A high‐safety, flame‐retardant cellulose‐based separator with encapsulation structure for lithium‐ion battery

The safety issues of lithium‐ion batteries have received attention because flammable organic electrolytes are used. Also, the commercial polyolefin separator will undergo severe thermal shrinkage when the internal temperature of the battery increases to 130–160°C, which increases the risk. Therefore, the development of a high thermal stability and high‐safety separator is an effective strategy to improve battery safety. Herein, we design a green, cellulose‐based separator (Cel@DBDPE) with a unique encapsulation structure for lithium‐ion batteries, in which functional flame retardants (DBDPE) are wrapped in microscrolls formed by the self‐rolling of 2D cellulose nanosheets upon freeze‐drying. This structure can firmly anchor DBDPE particles in the separator to prevent them from undergoing exfoliation and does not affect the properties of the separator, such as the thickness and the pore structure. Compared with commercial polypropylene, Cel@DBDPE has excellent thermal stability and flame retardancy. The former makes it less prone to thermal shrinkage and the latter can effectively prevent the combustion of the electrolyte, showing an efficient self‐extinguishing ability. Moreover, the Cel@DBDPE is only 15 μm in size and has competitive properties comparable to polypropylene. Thus, there is no sacrifice in the electrochemical performance of battery when the Cel@DBDPE is used as separator. This study provides a new structural design for the construction of a high‐safety separator.

To improve the safety of the batteries, researchers have proposed different solutions on the basis of the internal components of the battery, such as designing electrodes with a thermal-responsive function, [14][15][16][17] developing nonflammable electrolytes, [18][19][20][21][22] constructing high-safety separators, [23][24][25] and so forth.For example, Cui et al. 26 constructed a novel lightweight current collector with a Cu/PI/Cu sandwich structure, which can effectively improve the safety of batteries because of pre-embedding of flame-retardant components in the PI layer.The organic/metal composite current collector has significantly lower weight than the traditional metal foil, which can help to increase the energy density of batteries, but its current mechanical strength is not robust enough for industrial applications.Yamada et al. 27 used nonflammable solvents and special lithium salts to develop nonflammable high-salt-concentration electrolytes, which can drastically reduce the flammability of batteries.However, the properties (such as ionic conductivity, viscosity) of these nonflammable electrolytes are sacrificed to a certain extent compared with the commercial electrolyte and the cost is also higher.In fact, both the electrode and the electrolyte participate in the reaction of the battery, and the component changes of electrode and electrolyte will sacrifice their intrinsic properties, such as tensile strength, conductivity, viscosity, and so forth, which may further more or less affect the battery's performance.In other words, these strategies have to make a trade-off between flame retardancy and the electrochemical performance of batteries.
In contrast, introducing a flame-retardant function into the separator is a preferable and facile strategy.This is because the separator, as an "inert" internal component of the battery, does not participate in the chemical reaction process of the battery and has little direct influence on battery performance.It is well known that the currently used commercial polyolefin separator undergoes severe thermal shrinkage at a certain temperature (130-160°C), resulting in a short circuit between the cathode and the anode, which leads to fire or explosion.9][30][31] For example, Zhu et al. 32 prepared a safe separator based on hydroxyapatite nanowires with excellent thermal stability, which could still maintain structural integrity at temperatures as high as 700°C.Nevertheless, this separator has no flame-retardant function and therefore cannot extinguish the burning electrolyte.Recently, Cui et al. 33 constructed a 4-8 μm thick flameretardant coating on the surface of a commercial separator.When the electrolyte is ignited, this coating separator can effectively inhibit its combustion, so that the commercial separator has the function of flame retardancy.However, the coating method will increase the thickness and the internal resistance of the separator.In addition, there is also the possibility that the coating particles will fall off from the separator during battery operation, which will affect the performance of the battery.Therefore, it is of great interest to develop a new structural design that endows the separator with a flame-retardant function without a negative influence on the properties of the separator, like thickness, resistivity, and pore structure.
