Roll-to-Roll Functionalization of Polyolefin ... - ACS Publications

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Article Cite This: ACS Appl. Energy Mater. 2018, 1, 3292−3300

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Roll-to-Roll Functionalization of Polyolefin Separators for HighPerformance Lithium-Ion Batteries Ethan Rao,†,§ Brian McVerry,†,§ Arie Borenstein,† Mackenzie Anderson,† Robert S. Jordan,† and Richard B. Kaner*,†,‡ Department of Chemistry and Biochemistry and ‡Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States § Hydrophilix, Inc., Los Angeles, California 90025, United States Downloaded via UNIV OF SOUTH DAKOTA on July 25, 2018 at 05:24:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Modified polyolefin separators fabricated via a roll-toroll system exhibit markedly improved compatibility with lithium ion battery electrolytes. Zwitterionic molecules containing a perfluorophenyl azide functional group were synthesized and covalently bound to the surface of commercial polyolefin separators via UVactivated photochemistry. A roll-to-roll prototype system was constructed allowing for the functionalization of large areas of separator under ambient conditions at low cost. Lithium-ion battery cells containing the modified separators exhibit superior electrochemical performance using a common commercial electrolyte. The modified separators, both monolayer PE and trilayer PP/PE/PP, are wetted instantly upon contact with liquid electrolytes lacking linear carbonates. These electrolytes have been designed for use in batteries with advanced thermal stability properties and/or higher voltage windows, which have previously been hindered by incompatibility with commercial trilayer polyolefin separators. This scalable modification technique is able to meet the rapidly growing demand for low-cost, high-performance separators for safer lithium-ion batteries. KEYWORDS: lithium-ion battery, polyolefin separator, perfluorophenyl azide photochemistry, roll-to-roll process, surface modification

1. INTRODUCTION Lithium-ion batteries (LiBs) are the leading energy storage technology for portable electronic devices, electric vehicles, and smart-grid applications. LiBs offer several advantages over alternative battery materials including high energy density, low self-discharge, and zero memory effects. During discharge, lithium ions intercalated within a graphitic anode migrate into a metal oxide cathode material through a liquid electrolyte. A porous separator is used to prevent electrical contact between the electrodes, while enabling passage of the charged ions through the electrolyte medium. The separators are thin, porous polymer sheets that are mechanically and chemically robust, enabling the separators to perform with nominal change over the lifetime of the battery.1,2 Due to their robust mechanical properties, chemical stability in highly oxidative conditions, and low cost, polyolefin separators composed of polyethylene (PE) and polypropylene (PP) have emerged as the separators of choice in commercial LiBs. Additionally, polyolefin separators possess an inherent safety feature that prevents thermal runaway by acting as a fuse inside the cells. As a defective battery heats up from an electrical short, the porous separator melts into a dense film that breaks the circuit, preventing further electrochemical reactions within the battery.3,4 As several accounts of LiB fires © 2018 American Chemical Society

and explosions have come into the public spotlight, safety has become one of the most critical requirements when selecting a separator for commercial LiBs. This has led to an increased use of trilayer separators composed of a PE layer sandwiched between two PP membranes. This trilayer separator combines the chemical and thermal stability of PP with the thermal shutdown property of PE. Despite their superior properties, nonpolar polyolefin separators are incompatible with the polar cyclic carbonate electrolytes, ethylene carbonate (EC) and propylene carbonate (PC) due to insufficient wetting of the separators. Absorption of the electrolytic solution into the separator is essential for ion transport and low internal resistance of the overall battery.5,6 The poor chemical compatibility with these mixtures inhibits complete wetting of the liquid electrolyte into the separator directly affecting the overall power performance, cycling stability, and longevity of the battery.7 Less polar and less viscous linear carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and others are commonly used in combination with cyclic Received: March 28, 2018 Accepted: June 20, 2018 Published: June 20, 2018 3292

