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- ItemStructural and Electrochemical Properties of Layered P2-Na0.8Co0.8Ti0.2O2 Cathode in Sodium-Ion Batteries(Basel : MDPI, 2022) Pohle, Björn; Gorbunov, Mikhail V.; Lu, Qiongqiong; Bahrami, Amin; Nielsch, Kornelius; Mikhailova, DariaLayered Na0.8Co0.8Ti0.2O2 oxide crystallizes in the β-RbScO2 structure type (P2 modification) with Co(III) and Ti(IV) cations sharing the same crystallographic site in the metal-oxygen layers. It was synthesized as a single-phase material and characterized as a cathode in Na- and Na-ion batteries. A reversible capacity of about 110 mA h g−1 was obtained during cycling between 4.2 and 1.8 V vs. Na+/Na with a 0.1 C current density. This potential window corresponds to minor structural changes during (de)sodiation, evaluated from operando XRD analysis. This finding is in contrast to Ti-free NaxCoO2 materials showing a multi-step reaction mechanism, thus identifying Ti as a structure stabilizer, similar to other layered O3- and P2-NaxCo1−yTiyO2 oxides. However, charging the battery with the Na0.8Co0.8Ti0.2O2 cathode above 4.2 V results in the reversible formation of a O2-phase, while discharging below 1.5 V leads to the appearance of a second P2-layered phase with a larger unit cell, which disappears completely during subsequent battery charge. Extension of the potential window to higher or lower potentials beyond the 4.2–1.8 V range leads to a faster deterioration of the electrochemical performance. After 100 charging-discharging cycles between 4.2 and 1.8 V, the battery showed a capacity loss of about 20% in a conventional carbonate-based electrolyte. In order to improve the cycling stability, different approaches including protective coatings or layers of the cathodic and anodic surface were applied and compared with each other.
- ItemWater-Free SbOx ALD Process for Coating Bi2Te3 Particle(Basel : MDPI, 2023) Lehmann, Sebastian; Mitzscherling, Fanny; He, Shiyang; Yang, Jun; Hantusch, Martin; Nielsch, Kornelius; Bahrami, AminWe developed a water-free atomic layer deposition (ALD) process to homogeneously deposit SbOx using SbCl5 and Sb-Ethoxide as precursors, and report it here for the first time. The coating is applied on Bi2Te3 particles synthesized via the solvothermal route to enhance the thermoelectric properties (i.e., Seebeck coefficient, thermal and electrical conductivity) via interface engineering. The amorphous character of the coating was shown by the missing reflexes on the X-ray diffractograms (XRD). A shift from the oxidation state +III to +V of the Sb species was observed using X-ray photoelectron spectroscopy (XPS), indicating increased thickness of the SbOx coating layer. Additionally, a peak shift of the Sb 3d5/2 + O 1s peak indicated increased n-type doping of the material. Electrical measurements of spark plasma-sintered bulk samples confirmed the doping effect on the basis of decreased specific resistivity with increasing SbOx layer thickness. The Seebeck coefficient was improved for the coated sample with 500 cycles of SbOx, while the total thermal conductivity was reduced, resulting in enhancement of the zT. The results distinctly show that surface engineering via powder ALD is an effective tool for improving key properties of thermoelectric materials like electrical conductivity and the Seebeck coefficient.