Microstructure Design Of Titanium-Contained Carbon Nanofibers For High Performance Cathode In Li-S Batteries

Since the electric vehicle and portable electronic devices have been indispensable parts of human life,the traditional lithium-ion batteries would hardly meet the increasing needs of people for high energy density energy storage systems.Lithium-sulfur（Li-S）batteries have been considered as one of the most promising candidates for the next-generation energy storage technique due to its ultrahigh theoretical capacity(1675 m Ah g^-1)and earth abundance.However,there are also some inherent problems in Li-S batteries:（a）the low utilization of the active material caused by the poor conductivity of sulfur and its discharge product Li₂S or Li₂S₂;（b）the poor electrode structural stability arising from the large volumetric expansion（nearly 80%）during the charge-discharge process;（c）the irreversible capacity decay incurred from the shuttle effect of the intermediate lithium polysulfides（Li PSs）.Accordingly,two novel freestanding sulfur cathodes were prepared and studied based on the nanostructure design of titanium-containing carbon nanofibers in this paper.The details and conclusions are as follows:1.Integrated polar TiN layer and conductive carbon layer in-site over each fiber of TiO₂-CNFs,we synthesized an intact coaxial multilayered yolk-shell TiO₂/carbonnanofibers@void@TiN@carbon（TiO₂-CNFs@void@TiN@C）composite.And a freestanding TiO₂-CNFs@void@TiN@C/S cathode could be obtained after composited with sulfur by a modified melt-diffusion method.In this TiO₂-CNFs@void@TiN@C/S cathode,the TiN@C shell can not only increase the charge transfer efficiency and improve the reaction kinetics,but also serves as a physical barrier for Li PSs;The large specific surface area and hollow space can provide abundant active sites and reaction space for sulfur,thereby accommodating the volume expansion;the polar TiO₂ nanoparticles with strong absorbing ability for Li PSs can effectively inhibit the"shuttle"effect".Therefore,this TiO₂-CNFs@void@TiN@C/S cathode shows excellent electrochemical performance:The cathode shows a reversible discharge capacity of 675.8 m Ah g^-1 after 1000 cycles at 1 C and the average decay rate is as low as 0.054%per cycle;With the sulfur loadings of 3.6,6.2,and 9.5 mg cm^-2,the reversible discharge capacities maintain at,respectively,987.5,868.7,and668.5 m Ah g^-1 after 100 cycles.2.In this work,we synthesized a freestanding porous TiO/carbon nanofibers（TiO-CNFs）membrane and discussed its formation mechanism.After coating sulfur slurry onto the surface of TiO-CNFs,a freestanding TiO-CNFs/S cathode was obtained.Compared with the cathode coating on commercial aluminum foil,the freestanding TiO-CNFs/S cathode shows the following merits:（1）The TiO-CNFs membrane is lighter than aluminum foil,thus increasing the energy density of batteries;（2）The highly conductive CNFs network could enhance the conductivity and facilitate charge transfer of the cathode;（3）The large specific surface area and porous structure could provide sufficient reaction space for Li PSs;（4）The polar and conductive TiO nanoparticles can effectively adsorb and catalyze Li PSs.Therefore,this porous TiO-CNFs/S cathode exhibits excellent electrochemical performance:At the current densities of 0.1 C,0.2 C,0.5 C,1 C,2 C and 5 C,the specific capacities could reach1136.8 and 1105.3,1011.8,926.9,825.5,and 675 m Ah g^-1,respectively;After 500cycles at 1 C rate,the cathode still maintains a high specific capacity of 754.3 m Ah g^-1,showing an extremely high capacity retention rate.