(Keynote) Development of Novel Lithium and Sodium Salts Based on Huckel Type Anions
Authors: Wieczorek Wladyslaw 
Published: February 2020

Lithium salts are essential components of all electrolytes used in lithium-ion batteries. In our group in co-operation with CIC (Michel Armand) and Chalmers University (Patrik Johansson) various salts based on heterocyclic compounds with five-membered rings, such as 4, 5-dicyanoimidazole or 4, 5-dicyanotriazoles derivatives were obtained and investigated.

The novel, promising concept of the application of new anions is based on the application of so called “Hückel anions”. The name came from the transposition of the Hückel rule predicting the stability of the aromatic systems. One of the most common examples of this type of anions is 4,5-dicyano-triazole (DCTA). This particular structure is completely covalently bonded and shows very stable 6p (or 10p electron if CN bonds are involved in calculations) configuration. It can be produced from commercially available precursor and even more importantly does not comprise fluorine atoms. Salts of this type of anion were found to exhibit high (~300oC) thermal stability. LiDCTA was successfully tested in PEO matrices systems as a promising, improved electrolyte for lithium ion batteries. Unfortunately DCTA failed as a component of the EC/DMC (1:1) battery electrolyte.

Our idea was to design structure that would not have disadvantages of big bulky anions causing high viscosity when dissolved in organic solvents, therefore a decrease in conductivity. Also, ions of new salts should not form agglomerates after dissolution, due to ion pairs’ and triplet’s which negatively affect ionic conductivity of an electrolyte, and high transference number of a lithium cation. Molecular modelling studies showed that benzimidazolide and imidazolide anions show a typical behaviour of heterocyclic anion alternatives to PF6
-: if tailored correctly they offer more dissociative lithium ion pairs compared to LiPF6
-, but are not as electrochemically stable. The data obtained for the benzimidazolides, indicate that with very small alterations, such as the positioning of –CN substituent’s on the heterocycle, ion pairing can be changed drastically. With equal predicted properties for analogous imidazolides and benzimidazolides, the difference in anion size is an important variable for choosing an appropriate lithium salt.

Overall the potential of using cyano chemistry to create new lithium salts with excellent electrochemical stabilities and ion pairing properties is evident. The previously recognized improvement of both the electrochemical stability and the lithium ion pair dissociation ability, by increasing the number of cyano groups, is apparent also for the benzimidazole salts and further strengthened by the predictions made for P (CN) 6
-. However, an increase in the number of –CN groups leads to a larger anion size and require increased synthesis efforts.

Electrochemical properties of the systems doped with 4, 5-dicyano-2-trifluoromethyl imidazole lithium salt (LITDI) were most intensively studied. Crystallographic studies allowed for a better understanding of electrochemical properties of TDI salts. Despite of these studies more detailed examination of the interactions between [Li(glyme)]+ complex cations and weakly Lewis basic anions are still necessary to gain further insight into the ion coordination structure and transport mechanism in liquid and polymeric electrolytes containing LITDI as a dopant. In this presentation some recent results which enable us to design new electrolyte systems will be highlighted.

As an example the new class of ionic liquid systems in which lithium salt is introduced into the solution as a lithium cation−glyme solvate. This modification leads to the reorganization of solution structure, what entails release of free mobile lithium cation solvate and hence leads to the enhancement of ionic conductivity and lithium cation transference numbers. This method enables also even three-fold increase of salt concentration in ionic liquids or dissolution of salts previously insoluble in some ionic liquids.

As a further extension of the previously mentioned studies we have introduced a new family of fluorine-free solid polymer electrolytes for use in sodium battery application. Systems are based on three novel sodium salts: sodium pentacyanopropenide (NaPCPI), sodium 2,3,4,5-tetracyanopirolate (NaTCP) and sodium 2,4,5-tricyano-1,3-imidazolate (NaTIM) which were dissolved in poly (ethylene oxide) to simulate viable solid-polymer electrolytes for sodium-ion batteries. Salts were selected with respect to their fluorine-free composition and diffuse negative charges and in consequence offered special „tailored“ structure leading to better performance in electrolyte sytems. Due to this “liquid-like” high conductivities (> 1 mS cm-1) were obtained above 70 ˚C for solid-polymer electrolytes. All presented salts showed high thermal stability and suitable electrochemical stability windows up to 5V.

Finally some examples of the lithium-ion and sodium –ion batteries using newly design anions will be presented and discussed.