Superior Thermally Stable and Nonflammable Porous

Feb 21, 2017 - The electrolyte uptake (U) of the membrane was determined by measuring the weight of dry membrane (M0) and saturated membrane (Me) afte...
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Superior Thermally Stable and Nonflammable Porous Polybenzimidazole Membrane with High Wettability for High-Power Lithium-Ion Batteries Dan Li,+,† Dingqin Shi,+ Yonggao Xia,‡ Lin Qiao,+,† Xianfeng Li,*,+,§ and Huamin Zhang*,+,§ +

Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China † University of Chinese Academy of Sciences, Beijing 100039, China § Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, China ‡ Ningbo Institute of Materials Technology Engineering (NIMTE), Chinese Academy of Sciences, Zhejiang 315201, P. R. China S Supporting Information *

ABSTRACT: Separators with high security, reliability, and rate capacity are in urgent need for the advancement of high power lithium ion batteries. The currently used porous polyolefin membranes are critically hindered by their low thermal stability and poor electrolyte wettability, which further lead to low rate capacity. Here we present a novel promising porous polybenzimidazole (PBI) membrane with super high thermal stability and electrolyte wettability. The rigid structure and functional groups in the PBI chain enable membranes to be stable at temperature as high as 400 °C, and the unique flame resistance of PBI could ensure the high security of a battery as well. In particular, the prepared membrane owns 328% electrolyte uptake, which is more than two times higher than commercial Celgard 2325 separator. The unique combination of high thermal stability, high flame resistance and super high electrolyte wettability enable the PBI porous membranes to be highly promising for high power lithium battery. KEYWORDS: lithium-ion battery, polybenzimidazole, nanoporous membrane, thermal stability, flame resistance

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the lithium ions transport channels.1,11 Consequently, an ideal separator has to own the characteristics of high electrochemical, mechanical stability as well as high wettability with electrolyte and high ion conductivity.5,12 Most importantly, a large amount of heat could be produced during the charge/discharge process at a high C-rate, a dramatic increase in temperature of a battery will be induced, and therefore a high thermal stability is highly required to avoid short circuit, thermal runaway, and even explosion.13−15 At present, the commercial membranes for lithium-ion batteries are based on polyolefin membranes, such as polyethylene (PE), polypropylene (PP), and their compo-

INTRODUCTION The dramatically increased demand on electric vehicles promoted the development of advanced power batteries.1−4 Among the delivered battery systems, lithium-ion batteries have attracted great interest in recent years, mainly because of their attractive features like high energy density and high efficiency.5−8 However, the safety issues are becoming more and more serious with series of accidents occurring, which are derived from poor dimensional thermostability of separators and flammability of electrolyte.9,10 A rational design and assembly of different components are very essential to obtaining an acceptable power battery. Traditionally, a lithium-ion battery mainly composes of cathode, anode, electrolyte, and a separator. As one of the key components of a lithium-ion battery, a separator plays the role of preventing direct contact of the positive and negative electrode, providing © 2017 American Chemical Society

Received: December 20, 2016 Accepted: February 21, 2017 Published: February 21, 2017 8742

