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Cellulosic Biomass-Reinforced Polyvinylidene Fluoride Separators with Enhanced Dielectric Properties and Thermal Tolerance Lei Li,†,‡ Miao Yu,†,‡ Chao Jia,†,‡ Jianxin Liu,†,‡ Yanyan Lv,†,‡ Yanhua Liu,†,‡ Yi Zhou,†,‡ Chuanting Liu,†,‡ and Ziqiang Shao*,†,‡ †
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China Beijing Engineering Research Centre of Cellulose and Its Derivatives, Beijing 100081, China
‡
S Supporting Information *
ABSTRACT: Safety issues are critical barriers to large-scale energy storage applications of lithium-ion batteries (LIBs). Using an ameliorated, thermally stable, shutdown separator is an effective method to overcome the safety issues. Herein, we demonstrate a novel, cellulosic biomass-material-blended polyvinylidene fluoride separator that was prepared using a simple nonsolvent-induced phase separation technique. This process formed a microporous composite separator with reduced crystallinity, uniform pore size distribution, superior thermal tolerance, and enhanced electrolyte wettability and dielectric and mechanical properties. In addition, the separator has a superior capacity retention and a better rate capability compared to the commercialized microporous polypropylene membrane. This fascinating membrane was fabricated via a relatively eco-friendly and cost-effective method and is an alternative, promising separator for high-power LIBs. KEYWORDS: cellulosic biomass material, dielectric properties, thermal tolerance, separator, lithium-ion battery at room temperature.15 As noted by some references, polymer separators with high dielectric constants may prevent the electrolyte exudation upon storage, increase the charge carrier concentration, and enhance the ionic conductivity compared to separators with lower dielectric constants.16 However, the relatively high dielectric losses of PVDF may lead to dielectric heating and thermal breakdown,17−19 and its high crystallinity degree might restrict the conduction of lithium ions because the conduction mainly occurs in the amorphous region.20 Previous work has explored new approaches to conquer these drawbacks, including the introduction of low crystallinity or amorphous polymers,21 an electrode/separator assembly,16 surface coatings on the separators,22 composite membranes with organic or inorganic materials,2 and polymer structure modifications.23 However, some of these methods have negative effects, such as impurities from cross-linking or grafting, non-cost-effective preparation processes, and decreased mechanical properties.13 Alternative biomass materials have recently attracted substantial attention as an innovative solution for unparalleled advances in multiple energy storage devices.3,24−26 Among the various eco-friendly, natural, resource-based materials reported to date, cellulose and its derivatives have been highlighted as
1. INTRODUCTION Lithium-ion batteries (LIBs) possess advantages, such as a high energy density,1 a long cycle life,2 no memory effect, a low selfdischarge rate, and a low environmental impact, and they have been applied to areas in portable electronics.3 LIBs are believed to be promising for larger energy density power system, including electric/hybrid electric electronic products and renewable energy plants in the future.4 Nevertheless, a major concern for large-scale, commercialized applications of LIBs is a potential security issue because of the use of flammable carbonate electrolytes and the existence of a series of potential thermal runaway reactions.5−7 Thus, developing advanced LIB devices is crucial and urgent.8−11 As an integral part of LIBs, the separators in LIBs must have excellent integrated features, such as long-term mechanical and dimensional stability, uniform thickness, shutdown functionality at high temperatures (120− 180 °C), resistance to electrolyte impurities, and sufficient electrolyte wettability.12 The material options for separators mainly include polymers and polymer composites, such as poly(propylene) (PP), poly(ethylene) (PE), poly(vinylidene fluoride) (PVDF), poly(acrylonitrile) (PAN), and poly(ethylene oxide) (PEO).13 Among the polymers, the PVDFbased separators have been extensively applied in LIB devices because of their excellent aging tolerance, superior chemical resistance, high thermal stability, and membrane-forming properties.14 More importantly, the pristine PVDF possesses a relatively high dielectric constant of 5 at a frequency of 1 kHz © XXXX American Chemical Society
Received: April 8, 2017 Accepted: May 31, 2017 Published: May 31, 2017 A
DOI: 10.1021/acsami.7b04948 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
2.2. Preparation of the Composite Membranes. An NIPS wetprocess method (Figure 1a) was used to fabricate the composite
