This paper evaluates the influence of the morphology, surface area, and surface modification of carbonaceous additives on the performance of the corresponding cathode in a lithium–sulfur battery. The structure of sulfur composite cathodes with mesoporous carbon, activated carbon, and electrochemical carbon is studied by X-ray diffraction, nitrogen adsorption measurements, and Raman spectroscopy. The sulfur cathode containing electrochemical carbon with the specific surface area of 1606.6 m2 g−1 exhibits the best electrochemical performance and provides a charge capacity of almost 650 mAh g−1 in cyclic voltammetry at a 0.1 mV s−1 scan rate and up to 1300 mAh g−1 in galvanostatic chronopotentiometry at a 0.1 C rate. This excellent electrochemical behavior is ascribed to the high dispersity of electrochemical carbon, enabling a perfect encapsulation of sulfur. The surface modification of carbonaceous additives by TiO2 has a positive effect on the electrochemical performance of sulfur composites with mesoporous and activated carbons, but it causes a loss of dispersity and a consequent decrease of the charge capacity of the sulfur composite with electrochemical carbon. The composite of sulfur with TiO2-modified activated carbon exhibited the charge capacity of 393 mAh g−1 in cyclic voltammetry and up to 493 mAh g−1 in galvanostatic chronopotentiometry. The presence of an additional Sigracell carbon felt interlayer further improves the electrochemical performance of cells with activated carbon, electrochemical carbon, and nanocrystalline TiO2-modified activated carbon. This positive effect is most pronounced in the case of activated carbon modified by nanocrystalline TiO2. However, it is not boosted by additional coverage by TiO2 or SnO2, which is probably due to the blocking of pores.
Electrochemical performance of activated carbon/sulfur composite cathode in the Li-S cell with standard and TiO2-modified separator is evaluated by cyclic voltammetry and galvanostatic chronopotentiometry. The modification of the separator by TiO2 impregnation has beneficial effect on the charge capacity of the activated carbon/sulfur cathode in the Li-S cell. The specific capacity of the cathode in the cell with TiO2-modified separator is 632 mAh g-1 (calculated from cyclic voltammetry) and 673 mAh g-1 (determined from galvanostatic chronopotentiometry). Facile impregnation of the separator with nanocrystalline TiO2 results in the 10-20 % stable increase of the charge capacity of corresponding activated carbon/sulfur cathode as compared to its electrochemical performance in the system with non-modified separator.
For the first time, a spinel-type high entropy oxide (Zn0.25Cu0.25Mg0.25Co0.25)Al2O4 as well as its derivative lithiated high entropy oxyfluoride Li0.5(Zn0.25Cu0.25Mg0.25Co0.25)0.5Al2O3.5F0.5 and oxychloride Li0.5(Zn0.25Cu0.25Mg0.25Co0.25)0.5Al2O3.5Cl0.5 are prepared in the nanostructured state via high-energy co-milling of the simple oxide precursors and the halides (LiF or LiCl) as sources of lithium, fluorine and chlorine. Their nanostructure is investigated by XRD, HR-TEM, EDX and XPS spectroscopy. It is revealed that incorporation of lithium into the structure of spinel oxide together with the anionic substitution has significant effect on its short-range order, size and morphology of crystallites as well as on its oxidation/reduction processes. The charge capacity of the as-prepared nanomaterials tested by cyclic voltammetry is found to be rather poor despite lithiation of the samples in comparison to previously reported spinel-type high entropy oxides. Nevertheless, the present work offers the alternative one-step mechanochemical route to novel classes of high entropy oxides as well as to lithiated oxyfluorides and oxychlorides with the possibility to vary their cationic and anionic elemental composition.
The influence of five different inorganic additives with different composition and morphology, viz. nanofibrous rutile/TiOxNy, Li4Ti5O12 (LTS) from Altair, TiO2-P90 from Evonik, TiOxNy-anatase and nano-TiO2 on the electrochemical performance of the activated carbon/sulfur composite cathode is evaluated by cyclic voltammetry at the scan rate of 0.1 mV s-1 and by galvanostatic chronopotentiometry at the 0.1 C rate. The composites are prepared by facile mechanical mixing of activated carbon with the inorganic additive and subsequent melting-diffusion with sulfur. The addition of nano-TiO2 from titanium isopropoxide, TiOxNy-anatase, rutile/TiOxNy, and LTS Altair to the carbon/sulfur composite cathode increased the charge capacity of the corresponding Li-sulfur cell. The negative effect of the P90 additive can be attributed to the blocking of its surface by Al2O3 and SiO2, hindering the adsorption of polysulfides. TiOxNy-anatase additive exhibits the highest capacity improvement due to its surface-enhanced redox chemistry of conductive and sulphiphilic surface resulting in a 7 % increase of the voltammetric charge capacity. In addition, this additive has a beneficial effect on the cycling stability of the sulfur composite cathode during galvanostatic cycling.
Electrochemical impedance spectroscopy is used to study novel cathode materials for lithium-sulfur batteries, based on commercial carbon and titania. Coin cells with Li anode are investigated at various stages of galvanostatic cycling. For comparison, also symmetrical coin cells with a pair of positive (S/C/TiO2) or negative (Li) electrodes are studied. In addition to the application of titania as a barrier material impeding the polysulfide diffusion in the electrolyte solution, the inherent Li-insertion activity of TiO2 (anatase) and its contribution to the sulfur redox reactions is discussed.
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