Mechanism behind the Unusually High Conductivities of High Concentrated Sodium Ion Glyme-Based Electrolytes

Abstract

New highly concentrated electrolytes based on ether solvents were developed for sodium electrochemical cells. The investigated electrolytes use sodium triflate and glycol diether oligomers of different length to form the electrolyte. These electrolytes present conductivities that increase as a function of concentration even when the electrolyte is composed of a majority of ion pairs and aggregates. Correlation analysis between the electrolyte speciation and conductivity suggests the presence of two distinct mechanisms of charge transport, namely a traditional vehicular mechanism based on the diffusion of free ions and a hopping mechanism involving the making and breaking of ion-pairs and/or aggregates. The former mechanism represents the charge transport of glyme with 3 or 4 units, while the latter is observed in electrolytes composed of short chains, i.e., 1 or 2 units. The proposed mechanism of transport is corroborated via molecular dynamics simulations. In addition, our experiments demonstrate that the high concentration of the sodium salt not only increases the overall conductivity of the electrolyte but also does not affect its electrochemical window.

NaFSI and NaTFSI Solutions in Ether Solvents from Monoglyme to Poly(ethylene oxide)—A Molecular Dynamics Study

“…Two mechanisms of charge transport, diffusion of free ions and hopping between ion pairs, were postulated for concentrated Na electrolytes in ether solvents. 29 A similar effect of residence times decreasing with increasing salt concentration was also reported for sodium salt solutions in acetonitrile 45 and attributed to the transport mechanism.…”

“…However, at high salt concentration, an alternative mechanism of ion hopping between aggregates may become operative; 29 , 43 its effectiveness may be dependent on the solvent. 29 In this work, we showed that the interplay between ion concentration and mobility may be also affected by the anion of the salt, changing the degree of correlations. Through the analysis of contributions to the total conductivity, we found that ion–ion correlations in NaTFSI solutions are much more important than in NaFSI electrolytes.…”

“…Vibrational (infrared or Raman) spectroscopy is commonly used to experimentally assess interactions between ions and solvents in Na-conducting electrolytes. 20 , 21 , 24 , 27 − 29 , 31 , 33 , 34 , 37 These findings are supplemented by the results of quantum-chemical calculations. 13 , 20 , 21 , 25 , 28 , 38 Early works performing molecular dynamics (MD) for polymer electrolytes with Na + ions were focused on glyme–NaI 39 , 40 or PEO–NaI 41 , 42 systems.…”

“…Nowadays, classical and ab initio MD simulations are routinely applied to support and elucidate measurements of transport properties and structure of electrolytes. 15 , 24 , 29 , 31 , 36 , 37 , 43 − 45 …”

Dinâmica molecular fine-grained e coarse-grained de líquidos iônicos e sistemas de matéria macia

“…A adição de moléculas de PEO como solvente é amplamente utilizada em eletrólitos de metais alcalinos e, consequentemente, diversos estudos empregam diferentes tamanhos de PEO [36][37][38]40,41,[228][229][230][231] 187 Apesar disso, a maioria das simulações atomísticas reportam micelas de DPC ligeiramente elipsoidal 183,188,234 . No trabalho de Abel et al 183 Tabela 9.…”