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 6Li 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.
Highlights
Neutron Depth Profiling (NDP), developed in 1972 (Ziegler et al, 1972), allows the detection of a small concentration of a certain isotope as function of sample depth, via neutron capture reactions (Downing et al, 1993)
The stopping power calculation can be used to quantify the change in stopping power of a porous Li-ion battery electrode due to filling with a Li containing electrolyte, providing the opportunity to determine the depth resolved porosity, demonstrated for LiFePO4 electrodes
Spinel Li4Ti5O12 (LTO) shows almost no strain uponlithiation and its relatively high voltage prevents decomposition of typical Li-ion battery electrolytes (Tang et al, 2009; Odziomek et al, 2017; Wang S. et al, 2017). These properties ensure excellent rate capabilities, stable cycling and a long cycle life (Singh et al, 2013b). Previous measurements on another high rate material, LiFePO4 (Wang and Hong, 2007), revealed that at high currents, 20C, and larger (Liu et al, 2016), ionic conduction in the electrolyte is the limiting factor as indicated by the enormous differences in concentration and local current near the electrolyte compared to the current collector
Summary
Neutron Depth Profiling (NDP), developed in 1972 (Ziegler et al, 1972), allows the detection of a small concentration of a certain isotope as function of sample depth, via neutron capture reactions (Downing et al, 1993). Whitney et al (2009), first suggested using the 6Li capture reaction for NDP on lithium ion battery systems. Techniques allowing direct, non-invasive lithium detection operando are limited, as a result NDP was quickly adopted by a number of research groups in to order to study problems such as degradation (Nagpure et al, 2011) and charge transport limitations (Zhang et al, 2015). Interpretation of the results will be more challenging as the electrode properties are dynamic, expanding, and contracting under the influence of lithium content (Mohanty et al, 2013; Liu et al, 2015; Liu and Co, 2016)
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