In Situ Scanning Probe Microscopy of Li-Ion Battery Cathode Materials

Designing the next generation of high performance energy storage devices requires a deeper understanding of structural degradation. To this end, a wide variety of scanning probe microscopy techniques can help characterize charge transport and aging mechanisms in Li-ion battery materials and their interfaces. For example, we use in-situ probe microscopy and Kelvin probe force microscopy (KPFM) to investigate the mechanical and potentiostatic changes in MnO 2 cathodes as they undergo charge cycling. Topography monitors mechanical expansion during charging, and the dependence of expansion on scan rate reveals the separate roles of Mn 3+/4+ redox pseudocapacitance and faster, double-layer surface capacitance. On the same electrodes, KPFM observes the evolution of surface potentials with lateral resolution as low as 40 nm. At this length scale, inhomogeneous lithiation is clearly observed as a highly nonuniform, fractal growth or shrinkage of Mn 3+ and Mn 4+ phases. Via surface potential, KPFM also reveals "dead" zones that do not participate in charging, "hot" zones that charge or discharge most readily, and the evolution of each type as an electrode is repeatedly cycled. This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.