The reversible charge coupled insertion of active species such as Li ions into and out of an electrode is the central functional process in a battery. The atomic scale phenomena required for insertion, such as diffusion and interface reactions, determine to a large extent the performance and reliability of the battery. For example, the diffusion paths of the active species within the electrode, and the reaction steps that determine the rate of incorporation of the active species, all impact crucial battery performance parameters. Advanced experimental methods have recently evolved far enough to allow atomic scale investigation of these processes. This presentation will focus on the use of two powerful and complementary experimental methods to investigate atomic scale processes underlying Li ion insertion: in-situ transmission electron microscopy (TEM) and atom probe tomography (APT). The goal of such studies is to determine the atomic scale mechanisms controlling the various metrics of the battery (fade, capacity, (dis)charging rates, initial capacity loss) and to use this information to optimize their performance or to design new battery electrode materials.
We use in-situ TEM to track structural and electronic changes during electrochemical lithiation of LiMn2O4. A clear and sharp interface is observed to move through the lamella during lithiation, converting the original single crystal cubic spinel LiMn2O4 into a twinned tetragonal Li2Mn2O4 structure. EELS confirms the expected decrease in Mn valence and increase in Li content as a result of lithiation, however the exact chemical structure of the interface between the cubic and tetragonal phases and between the twin variants has not yet been resolved. Furthermore, the reason for the formation of the twin structure is not known. Therefore we turn to laser-assisted APT to determine the three dimensional position and chemical identity of individual atoms, making it the ideal tool to complement the dynamic studies in the TEM. The APT studies reveal defects and heterogeneous Li distributions in the starting and (de)lithiated materials. In addition, in-situ delithiation performed directly in the atom probe system shows that Li is intermittently and heterogeneously removed from the single crystalline tip. All in all, we attribute the complex observed kinetics and microstructures to the dominant role of defects as well as to strong coupling between Li composition and diffusion.