Cathodes for Li-Ion Batteries*

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 batteries and supercapacitors. We use in-situ probe microscopy and Kelvin probe force microscopy (KPFM) to investigate the mechanical and potentiostatic changes in MnO₂ 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+ pseudocapacitance and faster surface double-layer capacitance. KPFM observes the evolution of surface potentials with lateral resolution as low as 40 nm, at which scale inhomogeneous lithiation is observed as a highly nonuniform, fractal growth or shrinkage of Mn³⁺ and Mn⁴⁺ phases. Via surface potential, KPFM reveals “dead” zones that do not participate in charging, the “hot” zones that charge or discharge most readily, and the evolution of each type as an electrode is repeatedly cycled.