Direct observation of Sn crystal growth during the lithiation and delithiation processes of SnO2 nanowires

Tin (Sn) crystal growth on Sn-based anodes in lithium ion batteries is hazardous for reasons such as possible short-circuit failure by Sn whiskers and Sn-catalyzed electrolyte decomposition, but the growth mechanism of Sn crystals during battery cycling is not clear. Here we report different growth mechanisms of Sn crystal during the lithiation and delithiation processes of SnO2 nanowires revealed by in situ transmission electron microscopy (TEM). Large spherical Sn nanoparticles with sizes of 20–200 nm grew instantaneously upon lithiation of a single-crystalline SnO2 nanowire at large current density (j > 20 A/cm2), which suppressed formation of the LixSn alloy but promoted agglomeration of Sn atoms. Control experiments of Joule-heating (j ≈ 2400 A/cm2) the pristine SnO2 nanowires resulted in melting of the SnO2 nanowires but not Sn particle growth, indicating that the abnormal Sn particle growth was induced by both chemical reduction (i.e., breaking the SnO2 lattice to produce Sn atoms) and agglomeration of the Sn atoms assisted by Joule heating. Intriguingly, Sn crystals grew out of the nanowire surface via a different “squeeze-out” mechanism during delithiation of the lithiated SnO2 nanowires coated with an ultra-thin solid electrolyte LiAlSiOx layer. It is attributed to the negative stress gradient generated by the fast Li extraction in the surface region through the Li+-conducting LiAlSiOx layer. Our previous studies showed that Sn precipitation does not occur in the carbon-coated SnO2 nanowires, highlighting the effect of nanoengineering on tailoring the electrochemical reaction kinetics to suppress the hazardous Sn whiskers or nanoparticles formation in a lithium ion battery.

► Sn crystal growth during the lithiation and delithiation processes of individual SnO2 nanowires was studied with in situ transmission electron microscopy.
► During lithiation of the SnO2 nanowires, spherical Sn nanoparticles formed at large current density (over 20 A/cm2) due to Joule-heating-assisted agglomeration of Sn atoms produced by reduction of SnO2.
► During delithiation of the SnO2 nanowires coated with an ultrathin LiAlSiOx electrolyte, faceted Sn crystals grew out of the nanowire surface via the “squeeze-out” mechanism due to fast Li+ extraction through the electrolyte coating layer.