Understanding the mechanism of enhanced cycling stability in Sn-Sb composite Na-ion battery anodes: in-operando alloying and diffusion barriers

Sn-Sb composites are of great interest for high capacity sodium ion batteries due to their high stability, but because multiple phases and alloyed compositions are formed during cycling, the roles of each are challenging to deduce. In this work, two approaches were taken to investigate the importance of β-SnSb formation on the cycling stability of Sn-rich Sn-Sb composite sodium-ion battery (SIB) anodes. First, to tease out the role of each component, thin layers of amorphous silicon with thicknesses ranging from 0.5 to 10 nm, were incorporated between Sn and Sb layers, of equal thicknesses. Silicon has low solubility in both tin and antimony, and thus acts as a barrier layer that can interfere with the formation of Sn-Sb alloys. The equivalent composition of this sandwich structure was Sn<sub>53</sub>Sb<sub>47</sub>. Upon electrochemical cycling, a clear correlation between capacity retention and Si thickness was observed, and it was found that a 1 nm thick Si layer was sufficient to inhibit the formation of the β-SnSb intermetallic, resulting in loss of the capacity of the tin layer after a few tens of cycles. The second approach involved capping a Sn film with increasingly thicker Sb layers. Thicker antimony layers were found to have a large positive influence on cycling stability with a marked drop-off in the capacity retention when there is not enough Sb to fully convert the bilayer into β-SnSb. These results point to the necessity of the Sn and Sb being in intimate contact prior to cycling for the β-SnSb phase to form in-operando, which is necessary for the excellent capacity retention of the Sn-Sb system.