Herein, we use 2D bamboo cellulose nanosheets as the basic structural unit and integrate flame retardants (decabromodiphenyl ethane [DBDPE]) to construct a green and safety separator (Cel@DBDPE) with a unique encapsulation structure.The Cel@DBDPE separator shows an attractive morphology where the flame-retardant particles are highly dispersed and anchored in the cellulose microscrolls formed by the self-rolling of 2D cellulose nanosheets to 1D fiber-like scrolls during freeze-drying.In the Cel@DBDPE separator, the flame retardants are firmly and uniformly embedded in the interior of the separator rather than as a surface coating of the separator, neither affecting the separator's surface microstructure nor introducing additional binders.Also, this structure not only preserves the porous structure of the separator but also prevents the flame retardants from falling off the separator and into the electrolyte.Moreover, the Cel@DBDPE separator shows excellent thermal stability and flame retardancy.Therefore, it was quickly extinguished after burning for only 0.67 s in a combustion experiment, while commercial polypropylene (PP) was completely burned out, and maintained good structural integrity at 210°C.Furthermore, the thickness of Cel@DBDPE is only 15 μm and it also presents competitive properties compared with the commercial PP separator.On performing the electrochemical test, the batteries equipped with Cel@DBDPE and PP show similar performances.The unique structural design involved in this study provides a new perspective for the construction of a high-safety separator.Also, the Cel@DBDPE separator has great potential for application in high-safety lithium-ion batteries.

| Preparation of a cellulose@DBDPE separator
Cellulose nanosheets were prepared according to our previously reported work. 34,35First, a certain amount of flame-retardant particles, DBDPE, was dispersed in deionized (DI) water for a period of time, and then cellulose nanosheets were added in a ratio of 2:3 for ultrasonic processing.Then, the obtained dispersion was freeze-dried to obtain an aerogel.Next, 26 mg of aerogels was dispersed in 50 mL of DI water and sonicated for 30 s and filtered to obtain a wet membrane.Then, the obtained wet membrane was transferred to a Petri dish filled with absolute ethanol and soaked for 24 h.Finally, the wet membrane was sandwiched between two glass plates and transferred to an oven for drying at 70°C for 1 h and then transferred to a vacuum oven for drying at 80°C for 12 h to obtain the final membrane, namely, Cel@DBDPE; its density was about 2 mg/cm 2 .

| Preparation of the Cellulose@DBDPE separator
First, DBDPE and polyvinylidene difluoride (PVDF) were dispersed in N-methyl-2-pyrrolidone (NMP) at a mass ratio of 9:1 and stirred overnight.Then, the obtained slurry was coated on the surface of a commercial PP separator.The final PP/DBDPE was obtained by vacuum drying at 60°C for 12 h.

| Characterization
X-ray diffraction (XRD) was performed on a Bruker AXS D2 Advance with Cu Kα radiation (λ = 1.54178Å).A scanning electron microscope (SEM, FEI Quanta650 FEG) equipped with an energy-dispersive spectrometer was used to study the microscopic morphology and for the elemental analysis of the samples.The All-Electrodynamic Dynamic Test Instrument (INSTRON E1000) was used to study the tensile strength of the samples.The structural changes of the samples were investigated by Raman spectroscopy (Alpha 300R; WITec).

| The wettability test
The wettability of the sample surface to the electrolyte was characterized using a contact angle meter (OCA15EC; Dataphysics), and the volume of the electrolyte droplets added in each test was 3 µL.

| The electrolyte uptake test
The electrolyte uptake was calculated from the change in the mass of the sample before and after soaking in the electrolyte for 2 h, and the formula is as follows: where m 1 and m 2 are the masses of the sample before and after soaking, respectively.

| The thermal stability test
The changes in samples were recorded using an infrared thermal imager by placing the sample on a heating stage, and the temperature range was 90-200°C.

| The stress-strain test
The samples were cut into 1 cm × 3 cm sizes and measured using the All-Electrodynamic Dynamic Test Instrument at a tensile speed of 1.0 mm/min.

