wt% MADQ, the peak of heat release rate (PHRR) and total heat release (THR) of resultant BMI/MADQ-5 is reduced by 37.7% and 33.9%, respectively. Meanwhile, with modification of 1.07 wt% MADQ, BMI/MADQ-1 possesses UL-94V-0 rating. The relevant mechanism analysis reveals that the phosphaphenanthrene group in MADQ can exert flame retardancy effect both in condensed and gas phase. Besides, the impact strength of the BMI/MADQ is maximally increased by nearly 90.1%. Furthermore, the BMI/MADQ still maintains high tensile strength and thermal stability, which indicates the modification of MADQ did not deteriorate other properties of BMI. An innovative research idea and research basis for the preparation of intrinsic flame-retardant and toughened BMI is provided in this work.

High-performance flexible polyurethane foam based on hierarchical BN@MOF-LDH@APTES structure: Enhanced adsorption, mechanical and fire safety properties.

Improving resilience, enhancing fire safety and adsorption properties were the key points for the preparation of high-performance flexible polyurethane foam (FPUF). Here, MOF-derived petal-like Co/Mg-double metal hydroxide (Co/Mg-LDH) and 3-aminopropyltriethoxysilane (APTES) were selected to modify the hydroxylated boron nitride (BNNS-OH) to obtain a hydrophobic BN@MOF-LDH@APTES. Compared with the previous work, BN@MOF-LDH@APTES demonstrated extremely high filler efficiency in reducing the heat release per unit mass (THR/TM) (18.2 % reduction) and smoke production per unit mass (TSP/TM) (19.1% reduction) of FUPF during combustion. In addition, the obtained FPUF nanocomposite exhibited high absorption capacity while achieving remarkable thermal stability and fire safety. Moreover, the FPUF nanocomposite containing 1 wt% BN@MOF-LDH@APTES achieved a 71% increase in compressive strength, indicating excellent resilience. Therefore, this work provided a new material for the preparation of high-resilience FPUF with both flame retardancy and adsorption capacity.

The improvement of fire safety performance of flexible polyurethane foam by Highly-efficient P-N-S elemental hybrid synergistic flame retardant.

Herein, three different phosphorus-containing compounds (methyl phosphoryl dichloride, phenyl phosphoryl dichloride and phenyl dichlorophosphate) were reacted with 2-aminobenzothiazole respectively, and a series of synergistic flame retardants with phosphorus, nitrogen and sulfur elements were synthesized, named MPBT, PPBT and POBT respectively. Then, they were added to prepare flame-retardant flexible polyurethane foam (FPUF). Through the analysis of thermal stability, pyrolysis, heat release and smoke release behavior, the influence of different phosphorus-containing structures on the flame-retardant performance of FPUF was studied, and their flame-retardant mechanism was explored in detail. Among them, MPBT had the highest flame retardant efficiency with the same addition amount (10 wt%). The limiting oxygen index (LOI) value of PU/10.0% MPBT reached 22.5 %, and it successfully passed the vertical burning test. Subsequently, the addition amount of MPBT was increased and the best comprehensive performance of flame-retardant FPUF was explored. The results showed that the LOI value of PU/15.0% MPBT was increased to 23.5%. As for PU/15.0% MPBT, the peak heat release rate (PHRR) was 453 KW/m2, which was reduced by 46.64 %; and the flame retardancy index (FRI) value was also increased to 6.88. At the same time, the mechanical properties of flame-retardant FPUF were studied. The tensile strength of PU/15.0% MPBT reached 170 KPa, and the permanent deformation of FPUF/10% MPBT was only 4 %, showing its excellent resilience. The above results show that this phosphorus-containing element hybrid synergistic flame retardant (MPBT) has a very good application prospect in the field of flame-retardant polymer materials.

Exploration on structural rules of highly efficient flame retardant unsaturated polyester resins.

