Lithium Ion Distribution in Intercalation Electrode/Liquid Electrolyte Interfaces Determined By in-Situ Neutron Reflectometry

Abstract

Gaining a thorough understanding of electrochemical interfaces is critical for the development of next-generation energy storage/conversion devices with high energy density and long-life operation. Direct observation of structural changes provides invaluable information about charge transfer, ionic diffusion, and side reactions in electrochemical reaction processes. Neutron reflectometry is a powerful technique for gaining a nanoscopic understanding at solid/liquid interfaces as a function of the depth[22]. Neutrons are capable of prove electrochemical interfaces owing to the strong penetrating power through the substrate. In addition, lithium can be sensitively detected by neutron; because the scattering length of lithium is significant in comparison with other elements in the electrode for neutron, whereas it is small and almost invisible for X-ray. The depth profile of the neutron scattering length density (SLD) obtained by analyzing the reflectivity profile can reveal the distribution of diffusing ions at the interface. Previous studies on the electrode/electrolyte interfaces by neutron reflectometry have mainly focused on the electrode-side interface (surface structural changes and/or surface electrolyte interphases) [1]. There are no reports of neutron reflectometry being used for discussing the depth distribution at the liquid-electrolyte side. In this study, we analyzed the concentration gradient of ionic species from the layered rocksalt type cathodeto the organic electrolyte by in situneutron reflectometry. Epitaxial Lix M yOz(M= Mn, Co, Ni) film model electrodes with (104) and (001) orientations were fabricated on Nb:SrTiO3single crystals by pulsed laser deposition. In situ neutron reflectometry measurements were conducted on a time-of-flight reflectometer (J-PARC, BL16 SOFIA) with a Li counter electrode and deuterated carbonate-base electrolyte containing LiPF6 [2]. The neutron reflectometry analyses demonstrate that the lithium concentrations in the interfacejust after the cell-construction (prior to electrochemical cycling) which may be related to formation of electrical double layer. The interfacial structures changed with the cell voltages during the electrochemical cycling. We will discuss on the crucial structure-parameters which determine the stability and the reaction rate in the interfacial regions. The depth-profile of the lithium ion distribution could give insight into research directions for electrode materials with highly-functional interfaces.