Using molecular dynamics to quantify the electrical double layer and examine the potential for its direct observation in the in-situ TEM

Abstract

Understanding the fundamental processes taking place at the electrode-electrolyte interface in batteries will play a key role in the development of next generation energy storage technologies. One of the most fundamental aspects of the electrode-electrolyte interface is the electrical double layer (EDL). Given the recent development of high spatial resolution in-situ electrochemical fluid cells for scanning transmission electron microscopy (STEM), there now exists the possibility that we can directly observe the formation and dynamics of the EDL. In this paper we predict electrolyte structure within the EDL using classical models and atomistic Molecular Dynamics (MD) simulations. Classical models are found to greatly differ from MD in predicted concentration profiles. It is thus suggested that MD must be used in order to accurately predict STEM images of the electrode-electrolyte interface. Using MD and image simulation together for a high contrast electrolyte (the high atomic number CsCl electrolyte), it is determined that, for a smooth interface, concentration profiles within the EDL should be visible experimentally. When normal experimental parameters such as rough interfaces and low-Z electrolytes (like those used in Li-ion batteries) are considered, observation of the EDL appears to be more difficult.

Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species

“…Mechanistic understanding of molecular-level processes taking place at nanostructured electrode–electrolyte interfaces (EEI) is important to the development of clean and sustainable electrochemical technologies for applications in energy generation and storage as well as water and chemical separation and purification. − However, the complexity of operating EEI makes it difficult to obtain an accurate molecular-level description of charge-transfer kinetics, which affect the observed reduction and oxidation potentials. One of the major challenges is understanding the effect of different charge- and ion-transfer pathways (diffusion, migration, and convection) and ionic interactions on the activity and stability of the EEI. , Several in situ and operando electrochemical and physical characterization approaches combined with theoretical modeling have been used to study molecular mechanisms at operating EEIs. , For example, photon- and neutron-based scattering, nuclear magnetic resonance, infrared spectroscopy, and electron microscopy-based approaches − have been developed to study EEIs with a particular emphasis on investigating processes that are detrimental to the performance of EEI in electrochemical devices.…”

Nanoscale Electrochemistry Revisited

“…Using the ferro/ferricyanide redox couple, the authors demonstrated quantitative cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy measurements, thus establishing a foundation for future work concerned with simultaneous quantitative electrochemical measurements and TEM imaging. In the other recent study, the authors used molecular dynamics and image simulations to investigate the possibility of directly imaging the electrical double layer using in situ electrochemical TEM . The authors concluded that, for a high-contrast electrolyte such as CsCl and a smooth electrode–electrolyte interface, ion concentration profiles within the double layer should be visible to TEM.…”

“…In the other recent study, the authors used molecular dynamics and image simulations to investigate the possibility of directly imaging the electrical double layer using in situ electrochemical TEM. 214 The authors concluded that, for a high-contrast electrolyte such as CsCl and a smooth electrode−electrolyte interface, ion concentration profiles within the double layer should be visible to TEM. This is no doubt an intriguing result that highlights the potential of successfully applying the subnanoscale spatial resolution of TEM to dynamic electrochemical processes.…”

Micro-machining: An overview (Part II)

“…151 Electrical double layer (EDL) was modelled for electrochemical analysis of 'scanning transmission electron microscopy' by employing MDS. 152 In case of a high-contrast electrolyte like CSCl, the concentration profiles within the EDL were visible. On the other hand, the formation of the EDL seems to be invisible with low-contrast electrolytes like Li-ion.…”