Lithium-ion batteries have advanced enough to be a widespread technology, but future applications will demand higher-performing materials. Electrode materials that undergo conversion reactions are a class of high-performance materials that could meet this challenge, as they have the capability of storing larger amounts of energy compared to intercalation-based materials. However, conversion materials suffer from energy inefficiencies during electrochemical cycling due to issues with first-cycle reversibility and charge overpotentials.¹ These inefficiencies are especially hindering in conversion reaction anodes made from 3d transition metal oxides, because the anodes undergo significant structural changes during cycling.²  Further structural characterization is necessary to understand the link between cycling behavior and the energy inefficiencies of  the active material.

In this work, we study structure and composition changes in conversion anodes with the goal of understanding the lithiation kinetics, ionic diffusion, and nucleation and growth phenomena of such materials. Anodes with architectures of alternating Ni and NiO layers are used, as multilayer structures have been shown to control volume expansion and improve cycling lifetimes in related Li-alloy materials.³ However, the Ni/NiO multilayer architecture also enables more direct control over initial anode microstructure by tuning structural parameters, e.g. layer thickness and organization. Analysis of the layers and their interfaces with Scanning Transmission and Transmission Electron Microscopy (S/TEM) techniques and X-Ray reflectivity (XR) measurements help reveal structure-property relationships in the multilayer electrode. In particular, we elucidate the lithiation mechanism in the multilayer system in addition to changes in the layer nanostructure that have implications for understanding the effects of structural transformations on kinetics and overpotentials in the system. Complementary in-situ S/TEM lithiation experiments of model bilayer systems also present further insight into the reaction dynamics of this fascinating system.

            (1)        Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nature 2000, 407, 496.

            (2)        Luo, L.; Wu, J.; Luo, J.; Huang, J.; Dravid, V. P. Sci. Rep. 2014, 4.

            (3)        Fister, T. T.; Esbenshade, J.; Chen, X.; Long, B. R.; Shi, B.; Schlepütz, C. M.; Gewirth, A. A.; Bedzyk, M. J.; Fenter, P. Advanced Energy Materials 2014, 4.

Acknowledgment: This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This work was also supported by the NUANCE Center new initiatives, and made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the NSF MRSEC program (NSF DMR-1121262) at the Materials Research Center, The International Institute for Nanotechnology (IIN); the State of Illinois; and Northwestern University.