Li+ solvation in pure, binary and ternary mixtures of organic carbonate electrolytes. Ioannis Skarmoutsos†,*, Veerapandian calculations on small isolated clusters, the MD + Ponnuchamy†, ‡, Valentina Vetere ‡, simulations have predicted a preference of Li to interact with DMC molecules within its first Stefano Mossa†,* solvation shell and not with the highly polar † Univ. Grenoble Alpes, INAC-SPRAM, F-38000 EC and PC ones, in the binary and ternary Grenoble, France; CNRS, INAC-SPRAM, F-38000 mixtures. This behavior has been attributed to Grenoble, France and CEA, INAC-SPRAM, F-3800 the local tetrahedral packing of the solvent Grenoble, France Emails: [email protected] , molecules in the first solvation shell of Li+, [email protected] which causes a cancellation of the individual ‡ Univ. Grenoble Alpes, LITEN-DEHT, F-38000 Grenoble, molecular dipole vectors, and this effect seems France and CEA, LITEN-DEHT, F-38000 Grenoble, to be more important in the cases where France molecules of the same type are present. Due to Abstract these cancellation effects, the total dipole in the first solvation shell of Li+ increases when Classical molecular dynamics (MD) the local mole fraction of DMC is high. simulations and quantum chemical density functional theory (DFT) calculations have been Keywords: Lithium ion, solvation, organic employed in the present study to investigate carbonates, battery electrolytes, molecular the solvation of lithium cations in pure organic dynamics, density functional theory carbonate solvents (ethylene carbonate (EC), 1. Introduction propylene carbonate (PC) and dimethyl carbonate (DMC)) and their binary (EC-DMC, In recent years the demand for portable power 1:1 molar composition) and ternary (EC-DMC- applications has been highly increased, thus PC, 1:1:3 molar composition) mixtures. The giving a considerable impetus to the results obtained by both methods indicate that development of novel electrochemical devices, the formation of complexes with four solvent such as electric double layer capacitors and + molecules around Li , exhibiting a strong local lithium ion batteries. Lithium ion secondary tetrahedral order, is the most favorable. batteries are very common in consumer However, the molecular dynamics simulations electronics, such as laptop computers and cell have revealed the existence of significant phones, while they are also growing in structural heterogeneities, extending up to a popularity for automotive applications in order length scale which is more than five times the to decrease the green-house gas emissions in size of the first coordination shell radius. Due the atmosphere and, hence, to prevent global to these significant structural fluctuations in warming1-3. In general Li-ion batteries have the bulk liquid phases, the use of larger size been deployed so far in a wide-range of energy clusters in DFT calculations has been storage applications, ranging from energy-type suggested. Contrary to the findings of the DFT batteries of a few kilowatt-hour in residential systems with rooftop photovoltaic arrays to applications is to use mixtures of cyclic and multi-megawatt containerized ones, for the non-cyclic organic carbonates6. In such a way provision of grid ancillary services. the high dielectric constant of cyclic carbonates is combined with the low viscosity Li-ion cells mainly employ intercalation of acyclic carbonates ensuring good materials as positive and negative electrodes performances under low temperature and aprotic electrolytes to conduct Li+. The environments. On the other hand, the higher chemical nature of the electrodes determines thermal stability of cyclic carbonates ensures a the energy output, while the electrolyte affects reasonable operating temperature range for the the rate of the energy release by controlling mixed solvent. mass transport properties within the battery1. The interactions between the electrolyte and Although several experimental and theoretical the electrode materials are also very important studies devoted to the interactions of Li+ with and the formation of electrified interfaces pure and mixed carbonate-based electrolytes between them often dictates the performance have already been published, the solvation of the device. An electrolyte should meet a list structure and dynamics of lithium cations in of minimal requirements in order to be used in these solvents is still a subject of debate. such devices. In general it should be a good Interestingly, even the determination of the ionic conductor and electronic insulator, have a coordination number around the lithium ions in wide electrochemical window, exhibit pure carbonate-based solvents has not been electrochemical, mechanical and thermal definitely resolved7-18. While the generally