Maximilian Kaus

CuV2S4: A High Rate Capacity and Stable Anode Material for Sodium Ion Batteries

The ternary compound CuV2S4 exhibits an excellent performance as anode material for sodium ion batteries with a high reversible capacity of 580 mAh g-1 at 0.7 A g-1 after 300 cycles. A Coulombic efficiency of ≈99% is achieved after the third cycle. Increase of the C-rate leads to a drop of the capacity, but a full recovery is observed after switching back to the initial C-rate. In the early stages of Na uptake first Cu+ is reduced and expelled from the electrode as nanocrystalline metallic Cu. An increase of the Na content leads to a full conversion of the material with nanocrystalline Cu particles and elemental V embedded in a Na2S matrix. The formation of Na2S is evidenced by 23Na MAS NMR spectra and X-ray powder diffraction. During the charge process the nanocrystalline Cu particles are retained, but no crystalline materials are formed. At later stages of cycling the reaction mechanism changes which is accompanied by the formation of copper(I) sulfide. The presence of nanocrystalline metallic Cu and/or Cu2S improves the electrical conductivity, leading to superior cycling and rate capability.

Synthesis and electrochemical performance of nanocrystalline Al0.4Mg0.2Sn0.4O1.6 and Al0.25Mg0.38Sn0.38O1.5 investigated by in situ XRD, 27Al/119Sn MAS NMR, 119Sn Mössbauer spectroscopy, and galvanostatic cycling

Nanocrystalline Al0.4Mg0.2Sn0.4O1.6 and Al0.25Mg0.38Sn0.38O1.5 were prepared by a co-precipitation method from Al(NO3)3·9H2O, MgSO4, and SnCl4·5H2O, followed by calcination at different temperatures. We performed in situ X-ray diffraction measurements at temperatures between 307 K and 1173 K and transmission electron microscopy. The results reveal a crystal structure equivalent to that of SnO2 cassiterite, very small crystallite sizes of about 3–4 nm, and high thermal stability. The local structure around Al and Sn was investigated by 27Al/119Sn NMR and 119Sn Mossbauer spectroscopy. These measurements show that the calcination results in the formation of [AlO4] and [AlO5] units, in addition to the initial [AlO6] environment, and in local disorder around the Sn atoms. The electrochemical performance was studied by galvanostatic cycling against Li metal. These experiments were performed on bare and carbon coated materials. Sn is the active component in these materials and undergoes an alloying reaction. Both Al0.4Mg0.2Sn0.4O1.6 and Al0.25Mg0.38Sn0.38O1.5 exhibit good electrochemical performance with a very stable cycling and a discharge capacity of 522 mA h g−1 and 385 mA h g−1, respectively, after 100 cycles. Their performance is strongly improved in comparison with pure SnO2 prepared by the same synthesis route.