Design And Preparation Of Cathode,Barrier,and Separator For Lithium-Sulfur Batteries And Their Electrochemical Performance

Lithium sulfur batteries,one of the most promising energy storage methodologies for emerging electric vehicles,suffer from poor long-term cycling stability due to the shuttle effect caused by the dissolution of high order polysulfides.To enhance the cycling stability of sulfur cathode for high-energy lithium sulfur batteries,it is very critical to mitigating the diffusion of polysulfides to the anode.Therefore,this paper mainly studied the modification and mechanism of the cathode material,barrier,and separator to improve the electrochemical performance of lithium sulfur batteries.1.Caterpillar-like graphene confining sulfur by restacking effect for high performance lithium sulfur batteries:A caterpillar-like and reconfigurable graphene was designed to serve as the sulfur host.The caterpillar-like graphene highly expanded in solution and tightly restacked in dry condition due to the van der Waals force.Elemental sulfur was trapped and confined inside the restacked graphene layers.High mass loading of 63.8%sulfur in graphene was achieved after the caterpillar-like graphene was dried at 155?.The graphene-sulfur electrode has a good rate performance of 708 mAhg^-1 at 167.5 mAg^-1,582 mAhg^-1 at 335 mAg^-1,470 mAhg^-1 at 837.5 mAg^-1,400 mAhg^-1 at 1675 mAg^-1 and a stable cycling performance with small capacity decay of 0.16%per cycle over 200 cycles at 1675 mAg^-1.Moreover,the underlying mechanism of the restacking effect of caterpillar-like graphene on immobilizing the soluble lithium polysulfides was studied by density functional theory?DFT?,which clearly explained how the graphene immobilized the soluble lithium polysulfides by the restacking effect.2.Ad hoc solid electrolyte on acidized carbon nanotube paper improves cycle life of lithium-sulfur batteries:The performance of lithium-sulfur?Li-S?battery is greatly improved by using acidized carbon nanotube paper?ACNTP?to induce in situ polymerization of ether-based DOL/DME liquid to grow an ion-selective solid barrier,to seal in soluble polysulfides on the cathode side.The Li-S battery with the in situ barrier showed an initial specific capacity of 683 mAhg^-1 at a high current density of 1675 mAg^-1,and maintained a discharge capacity of 454 mAhg^-1 after 400cycles.The capacity decay rate was 0.1%per cycle and a high Coulombic efficiency of 99%was achieved.Experimental characterizations and theoretical models demonstrate the in situ polymerized solid barrier stops sulfur transport while still allowing bidirectional Li⁺transport,alleviating the shuttle effect and increasing the cycling performance.The soft and sticky nature of the solid electrolyte barrier makes it a good sealant,forming an enclosed catholyte chamber on the sulfur side.3.Absorption mechanism of carbon-nanotube paper-titanium dioxide as a multifunctional barrier material for lithium-sulfur batteries:We discover a multifunctional carbon nanotube paper/titanium dioxide barrier can effectively reduce the loss of active materials and restrain the diffusion of lithium polysulfides to the anode,thereby improving the cycling stability of lithium-sulfur batteries.Using carbon nanotube paper/titanium dioxide as the multifunctional barrier,the activated carbon/sulfur cathode with a high sulfur content of 70%delivers a stable cycling performance and high Coulombic efficiency?⁹9%?over 250 cycles at a current rate of 0.5 C.The improved electrochemical performance is attributed to the synergistic effects of the carbon nanotube paper and titanium dioxide,involving the physical barrier,the chemical adsorption due to the binding formation of Ti-S and S-O,and the other unique interactions between titanium dioxide and sulfur species.4.Thin Multifunctional Coating on Separator Improves Cyclability and Safety of Lithium Sulfur Battery:Lithium-sulfur battery is impeded by low utilization of insulating sulfur,dendritic growth of lithium metal anode,and transport of soluble polysulfides.By coating a layer of nano titanium dioxide and carbon black onto commercial polypropylene separator,we demonstrate a new composite separator that can confine the polysulfides within the cathode side,forming a catholyte chamber,and at the same time block the dendritic lithium in the anode side.Lithium-sulfur batteries using this separator show a high initial capacity of 1206 mAhg^-1 and a low capacity decay rate of0.1%per cycle at a current density of 0.5 C.Further theoretical analyses and experiments reveal the catalytic effects and the excellent dendrite-blocking capability of the⁷-mm thick coating.5.Nano-sized Titanium Nitride Functionalized Separator Improves Cycling Performance of Lithium Sulfur Batteries:We design a separator functionalized by a simple coating of nanosized titanium nitride and super C65.Lithium-sulfur batteries with the functional separator achieve a high specific capacity of 935 mAhg^-1 at a current density of 0.5 C,and show a super-long cycling performance at a small degradation rate of only 0.06%per cycle over 1000 cycles.The improvement mechanisms introduced by the new separator are further investigated by the electrochemical analysis and theoretical calculation.