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Novel Ceramic-Grafted Separator with Highly Thermal Stability for Safe Lithium-Ion Batteries Xiaoyu Jiang,† Xiaoming Zhu,‡,§ Xinping Ai,† Hanxi Yang,† and Yuliang Cao*,† †
Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China ‡ Hubei Collaboration Innovation Center of Non-power Nuclear Technology, Xianning 437100, China § School of Nuclear Technology & Chemistry and Biology, Hubei University of Science and Technology, Xianning 437100, China ABSTRACT: The separator is a critical component of lithium-ion batteries (LIBs), which not only allows ionic transport while it prevents electrical contact between electrodes but also plays a key role for thermal safety performance of LIBs. However, commercial separators for LIBs are typically microporous polyolefin membranes that pose challenges for battery safety, due to shrinking and melting at elevated temperature. Here, we demonstrate a strategy to improve the thermal stability and electrolyte affinity of polyethylene (PE) separators. By simply grafting the vinylsilane coupling reagent on the surface of the PE separator by electron beam irradiation method and subsequent hydrolysis reaction into the Al3+ solution, an ultrathin Al2O3 layer is grafted on the surface of the porous polymer microframework without sacrificing the porous structure and increasing the thickness. The as-synthesized Al2O3 ceramic-grafted separator (Al2O3−CGS) shows almost no shrinkage at 150 °C and decreases the contact angle of the conventional electrolyte compared with the bare PE separator. Notably, the full cells with the Al2O3−CGSs exhibit better cycling performance and rate capability and also provide stable open circuit voltage even at 170 °C, indicating its promising application in LIBs with high safety and energy density. KEYWORDS: ceramic-grafted separator, surface modification, lithium-ion batteries, safety, rate capability
1. INTRODUCTION Lithium-ion batteries (LIBs) have recently garnered a great deal of attention in portable electronics, electric vehicles, and gridscale energy storage systems due to their high energy and power density.1−4 In LIBs, the separator plays a vital role in determining the cycle life, energy/power density, and safety.5,6 Currently, the separators in commercialized LIBs are made of polyethylene (PE), polypropylene (PP), or their combinations. However, their poor thermal stability and intrinsically hydrophobic property limit their application in the large-scale energy storage for next-generation LIBs.7−9 To overcome these drawbacks, various separators have been designed, including ceramic-coated separators (CCS),10−13 nonwoven mat separators,4,14−16 and electrospinning fiber separators.17−20 The nonwoven and electrospinning separators show exceptional wettability and excellent thermal stability. However, the poor mechanical properties compared with commercialized polyolefin separator hinder their further applications.21 Ceramiccoated separators display sufficient mechanical strength and outstanding thermal stability for preventing internal short circuit. However, their increased thickness and blocked porous structure would lead to a decrease in energy and power density of the battery.22 Moreover, the heterogeneous distribution of ceramic particles and poor binding power of polymer binder would result in the detachment of particles from the separators during cell assembly and operation.23 © XXXX American Chemical Society
Recently, some new modification methods have been developed to improve the physical and electrochemical performance of polyolefin separator.24−28 Jung et al. prepared a thin Al2O3 layer-coated PP separator by atomic layer deposition.25 The ∼6 nm Al2O3 layer does not affect the pore structure while significantly suppressing thermal shrinkage of the PP separator. Xu et al. prepared a poly(acrylic acid) (PAA) and ZrO2 modified PE separator by a simple layer-bylayer self-assembly process, which can minimize the thickness increase of PE and preserve the porous structure to the maximum.26 Lee et al. applied radio frequency (RF) magnetron sputtering of Al2O3 on a porous PE separator for lithium ion batteries. The sputtered Al2O3 coating separator without polymeric binder materials can improve the rate capability compared to the bare PE separator.27 In our previous report, a novel facile approach was adopted to prepare an inorganic ceramic-grafted PE separator (CGS) by an ionization radiation grafted and hydrolyzed process.29,30 Compared to the ceramic coated separator (CCS), the grafting method is more beneficial to decrease the interfacial resistance of the separator and increase the ion transport capability.31 The SiO2/TiO2 grafted Received: April 20, 2017 Accepted: July 19, 2017 Published: July 19, 2017 A
DOI: 10.1021/acsami.7b05535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration and (b) proposed mechanism of the preparation process of the Al2O3−CGS. 2.3. Electrochemical Measurements. The ionic conductivity of the separators was measured using an electrochemical working station (Autolab PGSTAT128N) by electrochemical impedance spectroscopy (EIS) at room temperature. The electrolyte soaked separator was sandwiched between two stainless steel (SS) electrodes of 2.05 cm2 in area for ionic conductivity measurement. The measurement was carried out over the frequency range of 1 HZ to 100 kHz. The ionic conductivity (σ) was calculated using the following equation:
PE separators improve the electrochemical performance and safety of the battery. In the present work, we reported a novel ceramic-grafted separator (CGS) prepared by electron beam irradiation method through using the nonvolatile vinyltris(2-methoxyethoxy)silane (VTMEOS) monomer to bond the stable inorganic layer of Al2O3 nanoparticles on the surface of the PE separator. By immersion of the VTMEOS grafted PE separator and subsequent hydrolysis reaction into the Al3+ solution, an ultrathin Al2O3 layer is grafted to the porous polymer microframework to form the Al2O3 ceramic-grafted separator (Al2O3−CGS). This method using nonvolatile VTMEOS and stable Al3+ solution is relatively low cost, recyclable, and easily controllable, which can facilitate large-scale applications. The Al2O3−CGSs show not only similar thickness and pore structure to the bare separator but also excellent thermal stability and wettability to carbonate electrolyte, thus enhancing safety and battery performance.