Owing to the lack of research on structure-activity relationship and interaction mechanism between unsaturated polyester resins (UPR) and flame retardants, it has been a big challenge to prepare high-efficiency flame retardants for UPR in industry. In this research, to explore structural rules of high-efficiency flame retardants, several polymeric flame retardants were synthesized with varied main-chain, side-chain, phosphorus valence states and contents of flame retardant elements. The thermal stabilities of flame retardants and UPR composites were firstly assessed. It has been found the interaction existed between flame retardants and UPR, through transesterification reaction and ß scission pathway in polyester and polystyrene chains. With only 15 wt% of PCH3-S, UPR composites can reach V0 rating in UL-94. The PHRR and THR values can be maximumly decreased by 71.66 % and 77.67 %, with 20 wt% of PB-S. It has been found flame retardants with sulfone group and + 3 valence state of phosphorus in molecular backbone can release SO2 and phosphorus containing compounds in gaseous phase, which diluted fuel fragments and catalyzed H⋅ and HO⋅ radical removal. The mechanism for improved flame retardancy of UPR composites with various polymeric flame retardants were discussed in detail. Some general rules for highly efficient flame retardant UPR can be summarized: First, gaseous phase flame retardant mechanism plays the major role in improvement of flame retardant performance of UPR composites; Second, the combination of + 3 valence state of phosphorus structures, higher phosphorus contents and sulfone groups effectively improves the flame retardant efficiency of flame retardants.

Construction of graphite oxide modified black phosphorus through covalent linkage: An efficient strategy for smoke toxicity and fire hazard suppression of epoxy resin.

The black phosphorus (BP) can be compounded with other two-dimensional materials with flame retardant effect to achieve better synergistic effect. Herein, the multifunctional BP-RGO nanohybrids was fabricated by solvothermal strategy to improve the dispersion state of BP in epoxy resin (EP) and enhance its fire safety performance, where the reduced graphene oxide (RGO) was attached on the surface of BP via PC and POC bonds. With the incorporation of 2.0 wt% BP-RGO into EP matrix, 54.4 % reduction in total heat release (THR) was achieved along with 55.2 % decrease in peak heat release rate (PHRR) compared with neat EP. As a similar trend, the toxic CO and aromatic compounds were significantly inhibited, and the maximum decrease (28.5 %) in total smoke production (TSP) was achieved, indicating the enhanced fire safety performance of EP nanocomposites. These positive results is attributed to the synergistic effect of physical nano-barrier, free radicals trapping and char formation between BP and RGO components. Meanwhile, the EP/BP-RGO2.0 nanocomposites exhibited satisfying air stability even after being immersed in water for a month. This work enriches the strategies for enhancing the air stability of BP, and confirms its potential for smoke toxicity and fire hazard suppression in polymer nanocomposites.

Natural antioxidant functionalization for fabricating ambient-stable black phosphorus nanosheets toward enhancing flame retardancy and toxic gases suppression of polyurethane.

Herein, as a natural antioxidant, tannin (TA) is firstly used to functionalize black phosphorous (BP) nanosheets to improve the ambient stability and toxic suppression, thus decreasing the fire hazards of polymer materials. Compared to pure BP nanosheets, higher temperature for thermal oxidation decomposition is achieved for TA-BP nanosheets, directly confirming the ambient stability of TA-BP nanosheets. Meanwhile, from high resolution TEM and XPS results, TA-BP nanosheets after being exposed at air for 10 days present well-organized crystal structure and low POx bonds content. Cone calorimeter results illustrate that the incorporation of 2.0 wt% TA-BP nanosheets significantly decreases the peak value of heat release rate (-56.5 %), total heat release (-43.0 %), CO2 concentration (-57.3 %) of TPU composite. Meanwhile, with addition of low to 1.5 wt%, the release of highly-toxic CO gas is significantly suppressed, confirmed by lower peak value (0.52 mg/m3) and decreased total release amount (-55.1 %). The obviously enlarged tensile strength (36.7 MPa) and desirable elongation at break (622 %) are also observed. This strategy not only firstly adopts bio-based antioxidant to impart excellent environmental stability for BP nanosheets, but also promotes the promising potentials of BP nanosheets in the fire safety application of polymer composites.

Self-assembly followed by radical polymerization of ionic liquid for interfacial engineering of black phosphorus nanosheets: Enhancing flame retardancy, toxic gas suppression and mechanical performance of polyurethane.

