Oxide conversion reactions for batteries
Researchers using the XSD 33-BM-C beamline at the APS are exploring oxide conversion reactions as an alternative approach for high capacity lithium ion batteries.
Intercalation materials have long been the foundation for commercial lithium ion batteries (LIBs) but are intrinsically limited in charge capacity by the number of Li sites necessary to preserve the electrodes’ crystal structure during cycling. Conversion reactions of MX compounds such as the well known phase separation of a metal oxide into Li2O and reduced metal species provide an alternative storage mechanism that is not limited by the electrode crystal structure.
Oxide conversion reactions are an alternative approach for high capacity lithium ion batteries, but are known to suffer from structural irreversibility associated with the phase separation and reconstitution of reduced metal species and Li2O. In particular, the morphology of the reduced metal species is thought to play a critical role in the electrochemical properties of a conversion material.
Here we use a model electrode with alternating layers of chromium and chromium oxide to better understand and control these phase changes in real-time and at molecular length scales. Despite lacking crystallinity at the atomic scale, this superstructure is observed (with x-ray reflectivity, XR) to lithiate and delithiate in a purely one-dimensional manner, preserving the layered structure.
The XR data show that the metal layers act as nucleation sites for the reduction of chromium in the conversion reaction. Irreversibility during delithiation is due to the formation of a ternary phase, LiCrO2, which can be further delithiated at higher potentials. The results reveal that the combination of confining lithiation to nanoscale sheets of Li2O and the availability of reaction sites in the metal layers in the layered structure is a strategy for improving the reversibility and mass transport properties that can be used in a wide range of conversion materials.
Timothy T. Fister, Xianyi Hu, Jennifer Esbenshade, Xiao Chen, Jinsong Wu, Vinayak Dravid, Michael Bedzyk, Brandon Long, Andrew A. Gewirth, Bing Shi, Christian M. Schlepuetz and Paul Fenter, “Dimensionally-controlled Lithiation of Chromium Oxide,” Chemistry of Materials, Just Accepted Manuscript, DOI: 10.1021/acs.chemmater.5b01809, Published Online November 23, 2015.