Letter pubs.acs.org/journal/ascecg
High Performance and Biodegradable Skeleton Material Based on Soy Protein Isolate for Gel Polymer Electrolyte Ming Zhu, Chunyu Tan, Qun Fang, Liang Gao, Gang Sui,* and Xiaoping Yang State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: A kind of biodegradable and well-sourced soy protein isolate (SPI)/poly(vinyl alcohol) (PVA) composite nanofiber membranes was fabricated via electrospinning and used as skeleton material in gel polymer electrolyte (GPE) of lithium-ion batteries. This study revealed that the proper material proportion, low crystallinity, sufficient porosity and good affinity between nanofiber and the electrode/electrolyte resulted in high saturated electrolyte uptake and conservation rate of the nanofiber membranes. The GPEs based on the SPI/ PVA composite nanofiber membranes presented remarkable electrochemical performance, including high ionic conductivity, excellent compatibility with lithium electrode and good electrochemical stability. An ionic conductivity of 3.8 × 10−3 S cm−1 and interfacial resistance of 250 Ω for the GPEs can be obtained when the weight ratio of SPI to PVA in the spinning solution was 3:1, which was more superior to the existing biobased GPEs. In addition, the Li/GPE/LiCoO2 cells with GPEs based on the SPI/PVA (3:1) composite nanofiber membranes displayed the excellent initial discharge capacity and cycle performance. Therefore, the environmental and economical SPI/PVA composite nanofiber membranes with the optimized material proportion can be used as a green skeleton material in GPEs for high performance lithium-ion batteries. KEYWORDS: Gel polymer electrolyte, Soy protein isolate, Biodegradable, Nanofiber membranes, Skeleton materials
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INTRODUCTION In recent years, the preparation and application of safe and efficient power supply devices are developing constantly through the material innovation and structural design, especially involving some low cost and environmentally friendly materials. Among them, high performance lithium-ion batteries (LIBs) have been widely used in portable electronic devices and electric vehicles due to their many advantages including high energy density, memory-effect-free and long cycle life, etc.1−3 As an important constituent part of LIBs, the performance characteristics of electrolyte directly affect the application potential of the LIBs. In comparison with liquid electrolytes and solid polymer electrolytes, gel polymer electrolytes (GPEs) based on skeleton materials with polyporous structure show the comprehensive performance advantage in the battery capacity, security and ion transport, so that they can be applied in the commercial polymer LIBs. By now, the skeleton material for GPEs has also been prepared from some renewable sources. Xiao et al. reported that an environmentally friendly methyl cellulose casting membrane can be used as host matrix in GPE for LIBs.4 Later, polyvinylidene fluoride was coated on the surface of methyl cellulose membrane to improve the electrochemical performances.5 However, the resulting composite membrane had lost degradable properties. In the similar way, a GPE based on renewable hydroxyethyl cellulose was also prepared by Li.6 The electrochemical properties of GPEs © XXXX American Chemical Society
obtained in these works were not ideal because of the limited uptake ability of the casting membranes for liquid electrolytes. Electrospinning has been recognized as one of the most promising technology for the preparation of porous polymer skeleton materials in GPEs compared to other related technologies. It is easy to prepare porous membranes consisting of polymer nanofibers with the porosity of 30−90% via electrospinning, which can provide a large specific surface area and abundant channel for lithium ion transport, and improve the electrode/electrolyte interface property.7−9 Soy protein isolate (SPI) is one of the most abundant renewable resources from various soy products. And it has been regarded as candidate for applications in adsorption,10 hydrogel11 and antimicrobial material,12 owing to its extensive source, biodegradability and environmental friendliness. Moreover, the molecular structures in SPI with vast numbers of various functional groups can be expected to transfer lithium ions just as the ion transport through protein macromolecules in living systems. In recent research work, the potential advantages of SPI for favoring lithium salt dissociation and adsorbing lithium ion were found by Zhong et al.13 It is a pity that pure SPI nanofiber membrane was hard to obtain by electrospinning Received: June 2, 2016 Revised: August 7, 2016
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DOI: 10.1021/acssuschemeng.6b01218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
Figure 1. SEM images of (a) pure PVA, (b) SPI/PVA (1:1), (c) SPI/PVA (2:1), (d) SPI/PVA (3:1) naofiber membranes. The insets are the diameter distribution and the enlarged image of the nanofibers; TEM images of (e) pure PVA, (f) SPI/PVA (1:1), (g) SPI/PVA (2:1), (h) SPI/PVA (3:1) naofiber membranes.
