Chemical and Structural Mapping of Cathodes for Li-Ion Batteries From the Nano- to the Electrode-Scale

2019-08-01T00:00:00Z (GMT) by Mark F Wolfman
High-performance rechargeable batteries will play a key role in supplying our future energy needs. Electric vehicles, grid storage and portable devices place stringent demands on battery performance that cannot be fully satisfied with today's technologies. Bridging this gap requires an understanding of the chemistry occurring within a battery at multiple length-scales. The coupling of chemically sensitive spectroscopy and diffraction techniques with the spatial resolution provided by X-ray microscopy reveals heterogeneities at length-scales ranging from millimeters to nanometers, where kinetic limitations at one scale dictate the behavior at larger scales. Understanding the reaction inefficiencies underlying these heterogeneities is a key step in achieving next-generation battery performance. High-resolution ptychographic microscopy was used to probe the distribution of oxidation states within individual cathode particles, revealing incomplete oxidation unevenly distributed within particles, resulting in part from irreversible secondary reaction pathways. Reduced nickel states were found to dominate the outer atomic layer, even when the cell was subjected to highly oxidizing potentials. Full-field microscopy of secondary particles was conducted under applied current ("operando"), enabling observations that were not subject to relaxation effects but instead governed by the kinetic limitations present during real-world operation. Discrepancies between ensemble-average and particle-level measurements were found, demonstrating a high level of thermodynamic irreversibility in local redox reactions. Furthermore, a correlation was found between particle micro-structure and anomalous nickel reduction, clarifying the relationship between particle micro-structure and redox chemistry. A novel structural mapping technique was developed and used to measure heterogeneity within complete high energy density electrodes. The resulting maps reveal transport limitations through the solid matrix of the electrode and provide a comparison between solid and liquid diffusion. The multi-length-scale approach presented here provides a more detailed view of the underlying battery chemistry than is possible by conventional ensemble-averaged methods alone, and will provide a foundation for more targeted engineering solutions to performance limitations in Li-ion battery cathodes.