An important part of the effort to improve the performance of Li-ion batteries has been the development of in situ characterization methods designed to visualize the structural evolution of the active materials during battery operation and to directly correlate these measurements with electrical characteristics. A variety of optical, spectroscopic, and structural measurement techniques have been used for in situ characterization of Li-ion batteries, including optical microscopy, laser beam deflection, X-ray and neutron diffraction, Raman spectroscopy, X-ray absorption spectroscopy, scanning electron microscopy, nuclear magnetic resonance, and scanning probe microscopy. Although these techniques provide important insights into the inner workings of Li-ion batteries, they either lack the nanoscale spatial resolution commensurate with the morphology of the active battery materials and/or provide only limited information regarding the complex processes occurring during electrochemical cycling. This project uses a transmission electron microscope (TEM) with energy dispersive x-ray (EDX) spectroscopy and electron energy loss spectroscopy (EELS) that has the micrometer to atomic scale resolution combined with the chemical sensitivity needed to image the morphological and chemical changes taking place during lithiation and delithiation inside a battery.
We are fabricating and characterizing nanoscale Li-ion batteries, with overall dimensions of ≈1 µm indiameter and ≈7 µm in length — small enough to be imaged directly with a TEM without the need for thinning or ion milling. The nanobatteries have nanowire core-shell geometry with a core Ti/Pt/Ti current collector, a LiCoO2 cathode, LiPON electrolyte, and an amorphous Si shell anode. We are electrochemically testing individual nanowire Li-ion batteries (NWLIBs) inside a SEM and a TEM to determine the capacity and stability, and to observe real time changes in the battery microstructure during cycling. Noticeable changes in the microstructure, including void formation at the LiCoO2/LiPON interface, occur after the first charge/discharge cycle, and become more prominent as the cycling is continued.
We are also building an integrated characterization system that combines the ability to examine electrochemical properties within an inert atmosphere with surface and bulk spectroscopic and charge transport measurements performed in ultra-high vacuum. This system will be used to develop and characterize a range of electrochemical energy conversion and storage devices, including batteries, fuel cell electrocatalysts, electrochemical capacitors, and photoelectrochemical cells for hydrolysis. These measurements will provide the critical knowledge needed to improve the efficiency and performance of such devices.