| The combustion experiment
The samples were ignited after soaking in an electrolyte for several hours, and the changes in the samples were recorded using a camera during the combustion process.
The Cel@DBDPE separator and a PP microporous membrane (PP, Celgard 2400) were used as battery separators.The LiFePO 4 (LFP) cathode was prepared using the coating method, in which the mass ratio of the active material LFP powder, the conductive agent Ketjen black, and the binder PVDF was 8:1:1.The ionic conductivity of the separator was characterized using the high-precision electrochemical impedance spectroscopy (EIS, Solartron 1260 + 1287) test system after soaking it completely in the electrolyte, in which a stainless-steel sheet (SS) was used as the working and the counter electrode, and the amplitude and frequency range were set to 10 mV and 1.0 MHz-0.1 Hz, respectively.The formula for calculating the ionic conductivity (σ) is as follows: where L is the thickness of the separator, R b is the bulk resistance, and S is the contact area between the separator and the SS electrode.

| RESULTS AND DISCUSSION
Since the separator needs to be immersed in the electrolyte for a long time, the selected flame retardants should have good chemical stability and should be insoluble in the electrolyte.DBDPE, [36][37][38] as a new type of additive brominated flame retardant, has the advantages of a wide application range, high bromine content (81.5 wt%), good thermal stability (degradation temperature >300°C), low exudation rate, and so on; moreover, its solubility in most organic solvents is extremely low (<2 ppm).The molecular formula of DBDPE is C 14 H 4 Br 10 , and its molecular ball-and-stick model is shown in Supporting Information: Figure S1A.It consists of two benzene rings connected by a carbon chain, which does not contain oxygen atoms.Therefore, compared with other brominated flame retardants containing oxygen atoms, DBDPE will produce much lower concentrations of highly toxic compounds such as polybrominated dibenzo-p-dioxins and dibenzofurans 39 during the pyrolysis process.Also, previous work 33,40 has shown that DBDPE does not affect the electrochemical performance of the battery; therefore, DBDPE was selected as the flame retardant of the separator.In fact, halogenated flame retardants block the combustion process in the gas phase through a free radical scavenging mechanism.The combustion of the electrolyte is a chain-branching reaction that releases highly reactive H• and OH• radicals.Also, DBDPE will release Br• radicals, which can react with these highly reactive radicals to produce low-reactive halogen radicals to reduce the rate of energy generation, thereby slowing down or terminating the combustion reaction.The specific reaction process is shown in Supporting Information: Figure S1B.Moreover, the generated HBr gas can also dilute oxygen and cover the surface of the material, reducing the combustion speed to achieve self-extinguishing.The DBDPE powder particles are shown in Supporting Information: Figure S1C.
