Ten‐Minute Synthesis of a New Redox‐Active Aqueous Binder for Flame‐Retardant Li‐S Batteries

As a critical role in battery systems, polymer binders have been shown to efficiently suppress the lithium polysulfide shuttling and accommodate volume changes in recent years. However, preparation processes and safety, as the key criterions for Li‐S batteries' practical applications, still attract less attention. Herein, an aqueous multifunction binder (named PEI‐TIC) is prepared via an easy and fast epoxy‐amine ring‐opening reaction (10 min), which can not only give the sulfur cathode a stable mechanical property, a strong chemical adsorption and catalytic conversion ability, but also a fire safety improvement. The Li‐S batteries based on the PEI‐TIC binder display a high discharge capacity (1297.8 mAh g−1), superior rate performance (823.0 mAh g−1 at 2 C), and an ultralow capacity decay rate of 0.035% over more than 800 cycles. Even under 7.1 mg cm−2 S‐loaded, the PEI‐TIC electrode can also achieve a high areal capacity of 7.2 mA h g−1 and excellent cycling stability, confirming its application potential. Moreover, it is also noted that TG‐FTIR test is performed for the first time to explore the flame‐retardant mechanism of polymer binders. This work provides an economically and environmentally friendly binder for the practical application and inspires the exploration of the flame‐retardant mechanism of all electrode components.


Introduction
The serious environmental pollution and energy crisis have stimulated the search for advanced energy storage technology.Lithium-sulfur (Li-S) batteries, with an appealingly energy density (2600 Wh kg −1 ) and low cost, have been considered as one of the most promising substitutes to the current lithium-ion batteries for next-generation sustainable electrochemical energy storage technology. [1,2]Future Li-S batteries are expected to display a superior volumetric energy density of 700 Wh L −1 and a practical gravimetric energy density of 400-600 Wh kg −1 . [3,4]Until now, its commercialization has been challenging in sulfur utilization, rate performance, long cycle life, and safety concerns, due to the insulating sulfur species (including S, Li 2 S 2 , and Li 2 S), large volume expansion (∼80%), and undesirable "shuttle effect". [5,6][9] However, binder, an indispensable component in Li-S batteries to adhere and hold the conductive agent, active materials, and current collector together, still attracted less attention.Until now, polyvinylidene fluoride (PVDF), a linear polymer with good electrochemical stability and acceptable physical adhesion, has been the most typically used in Li-S batteries. [10,11]However, the Li-S batteries based on the PVDF binder still do not display satisfactory performance due to its intrinsic drawbacks: 1) PVDF with a weak interchain van der Waals forces is easy to swell in organic electrolytes and displays a low mechanical strength in cycling.2) PVDF is only soluble in expensive, toxic, and flammable organic solvents like N-methyl-2-pyrrolidone (NMP).3) The nonfunctionalized chain structure of PVDF is less effective in trapping the polar polysulfide intermediate and promoting polysulfide conversion, leading to a rapid capacity fade. [12,13]][21] Although these binders have contributed impressive electrochemical properties, their complicated preparation processes and sluggish kinetic behaviors still obstruct the practical application. [22,23]Besides, the other key criterion for Li-S batteries' practical applications is safe operation. [24]It is a promising strategy that flameretardant polymeric binders can be introduced into the battery system As a critical role in battery systems, polymer binders have been shown to efficiently suppress the lithium polysulfide shuttling and accommodate volume changes in recent years.However, preparation processes and safety, as the key criterions for Li-S batteries' practical applications, still attract less attention.Herein, an aqueous multifunction binder (named PEI-TIC) is prepared via an easy and fast epoxy-amine ring-opening reaction (10 min), which can not only give the sulfur cathode a stable mechanical property, a strong chemical adsorption and catalytic conversion ability, but also a fire safety improvement.The Li-S batteries based on the PEI-TIC binder display a high discharge capacity (1297.8mAh g −1 ), superior rate performance (823.0 mAh g −1 at 2 C), and an ultralow capacity decay rate of 0.035% over more than 800 cycles.Even under 7.1 mg cm −2 S-loaded, the PEI-TIC electrode can also achieve a high areal capacity of 7.2 mA h g −1 and excellent cycling stability, confirming its application potential.Moreover, it is also noted that TG-FTIR test is performed for the first time to explore the flame-retardant mechanism of polymer binders.This work provides an economically and environmentally friendly binder for the practical application and inspires the exploration of the flame-retardant mechanism of all electrode components.
to ensure safe operation. [25,26]On the basis of the above consideration, satisfactory binders should possess the characteristics of easy to prepare, stable mechanical properties, strong chemical adsorption ability or even catalytic conversion for polysulfides, environmentally friendly, safe operation to significantly enhance the battery performances and application potential of Li-S batteries.
