Revealing the complex layered-mosaic structure of the cathode electrolyte interphase in Li-ion batteries

Electrolyte decomposition on cathode surfaces is an irreversible reaction that creates a passivation layer known as the cathode electrolyte interphase (CEI). Since the CEI can lead to increased internal resistance, accelerated electrode decomposition, and loss of lithium inventory, a deeper comprehension is important for developing improved batteries. Although several models of the CEI have been proposed, the actual structure is unknown. Here we report three-dimensional tomography revealing the atomic-scale interface structure and micron-scale structure of a LiMn₂O₄ electrode. Atom probe tomography is employed to provide critical new information on CEI structure and composition: a complex layered-mosaic architecture is found consisting of inner homogeneous Mn_xO_y and MnF_x layers (~9nm- and ~4nm-thick) and an outer 3nm-thick mosaic structure containing a number of different inorganic and organic compounds. The results are used to develop a realistic quantitative model of the main electrochemical processes. This approach provides a new means to explore electrode systems including the effects of coatings and electrolyte additives.