Flame-Retardant Bilayer Separator with Multifaceted van der Waals

Jun 28, 2019 - The cells with the bilayer separator deliver excellent rate ... Figure 7a–c, all of the Raman spectra exhibit two typical bands of ca...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Flame-Retardant Bilayer Separator with Multifaceted van der Waals Interaction for Lithium-Ion Batteries Guangfeng Zeng,†,⊥ Junying Zhao,†,⊥ Chao Feng,†,⊥ Dongjiang Chen,† Yan Meng,‡ Bismark Boateng,†,§ Ning Lu,∥ and Weidong He*,†,§

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†

School of Physics, and §State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China ‡ College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China ∥ Department of Breast Cancer Medical Oncology, Tianjin Medical University Cancer Institute and Hospital, Tianjin Medical University, Ministry of Education, Tianjin 300060, PR China S Supporting Information *

ABSTRACT: Safety issues induced by a flammable organic electrolyte challenge the practical applications of high-specific energy lithium-ion batteries (LIBs). Here, we develop a robust bilayer separator by incorporating MoO3 and Al-doped Li6.75La3Zr1.75Ta0.25O12 (LLZTO). The bilayer separator is highly flame-resistive and manages to endure intense fire. Density functional calculations reveal that abundant hydrogen bonds and van der Waals forces within the bilayer separator greatly suppress the combustion with interfacial adhesion of MoO3 and LLZTO to poly(vinylidene fluoridehexafluoropropylene). With MoO3 and LLZTO, the graphitized carbon content of the carbon residues is increased, and the formation of molybdenum fluoride (MoFx) and lanthanum fluoride (LaFx) is induced during combustion, thus suppressing heat accumulation. The bilayer separator owns a large ductility (227%) and low thermal shrinkage (5%) after annealing at 160 °C for 4 h. Based on the bilayer separator, Li/LiFePO4 cells deliver a remarkable discharge capacity of 162 mA h/g at 0.5 C with a high capacity retention of 95% after 100 cycles. This work provides a new strategy for achieving safe LIBs. KEYWORDS: lithium-ion battery, bilayer separator, flame-retardant property, van der Waals interaction, high capacity

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fluoropropylene) (PVDF-HFP), and polysulfonamide/polypropylene display good thermal stability and flame-retardant performance. However, the complicated preparation process and insufficient electrochemical performance restrict its further development.6−9 Additives endow separators with flame retardancy through either radical mechanism or enhancing the thermal stability of the organic electrolyte.10 However, additives can react with electrode materials, lower the ion conductivity of the organic electrolyte, and deteriorate battery performance.11 Ceramic-modified separators show enhanced dimensional thermal stability but exhibit limited flameretardant properties.12−14 In addition, most flame-retardant polymer separators are monolayer-structured. The multilayer separator structure combines the advantages of each layer and gives rise to enhanced affinity to the electrolyte, higher ionic conductivity, and good electrochemical performances. Nevertheless, it is difficult to simultaneously own these optimal performances for a single multilayer separator.15

INTRODUCTION The steady conversion from fossil fuels to low greenhouse gas energy sources greatly boosts the development of advanced energy storage technologies. Among them, rechargeable lithium-ion batteries (LIBs) are expected to be the most promising energy storage devices because of their high specific energy density, long service life, and minimal memory effects.1,2 However, the uncontrolled safety issues related to the flammable organic electrolyte impede its large-scale application. Generally, LIBs must operate in a limited range of temperature and voltage.3 However, in some abnormal conditions such as short circuits and overcharge, the internal temperature of the battery rises quickly, decomposing the passivation film and solid electrolyte interphase on the lithium anode and thus triggering a series of exothermic reactions, leading to thermal runaway and increased internal temperature and pressure, which may give rise to electrolyte combustion and battery explosion.4,5 Developing flame-retardant separators (such as ceramicmodified separators) or using flame-retardant additives are effective for improving battery safety. Flame-retardant separators such as polyimide, poly(vinylidene fluoride-hexa© XXXX American Chemical Society