As one of emerging layered nanomaterials, the potential of black phosphorous nanosheets (BP) for fabricating high performance polymer composites was seriously confined by incompatible interface. Herein, interfacial engineering between BP nanosheets and polyurethane (PU) matrix was rationally designed, where employing polymerized ionic liquid as linking bridge between robust BP nanosheets and soft TPU. The ionic liquid (IL) was firstly confined onto the surface of BP nanosheets with the combination of electrostatic-driving self-assembly process and in situ radical polymerization was then performed. The successful preparation of IL-functionalized BP (IL-BP) nanosheets was confirmed by a series of analytic methods, incluing TEM, XPS, FTIR, and so on. The resultant IL-BP nanosheets imparted well mechanical performance and flame retardancy to TPU composites. Compared to the mechanical enhancement reported by other literatures, the break strength of TPU/IL-BP-1.0 was significantly increased by 50%, attributing to strong interfacial regulation of polymerized IL and mechanically robust BP nanosheets, generated by the similar polarity. Meanwhile, significant decreases of 38.2% and 19.7% in peak values of heat release rate and total heat release were achieved for TPU/IL-BP-2.0. With the investigation of combustion residue and pyrolysis products, it was found that a mass of pyrolysis products reacted with IL-BP nanosheets to form mechanically robust protective char and solid products, being no longer used as fuel to support combustion. Meanwhile, the maximum concentration of CO2 and highly toxic CO of TPU/IL-BP-2.0 were effectively decreased by 36.9% and 26.5%, compared to the pure TPU. Such a design route effectively regulates the interfacial interaction between BP nanosheets and polymer matrix and offers a practical route for preparing high performance materials.

Design of Hierarchical NiCo-LDH@PZS Hollow Dodecahedron Architecture and Application in High-Performance Epoxy Resin with Excellent Fire Safety.

Developing advanced performance epoxy (EP) resin with low flammability and light smoke has been an increasing focus of its research. Especially, it is crucial to reduce the emission of smoke and toxic gases generated during the burning of EP, so that it meets the green and safe industrial requirement. Therefore, a 3D NiCo-LDH@PZS hollow dodecahedral structure was designed and synthesized by using the ZIF-67 as both the precursor and an in situ sacrificial template and the amino group-containing polyphosphazene (PZS) as interfacial compatibilizer and flame retardant cooperative. The release behaviors of heat, smoke, and poisonous gases were carefully investigated. More precisely, the EP/NiCo-LDH@PZS4.0 is endowed with a decrease of 30.9% and 11.2% of the peak heat release rate and the total heat release, respectively. The emissions of smoke and poisonous gases including nitric oxides, aromatic compounds, carbonyl compounds, oxycarbide, and hydrocarbons are much less as well. Especially, the maximum release concentrations of HCN of EP/NiCo-LDH4.0 are reduced by 87.8%. With regard to styrene, methane, and ethane, the maximum release concentrations of EP/NiCo-LDH@PZS4.0 are reduced by 85.9%, 90.6%, and 93.1%, respectively. The total yield of CO and CO2 and the consumption of O2 of EP/NiCo-LDH@PZS4.0 are also reduced by 64.5%, 32.4%, and 33.6%. The fractional effective dose, an index of toxicity smoke, of EP/NiCo-LDH@PZS4.0 is reduced by 20.4%. The DMA tests were performed to study the mechanical properties of EP composites, and the storage modulus and Tg of EP composites are increased with the incorporation of NiCo-LDH@PZS. The possible mechanism of flame retardant was proposed based on the analysis of the condensed and gas phases of EP composites.

Hierarchical Structure: An effective Strategy to Enhance the Mechanical Performance and Fire Safety of Unsaturated Polyester Resin.

It is still a big challenge to prepare polymer/layered double hydroxide (LDH) composites with high performance, due to the strong agglomeration tendency of LDHs in the polymeric matrix. In this study, to avoid the agglomerated situation, the orientated LDH nanosheets were vertically grown on a ramie fabric surface, which was then embedded in unsaturated polyester resin (UPR) through the combination method of hand lay-up and vacuum bag. Due to the increased contact area and the restricted interfacial slip in the in-plane direction, the hierarchically LDH-functionalized ramie fabrics (denoted as Textile@LDH) significantly enhanced the mechanical performance of UPR composites. Then, the phosphorus- and silicon-containing coating (PSi) was used for the further improvement of the interfacial adhesion. The tensile strength of UPR/Textile@LDH@PSi composites increased by 121.67%, compared to that of neat UPR. The reinforcement mechanism was studied through analyzing the surface nano/microstructure and wetting properties of the raw and modified textiles, as well as the interfacial interaction between the ramie fabrics and UPR. Meanwhile, the thermal stability, thermal conductivity, and flame-retardant performance of ramie-reinforced UPR composites were improved. Particularly, as-prepared hierarchical Textile@LDH@PSi inhibited the heat release during the combustion process of fabric-reinforced UPR composites, and the peak heat release rate and total heat release values decreased by 36.56 and 47.57%, respectively, compared with the neat UPR/Textile composites. The suppression mechanism was further explored by analyzing the microstructure and chemical compositions of char residues. This research paved a feasible solution to improve the poor dispersion of LDHs in polymers and prepared the high-performance UPR composites with multifunctional applications.

