| Lithium-sulfur batteries are endowed with ultrahigh energy density(2500 Wh kg-1)in theory.Meanwhile,sulfur is earth-abundant,low-cost and largely friendly to the environment.These merits make Li-S batteries a highly promising next-generation energy storage system.However,the complex and multiple-phase electrochemical conversions in the working Li-S cells lead to the serious inherent limits for their practical application,containing the sluggish redox reactions of sulfur species,the dramatic volume change in the cathode during prolonged cycles,and the shuttling of dissolved intermediate polysulfides between cathode and anode.These problems result in the unconverted sulfur species,rapid capacity damping,and low energy conversion efficiency,especially under the conditions of high sulfur area loading and low electrolyte usage for practical applications.Currently,conductive and polar materials are mostly used to serve as sulfur hosts to enhance the dynamics of Li-S reactions and restrain polysulfides shuttling,on the aim of improving the electrochemical performance of Li-S batteries.In this paper,based on the graphene with lots of excellent features,we have focused on the design and construction of graphene-modified cathode for Li-S batteries.Sulfur atoms are doped in graphene lattice to increase the anchoring ability with polysulfides and provide highly active catalytic cites,and a multi-functional hierarchical cathode including the polysulfides intercept layer was constructed by graphene for solving the problems of the severe shuttle and slow transformation of high concentration polysulfides in Li-S full batteries.The main results are summarized as follows:?1?By using the commercial low-defect and highly conductive graphene as sulfur species framework,a porous composite cathode of graphene and sulfur?HCG/S?was prepared by a facile and efficient ball-milling technology.The high energy of ball-milling makes large sulfur particles into ultrafine nanodots on graphene nanosheets,simultaneously introduces a high quality of sulfur atoms into graphene lattice with C-S bonds.HCG/S cathode with high sulfur content?88.5 wt.%?could deliver a high full cathode capacity,a remarkable rate capacity and excellent cycling stability,along with a high area capacity of 4.4 m Ah cm-2 after 100 cycles at a sulfur loading of 6 mg cm-2.As demonstrated by computational simulation and adsorption experiments,graphene with sulfur dopants is effective to absorb polysulfides,leading to the much suppressive shuttle effect.The three-dimensional graphene networks also help to offer high-speed channels for both lithium ions and electrons,together with the porous structure and excellent mechanical properties to maintain the cathode integrality during long-term discharge and charge cycles.This work demonstrates that combining low-defect graphene and sulfur for composite cathode by ball-milling could efficiently resolve the problems of the insulation of sulfur and lithium sulfide,the collapse of cathode structure,and polysulfides shuttling in Li-S battery.?2?Vertical graphene arrays?VG?were grown on carbon nanofibers substrate by the CVD method,using ball-milling to effectively fabricate even mixing of graphene,carbon nanofiber,and sulfur composite while inducing adequate in situ doping of a large quality of sulfur atoms and sulfur-containing functional groups into graphene.This results in a three-dimensional porous S-doped graphene/S composite cathode?SG/S?.The influence of changed lattice structure and surface chemistry of VG for Li-S cells was studied.Adsorption experiments indicate that sulfur doped graphene has excellent affinity to entire sulfur species.The cyclic voltammetry of symmetric battery and the potentiostatic step discharge results found that SG could propel sulfur species redox reactions.Li-S cells with SG/S cathode show a high specific capacity of 1216m Ah g-1,together with remarkable rate capacity and cycling stability.SG/S cathode could also deliver high areal capacity above 5.2 m Ah cm-2 at 0.2 C at a sulfur loading of 8.0 mg cm-2,and high reversible areal capacities of 13.1 m Ah cm-2 at a sulfur load of 15 mg cm-2 and a low electrolyte/sulfur?E/S?of 5?L mg-1.In this work,sulfur doped graphene with both high conductivity and polarity as an effective metal-free electrocatalyst significantly promoted polysulfides conversions.?3?On the aim to further ameliorate the more severe shuttle effect and slow transformations of high concentration polysulfides in Li-S full batteries,the close-layered graphene for intercepting polysulfides is in cooperation with titanium dioxide catalytic nanodots decorated carbon nanofiber networks?CNFs/Ti O2?to construct a multi-functional hierarchical cathode?MFHC?towards the synergistic effect of polysulfides capturing,catalyzing and intercepting.RGO/S nanosheets were self-assembled into a layered structure on CNFs/Ti O2 for CNFs/Ti O2@RGO/S electrode.Comprehensive characterizations including in situ electrochemical investigations confirm that the migration of sulfur from RGO into the polar CNFs/Ti O2 interlayer was invoked during the solid-liquid-solid reaction,forming the MFHC with graphene intercepting layer.This cathode with both blocking effect and remarkable electrocatalysis could deliver a high sulfur conversion rate at the high sulfur area loading and low E/S.We further propose a general design for constructing the multi-functional hierarchical cathode which contains sulfur source layer,capturing-catalyzing layer and intercepting layer.Li-S full cells based on MFHC cathodes can provide high energy density of about 300 Wh kg-1 and 450 Wh L-1 at high sulfur loading(13 mg cm-2),low E/S(4?L mg-1)and anode/cathode capacity ratio?<4:1?.This research on the multi-functional hierarchical cathode paves a feasible pathway to solve the problems of Li-S full batteries for its practical fabrication. |
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