Received: May 16, 2019 Accepted: June 28, 2019 Published: June 28, 2019 A

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces As a traditional flame-retardant additive in poly(vinyl chloride) system, MoO3 shows good flame-retardant properties by reducing the formation of flammable benzene species and decreasing evolution of volatile hydrocarbon species.16 Li6.75La3Zr1.75Ta0.25O12 (LLZTO) as an excellent Li+ conductor has been added into the PVDF-HFP matrix. However, the mechanical properties of such separators still need to be improved.17,18 Herein, by using the flame-retardant properties of MoO3 and the excellent Li+ conductivity of LLZTO, we design a robust bilayer composite separator for safe LIBs. The bilayer separator unites merits of the MoO3−PVDF-HFP layer and LLZTO− PVDF-HFP layer and exhibits simultaneously excellent mechanical, wetting, thermal shrinkage, ion conductivity, thermal distribution, and electrochemical performances. In addition, the bilayer separator shows excellent flame-retardant performances. Both MoO3 and LLZTO form abundant hydrogen bonds and van der Waals forces with PVDF-HFP, which enhances the interfacial adhesion and reduces the combustion. In addition, MoO3 and LLZTO induce the formation of graphitized carbon, MoFx, and LaFx, which increases the thermal stability and suppresses the heat accumulation, thus improving the flame-retardant performances. Our work provides a new way to prepare multilayer flame-retardant separators and demonstrates its promising potentials for a variety of applications.

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σ=

L RS

(1)

where L is the thickness of the separator, R is bulk resistance, and S is surface area of the electrode. 2032 coin-type cells were assembled using the different separators to assess the electrochemical performance. LiPF6 (1.0 M) in EC/DMC (1:1 by volume) was used as an electrolyte. To prepare the LiFePO4 (LFP) cathode, the LFP powders were mixed with Super-P and PVDF with a weight ratio of 8:1:1, using N-methyl pyrrolidone as a solvent. The mixture was stirred for 6 h, then coated on Al foil, and dried at 80 °C overnight. The mass loading of the LFP cathode is 2 mg. The rate and cyclic performances were performed in the galvanostatic mode with a range of 2.5−4.2 V using a battery testing system (Neware, China). The electrolyte uptake (Eu) was calculated with the following equation Eu =

Wwet − Wdry Wdry

× 100% (2)

where Wwet is the mass of the wet separator and Wdry is the mass of the dry separator. The separator porosity (%) was determined with the following equation Porosity (%) =

Wwet − Wdry ρV

× 100%

(3)

where Wwet is the mass of the wet separator, Wdry is the mass of the dry separator, ρ is the density of the liquid, and V is the geometric volume of the separator. The MacMullin number was calculated with the following equation Nm =

EXPERIMENTAL SECTION

Preparation of MoO 3 −PVDF-HFP//LLZTO−PVDF-HFP Bilayer Separators. PVDF-HFP (Mw = 5.7 × 105 to 6.0 × 105), MoO3, and acetone were provided by Solvay Co. Ltd., Aladdin (Shanghai), and Ke Long (China), respectively. LLZTO was prepared through a solid-state reaction.19 Typically, the mixture with a stoichiometric ratio of LiOH·H2O (Aladdin), ZrO2 (Aladdin), La2O3 (Aladdin), Ta2O5 (Aladdin), and Al2O3 (Aladdin) was stirred overnight in isopropyl alcohol (Ke Long) to obtain a mixed slurry. The mixture was then dried at 80 °C and subsequently calcined at 900 °C for 6 h. The obtained LLZTO powders were then ball-milled in isopropyl alcohol with a planetary ball-milling machine (QM-3SP2, Zhengxian, China) for 13 h and dried at 80 °C for 12 h. The asprepared LLZTO powders were first dispersed in acetone to obtain a homogeneous suspension (0.1 g/mL) using a FRITSCH ball-milling instrument (Pulverisette 7 premium line, Germany). The bilayer separator was prepared in a two-step approach, first, the LLZTO−PVDF-HFP layer was prepared. Typically, certain amounts of PVDF-HFP were dissolved in a mixture solvent of acetone and N,N-dimethylformamide (3:7, w/w) and stirred at 50 °C, until a transparent colloidal solution was obtained. Next, LLZTO suspension was added into the colloidal solution and stirred at room temperature. The uniform solution was then coated on a clean stainless steel and dried at 50 °C in a vacuum oven for 30 min. Second, the MoO3− PVDF-HFP layer was prepared following the same procedure of the LLZTO−PVDF-HFP layer using acetone as a solvent. To investigate the optimal thickness of bilayer separator, the thickness of bilayer separator was set at 30, 40, 50, 60, and 70 μm. To investigate the optimal content of fillers in a bilayer separator, the content of MoO3 was set at 6.67, 12.5, 17.6, and 36.3%. The content of LLZTO in the PVDF-HFP separator was set to 6.67, 12.5, and 22.2%. The thickness of the MoO3−PVDF-HFP separator was 25 ± 2 μm, whereas that of the LLZTO−PVDF-HFP separator was 20 ± 3 μm. Electrochemical Measurement. Ionic conductivity of the separator was tested via a stainless/separator/stainless cell using the CHI760E electrochemical workstation (Shanghai Chenhua, China) over the frequency range from 0.1 Hz to 1 MHz. It was calculated according to the equation

