Development of high energy density solid-state batteries with Li metal anodes has been limited by uncontrollable growth of Li dendrites in liquid and solid electrolytes (SEs). This, in part, may be caused by a dearth of information about mechanical properties of Li, especially at the nano- and micro-length scales and microstructures relevant to Li batteries. We investigate Li electrodeposited in a commercial LiCoO2/LiPON/Cu solid-state thin-film cell, grown in situ in a scanning electron microscope equipped with nanomechanical capabilities. Experiments demonstrate that Li was preferentially deposited at the LiPON/Cu interface along the valleys that mimic the domain boundaries of underlying LiCoO2 (cathode). Cryogenic electron microscopy analysis of electrodeposited Li revealed a single-crystalline microstructure, and in situ nanocompression experiments on nano-pillars with 360–759 nm diameters revealed their average Young's modulus to be 6.76 ± 2.88 GPa with an average yield stress of 16.0 ± 6.82 MPa, ~24x higher than what has been reported for bulk polycrystalline Li. We discuss mechanical deformation mechanisms, stiffness, and strength of nano-sized electrodeposited Li in the framework of its microstructure and dislocation-governed nanoscale plasticity of crystals, and place it in the parameter space of existing knowledge on small-scale Li mechanics. The enhanced strength of Li at small scales may explain why it can penetrate and fracture through much stiffer and harder SEs than theoretically predicted.
Lithium is an ideal battery anode, with a theoretical specific capacity of 3860 mAh/g; replacing the conventional graphitic anode in Li-ion batteries with Li can increase energy density by ~50%. A significant drawback of Li anodes is dendrite formation during cycling, which can lead to short circuiting (a safety hazard and cell death) and to “dead Li,” which drastically reduces cycle life. Virtually all approaches to supress Li dendrite growth have not proven to be consistently successful. Preventing electrolyte and cell failure requires a more sophisticated understanding of Li dendrite growth kinetics and mechanics. Few experiments that probe mechanical behavior of electrodeposited Li exist. Most experiments on mechanical properties have focused on thin films, Li foils, and focused ion beam-carved Li. We developed in situ experimental methodology that allows one to electrochemically charge small-scale battery cells and to observe, in real-time, the formation of Li dendrites and to probe their mechanical response. Experiments reveal: (1) Li nano-deposits are single crystalline and typically shaped as faceted pillars with 300-800nm diameters, and (2) strengths of Li nanopillars are 16.0 ± 6.82 MPa, which is 24x greater than bulk. This strength enhancement can be explained in terms of the ubiquitous “smaller is stronger size effect” in nano-sized single-crystalline metals. This work expands the existing strength versus size property space for Li and helps explain why dendrites can penetrate through much stiffer and harder ceramic solid electrolytes, than what has been theorized.