Figure 1A shows the preparation process of the Cel@BDBDPE separator.First, the uniform dispersion obtained by ultrasonically mixing 2D sheet-like cellulose nanosheets (Figure 1B) and DBDPE powder particles was freeze-dried.After freeze-drying, a loose, porous, white aerogel (Supporting Information: Figure S2A) was obtained and cellulose nanosheets transformed into 1D fiber-like microscrolls during the freeze-drying process and encapsulated the DBDPE powder particles.Then, a certain amount of aerogel was ultrasonically dispersed in DI water and the obtained dispersion was filtered to obtain a wet membrane, which was then soaked in anhydrous ethanol for 24 h.The final separator was obtained by vacuum drying, denoted Cel@DBDPE, and is shown in Supporting Information: Figure S2B.The obtained separator is similar to white paper.From Supporting Information: Figure S2C, it is clear that the microstructure of the Cel@DBDPE separator is composed of fiber-like microscrolls with different diameters intertwined with each other, where DBDPE particles are anchored inside the microscrolls.Moreover, further characterization by XRD shows that the characteristic peaks of Cel@DBDPE are similar to that of DBDPE, indicating that the preparation process does not destroy the structure of DBDPE, as shown in Supporting Information: Figure S2D.It is worth emphasizing that the unique structure of the microscrolls originates from the topological deformation process of cellulose nanosheets during the freeze-drying process, as shown in Figure 1C-F.In the initial state, DBDPE particles are dispersed on the 2D cellulose nanosheets, as shown in Figure 1C.During the freeze-drying process, the 2D cellulose nanosheets gradually roll up and wrap DBDPE under the restraining action of anchored particles, and Figure 1D shows that the sheet-like cellulose is in the transition stage of rolling up and wrapping the DBDPE particles.On completion of topological deformation, the 2D sheet-like cellulose completely wrapped the DBDPE particles to form the final 1D fiber-like microscroll structure, as shown in Figure 1E.From the element mapping of Figure 1F, it can be seen that the DBDPE powder particles anchored in the cellulose microscrolls show a strong Br element signal.According to our previous work, 41 the formation mechanism of the transformation of cellulose nanosheets into cellulose microscrolls was described.The presence of micro/nanoparticles on the cellulose nanosheets will cause uneven shrinkage stress of cellulose nanosheets during freeze-drying, thus inducing self-rolling of the cellulose nanosheet.
To demonstrate the structural advantage of the asprepared Cel@DBDPE, we simultaneously constructed a DBDPE-modified PP separator (PP/DBDPE) for comparison, namely, a slurry mixture of DBDPE and PVDF was coated on the surface of the PP separator.The crosssection SEM images of Cel@DBDPE and PP/DBDPE are shown in Figure 1G,H.The former shows that the DBDPE is embedded inside the separator and the latter shows that the DBDPE coating is located on the surface of the PP.Furthermore, according to elemental mapping, Cel@DBDPE shows a strong Br signal originating from the DBDPE that is distributed throughout the bulk of the separator, and the elemental distribution of C and Br is basically consistent, showing integrity.In PP/DBDPE, there is no Br signal inside PP, and the elemental distribution of C and Br is quite distinct from each other, which is divided into two conspicuous layers.In addition, Cel@DBDPE has an ultrathin thickness of about 15 μm.Due to this unique structure of microscrolls, on the one hand, Cel@DBDPE preserves the porous internal structure of the separator; on the other hand, unlike the structure of the surface coating, Cel@DBDPE neither needs to introduce additional binder nor increases the thickness of the separator.However, the flame-retardant DBDPE coating of PP/DBDPE inevitably increases the thickness of the PP separator and a new interface is formed on the surface of the coating and the PP.This may block the pores on the surface on the surface of the PP and further affect the internal resistance of batteries.
To simulate whether DBDPE will fall off the separator and enter the electrolyte during long-term electrolyte immersion, Cel@DBDPE and PP/DBDPE were soaked in 1 mol/L LiFP 6 electrolyte and sonicated simultaneously, and the results are shown in Figure 2A,B.After ultrasonic treatment, a large amount of DBDPE coating on the PP surface fell off (as shown in the schematic illustrations and photo of Figure 2A), and the electrolyte containing PP/DBDPE became cloudy.In contrast, Cel@DBDPE maintained excellent structural integrity (as shown in the schematic illustrations and photo of Figure 2B), and the electrolyte containing Cel@DBDPE remained clear and transparent without precipitation.Furthermore, from the light transmittance of electrolytes containing different separators shown in Figure 2C, it is clear that the light transmittance of electrolyte containing Cel@DBDPE is almost maintained at 100%, while the light transmittance of electrolyte containing PP/DBDPE has dropped significantly to about 50%.The experiment shows that the unique microscroll structure of Cel@DBDPE enables the flame retardants to be firmly and uniformly confined within the separator and can effectively prevent the flame-retardant particles from falling off the separator and enter the electrolyte without peeling off like the DBDPE coating of PP/ DBDPE.