Triglycidyl isocyanurate (TGIC) is a commercially heterocyclic polyepoxide with three epoxy groups that can easily produce highdensity cross-linking with amino groups or hydroxyl groups. [27,28][29] Herein, we elaborately prepared a new type of multifunctional aqueous binder (PEI-TIC), which was constructed through triglycidyl isocyanurate (TGIC) and polyethyleneimine (PEI) by cross-linking modification.The as-prepared PEI-TIC possesses the following advantages in Li-S batteries: 1) Rapid (10 min) one-pot reaction: The fast one-pot reaction between epoxy group in TGIC and amino group in PEI lasts only 10 min to form a homogeneous aqueous solution.2) Environmentally friendly: Both material preparation and slurry formation are carried out in aqueous solution and low energy consumption environment (70 °C). 3) Strong adhesive strength and appropriate mechanical behaviors: The formation of the 3D cross-linked network endows the PEI-TIC binder with higher adhesive and mechanical properties. [30]In addition, accompanied by rapid solvent evaporation, aqueous binders will further form a 3D cross-linked network structure, which can favor electrolyte infiltration and accommodate volume changes during cycling. [31]4) Flame retardant: The PEI-TIC has a better thermal stability.Simultaneously, the PEI-TIC can release inert gases (such as CO 2 and NH 3 ) to impair the transfer of heat, fuel, and combustible gases.5) Anchoring or even catalytic conversion of lithium polysulfides: The functional groups like amino, carboxyl, and isocyanurate groups would contribute to chemical adsorption lithium polysulfides and even the redox kinetics.Therefore, the PEI-TIC electrode delivered an excellent rate capability of 823.0 mA h g −1 at 2 C and an outstanding areal capacity of 7.2 mA h g −1 even under 7.1 mg cm −1 S-loaded.Owing to these features that work cooperatively, this low-cost, easy-to-prepare, and environment-friendly binder (PEI-TIC) has great potential for mass production and practical application in high-performance Li-S batteries.

Results and Discussion
The epoxy-amine ring-opening reaction is one of the most widely used reactions in the preparation and modification of various materials because it is fast and occurs under mild conditions without the use of a catalyst. [11,27,32]As shown in Figure 1a, Figures S1 and S2, Supporting Information, TGIC, with highly reactive epoxy groups, can easily react with the amino groups of PEI at 70 °C for 10 min and form a hydrophilic monolith as an aqueous binder.Then, the hydrophilic monolith would form a highly cross-linked 3D network structure during the fabrication process of the electrode.Such in situ thermal cross-linking way not only gives the binder a stronger adhesion and mechanical strength but also can effectively improve the processability of highly crosslinked binders. [19,33,34]The cross-linking effect of TGIC and PEI was first studied by Fourier transform infrared (FTIR).Compared with TGIC sample, the characteristic peak of epoxy group in PEI-TIC at 926 cm −1 obviously disappeared, accompanied by the N-H bending vibration peak of 1598 cm −1 and stretching vibration peak of 3274 cm −1 intensity decrease, indicating that epoxy-amine ring-opening reaction occurred (Figure 1b).Besides, the peak centered at 1680 cm −1 of TGIC and PEI-TIC is attributed to C=O stretching vibration, which can further confirm that PEI has successfully cross-linked by TGIC crosslinking agent.To further confirm the surface chemical states, PEI-TIC binder was studied by X-ray photoelectron spectroscopy (XPS).Four different peaks in the XPS C 1 s spectra at around 284.7, 285.5, 286.1, and 287.9 are corresponds to the C 1s species of C-C, C-N, C-O, and N-C=O, respectively (Figure 1c).Besides, the O 1 s signals are from both N-C=O (531.6 eV) and C-OH/C-O-C (532.5 eV) groups. [35]hese experimental results further demonstrate that the epoxy groups in TGIC and amino groups in PEI can easily reaction with low energy consumption and generate a great deal of hydrophilic hydroxyl groups to obtain an environmentally friendly aqueous binder.
