Atomic Resolution Imaging of the Cathode-Electrolyte Interface on a LiMn2O4 Electrode

Abstract

Understanding the nature and composition of the cathode-electrolyte interface (CEI) layer in lithium-ion cells is a key element for the development of safer and longer lasting batteries. The CEI layer is a passivation layer formed, after electrolyte decomposition, on the surface of a positive electrode in lithium-ion batteries. Analogous to the solid electrolyte interface (SEI) on the anode, it is usually thinner (~5 nm vs 10-100 nm) and composed of poor ion-conductors that cause an additional resistance to the Li+ ions path [1]–[3]. Different surface sensitive techniques have been previously used to study the composition of the CEI layer and a model for its formation has been proposed [4]. However, its structure and composition are not fully understood yet and experimental techniques with sub-nanometer resolution are needed for an accurate characterization of this layer. Atom probe tomography (APT) is probably the best candidate, being a powerful technique for three-dimensional analysis of sub-nanometer structures and compositional variations that can achieve atomic resolution, along with chemical sensitivity for Li [5], [6]. In this work we successfully investigated for the first time the CEI layer formation on the surface of a LiMn2O4 (LMO) electrode using the APT technique. A commercial LMO cathode (from MTI) was calendared for ten days in a cell using standard 1M LiPF6 electrolyte in EC:DMC, and lithium metal as a counter and reference electrode. Three-electrode impedance measurements were performed at the OCV with regular intervals to observe and resolve the growth of the CEI layer on the cathode from the growth of the SEI layer on the lithium metal anode. After disassembling the cell, we prepared the sample for APT analysis and determined with atomic resolution the composition and structure of the CEI. A mixed layered- and mosaic-structure was observed, consisting of two inner Mn-rich layers adsorbed on the LMO surface and an external mosaic structure, mainly composed by LiF, Li2O and Li-carboxylates.