Then, to further evaluate the fundamental properties of the Cel@DBDPE separator for use as a separator, various characterizations were carried out.The wettability of the separator represents the affinity between the separator and the electrolyte.Excellent wettability is conducive to the rapid spread of the electrolyte on the surface of the separator and the uniform diffusion inside the separator to ensure uniform ion transport.Therefore, the contact angle test was performed on Cel@DBDPE and PP to characterize their wettability with the electrolyte, and the electrolyte used was 1 mol/L LiPF 6 dissolved in EC/EMC/DMC (1:1:1 v/v/v).From Figure 2D, it can be seen that the contact angle of the PP surface shows that the electrolyte droplets hardly spread within a few seconds, and the contact angle is 55.4°, while the electrolyte spreads rapidly on the surface of Cel@DBDPE, and its contact angle is only 17.8°, as shown in Figure 2E.Also, from the electrolyte diffusion experiment shown in Supporting Information: Figure S3, it can be seen that the electrolyte can only infiltrate the area in contact with the PP separator, while the electrolyte can continue to diffuse on Cel@DBDPE.Figure 2F exhibits the electrolyte uptake of PP and Cel@DBDPE.The former was only 66%, while the latter was as high as 244%.It can be seen that Cel@DBDPE shows better electrolyte wettability, due to its porous network structure and abundant polar functional groups.The tensile strength of Cel@DBDPE was also tested, found to be about 20 MPa, as shown in Figure 2G.In addition, as the separator needs to be immersed in the electrolyte for a long time, the chemical stability of the separator is very important.The stability of Cel@DBDPE was observed by soaking in 1 mol/L LiPF 6 electrolyte.The results are shown in Figure 2H; the appearance of Cel@DBDPE did not change during the long-term soaking process (14 days), and the electrolyte remained clear and transparent.Then, XRD of Cel@DBDPE before and after immersion in the electrolyte was performed to determine whether chemical changes occurred.As shown in Supporting Information: Figure S4, no characteristic peak of other impurities appears in the XRD diffraction peaks of Cel@DBDPE after soaking.Also, from the Raman spectrum shown in Figure 2I, it is clear that the characteristic peaks of pristine Cel@DBDPE and DBDPE powders are basically consistent, and no other characteristic peaks appear after soaking.The above experiments prove that Cel@DBDPE has good chemical stability.
Because of the poor thermal stability of the PP separator, it is prone to severe thermal shrinkage at high temperatures, which lays a hidden danger for the safety of the battery.To construct a high-safety battery, the separator should have excellent thermal stability.Therefore, the thermal shrinkage test was performed simultaneously on PP and Cel@DBDPE, and the change in the shape of the separator was recorded.From Figure 3A, it is clear that PP began to shrink significantly at 120°C, by about 32%.When the temperature increased to 150°C, PP shrank by about 82%, almost losing its overall integrity, and as the temperature continued to increase, PP completely lost its original structure.It can be seen that if the internal temperature of the battery increases due to unexpected conditions, the PP separator will shrink significantly at about 120°C, which may cause safety issues of the battery.In contrast, Cel@DBDPE did not experience any shrinkage changes during the entire thermal shrinkage test period and maintained excellent thermal stability at a high temperature of 210°C, as shown in Figure 3B. Figure 3C shows the shrinkage of PP and Cel@DBDPE with temperature.Moreover, the changes of PP and Cel@DBDPE with increase in temperature were recorded using an Infrared Thermal Imager, and the temperature of the separator surface was recorded in real time.Figure 3D-F shows the shape change and surface temperature of PP and Cel@DBDPE at ambient temperatures of 120°C, 150°C, and 180°C, respectively, and the surface temperature of PP is always close to the ambient temperature, while the surface temperature of Cel@DBDPE is always lower than the ambient temperature.Supporting Information: Figure S5 presents more pictures recorded by the Infrared Thermal Imager at different temperatures.PP showed a shrinking trend at 110°C, and when the temperature increased to 160°C, PP basically lost its initial circular appearance, and with the further increase of the temperature, the PP shrank more drastically.The above test results show that Cel@DBDPE has excellent thermal stability.This is because compared with the single-chain structure of PP, cellulose is a linear chain composed of ringed glucose molecules with chair conformation, 42 and there are a lot of hydrogen bonds between cellulose molecular chains, which can play the role of glue.In addition, the addition of DBDPE particles further improves the thermal stability.This indicates that Cel@DBDPE is advantageous for the construction of high-safety batteries.
