Sulfur Cathode Composites Design And Electrochemical Performance Evaluation For High-performance Lithium Sulfur Batteries

Lithium-sulfur?Li-S?batteries are desirable energy storage systems due to their overwhelming theoretical energy density(2500 Wh kg^-1),abundant resources,environmental friendliness and low costs.In spite of the unparalleled merits,commercialization of Li-S batteries is still hindered by low utilization of sulfur and sluggish electrochemical reaction kinetics.It is well accepted that nanocarbon hosts are the promising candidates to address the aforementioned problems through optimizing the conductivity and the electron transfer accesses during electrochemical reaction.However,reports have paid little attention to the modulation of ion transport accesses across the interface,and the accelerated electrochemical reaction kinetics by catalytic effect have not yet been analyzed systematically.Herein,we are mainly direct to the design of electron and ion transfer pathways of the hosts,modulation of the electrochemical reaction rate by catalyst application,so as to reduce energy barrier of polysulfide interconversions,improve the electrochemical reaction kinetics and boost the utilization rate of active material,realizing high-performance Li-S batteries.A TiN bridging conductive cathode construction is proposed.The bridging TiN is able to modulate the electron and ion transfer pathway along the radial direction of CNTs.Meanwhile,the introduced TiN also bonds with polysulfides attributed to the formation of S-Ti-N and S-N-Ti interactions,improving the polysulfide trapping ability of the host.After sulfur loading?63 wt%?,the 10-TiN@CNT-S cathode with TiN content of 10 wt%exhibits a reversible capacity of 594 mA h g^-1 at 1 C after 400 cycles,corresponding to capacity decay rate 0.047%per cycle.In order to advance the sulfur embedding capability and the sulfur areal density of the cathode,the polar hierarchical-porous carbon?PHPC?container with well-optimized S,N polar sites and large pore volume is realized through simultaneously modulating the ion transfer pathway,surficial polarity environment and pore volume by hydrothermal doping.The hierarchical-porous structure constructs interconnected porous channels in the cathode,pushing the electrolyte immersion and ion diffusion.The as-prepared cathode PHPC2@S?the mass ratio of dopant to carbon source of 2:1?with high sulfur loading of 75 wt%exhibits excellent electrochemical performance:it reveals a rate capacity up to 581 mA h g^-1 at 8 C and a stable cyclability at 2 C over 400 cycles.With the high sulfur areal density up to 7.5 mg cm-2,the areal capacity as high as 9.75 mA h cm-2 at 0.05 C is reached and the reversible areal capacity of 5 mA h cm-2 is remained at 1 C after 80 cycles.The cathode also can successfully works in a simulated pouch cell and stabled for 40 cycles with E/S ratio as low as 7 ? L mg^-1.Based on the above studies,it is found that to improve the life-time of the Li-S batteries at high rate,further boosting the conversion kinetics among sulfur species is a feasible method.Taking advantage of the fast ion and electron transfer pathways constructed by mesoporous carbon with carbon nanotubes?MPC/CNT?and simultaneously implanting the high-activity nano tungsten carbide catalyst,the electron and ion transfer rate of the cathode is promoted significantly.The nano WC is effective in anchoring and catalyzing the interconversion of the polysulfides,decreasing the delithiation voltage?2.18 V?,thus,the electrochemical reaction kinetics is boosted.Although the sulfur loading of the as-synthesized cathode MPC/CNT-WC10@S?WC content of 10 wt%?is raised to 76 wt%,the long-cycle cell at high rate is achieved:it delivers a high initial capacity of 707 mA h g^-1 at high rate of 5 C with a low capacity fading rate 0.039%per cycle after 500 cycles.Learning from the synthetical results,a cathode with fast ion/electron transfer accesses and effective catalytic effect is designed,which depends on the multi-functional carbon tunnels with diverse catalytic sites inheriting form biomass rice straws.The ionic tunnels ensure the fast ion/electron transfer access to reach the active material.And the large amount of polar heteroatom sites on the tunnel surface shows strong capability of anchoring and catalyzing the conversions of polysulfides,which decreases the delithiation voltage?2.22 V?,thus boosting the electrochemical reaction kinetics.The as-synthesized cathode?76 wt%?displays potential of fast charge-discharge and stable long-cycles:it shows high rate capacity of 564 mA h g^-1 at 6 C and a ultralong life of 2000 cycles at high rate 3 C with capacity decay rate as low as 0.029%per cycle.Even under high areal sulfur loading up to 5.8 mg cm^-2,the cathode at high current density of 9.7 mA cm^-2?1 C?can keep stable over 100 cycles.