σ0 σeff

(4)

where σ0 and the σeff are the ion conductivities of the pure electrolyte and electrolyte soaked with separator at 30 °C, respectively. The thermal shrinkage of the separator was performed in an oven at temperatures from 100 to 160 °C by heating the separators for 4 h. The thermal shrinkage was calculated with the following equation

Thermal shrinkage (%) =

Df − Di × 100% Di

(5)

where Df and Di are the diameters of the separators before and after heating. Material Characterization. Scanning electron microscopy (SEM) and energy-dispersive spectral (EDS) element mapping images were measured using Nova NanoSEM 450 (FEI, USA). Thermogravimetry analyses (TGA) were obtained using STA 449 F5 simultaneous thermal analyzer (Netzsch, Germany) in a N 2 atmosphere at a heating rate of 10 °C min−1. The Raman spectrum was performed using Renishaw InVia Raman microscopy with a 785 nm laser. X-ray diffractometer (XRD) measurements were conducted through the Bruker D8 (Bruker, Germany) with Cu Kα radiation (λ = 15 406 Å) at 40 mA and 40 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using K-Alpha (Thermo Fisher Scientific, USA) with Al Kα radiation at 5 kV. The Fourier transform infrared spectrum (FTIR) was tested with an IS50 (Thermo Fisher Scientific, USA). Stress and strain properties were investigated through a tensile testing machine (MTS Systems, CMT6104, China). The specific surface area and pore size were investigated with Brunauer−Emmett−Teller (BET) (TriStar II Plus, Micromeritics Instruments, USA). The mercury porosimetry technique was used to analyze the porosity of separators. The porosimetry measurements were done by using an automatic mercury porosimeter (PoreMaster-60, Malvern, British). Thermal Imaging Analysis. The separators, fixed on an Al foil substrate, were put on a heating device to evaluate their thermal distribution performances. The infrared light was detected by forwardlooking infrared (FLIR, A600-Series, Sweden) in the range of 7.5−13 μm at the frequency of 6.3 Hz. A 17 μm lens and noise-equivalent temperature difference mode were employed in thermal imaging B

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Cross-sectional SEM images of the bilayer separator and the schematic illustration for the bilayer structure. Top-view SEM images of (b) LLZTO-PVDF-HFP layer and (c) MoO3−PVDF-HFP layer in the bilayer separator, respectively. (d) XRD patterns of MoO3, LLZTO, PVDFHFP separator, and bilayer separator. Elemental mapping images of Zr, F, and Mo, F in a selective region of (e) LLZTO−PVDF-HFP layer and (f) MoO3−PVDF-HFP layer, respectively. The photographs of (g) LLZTO−PVDF-HFP layer, (h) MoO3−PVDF-HFP layer, and (i) PVDF-HFP separator. The diameter of the separator is 19 mm. analyses. When the temperature increased, the images showing heat transfer and distribution on the separator were captured. Density Functional Theory Calculations. Density functional theory (DFT) calculation was utilized to evaluate the interaction of MoO3, LLZTO, and PVDF-HFP, which was performed using the Cambridge Sequential Total Energy Package (CASTEP). MoO3 owns an orthorhombic structure, which shares a space group Pbnm with the cell parameters a = 3.963 Å, b = 13.855 Å, and c = 3.696 Å.20 In the cubic crystal structure Li7La3Zr2O12 (LLZO), lithium atoms that distribute on the 24 d and 96 h sites are partially occupied.21 Both the (010) surface of MoO3 and (001) surface of LLZO are the most stable surfaces for the two crystals.22,23 These structures were used in the evaluation of absorption energy calculation through the following equation E = Etot − (Ea + Ex )

