A great improvement in valence change memory performance has been recently achieved by adding another metallic layer to the simple metal-insulator-metal (MIM) structure. This metal layer is often referred to as oxygen exchange layer (OEL) and is introduced between one of the electrodes and the oxide. The OEL is believed to induce a distributed reservoir of defects at the metal-insulator interface thus providing an unlimited availability of building blocks for the conductive filament (CF). However, its role remains elusive and controversial owing to the difficulties to probe the interface between the OEL and the CF. Here, using Scalpel SPM we probe multiple functions of the OEL which have not yet been directly measured, for two popular VCMs material systems: Hf/HfO and Ta/TaO. We locate and characterize in three-dimensions the volume containing the oxygen exchange layer and the CF with nanometer lateral resolution. We demonstrate that the OEL induces a thermodynamic barrier for the CF and estimate the minimum thickness of the OEL/oxide interface to guarantee the proper switching operations is ca. 3 nm. Our experimental observations are combined to first-principles thermodynamics and defect kinetics to elucidate the role of the OEL for device optimization.

The continuous demand for improved performance in energy storage is driving the evolution of Li-ion battery technology toward emerging battery architectures such as 3D all-solid-state microbatteries (ASB). Being based on solid-state ionic processes in thin films, these new energy storage devices require adequate materials analysis techniques to study ionic and electronic phenomena. This is key to facilitate their commercial introduction. For example, in the case of cathode materials, structural, electrical and chemical information must be probed at the nanoscale and in the same area, to identify the ionic processes occurring inside each individual layer and understand the impact on the entire battery cell. In this work, we pursue this objective by using two well established nanoscale analysis techniques namely conductive atomic force microscopy (C-AFM) and secondary ion mass spectrometry (SIMS). We present a platform to study Li-ion composites with nanometer resolution that allows one to sense a multitude of key characteristics including structural, electrical and chemical information. First, we demonstrate the capability of a biased AFM tip to perform field-induced ionic migration in thin (cathode) films and its diagnosis through the observation of the local resistance change. The latter is ascribed to the internal rearrangement of Li-ions under the effect of a strong and localized electric field. Second, the combination of C-AFM and SIMS is used to correlate electrical conductivity and local chemistry in different cathodes for application in ASB. Finally, a promising starting point towards quantitative electrochemical information starting from C-AFM is indicated.

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, LixMn2O4 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.