Abstract
Tin anodes show a rich structure and reaction chemistry which we have investigated in detail. Upon discharge five plateaus are observed corresponding to -Sn, an unidentified phase (Na/Sn = 0.6), an amorphous phase (Na/Sn = 1.2), a hexagonal R-3m Na5Sn2, and fully sodiated Na15Sn¬4. With charging there are six plateaus, which are related to the formation of Na5Sn2 followed by the formation of amorphous phases and -Sn. Upon cycling the formation of metastable Na5Sn2 seems to be suppressed, leading to a single charge plateau at 0.2 V.
Theoretical voltages calculated from existing crystal structures using DFT provide a good match with constant current measurements, however, the voltage is more negative compared to quasi-equilibrium measurements (GITT). Search for additional (meta)stable phases using cluster-expansion method predicts many phases lower in energy than the convex hull, including the R-3m Na5Sn2 phase characterized experimentally. The presence of multiple phases in varying lattices with very similar formation energy suggests why the reaction mechanism is non-reversible. Interpretation of Mössbauer spectroscopy data is not yet elucidated due to the very low recoil-free fraction of the materials.
The electrode surface is terminated with a SEI layer rich in carbonates such as Na2CO3 and Na alkyl carbonates as evidenced by XPS. After a full charge at 2V, strong evidence for the formation of oxidized Sn4+ is obtained. Subjecting the electrode to a rest after a full charge at 2 V reveals that aging in the electrolyte reduces the oxidized Sn4+ into Sn2+ and Sn0, 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 Sn4+ and not by pure metallic Sn.