σ = d /(RA)
where d is the thickness of the separator, A is the area of the stainless steel electrode, and R is the bulk resistance, the high-frequency intercept of Nyquist curve on the x axis. The 2016 coin-cells were assembled by sandwiching the separator between the LiFePO4 (LFP) cathode (LiFePO4/acetylene black/ PVDF = 8/1/1, w/w/w) and the graphite (C) anode (graphite/ acetylene black/carboxymethyl cellulose/styrene−butadiene rubber = 8/1/0.5/0.5, w/w/w/w) and filling with liquid electrolyte (1 M LiPF6 in EC/DEC/EMC, 1/1/1, v/v/v). The Li/LFP half cells with 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in 1,3-dioxolane (DOL), dimethoxyethane (DME), and fluoroethylene carbonate (FEC) (5:5:1 in volume) or pure propylene carbonate (PC) as the electrolyte are also assembled. The cells were cycled between 2.0 and 4.0 V at room temperature on a LAND cycler (Wuhan, China) at various C-rates. For the open circuit voltage (OCV) drop measurement, the unit cell was charged to 4.0 V and placed in a drying oven at 170 °C, and its voltage drop was monitored as a function of elapsed time.
2. EXPERIMENTAL METHODS 2.1. Preparation of Al2O3 Ceramic-Grafted Separators (Al2O3−CGSs). The PE separators (16 μm, SK Energy) were immersed in the VTMEOS solution prior to radiation using an accelerator (Wasik Associates, U.S.A.) with energy of 1 MeV to doses of 60 kGy (1 Gy = 1 J/kg) at a does rate of 20 kGy/pass. Then the VTMEOS-grafted PE separators were immersed in the solution containing 0.15 mol L−1 KAl(SO4)2·12H2O and 0.36 mol L−1 HCl at 70 °C for 10 h to obtain the Al2O3 ceramic-grafted separator (Al2O3− CGS). 2.2. Materials Characterization. The surface chemical composition of the separator was analyzed by Fourier transform infrared spectra (FT-IR, Nicolet 6700) at ambient conditions and X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi) with an Al Kα X-ray source. The electrolyte wettability of the separators was determined by the sessile drop method with carbonate electrolyte (EC:DEC:EMC, 1:1:1, vol %, 2 μL) as a probe liquid on a Dataphysics OCA20 CA system. The surface and cross-sectional morphologies of the separators were characterized with a fieldemission scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP). The thermal shrinkage of the separator was evaluated by measuring its dimensional change after heating at 150 °C for 0.5 h and calculated using the following equation:
shrinkage(%) = (Wi − Wf )/Wi × 100%
(2)
3. RESULTS AND DISCUSSION Figure 1 displays the preparation process of the Al2O3 ceramicgrafted separator (Al2O3−CGS). First, PE separators were treated with high energy electron beams to produce free radicals to trigger the grafting reaction with VTMEOS.32 Then the VTMEOS-grafted PE separators were immersed in an acid KAl(SO4)2·12H2O solution. KAl(SO4)2·12H2O gives almost the complete recovery as hydrolysis product, Al3+, Al(OH)2+, Al(OH)2+, and Al(OH)4−. These ions are in equilibrium with amorphous Al(OH)3(am), which could react with VTMEOS on the PE separator to form the Al2O3 grafted PE separator.33,34 The mass of the PE separator increased by 25% after grafting Al2O3. The surface and cross-sectional FE-SEM images of the PE and Al2O3−CGS are shown in Figure 2. The surface morphology and overall thickness of the Al2O3−CGS remain almost unchanged, suggesting that the Al2O3 layer should be thin enough, which cannot be clearly observed by SEM. The separators were analyzed by FT-IR and XPS to confirm the existence of Al2O3. As shown in Figure 3a, for the Al2O3− CGS, two absorption bands at ∼750 and ∼550 cm−1 are related to the Al−O bond of AlO4, and the broad peak at 1060 cm−1 indicates the presence of Al−O−Si or Al−O−Al.35 This
(1)
where Wi and Wf are the areas of the separator before and after heat treatment, respectively. Thermal analysis of the separators was carried out on a DSC Q200 system of TA Instrument at a heating rate of 10 °C min−1 from 60 to 180 °C. The tensile strength was carried out at room temperature at a speed of 5 mm min−1 on a universal testing machine (CMT 6503, Shenzhen SANS Test Machine, Shenzhen, China). B