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

Research Article

ACS Applied Materials & Interfaces site.5,16 However, the poor thermal stability of polyolefin separator further hinders its application in high power battery (PE/130 °C, PP/160 °C). The hydrophobicity of polyolefin results in the poor wettability on electrolyte and low ionic conductivity, leading to the poor rate performance.6,17 Therefore, alternative separators with high thermal stability and good wettability are in urgent need for high power lithiumion batteries. Recently, many efforts have been taken to solve these issues. For example, introducing inorganic particles such as aluminum oxide (Al2O3),18,19 silicon dioxide (SiO2),20,21 and titanium dioxide (TiO2)22,23 into or onto the polyolefin separators could significantly improve their thermal stability and the wettability. Constructing the heat-resistant skeleton can also solve these issues, such as porous polyether imide separator,24 cellulosebased composite nonwoven,25 and the PMMA colloidal particles-embedded poly(ethylene terephthalate) (PET) composite nonwoven separator.26 Even though, the mentioned separators exhibited improved thermal stability. However, they rarely can retard the combustion. In addition, the complicated manufacturing process makes the membranes more expensive. Polybenzimidazole (PBI) is one kind of aromatic polymers possessed the highest thermal resistance and the best mechanical property among all the unfilled thermal plastics. Most importantly, PBI has very high inherent flame retardancy and is widely used for bunker clothing and other applications in aeronautics field, etc.27 In addition, because of the fact that the polar nitrogen atoms have strong interaction with electrolyte, the PBI separator is expected to have very good wettability with electrolyte, which can further improve the power density of the lithium-ion battery. Therefore, in this paper, the PBI-based spongelike porous membranes (PBI) with superhigh thermal stability and flame retardancy were designed and fabricated for high power lithium-ion batteries. A water vapor phase inversion method was employed to prepare highly symmetric spongy porous PBI membranes. The method is highly convenient to produce membranes with high porosity and easy for further upscaling. The high porosity together with its hydrophilic N−H groups will ensure the membranes with high rate capacity and will be highly beneficial to fabricating lithium-ion batteries with high power density, whereas the super high thermal stability, mechanical stability, and flame resistance can internally solve the critical safety issues of lithium-ion batteries.

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(BPS-100CL, Hangzhou Haxi) for about 10 min, where the temperature and relative humidity were kept at 50 °C and 100% respectively. Afterward, the membrane was completely immersed in water for 3 h to remove the residue DMAC. Finally, the prepared membranes were dried completely in a vacuum oven at 60 °C for 7 h. The average thickness of the membrane was 28 ± 2 μm. Characterization of Membranes. The 1H NMR was carried out on a BRUKER DRX400 with tetramethylsilane (TMS) as an internal standard and DMSO-d6 as solvent. Fourier transform infrared spectroscopy (FTIR) was collected with a BRUKER TENSOR 27. The morphologies of the membrane were detected by a scanning electron microscope (JSM-7800F). The porosity of the membranes was analyzed by immersing them in the n-butanol for 12 h; the weight of dry membrane (md) and nbutanol filled in membrane (mb) were recorded. Afterword, the porosity of the membrane was calculated according to eq 1

porosity = (mb /ρb )/(mb /ρb + md /ρd )100%

(1)

where md and mb are the weight of dry membrane and n-butanol filled in membrane, respectively. ρd and ρb represent the density of polymer and the n-butanol. The electrolyte uptake (U) of the membrane was determined by measuring the weight of dry membrane (M0) and saturated membrane (Me) after immersing the membrane in the electrolyte for 12 h. The electrolyte uptake (U) was calculated according to eq 2

U = (Me − M 0)/M 0100%

(2)

where M0 and Me are the weight of the membrane before and after immersion in the electrolyte, respectively. The swelling behavior of the membrane was measured by immersing the membrane into the electrolyte for 24 h at room temperature. Then the swelling was obtained by the changing area and thickness of the membrane after being immersed into the electrolyte. The area swelling (Sa) was calculated according to the eq 3

Sa = (A1 − A 0)/A 0100%

(3)

The thickness swelling (St) was calculated according to eq 4

St = (T1 − T0)/T0100%

(4)

where A0 and A1 are the area of the prepared membrane before and after being immersed in the electrolyte. And the T0 and T1 are the thickness of the prepared membrane before and after being immersed in the electrolyte. The mechanical property of membranes was conducted on the material test machaine (AG-2004,shimadzu) at a loading velosity of 5 cm min−1. The bulk impedance (Rb) of a prepared membrane was analyzed by the electrochemical impedance spectroscopy (EIS) (Solartron 1287 electrochemical workstation). The electrolyte-soaked membrane was sandwiched between two pieces of stainless steel (SS) and the spectra was recorded over a frequency range from 0.1 Hz to 1 × 106 Hz with the AC amplitude of 10 mV under open circuit potential condition at room temperature. The ionic conductivity (σ) was calculated according to the eq 5

