The Application Of Porous Biomass-based Carbon Materials In Li-S Batteries And Their Electrochemical Performances

Lithium sulfur batteries have a high theoretical capacity of(1675 mAh/g) and theoretical power density of(2600 Wh/kg), as sulfur is cheap, abundant in nature and environmentally friendly, so it is considered as the best choice for the next generation high energy density Li-ion batteries. But the commercial utilization of lithium sulfur batteries is still hindered by some problems: one aspect is the low electronic conductivity of element sulfur, which makes the efficiency of electrons transfer on the materials are very low, severely restricts the rate capability of the battery. In addition, the dissolution of long chain lithium polysulfides into the organic electrolyte during the discharging process, which can cause large capacity loss and low coulombic efficiency. Focusing on these problems, the biomass carbon materials is the object of study in this paper, with different biomass materials as carbon sources, through chemical pore-forming and high temperature pyrolysis method to synthesis different porous biomass carbon, then measure the conductivity and specific surface area and other related characters of the carbon, in order to select out high conductivity and high specific surface area porous biomass matrix to composite with sulfur, making a new superior cathode of lithium sulfur batteries. In this paper, the ingredient and microstructure analysis of samples were studied by X-ray diffraction(XRD), Raman spectra, BET, scanning electron microscopy(SEM) and transmission electron microscopy(TEM). By using the constant current charge and discharge technology and AC impedance test to analysis the electrochemical performance of composite materials. The research work in the paper mainly includes the following two aspects:(1) With biomass materials of bamboo as the carbon sources, potassium hydroxide solution as the pore-forming agent, the vacuum tube furnace was used to synthesis porous bamboo materials and composited with sulfur. The results showed that when the concentration of potassium hydroxide solution was 1.5 mol/L, carbonization temperature was 850 °C, the activated porous bamboo carbon acquired specific surface area of 1565.4 m~2/g, pore volume of 0.95 cm~3/g. The sulfur contents in composite was 86%, when the discharge current density is 0.1 C, the first discharge capacity was 1160 mAh/g, then cycled in the current density of 0.2 C, the first and the 100 th discharge capacity were 930 mAh/g and 710 mAh/g, respectively. The coulombic efficiency was more than 90%, and the electrochemical performance was superior to the battery with the pure sulfur as the cathode.(2) With biomass materials of wheat straw as the carbon sources, through the activation process and high temperature pyrolysis method to synthesis high conductivity and large specific surface area porous wheat straw carbon, the SSA and pore volume were 1066 m~2/g and 0.62 cm~3/g, respectively, the average pore size was 2.33 nm. The results showed that when the sulfur content was 74%, the current density was 0.1 C, the first discharge capacity was 1213 mAh/g, the 100 th discharge capacity was 870 mAh/g, the attenuation of a single cycle was about 0.11% and the cycling performance was stable.