Developing In situ Microscopies for the Model Cathode/Electrolyte Interface

Multiple processes occurring at the cathode/electrolyte interface such as metal dissolution and oxygen evolution are known to lead to long term cell life issues for lithium ion batteries. However, many questions remain about the precise mechanisms involved in some of these processes. As an example, while manganese (Mn) dissolution is known to occur with a variety of cathode materials, there is still present debate about the oxidation state of Mn that is involved in the dissolution process.[1],[2] Analysis of cells following failure as well as examination of species present in electrolyte samples from cycled cells, can tell us much about these degradation processes. However, these techniques may not capture information about chemical and/or electrochemical processes happening at or very near the cathode electrolyte interface. A need exists to develop diagnostic tools that can examine processes happening in situ immediately at the cathode/electrolyte interface.This project is focused on using in situ probe microscopy techniques to observe and measure processes happening at the cathode/electrolyte interface. We plan to employ three forms of scanning probe microscopes including atomic force microscopy (AFM), scanning electrochemical microscopy (SECM) and a combined system that utilized both techniques. The SECM shown in Figure II.3.F.1 is a scanned probe microscope that can be used to perform electrochemical analysis of species evolving from or being consumed at an electrode surface. Analysis occurs in an active four-electrode electrochemical cell containing an electrolyte solution. The applied potential at both the substrate (model cathode material) and the sensing “tip” electrode is controlled versus the same reference and auxiliary electrode by a bi-potentiostat. The use of a small “tip” electrode (typically consisting of a < 10µm dia. metal wire) allows high speed electrochemical analytical techniques to be employed. This enables examination of species that may evolve from the cathode/electrolyte interface before they undergo subsequent reactions such as electrolyte degradation or SEI formation.