Preparation And Application Of Tungsten-based Compounds For Lithium-Sulfur Batteries

With the rise of new energy electric vehicles on the market,people have higher requirements for the energy density of batteries.Compared with lithium-ion batteries,lithium-sulfur（Li-S）batteries are regarded as the next generation of electrochemistry due to their high theoretical specific capacity(1675 m A h g^-1)and energy density(2600 Wh Kg^-1).However,the low conductivity of sulfur in lithium-sulfur batteries and the shuttle effect of intermediate polysulfides have severely restricted its development.The modification of the cathode material and the separator are a important way to solve these problem.In this paper,the metal oxide and the metal carbide were designed as an efficient carrier for sulfur and separator modification due to the unique structure,then the polysulfides adsorption were explored.The details are as follows:（1）The flower-like and spherical structure of WO₃materials were prepared by hydrothermal and one-step pyrolysis method,then the WO₃/S cathode was obtained by mixing it with sulfur element by melt diffusion method.The flower-like and spherical structured cathode has initial discharge specific capacities of 1032 and 1002m A h g^-1at 0.1 A g^-1,respectively.Even after 500 cycles at a high current density of 1A g^-1,the capacity remained at 312 and 160 m A h g^-1,respectively.This shows that the flower-like and spherical structure of WO₃as a sulfur carrier improves the utilization rate of sulfur in the cathode to a certain extent,and improves the performance of the Li-S batteries.（2）WC and WC/Fe materials were prepared by hydrothermal method,then WC/S and WC/Fe/S composite cathode was obtained by melting and diffusing with sulfur in order to further explore the influence of metal compounds as sulfur carriers on the performance of lithium-sulfur batteries.The characterization of the material shows that Fe is uniformly dispersed in the WC microspheres.On the one hand,the excellent electrocatalysis and polar effect of WC can effectively adsorb polysulfides.On the other hand,the introduction of Fe improves the conductivity of the material.The synergy with Fe has improved the cycle stability of the battery to a certain extent.WC/Fe/S cathode at different current densities(0.1、0.2、0.5、1、2、3、5 A g^-1)showed the 1360、966、817、715、637、587、541 m A h g^-1initial discharge specific capacity,respectively.At the current density of 0.5 A g^-1,the first discharge capacity is 815 m A h g^-1.After 200 cycles,the capacity remains at 560 m A h g^-1,with an attenuation of0.16%per cycle;At a current density of 1 A g^-1,the cycle after 350 times,there is still a reversible capacity of 466m Ah g^-1,and the attenuation is only 0.09%per revolution.（3）The bimetallic Fe₃W₃C/WC composite material was prepared by hydrothermal and one-step pyrolysis,and coated on a commercial PP separator to obtain Fe₃W₃C/WC-PP separator.Experiments show that the inherent polarity of bimetallic carbides can effectively adsorb polysulfides and accelerate the kinetic process of the redox reaction of the battery.Therefore,the performance of the battery has been steadily improved through physical barriers and chemical combinations.Fe₃W₃C/WC-PP as separator has an initial specific discharge capacity of 1382 m A h g^-1at a current density of 0.1 A g^-1;at a current density of 1 A g^-1,the specific discharge capacity is 402 m A h g^-1after 1000 cycles,which only attenuates 0.059%per cycle.When the sulfur load is 4.8 mg cm^-2and 7.2 mg cm^-2,at a current density of0.5 A g^-1,there are still initial discharge specific capacities of 802 and 556 m A h g^-1,respectively.After 150 long cycles the capacity is maintained at 544 and 366 m A h g^-1,and the capacity retention rate is 67.8%and 65.9%,respectively.This results show that the Fe₃W₃C/WC-PP separator has a certain improvement in the rate performance and long-cycle stability of lithium-sulfur batteries.