Damage formation in Sn film anodes of Na-ion batteries

Sn anodes for Na-ion batteries exhibit a promising initial capacity of 847 mAh g^-1, which however, cannot be retained throughout continuous cycling due to the 420% volume changes that Sn experiences during sodiation. Previous experimental studies suggest that fracture does not occur in the submicron Sn particles during the formation of Na-Sn alloys; however, such colossal volume changes must result in microstructural damage. In the present work, the damage mechanisms during sodiation are isolated and accentuated by employing a Sn thick film of 0.5 mm as the anode. This simplified planar geometry allows to dispense with the influence of the binder and carbon additives that are required in porous electrodes. Post-mortem electron microscopy revealed new deformation mechanisms for anode materials, as multiple whiskers nucleated on the surface of the Sn, whereas pores formed within the Sn (over the Na-ion penetration distance) after electrochemical cycling. These mechanisms were in addition to the dry lake-bed fracture that was also observed. A comparative study on a Sn thin-film anode of 0.06 mm revealed the formation of fracture and pores after cycling, but no whiskers. The whiskers and pores observed in the thick Sn film anode may be more subtle at the nanoscale, and therefore have not been reported for submicron Sn particles in porous electrodes during sodiation.