The flame retardancy of Cel@DBDPE was further explored.PP and Cel@DBDPE were ignited with fire to observe the changes and to evaluate the flammability of the electrolyte.As shown in Figure 3G, PP started burning rapidly after being ignited, and both electrolyte and PP become violently burned at the same time.There is almost no residue in PP after combustion, indicating that PP did not have flame retardancy.Conversely, from Figure 3H, it is clear that Cel@DBDPE starts to burn after being ignited (6.10 s), but stops burning at 6.77 s, and Cel@DBDPE basically retains the original appearance.Generally, flame retardancy can be evaluated by measuring the self-extinguishing time 34 (SET) of the electrolyte, that is, normalizing the burning time to the electrolyte mass, and the SET of Cel@DBDPE is about 55 s/g.It is worth emphasizing that, according to the electrolyte uptake, the mass of the electrolyte absorbed by Cel@DBDPE (244%) is nearly four times that of PP (66%), indicating that the former contained more electrolyte than the latter in the combustion experiment and proving that Cel@DBDPE has excellent flame retardancy.Also, the flame retardancy of Cel@DBDPE is derived from the free radical scavenging mechanism of DBDPE powder particles, which can slow down or terminate the chain branching reactions of electrolyte combustion, thereby extinguishing the combustion of the electrolyte.The detailed combustion experiment is shown in Supporting Information: Videos 1 and 2.
The different behaviors of the two separators exposed to high temperatures are further shown in Figure 4.For the PP separator, when the internal temperature of the battery increases, it undergoes severe thermal shrinkage, which leads to an internal short circuit and generates a large amount of heat; this may cause the flammable electrolyte to be ignited, and thermal runaway occurs, as shown in Figure 4A.However, for the Cel@BDBDPE separator, when the internal temperature of the battery increases, first, it will not undergo severe thermal shrinkage like PP, which will lead to short circuit of the battery.Second, even if the electrolyte fire, the Cel@BDBDPE will release Br• radicals, which will react with the H• and OH• radicals generated by the combustion of the electrolyte to extinguish the combustion of the electrolyte through a radical scavenging mechanism, as shown in Figure 4B.Therefore, the Cel@BDBDPE separator with excellent thermal stability and flame-retardant property can effectively improve the safety of lithium-ion batteries.
The electrochemical performance of Cel@DBDPE was tested to evaluate whether it could be used as a separator for lithium-ion batteries.As the separator is exposed to a strong oxidizing or reducing environment during battery operation for a long time, it should have good electrochemical stability.Li//stainless-steel (SS) half-cells with PP and Cel@DBDPE separators were assembled and LSV tests were performed in 1 mol/L LiPF 6 electrolyte to evaluate the electrochemical stability of the two separators.From Supporting Information: Figure S6, it can be seen that the curves of PP and Cel@DBDPE basically overlap at 0-5 V (vs.Li/Li + ), and neither show obvious decomposition, which indicates that Cel@DBDPE has an electrochemical window comparable to that of PP.The SS/ separator/SS cell was further assembled and EIS of PP and Cel@DBDPE was performed; the ionic conductivity of both was calculated.From Supporting Information: Figure S7, it can be seen that the ionic conductivities of PP and Cel@DBDPE are 0.29 and 0.27 mS/cm, respectively, showing comparable values.Then, Li//Li symmetric cells were assembled and galvanostatic cycling tests were performed to evaluate the effect of Cel@DBDPE on long-term Li plating/stripping stability.As shown in Figure 5A,C, Li//Li symmetric cells with PP and Cel@DBDPE as the separator show similar EIS values (the corresponding equivalent circuit model is shown in Supporting Information: Figure S8), indicating that the transport behavior of Li + in Cel@DBDPE is similar to that of PP.Also, from Figure 5B,D, it can be seen that both Li/ PP/Li and Li/Cel@DBDPE/Li symmetric cells were stably cycled for over 300 h at 0.5 mA/cm 2 and 0.5 h.The polarization voltages of the two are higher in the initial stage, which may be caused by the formation of a solid electrolyte interphase.As the cycling progresses, they all enter a smooth operating state, showing a relatively flat plating/stripping voltage plateau (insets of Figure 5B,D).
