Abstract
Surface and interface play critical roles in energy storage devices, calling for operando characterization techniques to probe the electrified surfaces/interfaces. In this work, surface science methodology including electron spectroscopy and scanning probe microscopy has been successfully applied to visualize electrochemical processes at operating electrode surfaces in an Al/graphite model battery. Intercalation of anions together with cations is directly observed in surface region of the graphite electrode with tens of nanometers thickness, whose concentration is amazingly one order higher than that in bulk. An intercalation pseudocapacitance mechanism and a double specific capacity in the electrode surface region are expected based on the super-dense intercalants and anion/cation co-intercalation, which are in sharp contrast with the battery-like mechanism in the electrode bulk. The distinct electrochemical mechanism at electrode surface is well verified by performance tests of real battery devices, showing that surface-dominant nanometer thick graphite cathode outperforms bulk-dominant micrometer thick graphite cathode. Our findings highlight the important surface effect of working electrodes in charge storage systems.
Operando surface science methodology reveals surface effect in charge storage electrodes