Operando Neutron Depth Profiling to Determine the Spatial Distribution of Li in Li-ion Batteries

Abstract

Neutron Depth Profiling (NDP) allows determination of the spatial distribution of specific isotopes, via neutron capture reactions. In a capture reaction charged particles with fixed kinetic energy are formed, where their energy loss through the material of interest can be used to provide the depth of the original isotope. As lithium-6 has a relatively large probability for such a capture reaction, it can be used by battery scientists to study the lithium concentration in the electrodes even during battery operation. The selective measurement of the 6 Li isotope makes it a direct and sensitive technique, whereas the penetrative character of the neutrons allows practical battery pouch cells to be studied. Using NDP lithium diffusion and reaction rates can be studied operando as a function of depth, opening a large range of opportunities including the study of alloying reactions, metal plating, and (de) intercalation in insertion hosts. In the study of high rate cycling of intercalation materials the relatively low Li density challenges counting statistics while the limited change in electrode density due to the Li-ion insertion and extraction allows straightforward determination of the Li density as a function of electrode depth. If an electrode can be (dis)charged reversibly, data can be acquired and accumulated over multiple cycles to increase the time resolution. For Li metal plating and alloying reactions, the large lithium density allows good time resolution, however the large change of the electrode composition and density makes extracting the Li-density as a function of depth more challenging. Here an effective method is presented, using calibration measurements of the individual components, based on which the ratio of the components as a function of depth can be determined as well as the total Li-density. The same principles can be applied to insertion host materials, where the differences in density due to electrolyte infiltration yield the electrode porosity as a function of depth. This is of particular importance for battery electrodes where porosity has a direct influence on the energy density and charge transport.

Measurement of lithium diffusion coefficient in thin metallic multilayer by neutron depth profiling

“…Our intention was also to prove that NDP is a very suitable technique for studying thin Li‐ion batteries. Until now, only few attempts have been made in the field of thin LIBs research using the NDP technique in in situ and operando modes 10‐12 . The results carried out by NDP analysis have found confirmation with the previously published data, as for Rupp et al 4 Despite the different techniques in samples preparation, a similar value of Li diffusion in Cu has been estimated.…”

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

“…The reaction depth of the original isotopes can be deduced by measuring the residual energies of emitted particles, resulting in the Li concentration as a function of depth perpendicular to the surface of the electrode [204,205] . In situ NDP provides information on the Li reaction kinetics, transport and irreversible trapped Li behavior, described in detail in a recent review [206] . The maximum depth that can be probed depends on the stopping power of the material, which for battery current collectors and electrode materials typically amounts 30 to 40 m, with a resolution in the order of 10 to 60 nm.…”

Accelerating Battery Characterization Using Neutron and Synchrotron Techniques: Toward a Multi‐Modal and Multi‐Scale Standardized Experimental Workflow

“…[ 35 ] Neutron powder diffraction (NPD) has a long and successful track record in battery characterization, although great care must be taken in cell design to optimize signal‐to‐noise ratio. [ 36 ] Small angle neutron scattering (SANS), [ 35 ] neutron reflectometry (NR), [ 37 ] neutron imaging/computed tomography (NI and NCT), [ 38,39 ] and neutron depth profiling (NDP) [ 23,40 ] have all been successfully applied to provide structural information on battery materials, Li concentration gradients, and SEI growth mechanism, with yet further developments proposed in order to shed light on as yet inaccessible degradation mechanisms. Quasi‐elastic neutron scattering (QENS) is unique in providing a direct observation of ionic conduction mechanism on timescales ranging from picoseconds through to microseconds, yielding key information on the geometrical character of the ionic motion, distinguishing for example between continuous, liquid‐like diffusion, and discrete hopping between specific sites.…”