Synthesis And Regulating The Redox-Kinetics Of Li₂S/Carbon Cathodes For High-Energy Lithium Secondary Batteries

Lithium-secondary batteries have been recognized as one of the most powerful techniques for the storage of clean energy.The development of lithium secondary batteries with high performance but environmental benignity is essential to strengthen the electric vehicle industry and is the key route for the sustainable development of energy and envirorment in China.The performance of lithium-secondary batteries is heavily relied on the electrode materials.Specifically,the conventional intercalation-type cathode materials with limited theoretical capacities have been the bottle-neck issue for enhancing the specific energy of lithium-secondary batteries.Among available lithium-secondary batteries,lithium-sulfur batteries based on high-capacity sulfur cathode and metallic lithium anode have attracted extensive interests because of its very high energy density.One of the critical obstacles for its practical use is the potentially safety hazards and fast capacity decay caused by dendrites and pulverization of metallic lithium anode during the repeated cycling.In this study,we will devote to develop the new strategy for elegant design and controllable fabrication of high-performance but low-cost Li2S/carbon nanocomposites with novel structure and accelerated reaction kinetics,and overcome the obstacles in practical implementation of Li2S cathodes.The mechanism involved in the material fabrication,nanostructure engineering,components tuning and electrochemical performance will be synergistically understood in multiple scale and dimensions.Finally,the high-performance Li2S/C cathode materials will be exploited for the construction of rechargeable lithium batteries with high specific energy density but high safety by coupling with lithium-metal free high-capacity anode materials.With cheap air-stable inorganic lithium salt and commercial polymer as precursors,a controllable but low-cost strategy is developed for production of flexible Li2S/C composite paper cathode by combining the electrospinning technique and carbothermal reduction.The synergistic effect of three-dimensional porous conductive network and interfacial coupling between Li2S nanoparticles and N-doped carbon nanofibers enhance the redox activity,structural stability and charge/mass transport of the flexible Li2S/C cathode simultaneously.The flexible cathode could be directly applied in lithium-sulfur batteries without a metal current collector,conductive agent and binders in routine electrode.As a result,the cathode with Li2S loading of 3.0 mg cm-2 delivers superior capacity of 460-730 mAh g-1 and cycling stability at 0.2-2.0 C.Further increasing the L12S loading by layer-by-layer stacking,very high Li2S loading of up to 9.0 1g cm-2 could be achieved to deliver high areal specific capacity of 5.76 mAh cm-2.When paired with high-capacity Fe3O4 anode to construct lithium-secondary batteries,a high specific energy of 403 Wh kg-1 can be achieved,which is 2-3 fold higher than that of the now available lithium-ion batteries.A new porous ultrafine Li2S/C composite cathodic material is developed by taking cheap molecular inorganic lithium salt clusters encapsulated in metal-organic molecular cages as precursors and combining thermal reduction-chemical transformation reaction in the nano-confined space.The Li2S/C composites consist of metal-organic molecular cages derived N-doped porous carbon and nano-scale metal/metal sulfide enabled adsorption-catalytic interface.As the ultra-small size of Li2S naoparticles and adsorption-catalytic interface,the cathode exhibits high electrochemical activity/low overpotential for initial Li2S activation,and significantly enhanced conversion kinetics of polysulfides and reversibility.As a result,the Li2S/C-ZnS cathode with Li2S loading of 2.0 mg cm-2 delivers superior capacities of 570-820 mAh g-1 at 0.2-8.0 C and long lifetime of 1000 cycles.When coupling with Si anode,the rechargeable Li2S//Si full cells deliver high specific energy of 673 Wh kg-1 with good capacity retention of 57.7%for 200 cycles.Another case,the Li2S/C-Co cathode delivers a high capacity of 904 mAh g-1 with Li2S loading of 2.0 mg cm-2 at 0.2 C.When coupling with Si anode,the rechargeable Li2S//Si full cells deliver high specific energy of 651 Wh kg-1 and reversibly cycling for 500 cycles.A new strategy of introducing redox mediator is proposed for reducing the energy barrier and overpotential of Li2S activation.By introducing slight Na2S as redox mediator generator within Li2S cathodes based commercial Li2S,efficient Li2S activation and high reversible specific capacity is realized with soluble polysulfides as redox mediator from the preferential oxidation of Na2S in the charging process.This method greatly reduces the processing complexity and costs for complicated nanofabrication of highly active Li2S,and has been demonstrated to be effective in decreasing the inactive components(conductive agent and electrolyte)in Li2S cathode and enhancing the energy density of the cells.With 5 wt.%Na2S,efficient Li2S activation at 2.4 V with negligible overpotential is achieved,and the excellent specific capacity of 576 mAh g-1 at 0.1 C are enabled under challenging conditions of low electrolyte/Li2S ratio(4 μl mgLi2S-1),extremely high Li2S loading(14.0 mg cm-2)and 75 wt.%Li2S in the whole cathode with high areal specific capacity of up to 8.06 mAh cm-2.