Solid Electrolyte Interphase in Li-Ion Batteries: Evolving Structures Measured In situ by Neutron Reflectometry

Li-ion batteries are made possible by the solid electrolyte interphase, SEI, a self-forming passivation layer, generated because of electrolyte instability with respect to the anode chemical potential. Ideally it offers sufficient electronic resistance to limit electrolyte decomposition to the amount needed for its formation. However, slow continued SET growth leads to capacity fade and increased cell resistance. Despite the SEI's critical significance, currently structural characterization is incomplete because of the reactive and delicate nature of the SET and the electrolyte system in which it is formed. Here we present, for the first time, in situ neutron reflectometry measurements of the SEI layer as function of potential in a working lithium half-cell. The SEI layer after 10 and 20 CV cycles is 4.0 and 4.5 nm, respectively, growing to 8.9 nm after a series of potentiostatic holds that approximates a charge/discharge cycle. Specified data sets show uniform mixing of SEI components.