The Effect of the Enantiomeric Organization of the Solid Polymeric Electrolyte on the Ion Transport Properties Thereof
Authors: Kalita Michal Jan , Piszcz Michal , Poterala Marcin , Jankowski Piotr , Gajewska Agnieszka 
Published: February 2020

Solid Polymeric Electrolytes (SPEs) are systems which exhibit relatively high (in comparison to other solid-state systems) ionic conductivity at temperatures below 100ºC. The typical applications in which such systems can be used are electrochromic devices, fuel cells and lithium batteries.

Usually, the SPE for the last above-mentioned applications consists of lithium salt dissolved in poly(ethylene oxide)- PEO. Optionally, various compounds, like ceramic powders, plasticizers or anion receptors are added to the electrolyte in order to enhance its ion transport properties.

As the lithium cation conductivity of the solid-state electrolyte is several orders of magnitude lower in comparison to liquid systems (in which conductivity is at the level of about 10-2-10-3 S/cm), SPE-containing batteries can be used only in applications in which high energy density, low power density as well as low self-discharge of the battery is needed. To make the range of the possible applications for SPE-containing batteries, the lithium cation conductivity of such systems should be enhanced.

One of the approaches to this problem is orientation of the electrolyte. It was proved that mechano-, electro-, or magnetoorientation of the electrolyte results in improvement of the transport properties thereof. Similar observations was made when orientation of the polyoxythylene chains by synthesis of liquid crystals and synthesis of crystalline solvates of the salts was tested.

The enantioorientation of the electrolyte in the molecular level was proposed by Armand. This type of the organization of the membrane is realized by the use of the lithium salt bearing chiral anion, i.e. given enantiomer of the anion which has at least one asymmetric carbon Such application resulted in noticeable increase of the ionic conductivity, especially between 30 and 60°C. The studies conducted by Armand were, however, limited to PEO-potassium 10-camphorsulfonate system and the one in which asymmetric potassium salt of the assymeric imide of the triflic and 10-camphorsulfonic acid was used.

It is worth to stress that the increase of the conductivity in the 30-60°C temperature range is very interesting when application of the SPE-containing battery as power source for peacemaker or (bio)sensor is taken into consideration.

We decided to study systems containing amidates, being in which the amine part contain at least one assymeric atom of the carbon, and the acidic part comes from triflic (trifluoromethanesulfonic) acid. The tendency of such anion to ion pairing is similar to this of the triflate anion (CF3SO3
-).

The important argument for this decision was that part of the amine can contain aromatic group. Paillard proved that the aromatic group can interact with the polymer matrix. Due to this, use of salt with anion containing phenyl ring in its structure should lead to the system of high lithium transference number. Therefore, it should be interesting to compare properties of the systems containing salts which anion contain (or do not contain) phenyl ring in its structure.

In the presentation, the effect of chirality on the ion transport properties of the SPE will be presented. It will be shown that the organized systems (containing enantiomeric anion) exhibit better ion transport properties (lithium cation transference number and overall conductivity) in comparison to the ones which contain racemic mixtures of the enantiomers. Also the dependency between the chemical structure of the salt used and effect observed will be discussed.