Force field of tetrafluoroborate anion for molecular dynamics simulation: a new approach

Abstract

Design of new electrical energy storage devices including supercapacitors as well as an optimization of existing ones require not only new electrolytes, but also the deep and complete understanding of the processes occurring in the electrolyte solutions. Spectral techniques and classical molecular dynamics simulation (MDS) have gained a reputation as a reliable tool for such tasks. The starting point of any MDS is a choice or development of the force fields for all simulated particles. The combination of vibrational spectroscopy and molecular dynamics technique can provide a thorough understanding of the structure and dynamics of the ionic subsystem. In this connection, the reproduction of the vibrational spectra should be added to the requirements for the force fields of the most common electrolyte components. Many modern supercapacitors are based on organic electrolytes consisting of non-aqueous aprotic solvents such as acetonitrile, propylene carbonate and γ-butyrolactone and quaternary ammonium salts with tetrafluoroborate and hexafluorophosphate as anions. The purpose of the current work is to develop a new force field for tetrafluoroborate anion (BF₄^-) able to reproduce not only translational diffusion in acetonitrile medium, but also the spectral properties of this ion in a condensed phase. Since found in the literature force fields of BF₄^-, cannot satisfy these requirements, there were performed intensive quantum chemical calculations of BF₄^- at the M06-2X/6-311++G(d,p) level of theory to construct the potential energy surface with respect to the B-F bonds and F-B-F angles followed by evaluating corresponding intramolecular potential constants. Combining the obtained bond and angle force constants with partial charges on B and F atoms calculated at the same level of theory, and literature values of Lennard-Jones parameters, a new force field model for BF₄^- anion was created. Based on the carried out MD simulations of the BF₄^- ion in an infinitely diluted acetonitrile solution, it was proved that the obtained resulting model is capable to reproduce both transport and intra-ion vibrational properties of the tetrafluoroborate anion.