Critical size for the beta- to alpha-transformation in tin nanoparticles after lithium insertion and extraction

Tin nanoparticles can be transformed from the metallic beta-Sn structure to the semiconducting alpha-Sn structure after electrochemical lithiation and delithiation at room temperature. Here, we studied the influence of the size of the crystallites on the beta- to alpha-transformation in Sn nanoparticles. Differently sized Sn/SnOx nanoparticles were synthesized, processed in electrodes and cycled ten times in a lithium-ion cell at room temperature. X-ray diffraction (XRD) patterns before and after electrochemical lithium insertion/extraction reveal that samples with small particles contain the alpha-Sn structure. The critical size for this transformation is 17(4) nm. Smaller particles were transformed into the alpha-Sn structure while particles larger than 17 nm retain the beta-Sn structure. Temperature dependent XRD measurements show that this alpha-Sn structure is stable up to 220 degrees C before its reflections disappear. The formation of the alpha-Sn structure at room temperature in small particles and the unexpected high transition point can be explained by the substantial contribution of the surface energy (facilitating formation of alloys not observed in the bulk), lithium impurities in the alpha-Sn structure and the Li2O shell which is formed during lithium insertion.