The highly cross-linked 3D network structure endows the PEI-TIC binder with enhanced mechanical properties.The solid-state PEI-TIC copolymer was cut into a small disk for qualitatively bend and mechanical strength tests.As depicted in Figure 1d,e, the results indicate that the solid-state flexible PEI-TIC copolymer could lift a 100 g weight with no breaking.Besides, adhesion test in Figure 1f clearly shows that PEI-TIC binger can provide strong mechanical strength up to 500 g weight.To further evaluate the possibility of the PEI-TIC as the binder, the rheological behavior of the cathode slurries with PEI and PEI-TIC binders is first assessed by a cone-plate rheometer.As shown in Figure 2a, the measured viscosity at different shear rates clearly shows that both PEI and PEI-TIC cathode slurries display significant shear thinning behavior, which ensures the workability of the slurries during highshear-rate coating process.Compared with the PEI, the PEI-TIC binder displays higher viscosity due to the strong interaction of the plentiful oxygen-containing groups (C=O, C-OH, and C-O-C) in TGIC with other cathode components.And then, the viscoelastic effects were characterized by oscillatory rheological measurements.Variations of the storage modulus (G 0 , elastic part) and loss modulus (G 00 , viscous part) of PEI and PEI-TIC with shear stress are shown in Figure 2b and Figure S3, Supporting Information.In Region I, the value of G 00 is lower than that of G 0 , indicating that cathode slurries form network structure and display an elastic-dominant behavior. [36]With the strain amplitude increases, the G 00 is finally higher than G 0 , which indicates a liquiddominant behavior.PEI-TIC cathode slurry is easier to form a strong network and maintain the elastic-dominant behavior than that of PEI cathode slurry, as proved by its higher strain amplitude of G 0 -G 00 crossover point.The electrode slurries with PVDF, PEI, and PEI-TIC binders were coated on the carbon-coated Al foil and dried for 12 h.In particular, PEI-TIC will further form a 3D cross-linked network structure during the process of heating.The 3D cross-linked network and abundant oxygen-containing groups endow the PEI-TIC binder with enhanced adhesion force and mechanical properties.As shown in Figure 2c, the 180°peeling test results of PVDF, PEI, and PEI-TIC electrodes clearly revealed that PEI-TIC binder displays a higher adhesion force than that of PEI and PVDF.Nanoindentation measurements were performed to further characterize the enhanced mechanical properties (Figure 2d).At the same indentation depths, the PEI-TIC electrode represents the highest force imposed in three different binders, implying to its most robust.Clearly, these results imply that the PEI-TIC binder with 3D cross-linked network has flexibility, strong adhesive, and appropriate mechanical properties.As shown in Figure S4, Supporting Information, SEM was carried out to characterize the surface morphology of PEI-TIC electrode.At different scales, the electrode surface always shows integrity structure without cracks, indicating that the PEI-TIC binder has Energy Environ.Mater.2024, 7, e12572 strong bonding ability.The EDS mapping displays a uniform of N and O atoms on PEI-TIC electrode, which can effectively adsorb polysulfides.
