Theoretical Investigation of Ionic Liquids-Based Electrolytes for Sodium-ion Batteries: Understanding of Electrochemical Stability and Na+ Transport Mechanism
Tuanan C. LOURENCO1, Juliane FIATES1, Alex S. MORAES2, Gabriel A. PINHEIRO3, Mauro C. LOPES2, Rafael MAGLIA DE SOUZA5, Marcos G. QUILES3, Luís Gustavo DIAS5, Leonardo J. A. SIQUEIRA4, Juarez L. F. DA SILVA1
1São Carlos Institute of Chemistry, University of São Paulo, São Carlos, Brazil
2Chemistry Department, Central-West State University, Guarapuava, Brazil
3Institute of Science and Technology, Federal University of São Paulo, São José dos Campos, Brazil
4Department of Chemistry, Environmental, Chemical and Pharmaceutical Sciences Institute, Federal University of São Paulo, Diadema, Brazil
5Chemistry Department, FFCLRP, University of São Paulo, Ribeirão Preto, Brazil
The use of "green" and renewable energy sources is a crucial step in the pursuit of a more sustainable society. However, these energy sources are intermittent, which makes it necessary to use energy storage devices to ensure a continuous energy distribution. In this context, sodium-ion batteries (SIBs) are promising candidates because of the large abundance of Na on the planet, low production costs, the possibility to use cheap materials as battery components, and also the chemical similarities with Li. However, as well as in lithium-ion batteries (LIBs), improved electrolytes are developed to boost the battery's performance. Therefore, we haved used theoretical and computational methods to investigate the relationship between the ionic liquids (ILs) based electrolytes chemical structure, and their physicochemical properties, e.g., electrochemical stability windows (ESW), ionic transport, and viscosity, to improve our understanding of the electrolyte performances and develop new systems for SIBs. To reach our goals, we have used theoretical and computational methods, such as molecular dynamics simulations (MDs), density functional theory (DFT), and data mining techniques, which allowed us to cover a large number of possible systems with a reasonable cost and without expending chemical products. Moreover, with those techniques, it is possible to improve the atomistic understanding of the materials and also complement information from experimental measurements. For common IL electrolytes, we investigated several systems based on the combination of imidazolium or ammonium cations with fluorosulfonyl base anions. In this case, small and asymmetrical ions can be used to improve the ionic transport in the electrolyte. Furthermore, the MD simulations showed that the increase in Na+ concentration leads to the formation of ionic aggregates in the systems, which decreases the overall ionic transport and, at the same time, improves the sodium-ion diffusion mechanism. Another alternative to improve electrolyte performance is the use of weakly coordinated anions (WCAs), such as cyanoborate and fluoroalkoxyaluminate. Because of the large degree of charge delocalization, those anions present a low tendency to form large ionic aggregates upon Na+ addition, while displaying high ionic conductivities and low viscosities. In the case of fluoroalkoxyaluminate, the increase in the sodium-ion mole fraction leads to an increase in the ionic conductivities, which is different from what was seen in other ILs systems. In addition to the transport properties, the ESW has been investigated by various techniques. Using DFT calculations and data mining, we are able to cover a broad chemical space and obtain the ESW estimation for 108 possible ionic liquids. The calculations have shown that ammonium or other aliphatic cations can deliver ESW larger than 5 eV, while the addition of fluoroalkyl chains can improve the anion stability and widen the ESW. Therefore, ILs are a promising alternative to developing new and improved electrolytes for sodium-ion batteries. At the same time, theoretical and computational methods are powerful tools for seeking and understanding electrolyte behavior.