PVDF-HFP separator and bilayer separator (Figure 1g−i). SEM images illustrate that the filler disperses evenly in the separator (Figures 1b,c and S2a−c), which suggests the uniformity of MoO3 and LLZTO in the PVDF-HFP host. EDS mapping images of bilayer separator further confirm uniform dispersion of MoO3 and LLZTO, as shown in Figure 1e,f. From the XRD analysis of PVDF-HFP powders in Figure S1a, the diffraction peaks at 2θ = 18° and 20° are assigned to (100) and (110) plane of the α-phase property.24 In contrast, the PVDF-HFP separator exhibits weaker peak intensity at 20° (Figure 1d), which suggests low crystallinity. With the MoO3 and LLZTO, the peak intensity at 20° further decreases (Figures S1b,c and 1d), indicating that the modified PVDFHFP is highly amorphous. These results show that the MoO3 and LLZTO particles can weaken the crystalline phase and improve the movement of the PVDF-HFP chain, thus promoting the migration of Li ions in the separator25 and improving the electrolyte uptake. The electrolyte uptake of the bilayer separator is much higher than those of the PVDF-HFP (104.5%) and Celgard 2325 (91.8%) separators, reaching 372.6%. Moreover, the electrolyte retention of bilayer separators is also higher than those of Celgard 2325 and PVDF-HFP, as shown in Figure S3. The permeability of separators can be described with the MacMullin number (Nm, see Table S1),15 showing that the bilayer separator owns a low Nm value of 12.6, which is smaller than those of PVDF-HFP (55.2) and Celgard 2325 (19.2). Lower Nm values of the bilayer separator are due to the higher permeability, suggesting that the bilayer separator owns a high-porosity structure.26 As

(6)

where E is the adsorption energy, and Etot is the total energy of the relaxed model a and model x at the equilibrium state, Ea and Ex are the self-consistent field (SCF) calculation energy values of models a and x molecules (x = CH2CF2, CH3CHF2, CHF2CHFCF3 and CF2CFCF3), respectively. Ultrasoft pseudopotential with cutoff energy of 380 and 340 eV were used for MoO3 and LLZO, respectively. A generalized gradient approximation−Perdew−Burke−Ernzerhof function was employed in the SCF calculation with an SCF tolerance of 1 × 10−6.

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RESULTS AND DISCUSSION The bilayer separator was prepared through a two-step casting method. The cross-section SEM images of the bilayer separator and possible bilayer structure are shown in Figure 1a. Observation of the interface suggests that a unified bilayer is achieved. Optical images show a distinct color variation of the C

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) FTIR spectra of PVDF-HFP, MoO3−PVDF-HFP, LLZTO−PVDF-HFP, and bilayer separators, respectively. (b) Raman spectra of MoO3−PVDF-HFP, LLZTO−PVDF-HFP, and bilayer separators. (c) TGA curves of Celgard 2325, PVDF-HFP, and bilayer separators, respectively. (d) Stress−strain curves of PVDF-HFP, Celgard 2325 at a transverse direction, MoO3−PVDF-HFP, LLZTO−PVDF-HFP, and bilayer separators, respectively.