Construction of Hierarchical Natural Fabric Surface Structure Based on Two-Dimensional Boron Nitride Nanosheets and Its Application for Preparing Biobased Toughened Unsaturated Polyester Resin Composites.

It has been a big challenge to prepare the unsaturated polyester resin (UPR) composites with good fire safety, interfacial quality, and impact strength in an environmentally friendly way. In this study, to improve interfacial performance of fabric-reinforced UPR composites, nontoxic two-dimensional hexagonal boron nitride (h-BN) nanosheets were assembled on the surface of ramie fabrics, where sodium alginate acts as a green dispersant to disperse h-BN sheets during the process. Then, the biobased phosphorus-containing toughening agent (PCTA) was synthesized to simultaneously improve the impact strength and fire safety of the composite. With application of h-BN nanosheets-assembled fabric (AF) and 20 wt % of PCTA, the AF/UPR@PCTA20 composite presented the maximum 41.2% decrease in the value of peak heat release rate and a maximum 17.8% decrease in the value of total heat release, which also reached V-0 rating in the vertical burning test. Meanwhile, the AF/UPR@PCTA20 composite showed an obvious increase in limiting oxygen index, from 24.0 to 29.5% compared with RF/UPR. The flame retardant mechanism was investigated from gas phase and condensed phase. Furthermore, compared to neat RF/UPR composite, the AF/UPR@PCTA20 composite showed a significant 68.8% improvement in impact strength, implying an extreme toughening effect of PCTA on UPR composites. The research provides a viable green method for the development of environmentally friendly UPR composites in the future.

Rapid Synthesis of Oxygen-Rich Covalent C2N (CNO) Nanosheets by Sacrifice of HKUST-1: Advanced Metal-Free Nanofillers for Polymers.

A covalent oxygen-rich C2N (CNO) network derived from metal-organic framework (HKUST-1) was innovatively synthesized by a rapid and green microwave irradiation method. This method can produce CNO multilayers efficiently, which paves a way for practical application of the nanosheets. Structural characterization and synthesis processes of CNO nanosheets were investigated to further understand the key role of HKUST-1. The as-prepared CNO has a layered feature, which theoretically favors to improve flame retardancy and mechanical performance of polymers. Desirable results were obtained as expected: the fire safety, antitensile, and impact resistance of polylactic acid (PLA) were prominently enhanced after adding CNO nanosheets, which can be attributed to the excellent dispersion and compatibility. PLA/CNO nanocomposite was self-distinguished at 2 wt % content of CNO, whereas the tensile strength was increased more than 36% compared with that of pure PLA, as well as the impact strength. This work broadens the application fields of CNO and endows it a possibility of actual application.

Manganese phytate dotted polyaniline shell enwrapped carbon nanotube: Towards the reinforcements in fire safety and mechanical property of polymer.

High fire hazard of epoxy resin (EP) has been an unavoidable obstruction on its wide application. Here, a manganese phytate dotted polyaniline shell enwrapped carbon nanotube (MPCNT) is facilely constructed and employed as flame retardant for EP. By adding 4.0 wt% MPCNT, the peak heat release rate, total heat release values, peak CO yields and total CO yields are decreased by 27.2, 12.3, 44.8, and 23.3%, respectively. The decreased absorbance intensity of toxic aromatic volatiles is also observed. Then, a tripartite cooperative flame retardant mechanism (a continuous barrier network, catalytic charring function of phytate, and catalytic activity of MnP/C system) is proposed. Furthermore, the storage modulus of EP composites with 2.0 and 4.0 wt% MPCNT are increased by 23.0 and 25.8% at 40 °C, respectively. Thus, the simultaneous reinforcements in fire safety and mechanical performance of EP are successfully achieved. This work may represent a significant step forward in the facile construction of functionalized carbon materials for achieving their whole potentials in polymer-matrix composite.