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Li-S batteries are ideal candidates to replace current lithium-ion batteries as next-generation energy storage systems thanks to their high specific capacity and theoretical energy density. Composite electrodes based on carbon microstructures are often used as a host for sulfur. However, sulfur lixiviation, insoluble species formation, and how to maximize the sulfur-carbon contact in looking for improved electrochemical performance are still major challenges. In this study, a nitrogen doped mesoporous carbon is used as a host for sulfur. The S/C composite electrodes are prepared by sulfur melting-diffusion process at 155 °C. The effect of the sulfur melting-diffusion time [sulfur infiltration time] (1–24 h) and sulfur content (10–70%) is investigated by using XRD, SEM, TEM and TGA analyses and correlated with the electrochemical performance in Li-S cells. S/C composite electrode with homogeneous sulfur distribution can be reached with 6 h of sulfur melting-diffusion and 10 wt.% of sulfur content. Li-S cell with this composite shows a high use of sulfur and sufficient electronic conductivity achieving an initial discharge capacity of 983 mA h g−1 and Coulombic efficiency of 99% after 100 cycles.

ResumenSe evaluó el efecto de la adición de metano en un carbonizado de carbón sometido a reacción con concentraciones de oxígeno bajas. Desde un punto de vista cinético, dicho efecto es determinando por la energía de activación (Ea) y por el factor preexponencial para diferentes concentraciones de oxígeno y mezclas de oxígeno y metano. Se preparó un carbonizado con un tamaño de partícula de 75 a 150 µm a partir de un carbón bituminoso altamente volátil, y se determinaron los parámetros cinéticos de la oxidación empleando el método termogravimétrico de velocidad máxima. Las variables evaluadas en la cinética de oxidación fueron la concentración de oxígeno (5, 8 y 21 %), y el efecto de la adición de metano en el medio de reacción (5, 10 y 14 % de metano mezclado con 5 % de oxígeno). Los resultados mostraron que la Ea disminuía al aumentar la concentración de oxígeno, en tanto que al agregar metano se observó un efecto promotor cuando este correspondía al 5 % de la mezcla total; sin embargo, cuando el porcentaje de metano aumentaba, la Ea también lo hacía debido a la rápida reacción del oxígeno con el metano en fase homogénea comparada con la reacción heterogénea del oxígeno con el carbonizado. La Ea obtenida con 5 % de oxígeno fue de 122,9 kJ/mol y con 21 %, de 90,69 kJ/mol; en presencia de metano al 5 % y 5 % de oxígeno, la Ea fue de 110,7 kJ/mol, y aumentó hasta 170,8 kJ/mol cuando el metano representaba el 14 % de la mezcla. © 2017. Acad. Colomb. Cienc. Ex. Fis. Nat. Palabras clave: Parámetros cinéticos; Carbonizado de carbón pulverizado; Contenido de metano; Contenido de oxígeno. Effect of oxygen and methane on the oxidation kinetics of coal char AbstractIn this work we evaluated the effect of adding methane during the oxidation process of a coal char with low oxygen concentration. We analyzed the effect from a kinetic point of view and we determined the activation energy (Ea) and the pre-exponential factor (A) with different oxygen concentrations and oxygen and methane mixtures. We used a coal char from a highly volatile bituminous coal with particle size of 75 to 150 µm; the kinetic parameters were determined by thermogravimetric method (TGA) with the high-velocity model; oxygen concentration variations were studied with 5, 8 and 21% of oxygen, as well as the effect of adding methane (5,10 and 14%) when oxidation occurred with 5% of oxygen. The results showed that the Ea decreased when oxygen concentration increased, and that adding 5% of methane to the total mixture enhanced the effect. Instead, when we increased the methane percentage, the Ea increased due to the rapid reaction of oxygen and methane in the homogeneous phase as compared with the heterogenous reaction of oxygen and coal char. With 5% oxygen the Ea value was 122.9 kJ/mol and with 21%, it was 90.69 kJ/mol; when methane was added at 5%, the Ea value was 110.7 kJ/mol, and it increased to 170.8 kJ/mol when there was 14% of methane in the mixture.

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