MoO3−PVDF-HFP separator, however, does not show any features of the CC bond, where three peaks at 678, 830, and 1007 cm−1 ascribed to MoO3 are observed.30 In contrast, the bilayer separator exhibits all characteristic peaks of MoO3− PVDF-HFP and LLZTO−PVDF-HFP separators, indicating that a bilayer separator structure is formed. The thermal stability of the separators was investigated with TGA, and the results are shown in Figure 2c; the initial decomposition temperature of PVDF-HFP separator is 450 °C. In contrast, the bilayer separator decomposed at 375 °C, suggesting a reduced thermal stability, which is attributed to the interaction between PVDF-HFP and MoO3 and LLZTO.31 Nonetheless, the thermal stability of the bilayer separator is still higher than the Celgard 2325, making it attractive for LIB application. The tensile stress−strain curves of separators are shown in Figure 2d. The PVDF-HFP separator demonstrates a maximum tensile strength of 7.3 MPa with only 48% elongation observed. In contrast, the bilayer separator exhibits a much higher tensile strength of 22.3 MPa and an elongation of 227%. Moreover, the mechanical strength is observed to be better than that of the MoO3−PVDF-HFP, LLZTO−PVDFHFP, and Celgard 2325. The excellent mechanical properties are attributed to the large interfacial adhesion of MoO3, LLZTO to the PVDF-HFP matrix,32 which leads to the effective applied load transfer through the MoO3 and LLZTO contact to PVDF-HFP chains, thus protecting the PVDF-HFP polymer from breaking.33 The thermal shrinkage of separators was compared, as shown in Figure 3. The Celgard 2325 cannot withstand

demonstrated by the porosity data of separators (Table S2), the porosity of the bilayer separator is 39.8%, whereas the porosities of PVDF-HFP and Celgard 2325 are only 13.3 and 28.7%, respectively. Moreover, the bilayer structure owns a larger specific surface area of 1.492 m2/g, according to the BET data shown in Figure S4e, whereas those of Celgard 2325 (Figure S4a) and PVDF-HFP separators (Figure S4c) are only 0.883 and 0.640 m2/g, respectively. FTIR and Raman spectra of the separators were measured to investigate structure variation of PVDF-HFP. As shown in Figure 2a, the PVDF-HFP separator exhibits several peaks at 532, 610, 764, 975, and 1390 cm−1, which are assigned to the α-phase structure of the polymer.27 After incorporating MoO3 and LLZTO, there is striking variation in the vibrational modes and wavenumbers. Typically, the peak intensity at 1200 cm−1 (−CF2) is reduced, and a broader peak is obtained, which is attributed to the interaction of MoO3, LLZTO, and PVDFHFP.28 It also indicates that the amorphous content increases. Additionally, a new peak (1670 cm−1) assigned to the emerging CC bond is observed in the LLZTO−PVDFHFP and the bilayer separator, respectively, suggesting that PVDF-HFP suffers a dehydrofluorination reaction and forms a CC bond. LLZTO can create an alkaline-like condition, which leads to the dehydrofluorination reaction of PVDF-HFP, and the structure vibrations are consistent with those of PVDF in alkaline condition.23,29 Figures S1d and 2b show the Raman spectra of the separators. Compared with the PVDF-HFP (Figure S1d), the LLZTO−PVDF-HFP separator exhibits two new Raman peaks in 1137 and 1525 cm−1 (Figure 2b), and these peaks are also attributed to the CC bond.23 The D

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

degrees. The Celgard 2325 separator nearly melts completely, and the PVDF-HFP separator shrinks by 42.1%. In comparison, the bilayer separator exhibits higher thermal stability by shrinking with only 5.3%, suggesting that adding LLZTO and MoO3 into PVDF-HFP improves the dimensional thermal stability. The excellent thermal stability, decent mechanical strength, and low thermal shrinkage provide the bilayer separator with great advantages for practical applications. In the charge−discharge processes, the rising ohmic heat during short circuits induced by lithium dendrite can lead to overheating and melting of the battery separator. Hence, the thermal distribution performance of the separator plays a crucial role in battery operation. We investigate the thermal distribution on the surface of the separators, as shown in Figure 4a,b; there are noticeable shrinkage and nonuniform thermal distribution of the Celgard 2325 and PVDF-HFP separators, suggesting poor heat transport properties. Furthermore, the shrinkages associated with the PVDF-HFP and Celgard 2325 separators show possible ease of shorting at elevated temperatures during the cell operation. The timeresolved temperature curves of Celgard 2325 and PVDF-HFP separators show strong average temperature fluctuation, which would easily lead to local overheating. In contrast, the bilayer separator exhibits uniform thermal distribution and a mild average temperature fluctuation (Figure 4c), indicating that the