DOI: 10.1021/acsaem.8b00502 ACS Appl. Energy Mater. 2018, 1, 3292−3300

Article

ACS Applied Energy Materials

prototype roll-to-roll production system under ambient conditions. The prototype uses environmentally benign solvents, allowing for manufacturing of large areas of modified separator at low-cost. The modified separators could enable the commercialization of a wider range of electrolytes, opening opportunities for advances in battery safety or batteries that operate over larger voltage windows.

carbonates to reduce the overall viscosity of the electrolyte solution and improve compatibility with the polyolefin separators.8 However, the volatility of linear carbonates cause them to possess room temperature flash points, inducing high flammability.9 With growing concerns over LiB safety, thermally stable electrolytes such as lactones, sulfones, and ionic liquids and have gained attention, yet their use is limited by their incompatibility with traditional polyolefin separators.10−13 In particular, γ-butyrolactone (GBL) is an especially promising electrolyte solvent because of its ability to effectively solvate lithium salts, high flash point, and low cost.14−17 Beyond LiB performance or safety, assembly time is also constrained by the time necessary for the electrolyte to fully absorb into the separator.8,18 As the limits of battery performance are continuously pushed in academia and LiB commercial demand continues to rise, work to improve separator compatibility with organic electrolytes has gained both interest and importance. Current research has largely focused on improving the interaction between electrolytes and separators, while maintaining the superior physical and thermal properties of polyolefins by altering the surface properties of the separators. Surface modification of polyolefin separators has been accomplished using techniques such as exposure to plasmas,19,20 electron beams,7,21 γ-rays,22,23 physically grafting with hydrophilic molecules,24−29 or coating separators with nanoparticles.30,31 Comprehensive reviews by Xiang,2 Lee,32 Zhang,33 and Lu34 summarize these and other notable polyolefin separator surface modifications. Although many of these techniques demonstrate enhanced battery performance because of the surface modification, the ability to costeffectively scale these methods has proven difficult.32 In nearly every commercial application, it is necessary to produce several hundred meters of modified separator at low cost. Previous techniques have utilized expensive chemical additives or reaction conditions that stray from roll-to-roll fabrication, rendering them commercially impractical. Furthermore, previous techniques are primarily demonstrated on monolayer membranes composed of either PE or PP and very little data are presented on modifications of trilayer polyolefin separators. Trilayer separators are thicker and thus require longer periods to absorb electrolytes making the need for improved wettability even more significant. Here, we demonstrate a scalable method to modify both monolayer and trilayer separators for use in lithium-ion batteries through the functionalization of the polyolefin backbone with charged small molecules. Unlike commercial separators, our modified separators are rapidly and completely wetted upon contact with polar electrolyte liquids resulting in increased electrolyte uptake values, lowered resistance to Liion migration, and overall superior battery performance. Furthermore, the modified separators could allow for the commercial use of thermally stable, cyclic carbonate-based electrolytes that have been previously hindered by incompatibility with trilayer polyolefin separators. We employ perfluorophenyl azide (PFPA) photochemistry to covalently link PFPA containing molecules to the polyolefin chains comprising the separator, including interior pores, through exposure to low-power UV light. To develop a membrane separator with low internal resistance in addition to chemical compatibility with the electrolyte, a zwitterionic PFPA derivative (PFPA-sulfobetaine) was designed and synthesized. We demonstrate the scalability of our method with a lab-built

