Chapter 4
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Effect of Lithium Salt Content on the Performance of Thermoset Lithium Battery Electrolytes Markus Willgert,1 Maria H. Kjell,2 and Mats Johansson1,* 1KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, SE-100 44 Stockholm, Sweden 2KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemical Engineering and Technology, Applied Electrochemistry, SE-100 44 Stockholm, Sweden *Corresponding author. Tel. +46(0)8 790 92 87; fax: +46(0)8 790 82 83. E-mail address:
[email protected] (Mats Johansson)
Series of solid poly(ethylene glycol)-methacrylate electrolytes have successfully been manufactured in a solvent free process with an aim to serve in a multifunctional battery, both as mechanical load carrier as well as lithium ion conductor. The electrolytes have been studied with respect to mechanical and electrical properties. The thermoset series differs with respect to crosslink density and glass transition temperature (Tg). The results show that the conductivity increases, with salt content exhibiting similar trends, although at overall levels that differ if measured above or below the Tg of the system. The Tg transition on the other hand is more affected by the salt content for loosely crosslinked thermosets. The coordination of a lithium salt to the PEG-segments play a more important role for the physical state of the material when there are less restrictions due to crosslinking of the PEG-chains. The overall performance of the electrolyte at different temperatures will thus be more affected.
© 2012 American Chemical Society In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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Introduction There is an increasing demand of possibilities to store electrical energy since this energy is by a growing extent to be used in portable applications. As of today, lithium-ion batteries are considered to be the primary storage medium for electrical energy in for instance a Smartphone which, due to the rapid development of portable devices, typically uses significantly more electrical energy than its precursor because of the many new applications. This development calls for new solutions to store the energy, since batteries tend to possess a significant part of the total item weight, a highly disadvantegous property in a portable device (1). Additionally the batteries often occupy a considerable space in a product such as a mobile phone or an electric vehicle (EV) or hybrid electric vehicle (HEV). Earlier work has investigated the possibility to let the battery itself be a mechanical load bearing component in the total construction (1–3). This would significantly reduce the overall weight since the battery in this case not only provide for energy storage, but also replaces a load bearing component in the structure (4, 5). Previous work accounts for the possibility to build polymer electrolytes composed by poly(ethylene-glycol), (PEG) segments (3, 6). These are the most considered polymers within this context, since the glycol units in such a polymer are known to be able to coordinate to a Li+ ion (7–9). The PEG segments can then be crosslinked for instance by acrylate or methacrylate functional groups attached to these segments, forming a thermoset network. This action both enhances the mechanical properties dramatically, but also suppresses crystallization of the PEG, which would be detrimental for the ability to conduct lithium ions (10, 11). Extensive work within the field has for instance been conducted by Snyder et al, where PEG-based solid polymer electrolytes (SPEs) that are able to possess ionic conductivities (σ) of almost up to 10-5 S×cm-1 or a storage modulus (E′) of around 2 GPa have been manufactured (6). However, these two properties are counteracting one another, since a densely crosslinked thermoset may possess excellent mechanical properties at the cost of ion conductivity and vice versa. This is believed to stem from a number of reasons, short distances between the crosslinks and by that a low mobility of the backbone polymers which needs to be able to move to promote ion transportation throughout the electrolyte (10), but also a severe decrease in free volume in the network (12), which is a drawback since transport of the lithium-ion is coupled to the local mobility of the PEG chains (13). It can also be assumed that the polymer chain mobility itself is affected by the presence of lithium ions that coordinate to the chain segments. We have in earlier work showed that the concentration of the lithium salt in the electrolyte had little effect on either the ionic conductivity nor the mechanical properties for densely crosslinked systems when the electrolyte is below its Tg, as defined by the tanδ peak value, when using intermediate levels of salt content (14). It is however not fully clear how the balance between crosslink density, coordination strength, coordination sites per unit volume, and mobility of polymer segments between crosslinks affect the ion conductivity in relation to mechanical properties. There is thus a lack of full understanding in what mechanisms are critical in controlling the lithium ion diffusion in these systems, and the present work aims to investigate this further. Improved understanding of 56 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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these mechanisms and the main factors behind them is an essential objective on the way of designing structural batteries and to find advantageous trade-offs when designing the same. There are several polymerization techniques available for crosslinking of methacrylated or acrylated PEGs, where thermal initiation (3) or photoinduced initiation through UV irradiation (15) is the most common techniques used . Photoinduced polymerization reactions are known to be rapid, induce low thermal impact, and result in well defined networks (16). The technique is well established for solid polymer electrolytes (SPEs) (17, 18) and is the polymerization route used in this study. Previous studies on crosslinked PEG-methacrylate electrolytes have shown that increased crosslink density has a significant impact on the conductivity when the Tg transition increases above ambient conditions (6, 18) i.e. the temperature at which the ionic conductivity is measured plays a mayor role (19). It has also been shown that the lithium salt content has rather small effect on the conductivity below Tg for a specific thermoset (14). It is however not clear if the same relationship between conductivity and salt content is valid in the rubbery state i.e. above Tg for these systems and at what level of salt content that the SPE becomes conductive. The present study aim to reveal more details on this and how the lithium salt content affect the properties above Tg.
Experimental Materials Tetraethylene glycol dimethacrylate (SR209), and methoxy polyethylene glycol (350) monomethacrylate (SR550), both displayed as A and B respectively in Figure 1, were kindly supplied by Sartomer Company, Europe. 2,2-dimethoxy-2-phenylaceto-phenone (DMPA) was obtained by Ciba Specialty Chemicals (Switzerland). Lithium trifluoromethanesulfonate (lithium triflate) (97%) (Figure 1) was purchased from Chemtronica AB (Sweden). All chemicals were used as delivered. Techniques Procedures and Test Series, Photopolymerization Test specimens for two different test series were produced using a general procedure. The monomer/salt mixtures were weighed into small vials in proportions given in Table I. The vials were sealed and put on a shaking table overnight to allow complete mixing of the components. 1 weight-% photoinitiator relative to the monomer amount was then added to the mixture. 0.3 ml of the mixture was then transferred into a Teflon mould (15×20×1 mm) using a syringe and then cured under UV-irradiation at 15 cm distance from the UV light source using a total dose of 1.25 J×cm-2. The light source used for curing was a Blak Ray B-100AP (100 W, 365 nm) Hg UV lamp with an intensity of 5.2 mW×cm-2 as determined with an Uvicure Plus High Energy UV Intergrating Radiometer (EIT, 57 In Polymers for Energy Storage and Delivery: Polyelectrolytes for Batteries and Fuel Cells; Page, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
USA), measuring UVA at 320-390 nm. The temperature of the cured samples did not exceed 42 °C during cure. After curing, the cured solids were taken out of the moulds and cut into pieces of appropriate size, using a scalpel, for further characterization. The sample mixtures were prepared and UV-cured in a glove box under dry conditions, (
