Preparation and characterization of PEO-PMMA polymer composite electrolytes doped with nano-Al2O3
Introduction
Lithium-ion batteries are widely used in mobile phones, tablets, laptops and other digital electronic products because of their high energy density, high output voltage, long battery life, low level of environmental pollution and other features. Solid polymer electrolytes (SPEs) can potentially solve the safety issues of lithium-ion batteries by optimizing the polymeric film materials. SPE is an effective solution to the poor mechanical properties and low conductivity problems of lithium-ion batteries [1]. Compared to liquid electrolytes, SPEs have the following advantages: safer design, simpler stacking and hermetic sealing processes, superior in density, flame-resistance and shape suitability in the requirements of the application. Many polymer/lithium salt systems were developed and used [2], [3], [4].
One major limitation to the realization of SPE's applications is their mechanical and electrochemical properties. In order to obtain a balance for the compatibility of ionic conductivity and mechanical properties of the SPE, several investigations have been carried out such as synthesis of new polymer matrix, preparation of polymer single-ion conductor, and doping with nanoparticles. Through blocking [3], grafting [5], crosslinking [6], compositing and blending methods [7], [8], researchers can improve the performance of the polymer electrolyte by reducing crystallinity of the polymer, increasing the proportion of the amorphous region and the concentration of ions contained in the system, as well as decreasing glass transition temperature of the polymer electrolyte system and improving the capacity of lithium ion dissociation [9]. Adding liquid plasticizer is also a widely used technique to lower the operational temperature of polymer components, especially poly (ethylene oxide)(PEO), although this will hamper the mechanical properties of the polymer electrolytes [10]. Furthermore, it has also been demonstrated that room temperature ionic liquid can effectively improve the performances of the polymer electrolyte in terms of ionic conductivity and electrochemical stability [8], [11], [12], [13]. Various approaches have been used to improve the ambient temperature conductivity while retaining the mechanical properties and the stability of PEO by the addition of nano-scale ceramic fillers such as Al2O3, SiO2, TiO2 and BaTiO3 in PEO-based polymer electrolytes [12], [14], [15], [16]. These nanocomposite polymer electrolytes (NCPE) were stable up to 310 °C and showed good anti-thermal shrinkage performance [12]. The solid-state mix-salt polymer electrolytes were prepared and higher ionic conductivities were obtained for the mix-salt polymer electrolytes than that of the pure-salt counterparts [17]. PEO/LiTFSI (Bistrifluoromethanesulfonimide lithium) system was widely studied [18], [19], [20], [21]. LiTFSI has a good thermal stability with a decomposition temperature of 360 °C. The high dispersion of anionic charge of lithium salt makes it more easily ionized. LiTFSI and polymer matrix (a common substrate, such as PEO, PMMA, PVDF, etc.) were used as SPEs, which could have the characteristics of good conductivity, light weight, high flexibility and easy for molding [22], [23], [24], [25], [26], [27], [28].
Polymer blending was devoted to render the performance of SPEs because of its simplicity and efficiency [29], [30]. PMMA polymer has the advantages of being strongly hydrophilic and chemically stable, rich in raw materials and simple to synthesis. However, the film brittleness of PMMA polymer restricts its applications. PEO has good solubility for the lithium salt. It has been verified that best compatibility and maximum conductivity of the composite membrane were obtained when the mixture PEO-PMMA was mixed with a mass proportion of 1:4 [31], [32]. It would be interesting to take full advantage of these approaches, that is, to incorporate nano-scale fillers into a PEO/LiTFSI polymer salt complex, with a view to optimize the favorable physical properties mentioned above. PEO-PMMA composite polymer matrix can combine the advantages of both polymers.
In the present work, four composite polymer electrolyte systems, PEO-PMMA-LiTFSI, PEO-PMMA-LiClO4, PEO-PMMA-LiClO4-Al2O3 and PEO-PMMA-LiTFSI-Al2O3 were prepared by the solution casting technique. The polymer membrane morphology, ionic conductivity, interfacial properties, thermal stability and thermal shrinkage of the blends of solid polymer electrolytes were systematically studied.
Section snippets
Materials
Tetrahydrofuran (THF) was purchased from Aladdin and pretreated by 4A molecular sieves to remove water. LiTFSI (99%) and LiClO4 (99%) were both provided by Aladdin and vacuum dried at 120 °C for 24 h. PEO (average molecular weight: 300,000) and nano-Al2O3 (particle size: 10 nm) were purchased from Aladdin and vacuum dried for 24 h at 50 and 120 °C, respectively. PMMA (average molecular weight: 550,000) purchased from Alfa, was dried in a vacuum oven at 80 °C for 24 h.
Preparation of PEO/PMMA blending polymer membranes
PEO (0.18 g) and PMMA (0.72 g) were
Morphology characterization
Unlike the conventional method of adding lithium salt with the polymer matrix together, here we added lithium salt to the homogeneous polymer matrix solution. In this way the polymer chains can fully swell and stretch in the solvent so that the polar groups of polymer chain are more likely to react with the cations. Since evaporation of the solvent at the casting surface is much faster than the diffusion of solvent from inside to the surface, the drying of the liquid at the casting surface
Conclusions
In this paper, the polymer electrolyte membranes were prepared by solution casting technique. The ionic conductivity at room temperature for PEO-PMMA-LiTFSI and PEO-PMMA-LiTFSI-Al2O3 (EO/Li+ = 10) were 6.71 × 10−7 S/cm and 9.39 × 10−7 S/cm, respectively. Nano-Al2O3 can effectively improve the interfacial stability of the polymer electrolyte membrane. Good thermal stability of the samples indicated that the membranes could be adapted to support high temperature applications. Comprehensive data showed
Acknowledgments
The authors sincerely appreciate the financial support by the National Natural Science Foundation of China (50803008), Natural Science Foundation of Hunan (14JJ4035, 2011RS4067), the State Key Laboratory of Luminescent Materials and Devices at South China University of Technology (2013-skllmd-08), China Postdoctoral Science Foundation (201104508).
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