Identification of Sodiation Mechanisms in Graphite-base Negative Electrodes by Molecular Dynamics Simulations Combined with Potential Mean Force
Tuanan C. LOURENÇO1, Leonardo J. A. SIQUEIRA2, Luís Gustavo DIAS3, Juarez L. F. DA SILVA1
1São Carlos Institute of Chemistry, University of São Paulo, São Carlos, Brazil
2Department of Chemistry, Environmental, Chemical and Pharmaceutical Sciences Institute, Federal University of São Paulo, Diadema, Brazil
3Chemistry Department, FFCLRP, University of São Paulo, Riberão Preto, Brazil
One of the key steps in the application of renewable energy sources is the development of large scale energy storage devices, which are necessary due to the intermittent character of these sources. In this context, sodium-ion batteries (SIBs) are good candidates due to the large Na abundance, its relative low costs, and its chemical similarities with lithium. However, one of the main drawbacks of the SIBs is the poor intercalation rate of Na+ into the carbonbased electrodes, cathodes and anodes. One alternative to overcome this limitation is the usage of carbon-based materials with interlayer distances larger than the graphite distances, such as hard carbon and expanded graphite. The large interlayer distances decrease the repulsion between the Na+ and the graphite, and consequently improves the sodiation. Other approach widely investigated is the cointercalation, in which the Na+ is intercalated together with its solvation shell. To improve our understanding about the sodiation mechanism in carbon-based negative electrodes, we carried out classical molecular dynamics (MD) simulations with potential of mean force (PMF) calculations to obtain the sodiation energy barriers into carbon electrodes with different electrode morphology and electrolyte compositions. The electrode models were built using the stacking of eight flexible graphene layers with fixed atom charges, considering two different initial interlayer distances (L) and charges (Q), which resulted in four electrode structures: L0.405Q−13.00, L0.405Q−28.00, L0.445Q−13.20, L0.445Q−28.00. For the electrolyte, two systems were investigated; the binary mixture of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pyr14][Tf2N]) with 0.20 mole fraction of sodium bis(trifluoromethylsulfonyl)imide (NaTf2N), and ternary mixture considering the addition of 20% of triglyme molecules (G3) in the previous system. All simulations were carried out using the GROMACS package with the CLAP and OPLS force fields for the IL and triglyme molecules, respectively. Since the graphene layers are able to move in the y-axis, the electrolyte intercalation leads to the expansion of the intercalated pore and the compression of the others until all non-intercalated pores have interlayer distances close to the graphite. [Pyr14]+ have better intercalation rate than the Na+, which can be controlled by the interlayer distances, i.e. small distances decrease the [Pyr14]+ population within the pore. At the same time, the increase in the negative electrode charge increases the cations intercalation rate; however, the interlayer distances still the limiting factor. The addition of G3 molecules improves the sodiation due to the cointercalation mechanism in the system. However, looking deeply to simulations, we see that the interlayer distances also affects the cointercalation mechanism. For the larger values, the Na+ is intercalated within its solvation shell from the bulk, while in electrodes with the smaller interlayer distances, there is an exchange in the Na+ solvation shell, in which the ion leaves it solvation shell in interfacial region and acquires a new one inside the electrode. Therefore, the control of interlayer distance is necessary not only to improve the sodiation rate but also to control the selectivity of the ions intercalation and consequently reaches high battery performances.