2. MATERIALS AND METHODS 2.1. Materials. Pentafluorobenzenesulfonyl chloride, N,N-dimethylethylenediamine, and 1,3-propane sultone were purchased from Sigma-Aldrich and used as received. All lithium hexafluorophosphate/ carbonate electrolyte solutions and lithium tetrafluoroborate were purchased from Sigma-Aldrich, stored in an argon-filled glovebox, and used as received. γ-Butyrolactone was purchased from Sigma-Aldrich, stored over molecular sieves in an argon-filled gloved box, and used as received. Chloroform, ethanol, and acetone were purchased from Fisher Scientific and used as received. LiNi0.5Mn0.3Co0.2O2 (NMC) cathodes and graphite anodes were purchased from MTI Corporation and opened and stored in an argon-filled glovebox. Monolayer PE (Targray) and trilayer PE/PP/PE (Celgard 2320) separators were purchased and used as control separators. 2032 coin-cells were used for EIS and cycling testing. 2.2. Synthesis of PFPA-sulfobetaine. The synthesis of the PFPA-sulfobetaine derivative was completed in three steps (Scheme 1). Detailed synthesis and characterization (1H NMR, 19F NMR, MS) are provided in the Supporting Information.

Scheme 1. Synthesis of PFPA-sulfobetiane

2.3. Prototype Roll-to Roll Modification System. A roll of commercial separator was extruded at a rate of 2.5 cm min−1. The separator was rolled through a solution containing PFPA-sulfobetaine. The separator was then moved through a 254 nm UV light exposure zone. Two UV lamps were placed 1 cm apart facing the separator on each side. The separator was subject to UV exposure for 2 min and then immersed in a bath of 1:1 water:ethanol in order to remove any unbound PFPA-sulfobetaine. The 1:1 water:ethanol bath was continuously purified using an activated carbon filter which removes unreacted PFPA-sulfobetaine molecules and dimerization products from the bath. This process can be repeated with the same roll for increased functionalization. The separator was then rolled and dried in a vacuum oven overnight prior to use. 2.4. Separator Characterization. XPS studies were carried out on a Kratos AXIS Ultra DLD with a monochromatic Al Kα X-ray source operating at 10 mA and 15 kV. Survey spectra and individual high-resolution spectra were collected using pass energies of 160 and 10 eV, respectively. Data processing were performed using CasaXPS 2.3 software, and spectra binding energies were calibrated by assigning the hydrocarbon peak in the C 1s high-resolution spectra to 284.6 eV. Differential scanning calorimetry (DSC) (PerkinElmer differential scanning calorimeter) was used to determine the melting point of the untreated and modified separators. The analysis was performed under nitrogen atmosphere at a heating rate of 2 °C min−1. Contact angles 3293

DOI: 10.1021/acsaem.8b00502 ACS Appl. Energy Mater. 2018, 1, 3292−3300

Article

ACS Applied Energy Materials

Figure 1. SEM image of a PFPA-sulfobetaine modified polyolefin separator (left) and a schematic representation of PFPA-sulfobetaine molecules grafted onto the exterior and interior surfaces of a polyolefin separator (right). were measured using a first 10 Ångstroms contact angle goniometer. Sessile drop contact angles were taken using DI water on samples of separator that had been desiccated for at least 24 h prior to measurement. To prepare samples for post cycling contact angle measurements, we removed separators from coin cells after cycling, washed with DI water and acetone to remove residual electrolyte, and desiccated for at least 24 h prior to analysis. The electrolyte uptake of the separators was measured by immersing a sample of the separator in an electrolyte for 1 h. The weight of the separator was measured before and after immersion. Excess electrolyte on the separator surface was removed with filter paper prior to measuring the post immersion weight. Electrolyte uptake values were calculated according to eq 1. uptake = 100%(Wi − Wo)/Wo

AC impedance analysis was carried out using a Bio-Logic VMP3 potentiostat in the frequency range from 0.1 Hz to 1.0 MHz.