Figure 3. (a−d) Shrinkage of Celgard 2325, PVDF-HFP, MoO3− PVDF-HFP, LLZTO−PVDF-HFP, and bilayer separators at temperatures from 80 to 160 °C for 4 h, respectively. The diameter of the separator is 19 mm.

temperatures above 100 °C; however, the PVDF-HFP, MoO3−PVDF-HFP, LLZTO−PVDF-HFP, and bilayer separators barely changed after heat treatment at 130 °C. After heat treatment at 160 °C, these separators all shrink with various

Figure 4. FLIR thermal distributed images (left) and the corresponding time-resolved temperature curves (right) of (a) Celgard 2325, (b) PVDFHFP, and (c) bilayer separators. Optical photographs of burning test of (d) Celgard 2325, (e) PVDF-HFP, and (f) bilayer separators. E

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) Ion conductivities of Celgard 2325, PVDF-HFP, and bilayer separators. (b) Rate performance of Li/LFP cells with Celgard 2325, PVDF-HFP, and bilayer separators. (c) EIS of Li/LFP cells with Celgard 2325, PVDF-HFP, and bilayer separators. (d) Cyclic performances of Li/ LFP cells assembled with Celgard 2325, PVDF-HFP, and bilayer separators.

bilayer separator exhibits the highest ion conductivity (6.33 × 10−4 S cm−1 at 30 °C), which is higher than that of Celgard 2325 (4.16 × 10−4 S cm−1 at 30 °C). The improved ion conductivity can be attributed to the increased carrier mobility caused by the reduced crystallinity in PVDF-HFP.33−35 Moreover, the CC bond also improves ion conductivity by increasing the segmental motion of the PVDF chain.23 Electrochemical energy storage behavior of Li/LFP cells with the separators was tested. Batteries with a bilayer separator of thickness from 30 to 70 μm own discharge capacities of 161, 155, 159, 143, and 148 mA h/g at 0.5 C, respectively, as shown in Figure S6f. Batteries with thin separators have a high power density but tend to exhibit weak mechanical properties. Moreover, thin separators cannot effectively prevent dendrite formation. On the other hand, thick separators lead to low ion conductivity and Coulombic efficiency. Hence, the batteries with bilayer separators (50 μm) were further investigated. The cells with the bilayer separator deliver excellent rate performances of 159, 152, 132, 104, 91, 71, and 153 mA h/g at 0.5, 1, 3, 5, 7, 10, and 0.5 C, respectively, which are better than those cells with PVDF-HFP, Celgard 2325, MoO3−PVDF-HFP, and LLZTO−PVDF-HFP separators (Figures 5b and S6c). Electrochemical impedance spectroscopy (EIS) was conducted to investigate the charge transfer of Li/LFP cells with the separators, as shown in Figures 5c and S6d. Compared to those with PVDF-HFP, Celgard 2325, MoO3−PVDF-HFP, and LLZTO−PVDF-HFP, the Li/LFP cells with the bilayer separator exhibit the smallest semicircle in the high-frequency region, suggesting a lower charge-transfer resistance and faster reaction kinetics. Figures 5d and S6e show the cycling performance of cells with these separators at 0.5 C. The cells with a bilayer separator outperform the cells with PVDF-HFP, Celgard 2325,

bilayer separator owns excellent thermal transfer properties, which is also better than that of MoO3−PVDF-HFP and LLZTO−PVDF-HFP separators (Figure S5a,b). In addition, there is no observed shrinkage appearing in the range of experimental temperature. The enhanced thermal distribution performance may well be originated from the uniform dispersion of MoO3 and LLZTO, which improve thermal transfer and suppress the overheating of separator. A combustion test of the separators upon thermal trigger was conducted. Before the test, separators were wetted by a liquid electrolyte. As shown in Figure 4d,e, Movies S1, and S2, the Celgard 2325 and PVDF-HFP separators show immediate shrinkage and complete burning when exposed to the fire. The MoO3−PVDF-HFP separator shrank, but did not get ignited (Figure S5c). The LLZTO−PVDF-HFP separator initially got ignited, but the fire extinguished promptly after the lighter was removed (Figure S5d). The bilayer separator, on the other hand, suppresses the flame such that it cannot be ignited even when exposed to the fire for 30 s (Figure 4f and Movie S3). The flame-retardant properties of the bilayer separator suggest superior safety when utilized in battery applications. Figures S6a,b and 5a show the ionic conductivities of separators. The ionic conductivities of PVDF-HFP separators with various contents of MoO3 (6.67, 12.5, 17.6, and 36.3%) and LLZTO (6.67, 12.5, and 22.2%) are all higher than those of pure PVDF-HFP (1.45 × 10−4 S cm−1 at 30 °C). Among them, the separators containing 12.5% MoO3 and 12.5% LLZTO exhibit the highest ionic conductivities of 5.53 × 10−4 and 3.49 × 10−4 S cm−1 at 30 °C, respectively. However, excessive MoO3 and LLZTO contents lead to the aggregation of particles, causing reduction of ionic conductivity. Therefore, the PVDF-HFP separator with 12.5% MoO3 and 12.5% LLZTO were selected to fabricate the bilayer separator. The F

