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Effects of an Integrated Separator/Electrode Assembly on Enhanced Thermal Stability and Rate Capability of Lithium-Ion Batteries Seokhyeon Gong,†,§ Hyunkyu Jeon,†,§ Hoogil Lee,† Myung-Hyun Ryou,*,† and Yong Min Lee*,†,‡ †
Department of Chemical and Biological Engineering, Hanbat National University, 125 Dongseo-daero, Yuseong-gu, Daejeon 34158, Republic of Korea ‡ Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea S Supporting Information *
ABSTRACT: To improve the rate capability and safety of lithium-ion batteries (LIBs), we developed an integrated separator/electrode by gluing polyethylene (PE) separators and electrodes using a polymeric adhesive (poly(vinylidene fluoride), PVdF). To fabricate thin and uniform polymer coating layers on the substrate, we applied the polymer solution using a spray-coating technique. PVdF was chosen because of its superior mechanical properties and stable electrochemical properties within the voltage range of commercial LIBs. The integrated separator/electrode showed superior thermal stability compared to that of the control PE separators. Although PVdF coating layers partially blocked the porous structures of the PE separators, resulting in reduced ionic conductivity (control PE = 0.666 mS cm−1, PVdF-coated PE = 0.617 mS cm−1), improved interfacial properties between the separators and the electrodes were obtained due to the intimate contact, and the rate capabilities of the LIBs based on integrated separators/electrodes showed 176.6% improvement at the 7 C rate (LIBs based on PVdF-coated and control PE maintained 48.4 and 27.4% of the initial discharge capacity, respectively). KEYWORDS: separator-integrated electrode, electrode-integrated separator, integration, thermal shrinkage, lithium-ion batteries
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INTRODUCTION Since their introduction in the early 1990s, lithium-ion batteries (LIBs) have played an important role in realizing today’s information-rich and mobile society by powering consumer electronics, such as cell phones and laptops.1 Recently, fossilfuel depletion and environmental pollution have accelerated the development of next-generation energy-storage systems for various renewable energy sources (wind and solar energy). These systems are important in products such as electric vehicles, which require large-scale battery systems. However, LIBs are still regarded as the battery of choice for powering and developing large-scale battery applications because of their high energy density, low cost, safety, and long-lasting cycle life compared to those of other existing secondary battery systems.2−5 Safety is a critical factor that determines the success of the implementation of large-scale battery applications because malfunctioning large-scale batteries can provoke thermal runaway failure accompanied by fire and explosion.5,6 This can not only harm consumers’ health but also severely reduce the market values of the battery developers. Separators are microporous membranes made of polyolefin polymers, such as polyethylene (PE) and polypropylene.5,7 They prohibit the direct contact between the cathodes and the anodes, causing an internal short circuit, and serve as pathways © 2017 American Chemical Society
for the migration of the lithium ions within the pore structures. To fabricate a porous structure in the separators, polymer films undergo uniaxial or biaxial stretching processes. Consequently, when exposed to high temperatures, the separators tend to shrink along the opposite stretched direction to release the internal stresses within the polymer film. This is known as thermal shrinkage,5,8 which causes a direct contact between the cathodes and the anodes, resulting in an internal short circuit. To suppress the thermal shrinkage of the separators, ceramic and/or polymer-based composite coating layers are generally applied on one or both sides of the microporous membranes.9−15 In terms of LIB safety, thicker coating layers are advantageous. On the other hand, a high energy density is another key factor to consider for the development of largescale LIBs.1,4,16 From an energy density point of view, the thickness of the separator coating layer should be minimized while maintaining sufficient mechanical strength to prevent thermal shrinkage. In this study, to achieve both goals of improving the (i) safety and (ii) energy density of LIBs, we fabricated an integrated separator/electrode assembly using the spray-coating Received: January 2, 2017 Accepted: May 5, 2017 Published: May 5, 2017 17814
DOI: 10.1021/acsami.7b00044 ACS Appl. Mater. Interfaces 2017, 9, 17814−17821
Research Article
ACS Applied Materials & Interfaces
Additionally, the ionic conductivity (σ) values of the separators were evaluated according to the equation σ = l/RS, where l is the thickness of the separator, S is the contact area between the separator (radius = 1.6 cm) and the stainless steel blocking electrodes, and R is the bulk resistance measured using an impedance analyzer (VSP, BioLogics).14 The air permeability of the separators, represented by the Gurley number, was determined using a densometer (4110N, Thwing-Albert). To measure the Gurley number, the time required for a specific volume of air (100 mL) at a given pressure (6.52 psi) to pass through a specific area of the separator was measured.19 The adhesion properties of the separator/electrode assemblies were measured using a 180° peel test (AGS-X 500N, Shimadzu, Japan; sample size = 35 mm × 150 mm, displacement rate = 200 mm min−1). Cell Assembly. To evaluate the effect of the integrated separator/ electrode on LIBs, 2032-coin-type full cells consisting of LCO/ graphite were fabricated. The amount of liquid electrolyte (a mixture of 1.15 M LiPF6 in ethylene carbonate/ethyl methyl carbonate (EC/ EMC = 3:7 by vol., Panax Etec, South Korea)) was controlled by soaking the electrodes, separators, and integrated separator/electrodes in an argon-filled glovebox. Electrochemical Measurements. The electrochemical stability of the PVdF binder was evaluated using a linear sweep voltammetry (LSV) method. To evaluate the oxidation decomposition behavior (Li metal/separator/PVdF-coated stainless steel), the potential was swept from 0 to 6 V versus Li/Li+ at a scan rate of 1 mV s−1 and from the open-circuit voltage (OCV) to 0 V versus Li/Li+ at a scan rate of 0.05 mV s−1, to evaluate the reduction decomposition behavior (Li metal/ separator/PVdF-coated graphite). After cell assembly, the test cells were aged for 12 h, followed by cycling in a voltage range of 3.0−4.2 V in the constant current (CC) mode at C/10 (0.238 mA cm−2) for both charging and discharging processes, using a battery tester (PNE Solution, South Korea) at 25 °C. After the first cycling, the unit cells were cycled for three subsequent cycles at C/5 (0.476 mA cm−2) in the same voltage range in the CC/constant voltage (CV) mode and a CC mode for both charging and discharging processes at 25 °C. These series of cyclings were denoted precycling in our study. After precycling, the cycle performance and rate capability of the test cells were evaluated. To evaluate the cycle ability, the test cells were cycled in the voltage range of 3.0−4.2 V in the CC/CV mode for charging and the CC mode for discharging at C/2 (1.189 mA cm−2), at 25 and 60 °C, respectively. For testing the rate capability, the test cells were discharged by varying the discharging current density from C/2 to 20 C (C/2, 1 C, 3 C, 5 C, 7 C, 10 C, 20 C, and C/2) while keeping the charging current at C/2 (1.189 mA cm−2) at 25 °C. The alternating current (AC) impedance of the cell was measured by an impedance analyzer (VSP, BioLogics) across the frequency range of 1 MHz to 0.01 Hz. The impedance spectra were fitted using the z-fit program (BioLogics SAS, France). Battery Safety Measurements. To investigate the safety of LIBs under elevated temperature conditions, fully charged 2032-coin-type cells (4.2 V at CC/CV mode at C/2) after precycling were prepared and placed in an oven at 150 °C. The OCV changes of the test cells were monitored as a function of time.
technique. Despite the various advantages of the spray-coating technique, such as ease of operation, low cost, and scale flexibility (from small scale to industrial scale), there have been only few efforts to apply this technique to LIBs.17,18 We showed that the spray-coating technique can minimize the separator coating layer up to