3. RESULTS AND DISCUSSION PFPA photochemistry is a method designed to covalently attach molecules to the surface of polymeric materials while maintaining the material’s intrinsic bulk properties. PFPA molecules contain an azide functional group which, when activated by photoexcitation, produces a highly reactive singlet nitrene that can undergo C−H insertion within the hydrocarbon backbone of the polyolefins, thus creating a covalent bond between the polymer and PFPA molecule.35−37 Previously, we have demonstrated that attachment of PFPAterminated poly(ethylene glycol) to the surface of reverse osmosis membranes can improve fouling resistance.38 Toward a scalable production of our photoactive material, we have devised a facile synthesis of a novel compound, PFPAsulfobetaine. Typically, PFPA derivatives are synthesized starting with methyl pentafluorobenzoate. However, several reactions are required that involve the use of protection/ deprotection chemistry and expensive and rigorous carbodiimide activation39 that, in our opinion, is not a viable method to produce kilograms of material. Therefore, we have developed a novel synthetic route starting with pentafluorobenzenesulfonyl chloride. The synthesis is complete in three steps, involves no protection/deprotection chemistry, and can be accomplished under ambient conditions with common commercially available reagents enabling the synthesis of the desired product at low cost as shown in Scheme 1. Details of the synthesis are presented in the Supporting Information. A schematic representation illustrating the grafted PFPA-sulfobetaine molecules on the polyolefin separator is shown in Figure 1. The nature of the grafting process allows for PFPAsulfobetaine molecules to covalently attach to the inner pores of the separator in addition to the exterior surfaces. In this study, we employ PFPA-sulfobetaine as our chemical modifier for several reasons. Zwitterionic species are known to be much more oxidatively stable than other common hydrophilic side groups, such as ethylene glycol, which undergo hydrolysis.40 Furthermore, mixed charged compounds have greater hydrophilicity than ethers due to strong ionic interactions versus hydrogen-bonding interactions.41,42 The marriage of the hydrophilic sulfobetaine and hydrophobic PFPA moieties produces a surfactant, which, when in aqueous environments, drives the interaction of the PFPA to the

(1)

where Wi represents the weight of the separator post immersion and Wo represents the weight of the separator preimmersion. 2.5. Electrochemical Characterization. Ionic conductivity was measured by sandwiching the separator between two stainless steel electrodes (1.4 cm in diameter). The separator was soaked with the appropriate electrolyte in an argon atmosphere glovebox prior to cell assembly. The ionic conductivity was obtained using the bulk resistance measured by AC impedance analysis using a Bio-Logic VMP3 potentiostat in the frequency range from 100 Hz to 1.0 MHz. The ionic conductivity value was obtained using eq 2.

σ = d /(R sSA)

(2)

where σ represents the ionic conductivity, d is the separator thickness, Rs is the bulk resistance, and SA represents the surface area of the stainless-steel electrodes. 2.6. Preparation of LCO Cathodes. A binder solution was first prepared by adding the polymer binder polyvinylidene fluoride (PVDF, MW 50 000, Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP) solvent preheated at 90 °C. The solution was stirred until the PVDF was fully dissolved forming a clear solution. This binder solution was used in the preparation of the electrodes. Lithium cobalt oxide (LCO) cathode was made by forming a slurry consisting of lithium cobalt oxide (95%), carbon black (2.5%), and PVDF (2.5%) in NMP. This slurry was applied onto 12 μm-thick aluminum foil using a doctor blade technique and the coated electrodes were dried under vacuum at 120 °C. The blade height was adjusted to 140 μm in order to produce a cathode with a loading mass of 10.1 mg/cm2. 2.7. Battery Performance. 2032 coin-cells were tested on an 8Channel Battery Analyzer (MTI Corp). For NMC/graphite cells containing control and modified separators, the cells were cycled from 4.5 to 3.2 V at a specified constant current. For LCO/graphite cells containing the electrolyte 1.0 M LiBF4 in GBL, the cells were cycled from 4.2 to 2.7 V at a specified constant current followed by a period of constant voltage charging to a cutoff current of C/15. Post cycling 3294

