Real-time and direct observation of lithiation of ultra-small tin oxide nanoparticles

Understanding of the lithiation process of ultra-small nanoparticles is crucial for the development of lithium-ion battery electrodes with high-rate charging capability, but remains quite limited. In this work, we successfully investigate the lithiation process of ultra-small SnO₂ nanoparticles using in situ transmission electron microscopy (TEM). For the first time, lithiation stripes are observed in the lithiation of SnO₂ nanoparticles. Interestingly, rather than a composite structure with Li_xSn particles dispersed in matrix-like amorphous Li₂O reported for SnO₂ nanowires, a novel acorn-shaped configuration forms after lithiation of the nanoparticles, in which Sn/Li₁₃Sn₅ acts as the shell and Li₂O as the core. The lithiation mechanism of the nanoparticles is clearly revealed as two stages – an intercalation process which generates a volume expansion of <33%, and a conversion-alloying reaction which finally results in a volume expansion of ~100%. The total time for full lithiation of a nanoparticle scales linearly with the particle size, indicating that the lithiation process is reaction-controlled. The in situ TEM observations indicate that the ultra-small particles possess an ultrahigh rate-capability (>1000 C). This study strengthens the understanding of lithiation of nanoparticulate electrodes and provides guidance for the design of high-rate capable electrodes.