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces MoO3−PVDF-HFP, and LLZTO−PVDF-HFP separators by achieving a remarkable discharge capacity of 162 mA h/g at 0.5 C after 100 cycles, with a capacity retention of 95% and almost 100% Coulombic efficiency, further proving that the bilayer separator is much superior to the Celgard 2325 for LIB application. Moreover, compared with the reported ceramicmodified separators, the bilayer separator exhibits better electrochemical performances, as shown in Table S3. According to previous report,36 the strong interfacial adhesion can cause more carbon residues to form between polymer and flame-retardant additives upon combustion. This further evolves into a compact structure and suppresses the combustion of carbon residues. We supposed that the excellent flame-retardant properties of the bilayer separator are attributed to the increased interfacial adhesion of MoO3 and LLZTO to PVDF-HFP. The characteristic bonds of MoO3, LLZTO to the PVDF-HFP were then investigated through DFT calculation (Figure 6a−e). The adsorption energies of

(G band) indicate that the amorphous carbon and graphitized carbon coexist in the carbon residues. The ratio of intensity of the D band and G band (R = IG/ID) represents the graphitization. MoO3 and LLZTO increases the content of graphitized carbon, as proven by the R values (Table S2). The carbon residues of the PVDF-HFP separator exhibit an R value of 0.787, indicating a low content of graphitized carbon. With MoO3 and LLZTO, both MoO3−PVDF-HFP and LLZTO− PVDF-HFP separators show higher graphitization, and the R values of MoO3−PVDF-HFP (12.5% MoO3) and LLZTO− PVDF-HFP (12.5% LLZTO) separators are 0.950 and 0.977, respectively. In contrast, the bilayer separator shows the highest graphitization with an R value of 1.228, demonstrating that MoO3 and LLZTO significantly promote the formation of graphitized carbon. As also confirmed by the XPS spectra of C 1s of carbon residues (Figure S5), the peak at 284.1 eV is attributed to the sp2-bond carbon (CC bond).42 As mentioned above, a bilayer separator also has an intrinsic CC characteristic. However, in the combustion process, the pre-existing CC bond is destroyed, and the signals of CC bond are all derived from graphitized carbon. MoO3 and LLZTO may act as catalysts for the conversion of amorphous carbon into graphitized carbon, and the catalytic process can involve the dissolution of amorphous carbon by MoO3 and LLZTO followed by the precipitation of graphitized carbon.43,44 However, excessive MoO3 and LLZTO can cause the aggregation of particles and reduce the catalytic activity, leading to lower the content of graphitized carbon (Table S2). The graphitized carbon greatly improves thermal oxidation resistance and prevents the heat from transferring to carbon residues, thus increasing the flame resistance of the bilayer separator.45,46 Further, after burning, the binding energy of F 1s (−CF2, −CF)47 shifts from 687.8 eV to a lower value of 686.9 eV, as observed from Figure 7f,i. On the contrary, the binding energies of Mo 3d shift from 232.4 and 235.6 eV to higher values of 232.8 and 235.9 eV (Figure 7d,g), and La 3d binding energies shift from 831.6 and 835.8 eV to higher values of 832.1 and 837.3 eV (Figure 7e,h).48,49 Mo and La atoms in the bilayer separator suffer great electron loss in the combustion process, which is caused by the reaction with F species. The new F 1s peak at 685.0 eV attributed to the metal fluoride species (MoFx and LaFx) demonstrates the complex interaction of Mo, La, and F, which forms through a Lewis acid−base reaction.50,51 MoFx and LaFx also enhance flameretardant properties because of their high specific heat capacity, which avoids the heat accumulation in the carbon residues.52,53 Benefiting from the synergistic effects of the protective graphitized carbon, and the MoFx, LaFx species, this bilayer separator exhibits excellent flame-retardant properties.