EXPERIMENTAL SECTION

Materials. 3,3′-diaminobenzidine was purchased from Acros Organic, 4,4′- dicarboxydiphenylether was purchased from Peakchem. Other chemicals were purchased from Tianjing Damao Chemical Reagent Factory. All chemicals were used as received. Polymer Synthesis. The PBI was prepared through condensation polymerization process of 3,3′-diaminobenzidine and 4,4′- dicarboxydiphenylether as reported previously.28 Fabrication of Membranes. The sponge-like porous membranes were fabricated through the water vapor induced phase inversion method. First, the proper amount of PBI was dissolved in N,Ndimethylacetamide (DMAC) to prepare a 15 wt % casting solution with a dynamic viscosity of 94.7 Pa s (NDJ-8S viscometer, Shanghai Pingxuan Scientific Instrument Co., Ltd., China) at 25 °C. Afterward, the solution was casted onto a clean glass plate using a doctor blade with a thickness of 150 μm (Elcometer 3545 adjustable Bird Coater, Scraper, Elcometer 3545/8), and then the coated glass plate was quickly transferred to a constant temperature and humidity chamber

σ = d /(R bS)

(5)

where d and S are the thickness and effective area of a membrane, respectively. The thermal stability of a membrane was investigated by thermogravimetric analysis (TG), which was conducted in nitrogen at a heating rate of 5 °C min−1 from 50 to 800 °C using a thermal analyzer (PYRIS Diamond TG-DTA, High Temp 115). The dimensional thermostability of membranes at evaluated temperature was investigated by treating the membranes at different temperatures for 1 h. Electrochemical stability of a prepared membrane was recorded from 3 to 5.5 V Vs. Li+/Li using linear sweep voltammetry (LSV) on an electrochemical system (PARSTAT 2273, Princeton Applied Research, USA) and the voltage was swept at a scan of 0.5 mV s−1 8743

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic principle of the interaction between PBI membrane and the liquid electrolyte. (b) Schematic principle of the PBI membrane. (c) Morphology of the membrane cross-section. (d) Photographic image of PBI membrane exposing on fire. at room temperature. The membrane was sandwiched between the metallic lithium and SS, which were used as the reference and counter electrode, respectively. Electrochemical Performance of Batteries. Battery performance of the prepared membrane was studied by using half-cell method with CR2016 coin cells. The CR2016 coin cell was assembled in an argon-filled glovebox using a prepared membrane as a separator, 1 M LiPF6 in ethylene carbonate, diethyl carbonate and ethylmethyl carbonate (EC/DMC/EMC, 1:1:1 vol.) as electrolyte, LiFePO4 as the cathode (The active mass loading of the electrodes ranged from 1.5 to 1.8 mg cm−2) and metallic lithium as the anode. The galvanostatic charge−discharge was performed in the voltage window of 2.0−4.2 V on a Land automatic battery tester (Wuhan, China). The cycle performance of a LiFePO4/separator/graphite battery was studied by using a soft package battery (geometric area: 40 × 40 mm2), which was assembled by a prepared membrane, a cathode (LiFePO4), a anode (graphite) and liquid electrolyte (1 M LiPF6 in ethylene carbonate, diethyl carbonate and ethylmethyl carbonate (EC/DMC/EMC, 1:1:1 vol)), the mass ratio of cathodes and anodes were in the range of 1.5− 1.8. Which were based on the average value of three parallel test. The standard deviation of battery performance in this study was within 5%.

