Potential Dependent Ion Arrangement Near the Electrode/Electrolyte Interface

Abstract

The electric double layer (EDL) is the ion arrangement evolving at electrolyte interfaces. Despite its importance in many applications such as super-capacitors, lithium ion batteries (LIB), water purification, and stabilization of nanoparticles in solution, the structural motifs of the EDL are still under debate.In LIB, the molecular scale ion arrangement near the interface or EDL controls solvation and hence mass/charge transfer across the interface. Additionally, the interfacial speciation governs the reductive and oxidative processes at the interface. Thus, it has a great impact on the stability of the electrolyte. Hence, the near-interface structure plays a vital role in the solid electrolyte interphase (SEI) formation which is still poorly understood. The SEI protects the electrolyte from further decomposition and its stability, low electrical conductance, and good Li-ion transport properties are crucial for the cycling ability and long-term stability of the cell. Therefore, an atomic level elucidation of the electrolyte/electrode interface and its rearrangement dynamics due to potential changes is of utmost importance for understanding the behaviors of LIBs, and will ultimately lead to the minimization of ion transport resistance and improved long-term stability.We present an in-situ X-ray reflectivity (XR) study on the single crystalline boron-doped diamond – electrolyte interface. The metal-like electric conductivity and wide electrochemical window of the single crystalline boron-doped diamond allow us to apply a potential across the interface. We obtain structural information by modeling the potential dependent EDL to reproduce our experimental data. First proof-of-concept results on 1m CsCl(aq) show a clear trend of the XR profile on the applied potential of -1.2V – 1.2V vs AgCl. The length scale of π/qz = 4Å of the observed minima at qz = 0.75 Å−1 corresponds well to the expected length-scale of the inner-Helmholtz layer of the EDL. Our analysis suggests that the population of the inner-Helmholtz layer decreases with increasing potential.Our efforts allow to challenge the current theoretical models and simulations and will ultimately help to understand the mechanisms involved in the SEI formation and lead to the minimization of ion transport resistance as well as to an improvement of the LIB cycling ability. Figure 1