Visualization Of The Phase Propagation Within Carbon -Free Li4ti5o12 Battery Electrodes

The electrochemical reactions occurring in batteries involve the transport of ions and electrons among the electrodes, the electrolyte, and the current collector. In Li-ion battery electrodes, this dual functionality is attained with porous composite electrode structures that contain electronically conductive additives. Recently, the ability to extensively cycle composite electrodes of Li4Ti5O12 without any conductive additives generated questions about how these structures operate, the answers to which could be used to design architectures with other materials that reduce the amount of additives that do not directly store energy. Here, the changes occurring in carbon-free Li4Ti5O12 electrodes during lithiation were studied by a combination of ex situ and operando optical microscopy and microbeam X-ray absorption spectroscopy (mu-XAS). The measurements provide visualizations of the percolation of lithiated domains through the thick (similar to 40-mu m) structure after a depth of discharge of only 1%, followed by a second wave of propagation starting with regions in closest contact with the current collector and progressing toward regions in contact with the bulk electrode. These results emphasize the interplay between the electronic and ionic conductivities of the phases involved in a battery reaction and the formation of the phases in localized areas in the electrode architecture. They provide new insights that could be used to refine the design of these architectures to minimize transport limitations while maximizing energy density.