State-of-Charge Mapping with Spatially Resolved XPS in Patterned Thin Film Battery Electrodes

Nanostructures offer the opportunity to design-in higher power capability and cycling robustness, but their spatial distribution of active ion storage material can pose major challenges in transporting electrons to all regions of active material.  For example, high aspect ratio nanowire or nanotube electrodes on a planar current collector may require electrons to diffuse 10’s – 100’s μm though highly resistive oxide material before meeting an incoming lithium ion. While adding a conformal or coaxial current collector can improve performance at high rates, the characterization of this improvement must typically be inferred though bulk electrochemical measurements.

In this work, we develop a model system with which to directly measure the spatial distribution of state-of-charge (SOC) for a poorly conductive cathode material as a function of distance from a current collector and discharge rate using spatially resolved x-ray photoelectron spectroscopy (XPS). Our system includes a geometrically restricted electrode/current collector interface but a relatively unrestricted electrode/electrolyte interface. We fabricate electrodes with patterned planar current collectors using standard microfabrication techniques and deposit pinhole-free ultrathin films of the cathode material V₂O₅ using atomic layer deposition. These cathodes are then discharged in a liquid electrolyte under different rates and conditions, and the resulting SOC distribution is mapped using XPS with a lateral resolution as good as 15µm. Unlike microspot Raman or XRD, XPS provides a direct quantitative measurement of the SOC through the concentration of lithium ions and/or reduced vanadium ions (Figure 1). The latter is particularly appealing in that it is independent of surface contamination, providing an avenue towards SOC characterization even in the presence of a thin SEI layer. We also explore the depth dependence of the SOC using angle resolved XPS and ion beam depth profiling. Our measurements demonstrate that the electrode SOC decreases over a distance of order millimeters from the current collector at moderate rates, and that the shape and magnitude of the SOC gradient depends on the discharge rate. Finally, we compare our observations with simulations of the structure using COMSOL Multiphysics, and attempt to resolve discrepancies between the two. We believe this approach can provide design guidance for heterogeneous nanostructures applied to electrical energy storage.  We anticipate this technique to be broadly applicable to other electrode materials and active ions.