Characterization of sodium ion electrochemical reaction with tin anodes: Experiment and theory

Abstract Tin anodes show a rich structure and reaction chemistry which we have investigated in detail. Upon discharge five plateaus are observed corresponding to β-Sn, an unidentified phase (Na/Sn = 0.6), an amorphous phase (Na/Sn = 1.2), a hexagonal R-3m Na 5 Sn 2 , and fully sodiated I-43d Na 15 Sn 4 . With charging there are six plateaus related to the formation of Na 5 Sn 2 followed by the formation of amorphous phases and β-Sn. Upon cycling the formation of metastable Na 5 Sn 2 seems to be suppressed. Theoretical voltages calculated from existing crystal structures using DFT provide a good match with constant current and quasi-equilibrium measurements (GITT). Search for additional (meta)stable phases using cluster-expansion method predicts many phases lower in energy than the convex hull obtained from known structures, including the R-3m Na 5 Sn 2 phase. The presence of multiple phases in varying lattices with similar formation energy suggests why the reaction mechanism is non-reversible. 119 Sn Mossbauer spectroscopy results indicate a decrease of the isomer shift with increasing Na/Sn content, which is less pronounced than for Li–Sn compounds likely due to the lower electropositivity of Na. The electrode surface is terminated with an SEI layer rich in carbonates (Na 2 CO 3 and Na CO 3 R) as evidenced by XPS. After charge at 2 V, strong evidence for the formation of oxidized Sn 4+ is obtained. Subjecting the electrode to a rest after charge at 2 V reveals that aging in the electrolyte reduces the oxidized Sn 4+ into Sn 2+ and Sn 0 , and concomitantly suppresses the electrolyte decomposition represented by an anomalous discharge plateau at 1.2 V. Thereby, the catalytic decomposition of the electrolyte during discharge is caused by nanosized Sn particles covered by oxidized Sn 4+ and not by pure metallic Sn.