Neutron depth profiling technique for studying rechargeable lithium metal anodes

Abstract

The attention towards lithium (Li) metal anode (LMA) has been rekindled in recent years as it can augment the energy density of Li batteries due to its high theoretical specific capacity (3860 mAh∙g−1) and low electrochemical potential (−3.04 V versus standard hydrogen electrode), especially when paired with Li-free cathodes such as Li-oxygen and Li-sulfur. However, severe interfacial instability and safety concerns on rechargeable LMA, associated with Li dendrite formation, continuous side reactions, and infinite volume changes, extremely hinder its commercialization. Numerous strategies have been employed to modify LMA for realizing a uniform distribution of the Li ion flux through interface and dendrite-free Li deposits during repeated Li plating/stripping, which leads to a better cycling performance; however, to the best of our knowledge, a clear understanding of the Li deposition/dissolution behavior and the nucleation growth mechanism of Li dendrites is still lacking, which is conducive to more efficient modification studies on LMA. Therefore, it is critical to achieve considerable progress in the development of advanced characterization techniques. However, the high reactivity of Li metal, which leads to complexity of products and diversity in morphology, causes many difficulties in the characterization of in situ spectroscopy. Recently, some promising characterization techniques have been introduced to further investigate the evolution of LMA during cycling, such as cryoelectron microscopy, solid-state nuclear magnetic resonance technology, and neutron depth profiling (NDP) technique. Because of its high-penetration characteristics, quantitative and nondestructive merits, and highly selective sensitivity to6Li via the capture reaction with neutrons, the NDP technique shows a broad application prospect for obtaining real-time information of the electrochemical behavior of Li in Li metal batteries. The NDP results contain a wealth of information about time and space for Li. Accordingly, not only can the real-time distribution and migration of Li ions be detected, but also changes in the active sites of Li deposition/dissolution can be analyzed to understand the formation principle of Li dendrites and the failure mechanism of Li metal batteries. In addition, the NDP technique has shown its potential in the diagnosis and prediction of short circuit in Li metal batteries, which is confirmed through voltage curves. This review first briefly introduces the principle of the NDP technique and the methods for improving its space/time resolution; second, it summarizes the recent use of the NDP technique in the research on LMAs based on liquid or solid cell systems. Finally, it provides a prospect for the future development of NDP technique.