Direct imaging and manipulation of ionic diffusion in mixed electronic–ionic conductors

Abstract

Next generation Li-ion batteries require improved energy densities, power output and safety to satisfy the demands of emerging technologies. All solid state 3D thin-film batteries (ASB) based on nanoionics are considered as frontrunners to enable all this. In order to facilitate the introduction of this new architecture, a homogeneous electrochemical activity and a high ionic diffusivity of the electrodes is key. However, nanometer-resolved techniques to probe structural, electrical and electrochemical properties of the battery components are still limited. Here we propose a study that combines conductive atomic force microscopy (C-AFM) and secondary ion mass spectrometry (SIMS) for structural and electrical characterization. In addition, a novel concept called ion-modulated C-AFM (imC-AFM) is introduced to also sense the electrochemical activity of ions in confined volumes. Using the aforementioned methodologies, Li_xMn₂O₄ thin film cathodes are studied observing: (1) a direct correlation between electrical conductivity and local chemistry. (2) A non-uniform Li-ion electrochemical activity (i.e. ionic conductivity) on the cathode's surface with a clear enhancement in grain boundaries (GBs). Finally, (3) imC-AFM observes a high volume expansion associated with high Li incorporation. This work introduces a novel pathway for the rapid analysis of materials to be used in ASB.