conductivity of 0.98 × 10−3 S cm−1, which was still self-standing and mechanical flexible after swollen with liquid electrolyte.15 Nevertheless, the bad interfacial resistance and inferior ionic conductivity were shown because the active hydroxyl groups from PVA would react with lithium electrode. Combining SPI with PVA together, the composite material can not only obtain good processing performance in the electrospinning but also improve ionic conductivity and interfacial resistance due to the
because of its intrinsic molecular structure and brittleness. The application of poly(vinyl alcohol) (PVA) in LIBs has been studied over the past few years. Xiao prepared PVA separator that exhibited better electrolyte adsorption/retention capacity, lower thermal shrinkage, and higher ionic conductivity compared to the commercial polypropylene (PP) separator.14 In addition, Ma reported a GPE based on poly(methyl acrylateco-acrylonitrile)/PVA composite membranes with an ionic B
DOI: 10.1021/acssuschemeng.6b01218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering existence of functional groups of SPI and the formation of hydrogen bonding between SPI and PVA. Moreover, the SPI/ PVA composites would still remain a degradable property. In this study, the degradable SPI/PVA nanofiber membranes were prepared by electrospinning, and then the GPEs were produced after the activation process of stacked nanofiber membranes in liquid electrolytes. The GPEs based on the composite nanofiber membrane showed relatively high ionic conductivity, low interfacial resistance and excellent electrochemical performance. In addition, the preparation of this kind of composite skeleton material was not involved in any toxic or polluting chemicals, which made it full of green chemical technology advantage. Therefore, such an environmentally friendly skeleton material for GPEs has wide application potential in the field of safe and high performance Li-based power sources.
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A (%) = (W1 − W0)/W0 × 100%
(1)
where W0 and W1 are the mass of the dry and the saturated membrane, respectively. All the electrochemical properties were measured on Autolab PGSTAT 302N (Metrohm) at room temperature. The ionic conductivity was determined by AC impedance spectroscopy in the range of 0.1 Hz−100 kHz using the cell inserted GPEs into two parallel stainless steel (SS) discs. The ionic conductivity could be calculated by eq 2:
δ = L /(R b· S)
(2)
where δ is the ionic conductivity, Rb the bulk resistance, L and S the thickness and area of the films, respectively. The electrochemical stability window was examined using the method of linear sweep voltammetry in the cell Li/GPE/SS at a scan rate of 1 mV s−1, potential voltage ranging from 2 to 8 V. The lithium-ion transference number was measured using the method of chronoamperometry (CA) in the cell Li/GPE/Li with a polarization voltage of 5 mV. The impedance spectra were obtained by scanning in the frequency range of 0.1 Hz−200 kHz using symmetrical Li/GPE/Li cell. The charge/ discharge performance of the Li/GPE/LiCoO2 cells was galvanostatically measured on a LAND CT2001A battery tester in the potential range of 2.7−4.2 V at current densities of 0.1 C. The LiCoO2 electrode was a mixture of LiCoO2 (Aldrich Chemical Co., USA), acetylene black and poly(vinylidene difluoride) (PVDF) in the weight ratio of 8:1:1. The biodegradation behavior of SPI and PVA was carried out in a composting environment, and the composting medium was prepared by blending chicken manure and sawdust in a weight ratio of 50:50. Placed the compost in a plastic container and added water into the composting medium until the moisture content reached about 50%, and the water should be added periodically to maintain the moisture content.17 Dried nanofiber membranes wrapped by nonwoven and nondegradable polypropylene bags with high porosity were inserted inside the composting medium for microbial degradation tests, and the weights of the dried specimens were recorded as a function of biodegradation time.
MATERIALS AND METHODS
Materials. The commercial SPI powder (protein content, >90%; fat content,