the walls composed of lots of nanosized pores, which acting as the channels for lithium-ion transport. While the large quantities of cells can provide enough space to preserve the electrolyte and further improve ion conductivity. In addition, as described in Figure 1a, the polar ether bonds and nitrogen atoms in the PBI based matrix showed strong interaction with the electrolyte, which will induce an excellent compatibility with the electrolyte as well. Figure 2 showed the morphology of the PBI and Celgard 2325 membranes. Figure 2a, b were the cross-section and the

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RESULTS AND DISCUSSION The PBI with ether functional groups was synthesized via nucleophilic substation condensation polymerization of 3,3′diaminobenzidine (DABz) and 4,4′- dicarboxydiphenylether (DCDPE) at 140 °C as reported previously.28 The detailed reaction is shown in Figure S1a. Fourier transform infrared (FTIR) in Figure S1b verified the chemical structure of the prepared PBI. The broadened adsorption bands ranging from 2512 to 3589 cm−1 were assigned to the N−H groups in imidazole ring. The band at 1620 cm−1 was attributed to the stretching vibration of CN and CC in the N-containing five-membered heterocyclic rings. The bands at 1253 and 1195 cm−1 were assigned to phenyl ether groups. Figure S1c illustrated the 1H NMR spectra of prepared PBI polymers. The peaks at 7.25 and 7.50 ppm were attributed to H1 and H2 in Figure S1a, whereas the peaks in the range of 7.6−8.5 (δ = 7.63 (1H, s), 7.78 (1H, s), 8.33 (1H, s)) ppm were attributed to the H in benzimidazole ring. The peak at 12.93 ppm was assigned to the hydrogen in the imidazole ring, corresponded to H3 in Figure S1a. The results of the FTIR and 1H NMR spectra were in good agreement with the previous reports.27,29 Afterward, a sponge-like porous PBI membrane was fabricated by water vapor phase inversion method. As shown in Figure 1b−d, the designed membrane contained large quantities of micron-sized cells separated by ultrathin walls, and

Figure 2. (a, b) Cross-section morphology and the magnified crosssection morphology of a PBI membrane. (c, d) Surface morphology of the PBI membrane and the Celgard 2325 membrane.

magnified cross-section morphology of a PBI membrane. As can be seen, a typical symmetric sponge-like porous membrane was prepared, with uniformly distributed, interconnected porous structure. The surface morphology on glass plate side and air side is very different. Porous structures with polymer droplets coarsened on the air side and dense layer on glass plate side were clearly found, as shown in Figure 2c and Figure S4. The membrane morphology was largely determined by the dynamics and thermodynamics factors during phase inversion process. The relatively even concentration profiles, solvent, polymer and nonsolvent, was obtained due to the slower rate of precipitation compared with the other phase separation method, like water phase inversion method. Then the flat concentration leads to polymer solution solidification at similar 8744

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

Research Article

ACS Applied Materials & Interfaces Table 1. Physical Properties of PBI Membranes and Celgard 2325 Separators sample

thickness (μm)

porosity (%)

electrolyte contact angle (deg)

electrolyte uptake (%)

bulk resistance (Ω)

ion conductivity (S cm−1, 25 °C)