Figure 6. (a−c) Interaction of PVDF-HFP with the (010) surface of MoO3, with H atom on top of the O atom from the MoO3 surface. (d,e) Interaction of PVDF-HFP with the (100) surface of LLZO, with C atom on top of Li atom from the LLZO surface.

MoO3−CH2CF2 (−0.057 eV), MoO3−CH3CHF2 (−0.182 eV), MoO3−CHF2CHFCF3 (−0.082 eV), LLZO−CH2CF2 (−0.080 eV), and LLZO−CF2CFCF3 (−0.130 eV) are subsequently observed to be smaller than those of Mo−F (−4.35 eV), O−F (−1.16 eV), and La−F (−6.83 eV) bond strengths.37−39 In view of the small value of the adsorption energies, MoO3 may form hydrogen bonds (C−H···O) with PVDF-HFP because of the interaction of the Lewis acid in the C−H bond and Lewis base of the oxygen center, whereas the only force of attraction within LLZTO and PVDF-HFP is due to van der Waals forces.40 Large quantities of hydrogen bonds and van der Waals forces effectively increase the interfacial adhesion, facilitating MoO3 and LLZTO capture of carbon residues and improving the thermal stability of carbon residues.41 For a deep understanding of the flame-retardant mechanism, we performed Raman spectra to characterize the component of carbon residues of the separator. As shown in Figure 7a−c, all of the Raman spectra exhibit two typical bands of carbon materials. The bands at 1310 cm−1 (D band) and 1585 cm−1

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CONCLUSION A bilayer separator is synthesized using a facile casting method which demonstrates superior safety when exposed to flame. With MoO3 and LLZTO, large quantities of hydrogen bonds and van der Waals forces are generated, which increases the interfacial adhesion of MoO3 and LLZTO to PVDF-HFP. In addition, the graphitized carbon content of the carbon residues is increased, the MoFx and LaFx species are formed during combustion, which greatly enhance the flame-retardant performance. The bilayer separator also owns a high mechanical strength and low thermal shrinkage. With the robust bilayer structure, the Li/LFP batteries exhibit excellent G

DOI: 10.1021/acsami.9b08553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 7. Comparison of Raman spectra of the carbon residues of (a) MoO3−PVDF-HFP and (b) LLZTO−PVDF-HFP separators after burning. (c) Comparison of Raman spectra of the carbon residues of separators after burning. The comparison of XPS of Mo 3d (d−g), La 3d (e−h), and F 1s (f−i) spectra of bilayer separators before and after burning, respectively.

Notes

electrochemical performance. These results illustrate that the safe bilayer separator is promising for practical applications.

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The authors declare no competing financial interest.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS The work is supported by National Natural Science Foundation of China (grant no. 81572418), National Natural Science Foundation of Tianjin 15 (grant no. 15JCQNJC10100), the Sichuan Science and Technology Program (no. 2014RZ0041) and Science and Technology Development Foundation of Tianjin’s Colleges and Universities (grant no. 20140116).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08553. Video showing that the Celgard 2325 separator shows immediate shrinkage and complete burning when exposed to the fire (AVI) Video showing that the PVDF-HFP separator shows immediate shrinkage and complete burning when exposed to the fire (AVI) Video showing that the bilayer separator suppresses the flame that it cannot be ignited even when exposed to the fire for 30 s (AVI) XRD patterns, Raman spectra, SEM images, XPS spectra and FLIR thermal distributed images, ion conductivity of separators, rate and cyclic performances of Li/LFP cells with separators, EIS data, BET, porosity, digital photographs of combustion test, electrolyte uptake of separators, and R values of carbon residues (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weidong He: 0000-0001-8242-2888 Author Contributions ⊥

G.Z., J.Z., and C.F. contributed equally to this work. H

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J

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