Nanoscale characterization of ion mobility by temperature controlled Li-nanoparticle growth.

Detailed understanding of electrochemical transport processes on the nanoscale is considered not only as a topic of fundamental scienctific interest but also as a key to optimize material systems for application in electrochemical energy storage. A prominent example are solid state electrolytes, where transport properties are strongly influenced by the microscopic structure of grain boundaries or interface regimes. However, direct characterization of ionic transport processes on the nanoscale remains a challenge. For a heterogeneous Li$^+$-conducting glass ceramic we demonstrate quantitative nanoscopic probing of electrochemical properties on the basis of temperature controlled growth of nanoscopic Li-particles with conductive tip atomic force microscopy. The characteristic energy barriers can be derived from of the particle growth dynamics and are consistent with simultaneously recorded nano-voltammetry, that can be interpreted as an interplay between overpotentials, ion-conductivity and nanoscale spreading resistence. In the low temperature limit at around 170\,K, where the particle growth speed is slowed down by several orders of magnitude with respect to room temperature, we demonstrate ion-conductivity mapping with lateral resolutions only limited by the effective tip-surface contact radius. Our mapping measurements reveal the insulating character of the AlPO$_4$-phase, while any influence of grain boundaries is related to subsurface constrictions of the current paths.