Abstract
The surface of materials is one of the most important aspects of electrochemistry. The surface is where all critical charge transfers and catalytic interactions occur. For Li-ion batteries, the surface of the electrodes dictates the reactivity with the electrolyte, the ability for Li-ions to shuttle between the bulk and the electrolyte, and the rate at which the ions can transfer, all of which has an effect on rate-capability and cyclability. LiMn2O4 (LMO) is a promising cathode material with a high-energy density and a high-rate capability, but it is plagued with a cyclability problem based on a surface effect. In the LMO system the main contributor to cycling degradation is the Mn disproportionation reaction (2Mn3+ = Mn2+ + Mn4+) which creates soluble Mn2+ that is lost to solution. We have used a combination of high-angle annular dark-field (HAADF) aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to confirm the underlying spinel structure, and we have found, in as-processed LMO, a surface structure composed of Mn3O4 and a lithium-rich Li1+xMn2O4 subsurface layer which occurs as a result of surface reconstruction. We have also identified that oxygen loss is the mechanism by which the surface reconstruction occurs, but the question still remains, is the reconstruction of the material native or is it an effect of the electron beam? Oxide materials can be sensitive to electron-beam-induced phase changes so it is important to lower the electron dose low enough so that a determination can be made whether any impurity phases observed are an effect of the material or an effect of the electron beam. This research answers that question by imaging the surface of LMO with low-dose STEM by reducing the electron beam to pseudo-non-destructive levels. This work serves as an example of the importance of low-dose STEM as a critical technique for correctly identifying transformations and reconstructions in oxide or related materials.