PBI Celgard 2325

28 ± 2 25 ± 1

81 ± 2 41 ± 1

9±1 62 ± 1

328 ± 4 115 ± 4

1.11 ± 0.05 1.52 ± 0.05

13.4 (± 0.6) × 10−4 8.75 (±0.3) × 10−4

time over the entire film cross-section, leading to symmetric membrane morphology. While, the exchange of nonsolvent with solvent results in the initiation of a polymer lean phase as droplets by the nucleation and growth mechanism, leading to the separated micron-sized cellular structures, which displays the unique characteristic of interconnected voids separated by pore walls. In addition, when the casting solution was placed in the nonsolvent, a liquid layer emerges on the top of the cast solution. And this liquid layer brings about a solvent gradient through the cast film, leading to a higher concentration at the surface and a lower solvent concentration near the bottom. And the viscosity of the cast solution increased with increasing polymer concentration. As a result, the droplets were coarsened and a porous structure was formed at the top layer of a membrane, where the viscosity was relatively low and a dense surface skin was formed at the bottom. The prepared PBI membrane showed a porosity of about 81% (Table 1), which is two times higher than Celgard 2325 separator (41%). The high porosity of prepared membranes is expected to possess high electrolyte uptake and further high ion conductivity. The commercial Celgard 2325 separator was prepared via dry process by stretching step. The separator had slit-like pores with nanometer-size (Figure 2d), the pores on the separator surface were about 50 to 250 nm, which was much larger than prepared PBI porous membranes. As described in Figure 2c, there were no obvious visible pores on the surface morphology of PBI membrane. The average pore size of a PBI membrane was around 14 nm, which was calculated from the Poiseuille equation via pure water fluxes30 The smaller pores were beneficial to reduce self-discharge rate,31 prevent leakage of liquid electrolyte, and suppress the growth of the lithium dendrite.32,33,1 Because lower pore size of membranes would inhibit lithium dendrite from penetration through the membranes due to the stronger mechanical strength and then further avoid short circuit. In addition, the membranes, an electronic insulator, could stop the continued growth of lithium dendrite as well. Furthermore, the morphology of surface and cross-section before and the drying procedure have been studied and shown in Figure S4. As can be seen, the surface morphology of the PBI membrane before and after drying changed rarely, while some minor change occurred on the cross-section. To further study the stability of porous structure of PBI membranes, we detected the surface and cross-section morphology of a PBI membrane after 50 cycles at 1C, shown in Figure S4. The morphology of surface and cross-section of a PBI membrane was kept similar after cycling. Apart from the high porosity, the separator with good electrolyte wettability was in favor of superior electrochemical performance as well. In particular, uniform and fast wetting of the electrolyte through the whole separator was highly needed for fast transporting of lithium ions through a membrane. Therefore, the wetting property of the separator was evaluated by spreading test and liquid electrolyte contact angle measurement.34 When dipping the electrolyte on the surface of a prepared PBI membrane and a Celgard 2325 separator, as shown in Figure 3a, b, the electrolyte spread throughout the

Figure 3. (a, b) Photographs of the wetting behavior of the separator with liquid electrolyte. (a) Membranes without electrolyte. (b) Membranes with electrolyte. (c) Liquid electrolyte contact angles.

whole PBI membrane in two seconds, whereas the electrolyte on the Celgard 2325 kept the form of droplet for a long time. In addition, the liquid electrolyte contact angle of PBI membrane and Celgard 2325 were around 9 and 62°, respectively. All these results indicated the PBI membrane showed much higher electrolyte wettability than Celgard 2325 separator. This could be due to the existence of the polar ether bonds and amine groups in the PBI main chain, thus would generate strong interaction between the prepared PBI membranes with the polar electrolyte. However, Celgard 2325 separator was made from hydorphobic polyolefin, which resulted in the poor wettability with the electrolyte.12,17 To further confirm the wettability performance of the prepared membrane, we conducted an electrolyte uptake test. As expected, the electrolyte uptake of the prepared PBI membrane was about 328%, which was more than two times higher than a Celgard 2325 (115%). Combing high porosity, high electrolyte uptake and high wettability, the PBI was expected to have very impressive ionic conductivity.1,35 The mechanical properties of prepared PBI membranes were shown in Figure S2. The tensile strength of PBI membranes was 36.41 MPa. Although the tensile strength of PBI membranes was lower than the commercial Celgard 2325 separators, they were still enough for lithium-ion battery. And the mechanical strength of the prepared PBI membrane at the wet state (immersed in the electrolyte for 24 h) was also detected and shown in Figure S3. To our surprise, the mechanical strength of a PBI membrane at 8745