Then, the performance of Cel@DBDPE in half-cells was evaluated by assembling Li//LiFePO 4 (LFP) halfcells.Supporting Information: Figure S9 shows the CV test results of the Li/Cel@DBDPE/LFP half-cell; a pair of redox peaks appears at about 3.60/3.30V (vs Li/ Li + ), corresponding to the de-intercalation/intercalation of Li + in LFP, respectively, proving that Cel@DBDPE did not affect the charge/discharge behavior of the Li//LFP half-cell.Also, the EIS of Li/Cel@DBDPE/LFP and Li/ PP/LFP half-cells was tested, showing comparable charge-transfer resistance, as shown in Supporting Information: Figure S10. Figure 5E shows the charge/ discharge curves of Li/Cel@DBDPE/LFP and Li/PP/LFP half-cells at 0.3 C; the curves of the two almost overlap and a flat charge/discharge plateau appears at 3.50/ 3.35 V (vs.Li/Li + ).The charge/discharge specific capacity of Li/Cel@DBDPE/LFP is 144.6/144.3mAh/g and that of Li/PP/LFP is 147.4/147mAh/g, showing similar specific capacity.Then, cycling stability tests were performed on Li//LFP half-cells by cycling at 1.0 C, and from Figure 5F, it can be seen that Li/Cel@DBDPE/LFP and Li/PP/LFP half-cells show comparable performance at 1.0 C; the specific capacity of the former is 127.9 mAh/g after 100 cycles and that of the latter is 126.performance.From the above electrochemical tests, it is clear that Cel@DBDPE shows electrochemical performance comparable to that of commercial PP, and can be used as a lithium-ion battery separator.In addition, Cel@DBDPE also has excellent flame-retardant properties and can be used to construct high-safety lithium-ion batteries.

| CONCLUSION
In summary, we prepared a green separator with excellent wettability, thermal stability, and flame retardancy, which can be used to construct high-safety lithium-ion batteries.The designed Cel@DBDPE separator has a unique microscopic morphology, in which the DBDPE particles are anchored in the microscrolls formed by the self-rolling of cellulose nanosheets, and these cellulose microscrolls are intertwined and overlapped to form a 3D porous network structure, showing an ultrathin thickness of about 15 μm.The contact angle of the Cel@DBDPE separator is only 17.8°, which is better than that of the commercial PP separator (55.4°), and the electrolyte uptake of the former is as high as 244% and that of the latter is only 66%.The Cel@DBDPE separator shows good wettability with the electrolyte.In the thermal stability test, the Cel@ DBDPE separator maintained good dimensional stability at a high temperature of 210°C, while the PP separator began to experience significant dimensional shrinkage at 120°C.Also, in the combustion experiment, the Cel@DBDPE separator stopped burning only 0.67 s after being ignited, and had good flame-retardant property, while the PP separator was completely burned out after being ignited.In another electrochemical test, Li//Li symmetric cells with Cel@DBDPE and PP as separators, respectively, showed a cycle life of over 300 h at 0.5 mA/cm 2 and 0.5 h.Also, the Li/Cel@DBDPE/ LFP and Li/PP/LFP half-cells had specific capacities of 127.9 and 126.4 mAh/g after 100 cycles at 1.0 C, respectively, and showed similar rate performance.Therefore, the Cel@DBDPE separator shows comparable electrochemical performance to the PP separator and can be used as a lithium-ion battery separator.Our work proves that the integration of cellulose and flame retardants can effectively enable construction of green, high-safety separators, and provides a reference for constructing high-safety lithium-ion batteries from the perspective of separators.
