Contrary to previous assumptions, a new study finds that the needle-like lithium dendrites that grow in lithium (Li)-metal batteries are surprisingly strong and brittle, quite unlike soft bulk Li. According to the authors, understanding this brittle fracture behavior provides insights for suppressing dead Li formation and electrolyte cracking, enabling safer and more reliable Li-metal batteries. Li-metal anodes offer the highest specific capacity and the lowest electrochemical potential among all known anode materials, making them highly attractive for use in next-generation battery technologies. However, the use of Li-metal anodes poses significant safety challenges. During battery operation, lithium can grow into needle-like structures called dendrites that pierce the separator inside the battery, leading to internal short circuits and even explosions. It has been assumed that Li dendrites are soft and deformable, like bulk Li, leading to strategies focused on stiffening electrolytes. Yet, confusingly, recent experiments show that Li dendrites – supposedly soft – can fracture hard solid electrolyte materials that are much stronger. Understanding why this happens is central to improving Li-metal battery performance and safety.
Here, Qing Ai and colleagues developed a specialized technique to measure the mechanical properties of Li dendrites formed under realistic battery conditions. Using a nanomanipulator within a scanning electron microscope, Ai et al. carefully extracted dendrites grown in a coin cell and transferred them to a miniature mechanical testing device. The findings revealed that Li dendrites behave very differently from bulk Li metal. Rather than being soft and deformable, Li dendrites are strong and brittle, fracturing at tensile strengths greater than ~150 megapascals (MPa), far higher than the ~0.6 MPa strength of bulk Li. Imaging with cryogenic electron microscopy reveals that each dendrite consists of a single-crystal Li core surrounded by a thin solid electrolyte interface layer. Modeling and materials analyses show that this nanoscale structure is what gives dendrites their strength. According to Ai et al., the findings suggest that tailoring the solid electrolyte microstructures in future Li-metal batteries may be a viable approach to mitigate battery failure and improve safety.
