Lithium metal has re-emerged as an exciting anode for high energy lithium-ion batteries due to its high specific capacity of 3860 mAh g and lowest electrochemical potential of all known materials. However, lithium has been plagued by the issues of dendrite formation, high chemical reactivity with electrolyte, and infinite relative volume expansion during plating and stripping, which present safety hazards and low cycling efficiency in batteries with lithium metal electrodes. There have been a lot of recent studies on Li metal although little work has focused on the initial nucleation and growth behavior of Li metal, neglecting a critical fundamental scientific foundation of Li plating. Here, we study experimentally the morphology of lithium in the early stages of nucleation and growth on planar copper electrodes in liquid organic electrolyte. We elucidate the dependence of lithium nuclei size, shape, and areal density on current rate, consistent with classical nucleation and growth theory. We found that the nuclei size is proportional to the inverse of overpotential and the number density of nuclei is proportional to the cubic power of overpotential. Based on this understanding, we propose a strategy to increase the uniformity of electrodeposited lithium on the electrode surface.

Whereas standard transmission electron microscopy studies are unable to preserve the native state of chemically reactive and beam-sensitive battery materials after operation, such materials remain pristine at cryogenic conditions. It is then possible to atomically resolve individual lithium metal atoms and their interface with the solid electrolyte interphase (SEI). We observe that dendrites in carbonate-based electrolytes grow along the <111> (preferred), <110>, or <211> directions as faceted, single-crystalline nanowires. These growth directions can change at kinks with no observable crystallographic defect. Furthermore, we reveal distinct SEI nanostructures formed in different electrolytes.

We summarize the origins of lithium-ion battery safety issues and discuss recent progress in materials design to improve safety.

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SI Video S1. TiO 2-Au Micromotors Motion Remotely Triggered by UV Light and a Control. SI Video S2. TiO 2-Au Micromotors Moving Towards to TiO 2 Side in Water. SI Video S3. Motion of TiO 2-Au Micromotors in Different NaCl Concentration Environment under UV Light. SI Video S4. Motion of TiO 2-Au Micromotors with Different Coating Layer under UV Light. SI Video S5. Motion of Light-Driven Micromotors under Different UV Light Intensities in Water. SI Video S6. UV Light Triggered "Stop and Go" of a Micromotor. SI Video S7. Directional Control of TiO 2-Ni-Au Micromotors. SI Video S8. Motion of Light-Driven Micromotors in Different Conditions.