The linear sweep voltammetry (LSV) test of the Li//SS half-cell and the cyclic voltammetry (CV) test of the Li//LFP half-cell were conducted on the CHI 760E electrochemical workstation, where the voltage ranges of the former were 0-6 V (vs.Li/Li + ) and the scan rate was 1.0 mV/s, and the voltage range of the latter was 2.5-4 V (vs.Li/Li + ) and the scan rate was 0.1 mV/s.The EIS tests of the Li//Li symmetric cell and the Li//LFP half-cell were performed on a Solartron 1260 + 1287 with a frequency range of 1.0 MHz-0.1 Hz and an amplitude of 10 mV.The galvanostatic discharge/charge tests of the Li//Li symmetric cell and the Li//LFP half-cell were performed on a NEWARE battery testing system, where the voltage range of the latter was 2.5-4 V (vs.Li/Li + ).

F I G U R E 1
Preparation of the Cel@DBDPE separator.(A) Schematic illustrations of the preparation process.(B) Pristine cellulose nanosheets.Topological deformation process of cellulose nanosheets during the freeze-drying process, (C) cellulose nanosheets containing DBDPE in the initial state, (D) cellulose nanosheets rolling up and wrapping DBDPE particles in the transition stage, and (E) cellulose nanosheets transformed into Cel@DBDPE microscrolls in the final stage.(F) A single fiber-like Cel@DBDPE microscroll and corresponding Br elemental mapping.The cross-section SEM images of (G) Cel@DBDPE, in which DBDPE is embedded inside the separator, and (H) PP/ DBDPE with the DBDPE coating on the separator and the corresponding elemental mapping of Br and C. DBDPE, decabromodiphenyl ethane; PP, polypropylene; SEM, scanning electron microscope.
Characterization of the Cel@DBDPE separator.The ultrasonic treatment and the corresponding schematic illustrations of (A) PP/DBDPE and (B) Cel@DBDPE.(C) Light transmittance of an electrolyte containing different separators.Contact angle test of (D) PP and (E) Cel@DBDPE.(F) Electrolyte uptake.(G) Tensile strength of the Cel@DBDPE separator.(H) Soaking test of the Cel@DBDPE separator.(I) Raman spectra of the Cel@DBDPE separator before and after immersion in an electrolyte.DBDPE, decabromodiphenyl ethane; PP, polypropylene.

F I G U R E 3
Characterization of the Cel@DBDPE separator and the PP separator.Thermal stability test of (A) PP and (B) Cel@DBDPE.(C) Change of shrinkage with temperature.(D-F) Change of Cel@DBDPE (left) and PP (right) at different temperatures recorded using the Infrared Thermal Imager.Combustion experiment of (G) PP and (H) Cel@DBDPE.DBDPE, decabromodiphenyl ethane; PP, polypropylene.
4 mAh/g.Moreover, the rate performance of Li//LFP half-cells with different separators at different current densities was also tested, and the results are shown in Figure 5G.The Li/ Cel@DBDPE/LFP half-cell shows specific capacities of 157.4,157.7, 151.5, 142.9, 127.3, 104.6, and 154.2 mAh/g at 0.1 C, 0.2 C, 0.5 C, 1.0 C, 2.0 C, 3.0 C, and 0.1 C, respectively, and the Li/PP/LFP half-cell shows specific capacities of 150.5, 149.8, 143.6, 133.9, 122.1, 112.2, and 154.6 mAh/g at the same rate, showing similar rate F I G U R E 4 Schematic illustrations of the mechanism of different separators.(A) Thermal runaway mechanism of the PP separator.(B) Flame-retardant mechanism of the Cel@DBDPE separator.DBDPE, decabromodiphenyl ethane; PP, polypropylene.