potential building blocks because cellulose is naturally inexhaustible; has low cost and lightweight; is physicochemically steady, biocompatible, and recyclable; and has a low thermal expansion rate.27−29 Extensive research has been devoted to the application of cellulosic materials to a wide variety of rechargeable power sources, such as supercapacitors and LIBs,30−35 with a concentration on separator membranes,36 flexible electrode substrates,33 electrode binders,37 electrolyte adhesives,38 mechanical buffers for metallic anodes,29 and porous collectors. High-degree-of-substitution (DS) cyanoethyl cellulose (CEC), a type of cellulose synthesized from the alkalified cellulose and acrylonitrile via Michael addition, has a high dielectric constant and a low dielectric loss tangent and is considered to have the potential to compensate for the defects of PVDF-based separators.39 Meanwhile, its excellent mechanical and electrochemical properties, resistance to microbial attack and acid erosion, excellent thermal stability, and low moisture regain can also contribute to the safety and commercialization of separators. Moreover, another cellulosic material, cellulose nanofibers (CNFs), was deemed to be an effective additive for enhancing the mechanical properties and reducing the overall crystallinity of a material because of its high elastic modulus, hydrophilicity, large aspect ratio, low density, and small thermal expansion coefficient.40 Advanced separator membranes with improved dielectric properties and thermal resistance are being considered to manufacture next-generation, high-safety, and high-performance cells because of their excellent insulating properties and high-energy storage density. In particular, the theoretical understanding and interpretation of the comparative phenomenon for the design and synthesis of high-dielectric functional separators along with concurrent endeavors to promote ion transport kinetics and avoid interior short-circuit failures have not been reported. Herein, inspired by the facile functionalization of CEC and CNF, we propose a green material strategy for the development of novel ternary composited separators. A nonsolvent-induced phase separation (NIPS) method was adopted because the process can be easily controlled to avoid perforation, obtain a small and evenly distributed micropore size, and achieve a high tensile strength, biaxial and puncture strength, compared to other methods such as the dry-spinning/ hot-drawing process.41 This work sheds light on an ideal polyolefin substitute as an environmental energy storage material because of its specific functionality, easy processability, and reliability.
Figure 1. Schematic illustration of the experimental process (a) and structures of the PVDF separator before and after blending (b). The molecular chains of CEC were interspersed between the flexible molecular chains of PVDF, resulting in a decrease in the crystallinity, and CNF was uniformly filled in the composite system, which played a role in strengthening and toughening. membranes.41 The different precursor mixed solutions of PVDF/ CEC/CNF were designed with suitable compositions (Table S1). The total solid content of the different precursor solutions was 15 wt %, and CNFs were added in the form of a homogeneous suspension of DMF. First, PVDF, CEC, and CNF were mixed and stirred in a suspension of DMF for 180 min in a 40 °C bath at 500 rpm, and the homogeneous mixtures were cooled to room temperature and centrifuged at 10 000 rpm to completely remove the internal bubbles. Then, the suspensions were evenly cast on the surface of a glass plate at room temperature using a doctor blade with a 2.5 cm·s−1 velocity and 150 mm thickness. The plates were immediately immersed in a coagulation bath of deionized water at room temperature for 24 h. Finally, the resultant separators with a thickness of approximately 50 μm were dried at 60 °C for 24 h to remove any residual solvent. All prefabricated membranes were stored in a glovebox filled with argon gas (Lab 2000, O2 and H2O < 0.1 ppm) before use. Meanwhile, a commercialized PP separator was examined for comparison. In the following, the commercialized PP separator, the pristine PVDF membrane, and the PVDF separators composited with CNF-1%, CNF-1% and CEC-2%, CNF-1% and CEC-4%, CNF-1% and CEC-6% are referred to as PP, PVDF, PVDF/CNF, PVDF/CNF/2% CEC, PVDF/CNF/4% CEC, and PVDF/CNF/6% CEC, respectively. 2.3. Characterizations. The morphology of the different membranes was measured using a scanning electron microscope (SEM, Hitachi S-4800) together with the energy-dispersive spectrometry mapping. The contact angles of a 2 μL water droplet on the samples were determined using a dynamic contact angle analyzer (KRÜ SS DSA100). The pore distribution was evaluated through Brunauer−Emmett−Teller (BET, Quantachrome NOVA-4000e) nitrogen sorption−desorption measurements. The crystalline structure of the different membranes was verified using a wide-angle X-ray diffractometer (XRD, D8 ADVANCE). The thermal shrinkage behavior of the separators (diameter of 1.8 cm) was determined by storing them in an oven at every 20 °C from 120 to 240 °C for 1.0 h.36 The porosity of the membranes was analyzed by immersing them in nbutanol for 12 h.43 The thickness of the membranes was measured using a thickness gauge (CHY.C2). The mechanical properties of the separators were characterized using a universal testing instrument (Instron 3369) at room temperature. The electrolyte uptake measurements for the separators were recorded by soaking the separators in an electrolyte solution of 1 M LiPF6/EC/DEC/EMC
