Structural Studies of the Lithium and Sodium Salts with Heteroaromatic, Percyanated Anion
Authors: Jankowski Piotr , Gajewska Agnieszka , Poterala Marcin , Zukowska Grazyna Zofia , Dranka Maciej , Kalita Michal Jan , Zachara Janusz 
Published: February 2020

Lithium anode an Li-ion batteries are chemical power sources of high energy and power density. In other words, such batteries can be light, small and relatively efficient. Therefore, they can be used in several power-demanding devices, like laptops, tablets, smartphones as well as cameras. Despite of this commercial success, the scientific works oriented on development of these power sources are still conducted. Novel lithium batteries for special applications (extremely high and low temperatures, long-time-without-charging power sources) are being continuously elaborated. The most important problem, however, is stability of the battery and battery life length. This is related to the stability of the electrolyte-electrode interfaces.

Due to the huge demand for power sources from car industry, working out of the sodium and Na-ion batteries (analogs of lithium anode and Li-ion) is one of the biggest challenges for the electrochemists. In case of these batteries, problem of electrode-electrolyte stability (i.e. problem of corrosion of the electrodes) was not solved in the state-of-the art. The formation of SEI of good properties- stable in time and ion-conducting layer formed on the interface of the electrode is still a problem. If such layer was formed, majority of problems related to the construction of sodium-based batteries would be solved.

One of the possible approaches which may lead to overcoming of the above-mentioned problems is use of the novel types of the salt. In early 2000s, Armand proposed a novel type of the salts, with five-membered ring consisting of carbon and nitrogen, in which all the carbon atoms are substituted with cyano groups. The architecture of the proposed anion leads to distribution of the charge on all the nitrogen atoms. Moreover, the ion pairing in the lithium or sodium salts of such anions is relatively weak due to fact that cations are “hard” acids and nitrogen atoms are “soft” bases. The nitrogen-rich salt is also interesting due to fact that introduction of the nitrogen to SEI leads to the better properties of this layer.

In the recent years several heteroaromatic salts, derivatives of triazole, imidazole and pyrrole were synthesized and their crystallographic and spectral data were published.

In the present study we focus on the study of solvates of the salts shown above, and on the influence of the structure of the crystalline complexes on their vibrational spectra. The main impact is put on complexes of the salts with dimethyl ethers of ethylene glycol oligomers (glymes), due to their importance as model systems for poly(ethylene oxide) based electrolytes. Variety of the negatively charged sites available for cation complexation leads to the formation of adducts with different structure, being models for single ions, ionic pairs or aggregates. Also simulations of the formation of the ionic agglomerates in ether-based systems has good support when the presented results are taken into consideration.

It is worth to stress out that there is not any electrolyte based on organic carbonates which is stable against material of the anode in the state-of-the-art. Therefore, ethers are one of interesting candidates for solvents dedicated to battery with the sodium-type (metallic Na, Na-ion and similar) anode. What is also important, the compatibility (as well as solubility) of the studied salts with organic carbonates is weak and, due to this, solubility of the studied salts in carbonates is too small to use such solutions as electrolytes in real battery systems produced in the industrial scale.