Organization of the Polymeric Electrolytes Doped with Enantiomerically Pure Salts: Role of the Substituents of the Assymetric Carbon

Solid Polymeric Electrolyte (SPE) is a system in which salt is dissolved in the polymer matrix. These materials, discovered about 40 years ago by Wright, were widely studied by several research groups. Their properties predestine them to use in several application in which aprotic electrolyte need to be used. The typical ones are lithium and sodium batteries and electrochromic windows. Their mechanical stability (they are self-standing) gives them several advantages when their application is considered in comparison to the liquid systems.

The main problem which limits use of SPEs in the commercial systems is related to poor ion transport properties of such electrolytes. Due to low diffusion coefficients in the solid system in comparison to the liquid one results in insufficient overall conductivity, strong lithium cation-polymer matrix interaction gives low lithium transference number. Moreover, the conductivity of SPEs falls rapidly below the melting point of PEO (for example, in the PEO-LiCF₃SO₃ system, the difference in conductivity at 35°C and 65°C exceeds two orders of magnitude). In consequence, application of the SPE even in the systems in which no high power density is needed (like pacemakers or biosensors), was not introduced into the industrial scale. This is despite of the advantages of SPE-containing power sources like stable in time inner resistance and low self-discharge rate. Therefore, various additives were introduced into the electrolyte in order to improve ion transport properties of the system. Amongst others, ceramic powders, plasticizers or anion receptors were tested.

It is commonly known and agreed that the ionic transport of the cations in SPEs is based on the "hopping" of the positively charged ions which are coordinated by anions and oxygen atoms of the polyethers. despite from that, interestingly, there are two opposite approaches to enhancement of the SPE properties. One of them is based on the assumption that the "hopping" of the ions will be faster when segmental motions of the polymeric matrix are intensive. This leads to the conclusion that plasticization of the polymeric matrix should enhance the ion transport properties of the membrane. This approach encouraged various scientists to study systems which contain low molecular weight plasticizer and SPE matrices of branched architectures (e.g. star-, brush- or network-like matrices) The opposite one base on the fact that conductivity of the SPE is strongly dependent on the local properties of the electrolyte and "fast conduction paths" are crucial when conductivity of the electrolyte is taken into account. Therefore, polymer-ceramic filler composites (in which conductivity near the polymer-ceramic filler interface is high) and highly organized systems were tested. This last was practically realized in several various ways. Studies on systems which are salt solvates, application of matrices being liquid crystals, stretch of the matrix, use of magnetically or electrically active ceramic powders together with putting the electrolyte inside the proper force field).

Armand studied the enantiomeric organization of the electrolyte. This effect can be achieved by use of the salts with anion containing assymeric carbon atom in its structure. He proved that the ion transport properties of the system containing enantiomericaly pure salt are much better in comparison to the same system but containing salt in the form of the racemic mixture. Their research, however, was limited to the potassium salts of the camphorosulfonic acid and the assymetric imide in which one part came from the camphorosulfonic acid and the other was CF₃SO₂ group.

In our studies, we concentrated on N-substituted amidates of perfluorinated sulfonic acids (like CF₃SO₃H or C₆F₅SO₃H). The assymetric carbon atom is present in the aminic part of the anion. Such salts can be easily obtained from commercially available chiral amines (methylbenzylamine, methylphenethylamine, sec-butylamine, 3,3-dimethyl-2-butylamine, 1,2-dimethylpropylamine, etc.) and acidic chlorides. Therefore, various salts of different structures can be synthesized.

In the presentation, role of the structure of the acidic and aminic part of the anion on the properties of the electrolyte containing such salt will be discussed. It will be shown that use of the enantiomerically pure salt can lead to the enhancement of the lithium transference number and overall conductivity. It will be also proved that the electrochemical stability of the system is sufficient enough to build a lithium battery containing such electrolyte.