DOI: 10.1021/acsami.7b05535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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which is expected to play a key role in improving the ionic conductivity of the separators.36−38 Figure 4 shows photographic images of the PE and Al2O3− CGS before and after heating at 150 °C for 0.5 h. The Al2O3−
Figure 2. SEM images of the surface and cross-section of the (a,c) PE and (b,d) Al2O3−CGS.
Figure 4. Photographs of the (a,c) PE and (b,d) Al2O3−CGS before/ after storage at 150 °C for 0.5 h, respectively.
Figure 3. (a) FT-IR spectra, (b) XPS spectra, and (d) electrolyte (EC/ DEC/EMC) contact angle images of the PE and Al2O3−CGS. (c) SEM-EDS linear scan profile obtained along the yellow line on the Al2O3−CGS shown in the inset.
CGS showed almost no shrinkage at 150 °C while the PE separator exhibited a shrinkage of 84%. This result indicates that the grafted Al2O3 layer acts as a thermostable framework to resist the dimensional variation of the separator at elevated temperature. The DSC thermograms of the bare PE and Al2O3−CGS are shown in Figure 5a. The melting temperature of the bare PE is around 135 °C, lower than that (138 °C) of the Al2O3−CGS. The experimental result should be ascribed to the introduction of the heat-resistance Al2O3 layer, which causes an increase in
observation is consistent with the XPS spectrum of the Al2O3− CGS, where the representative bonding configurations of Al (Al 2s and 2p) and O (O 1s) were clearly observed (Figure 3b). To further verify the distribution of Al2O3 inside the separator, an energy-dispersive X-ray spectroscopy (EDS) linear scan was performed across the Al2O3−CGS. As shown in Figure 3c, Al, Si, and O elements are uniformly distributed along the crosssection of the separator. The results confirm that the ultrathin alumina layer had been grafted successfully throughout the entire separator. Contact angle measurements with the carbonate electrolyte (EC/DEC/EMC) were conducted to investigate the effect of alumina layer grafted on the surface on the electrolyte wettability. As shown in Figure 3d, the electrolyte contact angles were 46° and 21° for the PE and Al2O3−CGS, respectively, indicating better wettability for the Al2O3−CGS with the electrolyte. The improvement on the electrolyte wettability can be attributed to good electrolyte affinity of the grafted Al2O3 layer to avoid the formation of the microbubble to hinder the pore structure of the PE framework,
Figure 5. (a) DSC curves, (b) tensile curves, and (c) Nyquist plots of the PE and Al2O3−CGS. C
DOI: 10.1021/acsami.7b05535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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FEC electrolytes are also examined. As shown in Figure 7, both cells in various electrolyte show similar discharge capacity at 0.5
the melting temperatures to some extent. Figure 5b shows the tensile strength and elongation rate of the bare PE and Al2O3− CGS. The Al2O3−CGS shows higher tensile strength (131 MPa) but lower break-elongation (25%) than the bare PE separator (114 MPa, 43%). The reasons for that are due to the decomposition of the PE framework by electron beam radiation to some extent. The ionic conductivity of separators is tested by electrochemical impedance spectrum. Figure 5c shows the Nyquist plots of SS/separator/SS cells. According to eq 2, the Al2O3−CGS displays higher ionic conductivity (0.53 mS cm−1) than that (0.40 mS cm−1) of the bare PE separator, which can be ascribed to the well-preserved porous structure and better electrolyte wettability. The electrochemical performance of separators is characterized by cycling LiFePO4 (LFP, cathode)/graphite (C, anode) full cells between 2.0 and 4.0 V at room temperature. Figure 6a compares the discharge capacity of the cells with the
Figure 7. Rate capabilities of Li/LFP half cells assembled with bare PE and Al2O3−CGS in (a) DOL/DME/FEC and (b) PC electrolyte.
