Identification of Lithiation Mechanisms in Li-Ion Batteries: Multilayer Electrodes and Neutron Reflectometry

The reversible incorporation of lithium into electrodes is a key process in energy storage in Li-ion batteries. A detailed understanding of lithiation and delithiation mechanisms acting in electrodes is therefore essential for both fundamental and application-oriented research. We introduce an approach that combines isotope multilayer electrodes with neutron reflectometry to directly probe the lithiation mechanism in operando during electrochemical cycling. The core idea is that the isotope multilayer transforms complex reflectivity patterns into simpler patterns characterized by a single, well-defined Bragg peak. The evolution of the peak (scattering vector position and intensity) as a function of state-of-charge provides a signature of the underlying mechanism. This drastically simplifies data analysis and, in some cases, makes it possible at all. Proof-of-concept experiments on 73Ge/Ge multilayer electrodes demonstrate the effectiveness of the method. A direct and easy comparison between experimental results and simulations indicates a homogeneous lithiation and delithiation mechanism, independent of cycle number and cycling rate.