Effects of functional groups and anion size on the charging mechanisms in layered electrode materials

Highlights

• We have conducted molecular simulations on 20 model supercapacitors systems based on graphene or MXene electrodes, with different functional groups, and 5 neat ionic liquids, with different anions.

• When charging negatively, the electrodes expand and no major dependence on the anion type is observed. When charging positively, the volume changes are more complex.

• Volume changes are generally very well correlated with the quantities of adsorbed ions, except for the largest anions for which ion reorientations affect the interlayer spacing.

• We demonstrate that the charging mechanisms are largely correlated with the anion sizes and the surface charge of the electrode material in the uncharged state.

Abstract

We report on an extensive molecular simulations study about the influence of the nature of functional groups and anion size on the charging mechanisms and volume expansion/contraction in layered materials used as electrodes for energy storage applications. The study of the electrochemical behavior of graphene and Ti₃C₂ MXene (with three different functional groups) in five ionic liquids shows that the electrodes expand when charged negatively and no major dependence on the anion type is observed. When the electrode is positively charged, the volume changes are more complex and no specific trend could be observed depending on the anion size. The volume changes are in most cases, very well explained by the quantities of adsorbed ions. In specific cases, e.g., for FSI⁻ and TFSI⁻ anions, the reorientation of the ions can also affect the interlayer volume. We demonstrate that the charging mechanisms are changing consistently with the anion sizes and are largely correlated with the surface charge of the electrode material in the neutral state.

Introduction

Electrical energy storage (EES) has attracted huge attention from the scientific community in the past decades due to its widespread usage and irreplaceable function in our daily life [1]. Batteries have played a dominant role for years due to their high performance and moderate price. However, supercapacitors, also known as electrical double-layer capacitors, have played an increasingly important role in the EES market and research field. Supercapacitors show their superiority in fast energy delivery or harvesting but suffer from moderate energy densities [2]. It is therefore essential to seek ways to increase their energy density without compromising their power density.

Like batteries, the electrochemical performance of a supercapacitor device is mainly determined by its active components, i.e., the electrodes and electrolyte. On the electrode side, recent studies have demonstrated that transition metal-based, low-dimensional systems, could be promising excellent electrode materials for EES, due to their superior electronic properties, multiple oxidation states, spin states, etc [3], [4], [5], [6]. Two-dimensional transition metal carbides and nitrides, also called MXenes, are one of the typical representatives of these transition metal-based low dimensional systems [7], [8]. Due to the relatively weak attractive interaction between MXene layers and the existence of hydrophilic groups on the layer surface inherent to the synthesis process, the electrolyte ions can spontaneously intercalate into MXenes, which significantly enhances the electrochemical performance, leading to high gravimetric and volumetric capacitances along with remarkable cyclability [9], [10], [11], [12]. Compared to some other conventional pseudocapacitive materials, including transition metal oxides, conductive polymer, and redox-active organic molecules, MXene electrodes not only exhibit a promising capacitance but also have much better cyclability [13].

Selecting an appropriate electrode/electrolyte system is critical to achieving a high energy density EES device. Extensive experimental works have focused on understanding the charging mechanisms for MXenes combined with various electrolytes. In neat ionic liquids or neutral and alkaline aqueous solutions, MXenes mainly exhibit double layer capacitance resulting from electrostatic adsorption. In aqueous H₂SO₄, MXenes show a pseudocapacitive behavior, due to the fast surface reversible redox reactions of titanium [11], [14], [15]. In lithium or sodium-based organic electrolytes, MXenes show intercalation pseudocapacitance [10], [16]. Wang et al. observed a drastic influence of the solvent on the pseudocapacitive charge storage of the Ti₃C₂ MXene material [17]. With lithium-based electrolytes and Ti₃C₂, one can obtain twice larger charge storage when using carbonate solvents compared with nitrile- or sulfoxide-based solvents. Overall, the chemical nature of the electrolyte ions and the solvent has an important effect on the molecules/ions arrangement in MXenes, which is in direct correlation with the total charge storage ability.

Understanding the charge storage mechanisms and designing the right combination of electrode and electrolyte is therefore essential and meaningful. However, due to the large number of possible MXene/electrolyte associations, and the technical difficulty, as well as the cost, of synthesizing some MXenes, a systematic experimental exploration of the charging mechanism and capacitive performance of the MXene family would be very costly and time-consuming [18]. Nowadays, among different computational methods, molecular dynamics (MD) simulations are playing a pivotal role in investigating mechanisms in-depth and designing supercapacitors with improved capacitive performance in a more effective way [19], [20], [21].

MD simulations can provide detailed and realistic ionic diffusion coefficients, intercalation and electrode structural information of such 2D layered materials and systems. Muckley et al. reported the use of MD simulations for exploring the driving force inducing the volumetric changes during hydration and dehydration cycles [22]. Osti et al. demonstrated that the presence of K⁺ ions could reduce the self-diffusion of water molecules and enhance the stability of hydrated MXene, in agreement with their experimental results [23]. The dynamical structural response of MXenes to the ion intercalation during the electrochemical process is crucial, Berdiyorov et al. observed an increase and a decrease of the MXene interlayer distance when intercalating anions and cations, respectively [24]. Xu et al. employed a multi-layer MXene / ionic liquid simulation system with flexible interlayer spacing to obtain an atomic scale image of the molecular changes occurring during charge and discharge [25]. These simulations reproduced the trend observed in electrochemical and X-ray diffraction experiments [26], [27] and allowed them to propose mechanisms for the volume expansion, at negative polarization, and contraction, at positive polarization.

In this work, we conduct molecular dynamics simulations of a range of model supercapacitors based on layered electrode materials and neat ionic liquid electrolytes. On the electrode side, graphene and Ti₃C₂ MXene with three functional groups (Ti₃C₂F₂, Ti₃C₂O₂, Ti₃C₂(OH)₂) are selected for investigating the effect of the nature of the electrode material and functional groups on charge storage mechanism. On the electrolyte side, we vary the anion type and analyze the changes incurred on the relative volume expansion. The results are then interpreted through a detailed description of the charging mechanisms, ion reorientations, and free energies.