New polymer electrolytes based on ether sulfate anions for lithium polymer batteries: Part I. Multifunctional ionomers: Conductivity and electrochemical stability
Introduction
With regard to liquid solvents, the use of polymeric electrolytes, gelled or not, provides specific advantages to lithium batteries. First of all and, insofar as the functional polymer has enough mechanical strength, they make macroporous separator made of apolar polyolefins unnecessary. This mechanical strength has often been achieved by the polymer cross-linking [1], [2], [3], [4] while recently a huge increase in storage modulus was obtained, by addition of cellulosic nanofibers without compromising conductivity or electrochemical stability [5], [6], [7]. It is well known that these functional polymers may consist of solvating ether units and anionic functions bonded to the macromolecular backbone [8], [9] that provide single-cation conducting network electrolytes [10]. It is also well known that the ionic conductivity of most of the lithium salts is mainly governed by anion mobility [10], the latter probably being detrimental to the operation of the lithium battery [11]. Until now, most ionomers have been prepared by free-radical [8], [9] or ring-opening copolymerization [12], both involving a non-commercial ionic monomer. An alternative to this copolymerization route may be the chemical modification of polymers, a process that could lead to cost reductions, provided the starting polymers are commercially available. In addition, the use of specific functional monomers may, during the copolymerization, lead to transfer or termination reactions that affect the final properties of the resulting copolymer. However, it should be emphasized that transposition of reactions on organic molecules to chemical modifications of polymers is often not a trivial matter and special care must be taken to avoid or at least limit degradation of the macromolecular backbone [13].
The present paper deals with the design and synthesis of new multifunctional copolymers consisting of (i) polar protic and/or aprotic groups, (ii) solvating polymeric units based on polyethers and (iii) ionic ether sulfate functions. Previous papers reported the synthesis of oligomeric lithium ether sulfates from oligo(oxyethylene) end-capped by one or two OH groups [14], [15], [16]. This paper deals with new ionomers, based on a poly(oxyethylene) solvating matrix and on ether sulfate anions, prepared from a poly(oxyethylene)-co-poly(epichlorohydrin) copolymer. The ionomer intrinsic conductivities and their dependence on cation radius and the nature and content of the polar groups were thoroughly investigated. Thereafter, changes in their conductivity in the presence of a non-volatile plasticizer and complexing agents were determined. Ether sulfate anions were selected because of their (i) low basicity, (ii) expected electrochemical stability, (iii) low cost and (iv) acceptable environmental impact. In fact, subsequent treatment by alkaline aqueous solutions should easily remove alkaline sulfates and eventually allow the polyether to be recovered.
Section snippets
Products
The poly(oxyethylene)-co-poly(epichlorydrine), P(OE/ECl), copolymer (90/10 mol%) was purchased from Zeon and used as received. It is rather a random copolymer.
Poly(ethylene glycol dimethylether), PEGDME 500, , 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, TMTAC and sparteine from Aldrich were used as received.
CH3(OCH2CH2)3OSO3Li was obtained as previously described [15], dried in vacuum and stored in a glove box. The “battery grade” solvents ethylene carbonate (EC) and
Ionomer synthesis
The first chemical modification, namely a nucleophilic substitution of chlorine by a hydroxyl function, brings to the polyether a versatile function that in subsequent steps can induce polar and ionic functions as well as curable moieties to be incorporate. From these OH groups, several ionic groups might be grafted onto the macromolecular backbone.
The synthesis was carried out in several steps. First, the chlorine function groups of commercial poly(oxyethylene-co-epichlorydrine), P(OE/ECl),
P(OE/SLi) copolymers
The composition of the POE-derivatives was determined by elementary analysis. The yield of the first chemical modification of P(OE/ECl) into P(OE/OH) is quantitative since elementary analysis did not detect any chlorine. However, the esterification of the alcohol functions was partial, with conversion yields ranging from 60 to 80%. This disagrees with previous experiments performed on POE oligomers [15], where esterification was quantitative and was probably related to problem of efficiently
Ionomers based on terpolymers P(OE/SLi/OH) and P(OE/SLi/CN)
Conductivities versus reciprocal temperatures are plotted for ionomers at several salt concentrations in Fig. 2. Below the melting temperature of the crystalline phase, conductivities were fairly low, in accordance with DSC measurements, the melting points of these electrolytes ranging from 30 to 50 °C. The ionomer with the lowest cationic exchange capacity (cec), P(OE/SLi) (30), exhibited the highest conductivities with 6 × 10−5 S/cm at 70 °C and 10−4 S/cm at 87 °C. This result cannot be explained by
Conclusion
The modification of commercial polyethers opens up significant opportunities for designing and synthesizing a variety of multifunctional ionomers. In addition, chemical modification should allow the drawbacks related to copolymerization of functional monomers, i.e. transfer and termination reactions, to be overcome. Optimization of the experimental conditions should enable reaction yields to be increased and the problem of chain breaking to be resolved.
Despite the choice of environmentally
Acknowledgments
The authors thank our partner Batscap, l’ANRT (Association nationale de la recherche technique) for financial support and Ms Denise Foscallo for help with film processing and electrolyte characterizations.
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