Methacrylic-based solid polymer electrolyte membranes for lithium-based batteries by a rapid UV-curing process

https://doi.org/10.1016/j.reactfunctpolym.2010.12.007Get rights and content

Abstract

The present communication describes solid polymer electrolyte (SPE) membranes prepared by the direct free radical photo-polymerisation (UV-curing) of poly(ethyleglycol)methacrylic oligomers in the presence of a lithium salt. The highly mobile pendant ethoxy chains, constituting a considerable fraction of the whole polymer matrix, can provide a large density of coordination sites for the Li+ ions and make the material an interesting solvent-free ion conducting medium for high temperature application. The production process is simple and versatile; the resulting membranes demonstrate mechanical integrity due to the cross-linked nature of the polymer network, and wide thermal stability. The electrolytes produced are extra soft, non-crystalline, transparent solids, do not contain volatile matter and show sufficient ionic conductivity along with a wide electrochemical stability window. Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR), and electrochemical impedance spectroscopy (EIS) are employed to characterise the polymers, monitor its phase changes and control the conductivity of the electrolytes as a function of lithium bis(trifluoromethane)sulfonimide salt (LiTFSI) concentration. The temperature dependence of the ionic conductivity follows the Vogel–Tamman–Fulcher (VTF) equation, and the ionic conductivity at 60 °C reaches values higher than 10−4 S cm−1.

Introduction

Nowadays, Li-ion batteries are compact and lightweight rechargeable power sources, stable to over 1000 cycles, operating at about 4 V with energy densities ranging between 200 and 300 Wh kg−1 [1]. The lithium-based battery technology is continuously evolving and has gained an unprecedented significance since the demand for portable electronic devices, such as cellular phones, notebooks and Mp3 players, has been continuously increasing [2], [3], [4]. In the last few years, Li-based polymer batteries have been identified as a very promising technology to meet the requirements of upcoming applications, such as standby power and electric vehicles; much work has been done to discard the unsafe, conventional liquid organic electrolyte solutions made of a lithium salt (e.g., LiClO4 or LiPF6) dissolved in organic carbonates. In fact, inert and solvent-free lithium ions conducting membranes are ideally needed for the development of all-solid state batteries [5], [6], [7], [8]. The benefits are substantial: besides excellent processability and flexibility, switching to a fully solid configuration gives concrete promise of higher safety, due to the absence of flammable organic solvents, possible prevention of short circuits due to the growth of lithium dendrite crystals upon cycling, large modularity in design and ease of handling [9].

Solid polymer electrolyte (SPE) membranes formed by the dissolution of suitable lithium salts in suitable polymer matrixes were first studied by Wright [10], but it was the work of Armand et al. [11] that focussed on polyethers, namely poly(ethylene oxide) (PEO) which has been the subject of lithium-based electrolyte research for more than a decade. SPEs hold a potential to be used in high energy density batteries, to replace the liquid and gel-based electrolytes of today and boost the performances even further [5], [6], [12], [13]. They serve two principal roles in rechargeable Li-based batteries: not only do they function as the traditional electrolyte (i.e., the medium for ionic transport), but also as the separator which insulates the positive electrode from the negative. Consequently, they are requested to have certain level of physical properties (e.g., sufficient mechanical strength) in order to withstand the electrode stack pressure and stresses caused by dimensional changes which the rechargeable electrodes undergo during charge/discharge cycling, as well as elasticity and thermal stability [14], [15].

At present, the main limits of SPEs are the low ionic conductivity at ambient temperature and the low Li+ transference number under equilibrium conditions, which is due to the low segmental mobility of the polymer chains complexed with the lithium salt. Typically, the conductivity of bulk polymeric systems used nowadays, independently of the chemical nature of the polymer matrix, remains in the range of 10−7–10−5 S cm−1 at ambient temperature, whereas a conductivity of the order of 10−3 S cm−1 or higher is considered as the target for a commercial applicability: appreciable conductivities can be achieved only at temperatures above 70 °C [16], [17], [18], [19]. However, if applications of SPEs in electrochemical devices are remote, excluding the automotive fields where temperature is not a crucial parameter, the concept of a fully solid lithium metal polymer battery is still very appealing and is presently being considered in industrial laboratories involved in electric transportation [9]. Therefore, the study of novel yet efficient solid polymer electrolytes is important. Another drawback of present polymer electrolytes is the time requested for preparing the membrane, while fast, easy and efficient processing in batteries production are key factors for their development.

In this respect, the present work describes the preparation of SPEs by free radical photo-polymerisation (UV-curing) of multi-functional methacrylic oligomers containing polyethylene oxide chains, in the presence of a lithium salt. The process is fast and takes few seconds for producing fully-solid polymer electrolyte membranes without the use of solvents and volatiles [20], [21], [22], [23], [24]. The process is also versatile: in this paper we show that by appropriately choosing the oligomers to modulate the concentration of ethoxy units and changing the average methacrylic functionality of the systems one can vary the SPE performances and guarantee an appreciable ionic conductivity along with a wide electrochemical stability window. This approach could lead to solid polymer electrolytes which can be applied for high temperature applications.

Section snippets

Experimental section

Unless otherwise stated, all starting materials and reagents were purchased from commercial suppliers and used without any further purification. Experiments were repeated two to three times and were found to be reproducible. All samples were stored in ambient laboratory conditions.

Characterisation of the prepared membranes

Different kinds of variables (i.e., reactants ratio, their molecular weight, chain length, lithium salt concentration) were considered in the different stages of this work. Firstly, various salt-free solid polymer membranes (SPMs) were prepared by changing the ratio of the di-functional methacrylic oligomer BEMA to the mono-functional PEGMA and changing the molecular weight of PEGMA. Table 1 shows the composition of the various salt-free solid polymer membranes. Sample SPM-1 is made of BEMA:

Conclusions

Solid polymer electrolyte membranes were prepared by a solvent-free rapid photo-polymerisation procedure and thoroughly characterised in terms of kinetics, thermal and morphological properties, ionic conductivity and electrochemical stability. The reported findings show that lithium salt doped highly cross-linked methacrylates can be used as solid polymer electrolytes in lithium-based rechargeable batteries for high temperature applications. Their ionic conductivity (>10−4 S cm−1) at above 60 °C

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