Insight on the Failure Mechanism of Sn Electrodes for Sodium-Ion Batteries: Evidence of Pore Formation during Sodiation and Crack Formation during Desodiation

The development of Sn based anode materials for sodium ion batteries is mainly hindered by the limited understanding of sodiation/desodiation mechanisms inside the active material, which typically results in electrode damage. Herein, we report a post-mortem ex situ scanning electron microscopic analysis of Sn thin film motivated by the intention to elucidate these structural mechanisms. Our results reveal for the first time that the surface of Sn electrode film becomes highly porous during sodiation with no presence of obvious cracks, a surprising result when compared to previous reports performed on Sn particles. Even more surprisingly, sequential ex-situ SEM observations demonstrate that, once the desodiation starts and reaches the second desodiation plateau (0.28 V), obvious cracks in the Sn film are instead observed along with porous islands of active material. These islands appear as aggregated particles which further split into smaller islands when the desodiation potential reaches its maximum value (2.0 V). Finally, for the first time, the experimental value of the sodium diffusion coefficient inside Sn was measured (3.9 X 10(-14) cm(2) s(-1)) using electrochemical impedance spectroscopy.