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

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ACS Applied Materials & Interfaces

temperature) of PBI polymer was around 392 °C (Figure S10). All this results indicated a much better thermal stability of a PBI membrane than a Celgard 2325. Dimensional thermostability of the separator is a significant factor for high power density batteries, particularly at elevated temperature. A good thermal stability for separator can effectively overcome the safety issues of the battery. Thus, we studied the thermal shrinkage behavior of PBI membranes and Celgard 2325 by treating them at certain temperature for 1h in air and observing the dimensional change. The results are shown in Figure S5. Celgard 2325 showed no change when the temperature was below 130 °C, whereas Celgard 2325 generated discernible one-way volume shrinkage and the thermal shrinkage was up to 60% when the temperature increases to 160 °C. This was because the Celgard 2325 separator was prepared by uniaxial stretching step, the lamellar aparted from each other were oriented in the machine direction after the stretching step. The mobility of the polymer chains enhanced with the elevated temperature.32 The melting point of polyethylene is about 130 °C. When the temperature rised to 200 °C, the separator melted instantly. While for the prepared PBI membrane, it exhibited superior thermal stability with no thermal shrinkage even at around 400 °C, which demonstrated an excellent thermal stability. The above results thus made the PBI membrane as a very promising candidate for high power density battery even at extremely high temperature. The flame retardancy of the separator was another significant factor for the lithium-ion batteries. 36 The commercial polyolefin membrane was flammable, thus the battery assembled with the polyolefin membrane as the separator was easily burned at elevated temperature and further led to serious fire accident. In our system, the selected PBI polymer had a very good flame retardant, therefore, can immediately terminate the fire accident to internally avoid this problem. To further confirm this, we carried out combustion tests on a PBI membrane and a Celgard 2325 separator, and the results are shown in Figure S6. The Celgard 2325 separator was immediately melted and fired when exposed to fire. However, when exposing the prepared PBI membrane on the fire, the PBI membrane just curled and will not burn.The flame retardancy of electrolyte-soaked membranes was also studied and shown in Figure S7. The electrolyte-soaked Celgard 2325 separator and PBI membrane were both burned immediately when exposed on fire, which was attributed to the flammability of organic electrolyte, whereas the burning rate of PBI membranes was much lower than that of Celgard 2325 separators, which resulted from the high thermostability of prepared PBI membranes. After burning, the PBI membrane still kept very well, further confirming its super nonflammable property. The super nonflammable property rendered a PBI membrane as a highly promising candidate to improve the safety of lithium-ion batteries. Electrochemical stability of prepared membranes was investigated by the linear sweep voltammetry (LSV) method. The electrochemical operating window of the commercial Celgard 2325 separator and PBI membrane was described in Figure 6. It was clear that the electrolyte soaked PBI membrane was stable under around 4.5 V vs Li+/Li, which was almost the same with the commercial Celgard 2325 separator. And the electrochemical stability of prepared membranes at 55 °C were also investigated and the results are shown in Figure S8, which shows that the electrolyte soaked PBI membrane was still stable under around 4.5 V vs Li+/Li. Consequently, it implied that the

the wet state was better than a dried membrane, the tensile strength of PBI membranes was 50.33 MPa. The mechanical stability might attribute to the high affinity between the PBI membrane and electrolyte. To further study the stability of the PBI membrane at wet state, we detected the swelling, area swelling, and thickness swelling. The area swelling of a PBI was around 19. 4%, whereas the thickness swelling of that was 3.0%. The ionic conductivity of membranes was investigated by electrochemical impedance spectroscopy. From Figure 4, the x

Figure 4. Electrochemical impedance spectroscopy (EIS) of the symmetrical cell with the PBI membrane and Celgard 2325 separator sandwiched between two stainless steels.