DOI: 10.1021/acsaem.8b00502 ACS Appl. Energy Mater. 2018, 1, 3292−3300

Article

ACS Applied Energy Materials

modification. The contact angle remained relatively unchanged after cycling, indicating that the coating remained stable under the electrochemical conditions of the cell (Table S1). It can be reasonably assumed that the PFPA-sulfobetaine is more easily incorporated into PE than PP, as PP is more chemically resistant. The data suggest that the monolayer PE separators have a greater degree of functionalization; however, because contact angle and XPS both solely reflect the outermost surface, this does not account for the functionalization of the inner-PE layer of the trilayer separator. Because of the insulating nature of the separator, and the low atomic number of the atoms specific to PFPA-sulfobetaine, attempts to obtain elemental mapping within the separator interior using energydispersive X-ray spectroscopy (EDX) were inconclusive. To confirm that the thermal shutdown ability remains unchanged due to the modification procedure, differential scanning calorimetry (DSC) was used to determine the melting point of the modified separators (Figure S1). Both the unmodified commercial and modified monolayer PE separators exhibit a melting point close to 135 °C. The unmodified commercial and modified trilayer separators exhibit two melting points at approximately 133 °C (PE) and 158 °C (PP). The minimal change in melting point demonstrates that the PFPA-sulfobetaine surface coating only nominally alters the thermal properties of the separator and the thermal shutdown ability is retained. Given this result and the electrochemical stability of PFPA-sulfobetaine over the relevant voltage windows, we do not expect any undesired side effects due to the introduction of PFPA-sulfobetaine on the surface of the separator. As designed, the modified separators exhibit significantly improved electrolyte uptake with each of the electrolytes tested. The introduction of charged functional groups on the polyolefin surfaces increases the surface energy of the separator allowing for better absorption of the highly polar electrolytes. Thus, the incorporation of PFPA-sulfobetaine into the polyolefin backbone allows for greater absorption of the electrolyte into the separator. Ten μL of six different electrolytes were pipetted onto samples of polyolefin separator with radii of 8 mm as shown in Figure 3. 1.0 M LiPF6 in 1:1 EC:DMC, 1:1 EC:EMC, and 1:1 EC:DEC were selected for their commercial applicability. Electrolytes lacking linear carbonates, 1.0 M LiPF6 in 1:1 EC:PC, 1.0 M LiBF4 in 1:1 EC:GBL, and 1.0 M LiBF4 in GBL were also tested. Samples of PFPA-sulfobetaine modified separators with radii of 8 mm are wetted completely by 10 μL aliquots of electrolyte. The wetting of the trilayer separator with electrolytes without linear carbonates (1:1 EC:PC, 1.0 M LiBF4 in 1:1 EC:GBL, 1.0 M LiBF4 in GBL) is especially notable given the incompatibility with these electrolytes and commercial trilayer polyolefin separators. GBL electrolytes have received attention recently because of their flame-resistant nature (Figure S2), which could potentially be used in LiBs with advanced safety characteristics. Table 2 displays the electrolyte uptake values

surfaces of the hydrophobic polyolefin separators. Our attempts to modify separators with PFPA derivatives in organic solvents have been unsuccessful due to side reactions between the generated nitrenes and solvent molecules43 and solvent competition with the hydrophobic separator surfaces. For this work, samples of modified polyolefin separator were fabricated using a lab-built, roll-to-roll modification system depicted in Figure 2. This system allows for the continuous

Figure 2. Schematic of a roll-to-roll system designed to graft PFPAsulfobetaine molecules onto a roll of polyolefin separator.

modification of commercial separators, enabling large areas of modified separator to be produced under ambient conditions at low cost. The commercial separators are exposed to a solution containing PFPA-sulfobetaine, subject to UV light to drive the surface modification, then dipped in a rinse bath to remove any unreacted material or dimerized PFPA molecules resulting from the reaction. After the roll-to-roll modification, samples of the modified roll were then cut out for characterization, electrolyte uptake experiments, and electrochemical performance testing. 3.1. Characterization. XPS was used to confirm the presence of the PFPA-sulfobetaine molecules covalently bound to the polyolefin surfaces, with the atomic percentage relative to carbon present on each sample’s surface displayed in Table 1. The commercial hydrocarbon separator surfaces contain mostly carbon with nominal fluorine content (99.9 99.9 99.9