Moreover, it is expected that PEI-TIC will be beneficial to the adsorption and reaction kinetics of lithium polysulfides.As shown in Figure 2e inset, visualized adsorption experiments were carried out by mixing 300 mg different binders with 8 mL Li 2 S 6 solution (10 mM) to study the adsorption capacity of PVDF, PEI, and PEI-TIC binders to polysulfides.Significantly, the Li 2 S 6 solution exposed to PEI and PEI-TIC has turned colorless after adsorption while the solution with PVDF still displays slight discoloring, visually demonstrating the strong adsorption ability of PEI and PEI-TIC.Ultraviolet-visible spectroscopy was performed to quantitative tracking the Li 2 S 6 adsorption ability of PVDF, PEI, and PEI-TIC binders.As witnessed in Figure 2e, the Li 2 S 6 peak at 420 nm obviously decreases and almost disappears after treated with PEI and PEI-TIC binders.The PEI and PEI-TIC binders display strong adsorption ability due to the strong interaction between the functional groups (like amino, carboxyl, and isocyanurate groups) and polar Li 2 S 6 .And then, to elucidate the possible interactions between PEI-TIC and polysulfides, XPS analysis was performed after Li 2 S 6 adsorption.As depicted in Figure 2f, the S 2p peak reveals six peaks located at 161. ), and polythionate [O 3 S 2 -(S) x-2 -S 2 O 3 ], respectively. [36,37]The formation of surface S 2 O 3 2− /O 3 S 2 -(S) x-2 -S 2 O 3 species is probably attributed to reactions between Li 2 S n and PEI-TIC, manifesting their strong interaction.[40] In order to describe this interaction, the binding energies (E b ) were estimated using the equation: E b = E ad-sub -E ad -E sub .As shown in Figure 2g,h and Figure S5, Supporting Information, the binding energy of PEI-TIC-Li 2 S 8 (2.30 eV), PEI-TIC-Li 2 S 6 (2.66 eV), and PEI-TIC-Li 2 S 4 (3.09eV) is higher than that of PEI-Li 2 S 6 (1.26 eV), PEI-Li 2 S 6 (1.14 eV), and PEI-Li 2 S 4 (1.45 eV), substantiating that the adsorption performance is distinctly improved after the introduction of TGIC.
Besides the adsorption effect, the potential kinetic improvement ability is also an important evaluation index for Li-S batteries binders.First, to exclude any influence from Li metal electrode, the symmetric cells with Super P and three different binders were assembled to conduct electrochemical kinetics measurements.As shown in Figure 3a, PEI-TIC binder reflects a smaller potential hysteresis and higher redox peaks than those of PEI and PVDF at 2 mV s −1 , indicating that the conversion between Li 2 S 6 and Li 2 S on PEI-TIC binder is pronouncedly enhanced.The fast reaction of polysulfides at the liquid-solid boundary is a key factor for improving the reaction kinetics.Therefore, the Li 2 S nucleation behaviors were investigated by potentiostatic discharge measurements, as presented in Figure 3b,c.PEI-TIC binder displays earlier current response time, higher current intensity, and precipitation capacities (171.9 mAh g −1 ) than that of PEI (precipitation capacities is 155.1 mAh g −1 ), demonstrating that the PEI-TIC binder is facilitating to the rapid ion transfer and fast Li 2 S nucleation.And then, the Gibbs free energies in each step conversion reaction were calculated based on DFT calculations to further reveal the intrinsic catalytic effect of PEI-TIC.As shown in Figure 3d, the reaction of S 8 to Li 2 S 8 on PEI and PEI-TIC binders shows negative Gibbs free energies, implying that the reaction is spontaneous.In all involuntary reaction processes from Li 2 S 8 to Li 2 S (Li 2 S 8 → Li 2 S 6 → Li 2 S 4 → Li 2 S 2 → Li 2 S), the Gibbs free energies of PEI-TIC are lower than that of PEI in each step, suggesting that the reduction process on PEI-TIC is more thermodynamically favorable.
The reaction from Li 2 S 2 to Li 2 S with the highest Gibbs free energies is the rate-limiting step and the origination of the sluggish sulfur redox reaction of Li-S batteries.PEI-TIC displays a lower Gibbs free energy of 1.187 eV in the rate-limiting step than that of PEI (1.364 eV), indicating a greater Li 2 S nucleation efficiency (consistent with the abovementioned experimental results in Figure 3a-c).In addition, the decomposition energy barriers of the adsorbed Li 2 S on PEI and PEI-TIC were also investigated.As depicted in Figure 3e,f and Figure S5, Supporting Information, the energy barrier for Li 2 S decomposition on PEI-TIC substrates (0.72 eV) is lower than that on PEI (0.98 eV), demonstrating the intrinsic ability of PEI-TIC substrates to promote the charging process.