and 1 C rate. In the DOL/DME/FEC electrolyte, the cell with Al2O3−CGS displays much higher discharge capacity than that with the bare PE separator at 10 C rate. In the PC electrolyte, the cell employing Al2O3−CGS displays a slightly better rate capability at above 2 C. Therefore, the improvement of the discharge rate capability by ceramic-grafted separator is universal in various electrolytes due to the improved wettability of the Al2O3−CGS. In order ensure the thermal stability of the Al2O3−CGS in real cell system, the open circuit voltage (OCV) of the fully charged LFP/C full cells with different separators was monitored continuously at 170 °C. As shown in Figure 8a, the OCV of the cell with the bare PE separator drops sharply only after 70 s, which is due to the thermal shrinkage of the separator and causes internal short-circuits in the cell. However, the cell with the Al2O3−CGS shows only a slow voltage drop during the heating progress, caused by the increase of the selfdischarge at the high temperature. When the cells are cooled to room temperature, the cell with the Al2O3−CGS can gradually recover back the initial voltage while the voltage of the cell with the bare PE separator drops to 0 V. Figure 8b shows the photographic image of the separators after the OCV test. It is obvious to be found that the bare PE separator displays serious shrinkage and melts to bind with the electrode together, whereas the Al2O3−CGS still maintains its original dimensions. Overall, the excellent thermal stability of the Al2O3−CGS could contribute to the safety of lithium-ion batteries.
Figure 6. (a) Cycle performance and (b) rate capabilities of LFP/C full cells assembled with bare PE and Al2O3−CGS.
bare PE and Al2O3−CGS. The cell with the Al2O3−CGS exhibits higher discharge capacity and capacity retention (103.0 mAh g−1 after 100 cycles, 85.6% capacity retention) at 1 C (1 C = 170 mA g−1), compared with the cell with the bare PE separator (91.2 mAh g−1, 74.7% capacity retention). The improvement of cycling performance of the LFP/C full cell may result from the better affinity of the Al2O3−CGS with electrolyte. Figure 6b shows the discharge C-rate capabilities of both separators. The discharge capacities for the LFP/C full cells with both separators decrease as the discharge current density increases from 0.2 to 5 C. However, the cell with the Al2O3−CGS exhibits higher capacity retention at relative high current rate compared with the cell with the bare PE separator. The superior rate performance of the cell with the Al2O3−CGS can be attributed to the higher ion conductivity of separator, which lowers the ohmic polarization significantly at high current density. To further verify the improvement of discharge rate capability using Al2O3−CGS, the rate performance of Li/ LiFePO4 half cells with PC liquid electrolyte and DOL/DME/
4. CONCLUSION In this work, we have successfully prepared the ultrathin Al2O3 layer grafted PE separator by simply electron beam radiation and a hydrolysis process. The Al2O3 ceramic-grafted separator (Al2O3−CGS) displays a similar porous structure and thickness compared with the bare PE separator but significantly enhanced D
DOI: 10.1021/acsami.7b05535 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 8. (a) OCV test of LFP/C full cells assembled with the bare PE and Al2O3−CGS. (b) Photographic image of these separators after the OCV test.
electrolyte wettability, ionic conductivity, and thermal stability. This is an important advantage of the ceramic-grafted method compared with conventional inorganic-polymer binder/polymer separators. In addition, the hydrophilic Al2O3 is evenly distributed across the separator, so as to increase the internal hydrophilicity of separator, which is really different from the previously reported surface magnetron sputtering deposition approach and ceramic-coating method. The LFP/C full cells with the Al2O3−CGS show better electrochemical performance and excellent thermal stability. The simple and effective electron beam radiation approach has promising applications in the developments of highly safe and high-performance lithium-ion batteries. Moreover, it is anticipated that this method could be extended to prepare the multifunctional separator for next-generation LIBs that are in strong pursuit of selective sieving for soluble ions in the electrolyte.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xinping Ai: 0000-0002-8280-0866 Yuliang Cao: 0000-0001-6092-5652 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank financial support by National Key R&D Program of China (No. 2016YFB0100400). REFERENCES
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