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. PVDF (Mw = 3.6 × 105, HSV 900) was purchased from Aladdin with 97% hydrolysis and a molecular weight of 72 000 g mol−1. Acrylonitrile and N,N′-dimethylformamide (DMF) were provided by Sinopharm Chemical Reagent. CEC was prepared in our laboratory using cotton (M30) and acrylonitrile.15,39 The DS was 2.46 [see the Fourier transform infrared (FTIR) spectrum in Figure S1 and the calculation method in eq S1]. CNFs were prepared via oxalic acid hydrolysis and high-pressure homogenization of the bleached eucalyptus pulp (see the transmission electron microscopy (TEM) images and the FTIR spectrum in Figures S1 and S2).42 Lithium (Li), lithium cobalt oxide (LiCoO2), and steel slices (SS) were purchased from Beijing Nonferrous Metals Research Institute (battery grade). The electrolyte used in the half cell consisted of LiPF6 in a solvent mixture of ethylene carbonate (EC, AR), diethyl carbonate (DEC, AR), and ethyl methyl carbonate (EMC, AR) (LiPF6/EC/DEC/EMC, 1:1:1:1, v/v/v/v) obtained from Dongguan Shanshan Battery Materials Co. Ltd. B
DOI: 10.1021/acsami.7b04948 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. SEM images of pristine PVDF (a), PVDF/CNF (b), PVDF/CNF/2% CEC (c), PVDF/CNF/4% CEC (d), and PVDF/CNF/6% CEC and (e) membranes and their pore size distribution (f).
Figure 3. Characterization of different membranes. Dependence of the dielectric permittivity (a) and dielectric loss (b), XRD patterns (c), temperature dependence of the ionic conductivity of the separator/electrolyte system (d), stretching curves (e), and thermal shrinkage properties (f). (1:1:1:1, v/v/v/v) in a glovebox.44 The bulk impedance and the ionic conductivity of the prepared separator were recorded using electrochemical impedance spectroscopy (EIS) (CHI660E electrochemical workstation) at different temperatures from 0 to 80 °C. Lithium-ion transference numbers were determined via ac impedance measurements in combination with a dc polarization step.45 The dielectric properties of the blend separators were investigated using an impedance analyzer (Agilent 4294A).39 The interfacial stabilities of the electrolyte-soaked membranes and the lithium metal electrode were characterized using the interfacial impedance.46 The chemical compositions of the membrane surface were examined using an X-ray photoelectron spectroscopy (XPS) analyzer (Amicus Budget) and an FTIR (Spectrum Two) spectrometer. The melting points of the membranes were determined by differential scanning calorimetry (DSC) (NETZSCH STA 449F3). The open-circuit voltages (OCVs) at different temperatures were measured using an electrochemical workstation with a thermometer (YC-727UD) by attaching a thermocouple to the battery surface.47 Nail penetration tests were conducted by penetrating a 2.5 mm steel nail through a cell with a thickness of 4.2 mm, a width of 61 mm, and a length of 88 mm (model-426188), and the temperature and voltage of the cells were recorded using data recorders (HIOKI 8430-21)48 (all details are shown in the Supporting Information). 2.4. Battery Assembly and Electrochemical Measurements. The button-type half cells (LIR2025) were assembled by sandwiching
the blend membranes between the LiCoO2 anodes and the Li metal cathodes after immersing them in a liquid electrolyte. All steps were executed in a glovebox filled with argon gas. The Shenzhen Neware battery testing equipment (CT-3008W) was used to evaluate the cycling stability and discharge rate performance of the cells. The cells were cycled at a constant charge/discharge current density of 0.2 C/ 0.2 C to determine the cycling performance. The discharge rates were varied from 0.2 to 8.0 C at a charge current of 0.2 C and at a voltage range of 2.8−4.2 V to measure the discharge rate performance.49
3. RESULTS AND DISCUSSION The morphology and pore size distribution of the pristine PVDF, PVDF/CNF, and PVDF/CEC/CNF separators were first investigated via SEM and BET (Figure 2a, see the crosssectional images in Figure S3). The SEM observation confirmed that the surfaces of the separators were highly homogeneous without obvious defects, and the hollow, 3D structure with linked micropores (see the model of the cross section in Figure 1b) was eventually formed through the NIPS process. Furthermore, all membranes possessed an approximate thickness of 50 μm and an average pore size of