The batteries with three different binders were characterized by using C/S complex as the active material and lithium metal as the counter/reference electrode to reveal the effectiveness of PEI-TIC binders in practical Li-S batteries.Figure 4a exhibits typical cyclic voltammetry (CV) curves of the PVDF, PEI, and PEI-TIC electrodes.The CV curve of PEI-TIC electrode displays two reduction peaks (at 2.29 and 2.00 V) in the cathodic scan and an oxidation peak (at 2.39 V) in the anodic scan, corresponding to the reduction processes of S to Li 2 S 4 (I c1 ), Li 2 S 4 to Li 2 S (I c2 ), and the oxidation process of Li 2 S to Li 2 S 8 /S (I a1 ), respectively.Compared with PEI-TIC electrode, the PEI and PVDF electrodes show lower peak potentials for I c1 and I c2 peaks and a higher peak potential for Ia 1 , implying that electrochemical polarization can be efficiently reduced by PEI-TIC binder (Figure 4b).Tafel slopes calculated from the CV curves are used to assess the conversion kinetics of PVDF, PEI, and PEI-TIC electrodes.As shown in Figure 4c, Figures S6 and S7, Supporting Information, the fitting slopes of PEI-TIC electrodes at I c1 , I c2 , and I a1 are 82.5, 46.8, and 82.3 mV dec −1 , which are lower than that of PEI (84.7, 102.5, and 83.9 mV dec −1 ) and PVDF (117.4,104.2, and 181.9 mV dec −1 ), indicating that PEI-TIC can improve the conversion kinetics of polysulfides.Moreover, galvanostatic intermittent titration (GITT) experiments and electrochemical impedance spectroscopy (EIS) measurements were conducted to further explore the reaction and diffusion kinetics of PVDF, PEI, and PEI-TIC electrodes.The internal resistances (ΔR) of three different binders are examined by GITT measurements at a 0.1 C-rate to quantify the polarization degree during electrochemical operation (Figure 4d, Figure S8a,b, Supporting Information).The ΔR is calculated as follows: [41] IΔR where I and ΔV in formula (1) represent the applied current and the voltage difference between the points of quasi-OCV and close circuit voltage, respectively.PEI-TIC electrode displays smaller ΔR values than PVDF and PEI electrodes during Li 2 S nucleation and Li 2 S activation and even all the charge and discharge processes, which suggests the PEI-TIC electrode had accelerated redox kinetics and a higher Li + diffusion rate (Figure 4e,f).Besides, the detailed analysis from GITT curves in Figure S9a-c, Supporting Information also shows that the high voltage platforms (Q H ) of PEI-TIC, PEI, and PVDF electrodes occupy 25.7%, 26.9%, and 28.0% of the entire discharging time, corresponding to the Q L /Q H values of 2.89, 2.70, and 2.57 (theoretical value is 3).The higher the value of Q L /Q H indicates the significantly promoted conversion in the PEI-TIC electrode of Li 2 S 4 to Li 2 S (Figure 4g).Benefiting from water evaporation and in situ thermal cross-linking way, the PEI-TIC electrode can easily form a highly cross-linked 3D network structure, which is beneficial to Li + diffusion properties.Besides GITT measurement, EIS can also be used to obtain the information about the Li + diffusivity property.The EIS plots of PVDF, PEI, and diffusion coefficient, can be determined at low frequencies. [42,43]s shown in Figure 4i, PEI-TIC electrode exhibits a lower Warburg impedance coefficient (σ) of 1.53 Ω cm 2 s −0.5 than those of PVDF (3.81 Ω cm 2 s −0.5 ) and PEI (3.78 Ω cm 2 s −0.5 ), again demonstrating the fast Li + diffusion of PEI-TIC, in line with the CV and GITT results.).j) Compare the integrated electrochemistry performance with other current reported binders, such as AFG, [35] D-PAA/ C-EA, [49] PVP-PEI, [17] SPP, [50] CMC-CA, [19] PDAT, [51] Maleate-PEG, [52] CSEG, [48] PAA-HPRN + . [53]lectrochemical performance evaluations were conducted to investigate the superiority of PEI-TIC binder.Figure 5a displays the cycling performance of PVDF, PVDF + TGIC (physical mixture), PEI, and PEI-TIC electrodes at 0.5 C, among which PEI-TIC electrode displays the highest specific capacity of 1055.7 mAh g −1 and preserves a capacity of 867.5 mAh g −1 after 300 cycles, corresponding to a capacity retention of 82%.By contrast, PVDF, PEI, and PVDF + TGIC electrodes only exhibit the final capacities of 483.9, 596.2, and 593.6 mAh g −1 and worse capacity retention rates of 53.8%, 63.3%, and 65.4%.The excellent cycling performance of PEI-TIC electrode is mainly attributed to its superior polysulfide adsorption and fast Li 2 S redox reaction.It is also noted that the average Coulombic efficiencies (CE) in PVDF, PEI, and PEI-TIC electrodes are over 98.5% in the last cycle, higher than that of PVDF + TGIC (94.6%), demonstrating that TGIC added into electrode through physical mixture may increase the shuttle effect.Furthermore, the rate capabilities under various current rates were investigated, as presented in Figure 5b.Due to the low Li + diffusion resistance and fast reaction kinetics, the PEI-TIC electrode shows a superior rate performance of 1297.8, 1147.3,1049.4,941.3, and 823.0 mAh g −1 at current rates of 0.1, 0.2, 0.5, 1, and 2 C, respectively, which are higher than those of PVDF and PEI electrodes.Figures 5c, Figures S10 and S11, Supporting Information, show the corresponding charge-discharge curves of PVDF, PEI, and PEI-TIC electrodes under various current rates.The profiles of PVDF and PEI electrodes display a significant shift and a lower discharge plateau at 2 C, which is attributed to their high polarization and sluggish redox kinetics.In comparison, the profiles of the PEI-TIC electrode show clear and flat plateaus from 0.1 to 2 C.Then, the cycling durability of PEI-TIC electrode at 1 C is shown in Figure 5d.The PEI-TIC electrode displays a superior initial reversible capacity of 817.0 mAh g −1 and 76% capacity retention even after 800 cycles, corresponding to a negligible capacity decay rate of 0.035%.][46][47][48] In addition, the PEI-TIC electrode displays an excellent rate performance of 1137.7,960.0, 900.7, 807.1, and 715.2 mAh g −1 at current rates of 0.1, 0.2, 0.3, 0.5, and 0.8 C, respectively, even at 4 mg cm −2 S-loaded (Figure 5g).Stable cycling behavior of PEI and PEI-TIC electrodes with S loading of 4.1 and 4.3 mg cm −2 is presented in Figure 5h.The PEI-TIC electrode with 4.3 mg cm −2 delivers a high specific capacity of 888.0 mAh g −1 with satisfactory Coulombic efficiency over 96% and 80% capacity retention after cycling, which is higher than that of PEI electrode (with a final capacity of 648.3 mAh g −1 and 64% capacity retention).To further satisfy the demands of practical batteries, the high sulfur loading and lean electrolyte conditions have attracted a great deal of attention.Therefore, PEI-TIC electrode was further used to assemble the battery with high sulfur loading of 7.1 mg cm −2 (Figure 5i).Even at 7.1 mg cm −2 S-loaded and a relatively low E/S ratio of 9 μL mg −1 , the PEI-TIC electrode can still provide a high area capacity of 7.2 mAh cm −2 and stabilize 6.3 mAh cm −2 after cycling (CE > 96%).The high area capacity and superior cycling performance can be attributed to superior Li + diffusion properties, polysulfide adsorption and reaction kinetics of PEI-TIC binder with 3D cross-linked network structure and numerous active functional groups (Figure 5f).[50][51][52][53] Besides enhancing electrochemical performance, flammability is also a crucial safety hazard, which needs to be mitigated, for Li-S batteries' practical applications.As shown in Figure 6a, the PEI-TIC binder started to burn upon exposure to flame about 6 s.Subsequently, the PEI-TIC binder effectively suppressed with greatly weakened flame and even self-extinguished the flame, which may benefit from the highly crosslinked copolymer structure of PEI-TIC binder that can generate Ncontaining radicals and noncombustible NH 3 /CO 2 /water gas to impair the transfer of heat, fuel and oxygen. [54,55]To further investigate the gas phase pyrolysis products during PEI-TIC combustion process, [56,57] TG-FTIR was performed, as presented in Figure 6b,c.The 3D TG-FTIR spectra of PEI-TIC clearly show that the pyrolysis had not occurred before 24 min (corresponding to 270 °C), but only a small amount of CO 2 was released.PEI-TIC displays a maximum decomposition rate at 350 °C, and the main peaks of 3D TG-FTIR spectra can be assigned as follows: NH 3 (926 and 962 cm −1 ), -OH (1140 and 3487 cm −1 ), C=O (1756 cm −1 ), CO 2 (2298 and 2378 cm −1 ), and -CH 3 /-CH 2 CH 3 (2774-3009 cm −1 ).Some of the noncombustible gases (NH 3 and CO 2 ) with strong flame-retardant potential are unambiguously  6d,e).By contrast, the peak intensity of C=O shows a maximum decomposition rate at 375 °C, implying that TGIC has a better thermal stability.These results clearly verify that the apparent flame-retardant property of the PEI-TIC binder stems from the release of noncombustible gas and the formation of the cross-linked thermostable polymer.The above results provide the strong evidence that PEI-TIC binder with the characteristics of easy to prepare, environmentally friendly, superior adhesive strength, appropriate mechanical behaviors, flameretardant, strong chemical adsorption, and catalytic conversion ability holds promise for applications in Li-S batteries.

