Abstract
Electrical mobility demands an increase of battery energy density beyond current lithium-ion technology. A crucial bottleneck is the development of safe and reversible lithium-metal anodes, which is challenged by short circuits caused by lithium-metal dendrites and a short cycle life owing to the reactivity with electrolytes. The evolution of the lithium-metal-film morphology is relatively poorly understood because it is difficult to monitor lithium, in particular during battery operation. Here we employ operando neutron depth profiling as a noninvasive and versatile technique, complementary to microscopic techniques, providing the spatial distribution/density of lithium during plating and stripping. The evolution of the lithium-metal-density-profile is shown to depend on the current density, electrolyte composition and cycling history, and allows monitoring the amount and distribution of inactive lithium over cycling. A small amount of reversible lithium uptake in the copper current collector during plating and stripping is revealed, providing insights towards improved lithium-metal anodes.
Highlights
Based on their large gravimetric and volumetric energy densities, Li-ion batteries are the technology of choice for portable electronics and electrical mobility[1,2,3]
Pei et al.[15] demonstrated with scanning electron microscopy (SEM) that the Li-metal nuclei density is proportional to the cubic power of the overpotential, consistent with classical nucleation and growth theory
Neutron Depth Profiling (NDP) of Li relies on the capture reaction of a thermal neutron with a 6Li atom, which results in two charged particles that, based on conservation of momentum and energy, have a well-defined energy: 4He (2044 keV) and 3H (2727 keV)
Summary
Based on their large gravimetric and volumetric energy densities, Li-ion batteries are the technology of choice for portable electronics and electrical mobility[1,2,3]. SEM and optical studies have shown a correlation between the current density and the plated Li-metal microstructure in line with the Chazalviel model[17] This model predicts that at current densities that deplete the electrolyte concentration, at the surface of the anode, the anion concentration will eventually drop to zero after a characteristic Sand’s time time[18]. The microstructural evolution during stripping may leave isolated regions of Li-metal This so called “dead” Li-metal contributes to capacity loss, and may have an important role in the penetration of Li-metal though the electrolyte towards the positive electrode over repeated cycling, causing an internal short-circuit and the consequential safety risks. The evolution of the complex SEI/Li-metal microstructure should be considered to assess the safety risks and energy storage efficiency of Li-metal batteries
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