intercept of the EIS was the bulk resistance of the samples. As can be seen in Table 1, the ionic conductivity of the PBI membrane was around 13.4 × 10−4 S cm−1 at 25 °C, which was much higher than the Celgard 2325 separator (8.75 × 10−4 S cm−1). The thermal properties of the PBI membrane and Celgard 2325 were characterized by thermogravimetric (TG) curves under an atmosphere of nitrogen with temperature range of from 50 to 800 °C (Figure 5). A dramatic fall at about 340 °C in the weight of Celgard 2325 was attributed to the decomposition of the polyolefin backbone. While, for a PBI membrane, with the increment of the temperature, the weight loss at around 540 °C is attributed to the decomposition of the PBI polymer backbone, which was in accordance with the reported results in previous literature.24 The Tg (glass transition

Figure 5. Thermogravimetric curves of the PBI membrane and the Celgard 2325 separator under an atmosphere of nitrogen, the rising rate was 5 °C min−1. 8746

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

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the PBI membrane played higher capacity at each current density because of the strong affinity between the electrode and the electrolyte and high uptake of the electrolyte. The capacity vs cycles curves are shown in Figure 8. The battery assembled with a PBI membrane showed a very stable performance. The tenth cycle (the cell performance was at steady condition) discharge capacities of LiFePO4/Li cells for a PBI membrane and a Celgard 2325 separator were 151.0 and 145.9 mAh g−1, respectively, after 100 cycles, the discharge capacity of the cells with a PBI membrane changed rarely (145.9 mAh g−1). However, the discharge capacity for a Celgard 2325 separator decreased to 123.6 mAh g−1. The results indicated that the PBI membrane showed higher capacity retention (96.6% in 100 cycles). While the capacity retention of the commercial Celgard 2325 separator was 84.7%. The cycle performance of LiFePO4/graphite full battery with PBI membranes was similar to that of the LiFePO4/Li battery (shown in Figure S9).The good cycle performance of the PBI membrane was assigned to the high electrolyte uptake of the PBI membrane, which was beneficial for reducing electrolyte leakage. In addition, the unique microstructure of the PBI membrane was also in favor of increasing cycle stability, due to the fact that it would prevent the growth of the lithium dendrite and interception of the active materials in the electrode by the formed membrane framework.1 To further evaluate the cell performance at relatively high temperature, a battery with a PBI membrane was operated at 55 °C. As shown in Figure 9, the battery with a PBI membrane exhibited 155 mAh g−1 at 55 °C at a 3C rate, and with capacity retention of 96.5% after 100 cycle, which was better than the performance of the battery with a Celgard 2325. On the one hand, the battery performance normally increases with temperature due to the higher ionic conductivity and the good ability to retain the electrolyte solution, especially at a high C-rate, at which the rate of ion transport is much lower than rate of electron conduction, whereas the higher temperature was more favorable to ion transport.36,39 On the other hand, the time needed during cycling of a battery was shorter at a higher C-rate, and thus the lower impact on the battery performance would be obtained because of the less time for side reaction. In addition, as shown in Figure 10a, a large-area, uniform, and flexible porous membrane was prepared by a simple water vapor induced phase inversion process. Moreover, a charged (4.2 V) soft package battery (geometric area: 77 × 50 mm2) with a PBI membrane, LiFePO4 cathode and metallic lithium

Figure 6. Electrochemical operating window of the PBI membrane and the Celgard 2325 separator.