Conclusion
With environment-friendly process, an aqueous binder (PEI-TIC) has been successfully prepared through an easy and fast epoxy-amine ringopening reaction (10 min) for high-performance Li-S battery.The 3D cross-linking network structure in PEI-TIC binder can not only provide superior adhesive strength and considerable mechanical behaviors to build a robust electrode, but realize fast Li + diffusion.Moreover, numerous functional groups like amino, carboxyl, and isocyanurate groups are beneficial to the chemical adsorption and reaction kinetics of lithium polysulfides.Attributed to these advantages, the Li-S batteries based on the PEI-TIC binder deliver a high initial discharge capacity of 817.0 mAh g −1 and an outstanding cycling stability with 0.035% capacity decay rate over 800 cycles.Even under 7.1 mg cm −2 S-loaded (9 μL mg −1 ), a high areal specific capacity of 7.2 mAh cm −2 and a superior cycling stability, implying its great application prospects.In addition, as proved by TG-FTIR, the PEI-TIC binder with flameretardant property would significantly improve the safety of Li-S batteries.Such an environment-friendly and flame-retardant binder epitomizes a significant step toward realizing a practical low-cost, highperformance, and safer Li-S battery.

Experimental Section
Preparation of the PEI-TIC aqueous binder: 400 mg polyethyleneimine (PEI) and 200 mg triglycidyl isocyanurate (TGIC) were added into 9 mL distilled water and stirred at 70 °C for 10 min to prepare the aqueous binder (PEI-TIC).After filtering, the resulting water solution with a mass ratio of 1 mg per 15 μL solvent.
Synthesis of Li 2 S 6 solution and adsorption test: The Li 2 S 6 catholyte was prepared by mixing sulfur and lithium sulfide (Li 2 S) at a molar ratio of 5:1 into a DOL/DME mixture (1:1 Vol%) and stirring overnight at 70 °C in an Ar-filled glovebox.Then, 300 mg of PVDF, PEI, or PEI-TIC was added to 8 mL of 10 mM Li 2 S 6 solutions for observation.
Measurement of the Li 2 S nucleation: The Li 2 S 8 catholyte was prepared by mixing sulfur and Li 2 S at a molar ratio of 7:1 into a DOL/DME mixture (1:1 Vol%) and stirring overnight at 70 °C in an Ar-filled glovebox.The working electrodes were prepared by mixing conductive agent and binder (PEI and PEI-TIC) by a weight ratio of 6:4.Then, 20 μL Li 2 S 8 catholyte (0.5 M) and 20 μL blank electrolyte were added into the working electrodes and the Li anode side, respectively.The cells were discharged 2.09 V at 0.1 C and then discharged at 2.08 V until the current is lower than 0.03 mA for Li 2 S nucleation and growth.
Symmetrical cell assembly and measurements: The electrodes were prepared by mixing conductive agent and binder (PEI and PEI-TIC) by a weight ratio of 6:4.Two identical electrodes were used as working electrode and counter electrode, respectively, to prepare the symmetrical cells.Then, the symmetrical cells with 40 μL Li 2 S 6 catholyte (0.2 M) were tested at scan rate of 2 mV s −1 (voltage range from −1.0 V to 1.0 V).
