Rational Optimization of Cathode Composites for Sulfide-Based All-Solid-State Batteries.

All-solid-state lithium-ion batteries with argyrodite solid electrolytes have been developed to attain high conductivities of 10-3 S cm-1 in studies aiming at fast ionic conductivity of electrolytes. However, no matter how high the ionic conductivity of the electrolyte, the design of the cathode composite is often the bottleneck for high performance. Thus, optimization of the composite cathode formulation is of utmost importance. Unfortunately, many reports limit their studies to only a few parameters of the whole electrode formulation. In addition, different measurement setups and testing conditions employed for all-solid-state batteries make a comparison of results from mutually independent studies quite difficult. Therefore, a detailed investigation on different key parameters for preparation of cathodes employed in all-solid-state batteries is presented here. Employing a rational approach for optimization of composite cathodes using solid sulfide electrolytes elucidated the influence of different parameters on the cycling performance. First, powder electrodes made without binders are investigated to optimize several parameters, including the active materials' particle morphology, the nature and amount of the conductive additive, the particle size of the solid electrolyte, as well as the active material-to-solid electrolyte ratio. Finally, cast electrodes are examined to determine the influence of a binder on cycling performance.

Characterization of slow pyrolysis biochars: effects of feedstocks and pyrolysis temperature on biochar properties.

Biochars are increasingly used as soil amendment and for C sequestration in soils. The influence of feedstock differences and pyrolysis temperature on biochar characteristics has been widely studied. However, there is a lack of knowledge about the formation of potentially toxic compounds that remain in the biochars after pyrolysis. We investigated biochars from three feedstocks (wheat straw, poplar wood, and spruce wood) that were slowly pyrolyzed at 400, 460, and 525°C for 5 h (straw) and 10 h (woodchips), respectively. We characterized the biochars' pH, electrical conductivity, elemental composition (by dry combustion and X-ray fluorescence), surface area (by N adsorption), water-extractable major elements, and cation exchange capacity (CEC). We further conducted differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffractometry to obtain information on the biochars' molecular characteristics and mineralogical composition. We investigated trace metal content, total polycyclic aromatic hydrocarbon (PAH) content, and PAH composition in the biochars. The highest salt (4.92 mS cm) and ash (12.7%) contents were found in straw-derived biochars. The H/C ratios of biochars with highest treatment temperature (HTT) 525°C were 0.46 to 0.40. Surface areas were low but increased (1.8-56 m g) with increasing HTT, whereas CEC decreased (162-52 mmol kg) with increasing HTT. The results of DSC and FTIR suggested a loss of labile, aliphatic compounds during pyrolysis and the formation of more recalcitrant, aromatic constituents. X-ray diffractometry patterns indicated a mineralogical restructuring of biochars with increasing HTT. Water-extractable major and trace elements varied considerably with feedstock composition, with trace elements also affected by HTT. Total PAH contents (sum of EPA 16 PAHs) were highly variable with values up to 33.7 mg kg; irrespective of feedstock type, the composition of PAHs showed increasing dominance of naphthalene with increasing HTT. The results demonstrate that biochars are highly heterogeneous materials that, depending on feedstock and HTT, may be suitable for soil application by contributing to the nutrient status and adding recalcitrant C to the soil but also potentially pose ecotoxicological challenges.

Dynamic interplay of alkali cations and a natural organic binder in the microstructural evolution of Cu2ZnSnS4 thin films prepared from Cu2ZnSnS4 powder-containing inks.

The use of pre-synthesised Cu2ZnSnS4 (CZTS) sub-micron powders as a raw material for preparing CZTS thin films for photovoltaic absorber applications is examined. A challenge in preparing photovoltaic device-relevant CZTS films from submicron powders is producing a dense CZTS film by a sintering process. This is due to the nature of non-unimodal particle size and morphology that typically lead to the formation of pores after sintering. This work aimed to study the sintering behaviour of CZTS films that were prepared from a CZTS powder-containing ink. Complementary DT-TGA and in situ X-ray powder diffraction studies at elevated temperature reveal that the tetragonal kesterite phase in the as-sintered CZTS film is stable until 620 °C. An effective tendency of CZTS powder towards film recrystallisation occurs when alkali cations (Na and/or K) are added to the ink. For the first time, effects of additional natural gum as a binder in the CZTS powder-containing ink on the CZTS film sintering behaviour were also investigated. Contrary to the positive effects of alkali addition, the binder inhibits recrystallisation of CZTS. Therefore, the amount of binder was controlled in a quantity large enough to modify the ink viscosity, but low enough to allow large CZTS grain growth during sintering. A dense and compact as-sintered CZTS film can be produced from a CZTS powder-containing ink with 10 mol% Na and 2 mol% K alkali addition along with 3 wt% binder addition.