stability, be environmental friendly and inert to accepted picture comprises a tetrahedral other cell components such as cell separators, coordination of the carbonyl oxygen atoms electrode substrates, and cell packaging around Li+, some experimental and theoretical materials. studies propose the existence of local structures exhibiting slightly higher As mentioned above, the transport of Li+ ions coordination numbers. The dependence of this inside the electrolyte determines the rate of the local coordination number on the ion energy transfer, which has been stored on the concentration is also somehow controversial. electrodes4. According to the literature the However, it should be emphasized that transport of Li+ ions is controlled by a two-step designing experimental methods or theoretical mechanism involving the solvation of the ions models to analyze the experimental data in by the solvent molecules, followed by the order to provide a direct measurement of the migration of the solvated ions5. A deeper coordination number is an extremely understanding of the solvation of Li+ ions may complicated task7,19,20. On the other hand, since therefore act as a springboard towards the the validation of molecular simulation results rational design of novel electrolytes with strongly depends on the direct comparison improved Li+ conductivity. with experimental data, the development of By now, the most commonly employed experimental methods proving a direct strategy towards the rational design of determination of the coordination number electrolytes with optimal properties for battery becomes indispensable in order to obtain a clear picture about the local structural effects carried out for each structure ensuring the in liquid solvents. absence of imaginary modes, and confirming each structure as a minimum on the potential On the basis of the above considerations the energy surface. Zero-point energy (ZPE) aim of the present study is not to give a final corrections have been also taken into account. answer to this particular problem. The main Thermodynamic quantities such as the entropy, purpose is to provide some general insight enthalpy and free energy of the investigated concerning the differences in the solvation clusters have been estimated at the temperature mechanisms of Li+ in pure and mixed binary of 298.15 K. and ternary carbonate-based solvents, by employing molecular dynamics simulations 2.2 Molecular Dynamics Simulations and quantum chemical calculations. Also, In the present study the solvation structure of recent advances in the investigation of the Li+ at infinite dilution in pure and mixed local solvation structure by means of both carbonate-based solvents has been investigated experimental and theoretical techniques will be via classical molecular dynamics (MD) discussed and compared with the findings of simulations. The selected pure solvents were the present study. Such a discussion might be ethylene carbonate (EC), propylene carbonate used as a springboard towards a better (PC) and dimethyl carbonate (DMC). The understanding of the solvation phenomena in binary EC-DMC mixture with a molar these electrolytes, significantly improving the composition 1 EC : 1 DMC and the ternary rational design of electrolytes for battery EC-DMC-PC one with 1 EC : 1 DMC : 3 PC applications. molar composition were also studied. 2. Computational Methods and Details Inside each simulation cubic box, one lithium 2.1 Density Functional quantum chemical cation was placed among 215 solvent calculations molecules in the case of the pure solvents and the ternary mixture, whereas in the case of the Quantum chemical calculations for several binary mixture it was placed among 214 clusters of pure, binary and ternary electrolytes solvent molecules. Trial runs with larger including a lithium cation have been performed system sizes have verified that the employed using the ADF software21. The Density number of molecules are sufficient in Functional Theory (DFT) has been employed describing the solvation structure of Li+ , for the optimization of structures, using the avoiding in this way any possible artifacts PBE GGA22 functional and a TZP (core double arising from finite size effects. The initial zeta, valence triple zeta, polarized) slater type configurations of the simulated systems were orbital (STO) basis set. Our calculations for prepared by using the Packmol software26. some representative clusters have revealed that the basis set superposition error (BSSE) The force fields employed in the simulations corrections23 are negligibly small. A similar were adopted from previous studies27,28. The observation has also been pointed out in recent intermolecular interactions in these models are DFT studies24,25. Frequency analysis