Preparation of PEI-TIC electrode and electrochemical measurement: The S/ C composite was prepared by adding commercial sulfur powder, graphene, ethanol, and CS 2 into a glass vial.Then, the mixture was sonication for 1 h and stirred at 60 °C to remove ethanol and CS 2 , followed by heating to 155 °C for 10 h.70 mg S/C composite, 15 mg conducting agent, 15 mg binder (PEI or PEI-TIC), and a few drops of water were mixed and ground to form a uniform slurry.Then, the slurry was coated on a carbon-coated Al foil and dried at 70 °C for 12 h in a vacuum oven.The standard CR2032 coin-type cells were assembled using circular cathode sheets with a diameter of 11 mm and 1.0-1.5 mg cm −2 S-loaded, Li metal anode and commercial electrolyte (1.0 M LiTFSI in DME: DOL = 1:1 Vol% with 2.0% LiNO 3 ) in an Ar-filled glovebox.The coin cells have a relatively low electrolyte/sulfur (E/S) ratio of 15:1 μL mg −1 .For the thick cathode sheets with 4.0-4.3 and 7.1 mg cm −2 S-loaded, the E/S ratios of 12 and 9 μL mg −1 were applied, respectively.The CV tests and EIS (10 mHz-100 kHz) were measured on a VMP-3 multichannel workstation, while the long-term cycling, rate performance, and GITT were performed using LAND-CT2001C and LAND-CT3002C test systems.The GITT was cycled with 20-min current pulses at 0.1 C and the open-circuit voltage periods (20 min).

Figure 1 .
Figure 1.a) Schematic illustration of PEI-TIC binder.b) FTIR spectra of PEI, TGIC, and PEI-TIC solid-state copolymer.c) XPS C 1 s and O 1 s spectra of solidstate PEI-TIC copolymer.d) Bend property, e) adhesion property, and f) mechanical strength tests of PEI-TIC.

Figure 2 .
Figure 2. a) Variation of viscosity with shear rate for PEI and PEI-TIC cathode slurries.b) Variation of G 0 and G 00 with shear stress for PEI-TIC cathode slurry.c) Peeling strength of the PVDF, PEI, and PEI-TIC electrodes.d) Load-indentation depth curves of the PVDF, PEI, and PEI-TIC electrodes.e) UV-vis spectra of Li 2 S 6 solution after static adsorption.The inset is the digital photos after static adsorption.f) XPS S 2p spectrum of PEI-TIC binder after static adsorption.g, h) Binding energy of PEI-TIC and PEI with polysulfides.

Figure 3 .
Figure 3. a) CV curves of PVDF, PEI, and PEI-TIC symmetrical cells with Li 2 S 6 and PEI-TIC symmetrical cell without Li 2 S 6 at 2 mV s −1 .Precipitation profiles of Li 2 S with b) PEI-TIC and c) PEI electrodes.d) The Gibbs free energy profiles of sulfur species on PEI and PEI-TIC.e, f) Energy barriers profiles of Li 2 S decomposition on PEI and PEI-TIC.

Figure 4 .
Figure 4. a) CV curves of PVDF, PEI, and PEI-TIC electrodes in Li-S batteries at 0.1 mV s −1 .b) The peak potential in the oxidation (I a1 ) and reduction process (I c1 and I c2 ).c) Tafel plots of PVDF, PEI, and PEI-TIC electrodes at I c2 .d) GITT plots on PEI-TIC electrode.Internal resistances of PVDF, PEI, and PEI-TIC electrodes during the e) discharge and f) charge process.g) The corresponding histogram of Q L /Q H . h) Nyquist plots of PVDF, PEI, and PEI-TIC electrodes.i) Variation of Z' with ω −0.5 at low frequencies.

Figure 6 .
Figure 6.a) Flame-retardant tests of PEI-TIC copolymer.b, c) 3D FTIR spectra of decomposed products.d) FTIR spectra of decomposition products at different temperatures.e) Relationship between absorbance and temperature for NH 3 , CO 2 , and -CH 3 /CH 2 CH 3 .