electrolyte soaked PBI membrane was highly stable under lithium-ion battery operating condition. The battery performance was one of the most important parameters that evaluate the availability of separator. CR2016 coin cells were assembled in an argon-filled glovebox using prepared membrane as a separator, LiFePO4 as the cathode and metallic lithium as the anode. The galvanostatic charge− discharge was measured in the voltage window of 2.0−4.2 V at a charge/discharge rate of 0.5 C/0.5 C (Figure 7a). The rate performances of the batteries with a PBI membrane and a Celgard 2325 separator were exhibited in Figure 7b. The current densities of 0.2, 0.5, 1, 2, and 5C were used in our experiments. The discharge capacity of the cell decreased with the increasing current density. The discharge capacity of the battery using the Celgard 2325 separator as a separator was 152.9 mAh g−1, the capacity retention ratios were 95.8, 89.3, 80.1, and 63.9% at 0.5, 1, 2, and 5C, respectively. In contrast, the battery using a PBI membrane as a separator exhibited the discharge capacity of 154.4 mAh g−1 at 0.2C, and the capacity retention kept 96.3% at 0.5C, 92.7% at 1C, 88.0% at 2C, and 83.2% at 5C. The results indicated that batteries assembled with a PBI membrane exhibited much better rate capabilities than that of a Celgard 2325 separator. The good rate capability of a PBI membrane was ascribed to the high porosity, high electrolyte uptake, high electrolyte wettability and further higher ionic conductivity,37,38 whereas for the Celgard 2325 separator, the relatively lower rate capacity was attributed to the inherent hydrophobicity of the polymer backbone. In particular,

Figure 7. Electrochemical performance of the batteries assembled with a PBI membrane and a Celgard 2325 separator. (a) Cycle performance at 0.5C at the cutoff voltage of 2−4.2 V. (b) Rate performance battery with a PBI membrane and a Celgard 2325 separator at the rate of 0.2, 0.5, 1, 2, and 5C under the voltage range of 2.0−4.2 V. 8747

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

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Figure 8. Discharge capacity (at 0.5C) of the batteries assembled with (a) a PBI membrane and (b) a Celgard 2325 separator.

anode could light up a logo of “DNL17”, which consisted of about 50 LED lights. Figure 10b shows the charge−discharge profile of the soft package battery, the discharge capacity of the battery using a PBI membrane as a separator was 144.5 mAh g−1 at current density of 0.2C, which was similar to the performance of coin cells. All these results further proved the applicability of the PBI membrane in a lithium-ion battery system.

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CONCLUSION In summary, we prepared a spongelike PBI porous membrane by water vapor induced phase inversion process. The membranes own attractive features of super high porosity and electrolyte wettability, and as a result, induced low bulk resistance and high ionic conductivity. More significantly, the prepared porous membranes showed super flame retardancy and high thermal stability. The PBI membrane kept stable even

Figure 9. Cycle performance of batteries assembled with a PBI membrane and a celgard 2325 separator at 55 °C at 3C.

Figure 10. (a) Flexibility of the PBI membrane. (b) Charge−discharge profiles at 0.2C. Optical images of LED logo (c) before and (d) after lighting by a soft package Li-ion battery assembled with PBI membrane. 8748

DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750

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ACS Applied Materials & Interfaces at 400 °C, whereas the Celgard 2325 separator would melt just at 200 °C. In brief, the prepared porous membranes perfectly combined excellent wettability and super high thermal stability and super flame retardancy, which resulted in much better cycle stability and rate capacity performance than the currently used commercial polyolefin membranes. Most importantly, the membranes could overcome the critical safety issues because of their excellent dimensional stability and nonflammable property. The results indicated that the spongelike porous PBI membrane holds great promise as the separator for high power battery.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16316. Scheme of PBI polymer synthesis and characterization, mechanical properties, SEM images of PBI membranes, dimensional thermostability, combustion test, electrochemical operation window of membranes at 55 °C, cycle performance of LiFePO4/separator/graphite and DSC curve (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xianfeng Li: 0000-0002-8541-5779 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the financial support from the Outstanding Young Scientist Foundation, Chinese Academy of Sciences (CAS), Science and Technology Service Network Initiative (KFJ-EW-STS-108), and National Youth Top-notch Talent Program.

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ABBREVIATIONS PBI, polybenzimidazole PE, polyethylene PP, polypropylene SS, stainless steel TG, thermogravimetric analysis LSV, linear sweep voltammetry EIS, electrochemical impedance spectroscopy Rb, bulk impedance

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REFERENCES

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DOI: 10.1021/acsami.6b16316 ACS Appl. Mater. Interfaces 2017, 9, 8742−8750