Probing Lithium Metal Microstructures with Dynamic Nuclear Polarization from Lithium Metal

Abstract

Lithium metal batteries (LMBs) show strong potential for next-generation energy storage, because of their high theoretical energy density. However, their practical deployment is limited by microstructure and dendrite formation, which raises serious safety and efficiency concerns. A key factor influencing dendrite growth is the passivating film, or solid electrolyte interphase (SEI), that forms at the lithium metal surface. Gaining insight into the SEI is essential for controlling dendrite formation, yet this remains challenging due to its complex composition and disorder. Lithium metal endogenous Dynamic Nuclear Polarization (DNP) offers opportunities to probe the SEI with enhanced sensitivity. In this work, we investigate how dendrite morphology influences both DNP enhancement and polarization transfer mechanisms. We develop a protocol to produce lithium dendrites with controlled morphologies on copper electrodes, and find that DNP efficiency is strongly affected by dendrite morphology and the substate they grow on, primarily through variations in the electron spin relaxation times. For Li dendrites grown on a copper substrate, extremely short relaxation times are seen for some morphologies, which we suggest is due largely to the Cu impurities present at the Li/SEI interface since the effect is more pronounced for the thin dendrites, where electrons travel for shorter distances before encountering a surface impurity. The enhancements also depend on the frequency of the applied microwave radio frequency (rf) irradiation, and the extent of rf penetration into the lithium microstructures. Finally, we explore polarization transfer from lithium metal to surrounding salt species in the SEI, providing insight into the underlying mechanisms.