Activated Carbon/Sulfur Composite Cathode Materials For Lithium-Sulfur Batteries

Lithium–sulfur(Li–S)batteries are considered one of the most promising next-generation secondary batteries because of their high theoretical specific capacity(1675 mA h g-1)and theoretical energy density(2600 W h kg-1).However,sulfur cathodes are still associated with several inherent hurdles:(1)the poor electronic/ionic conductivity of both elemental sulfur and discharge product Li2S;(2)the high solubility of long-chain lithium polysulfides(Li2Sn,4 ? n ? 8)in conventional organic electrolytes;(3)the volume expansion of sulfur during discharge process.One of the most effective solutions to the above mentioned problems is loading sulfur into electronically and ionically conductive frameworks that can trap polysulfides.KOH–activated carbon materials are a promising framework for sulfur cathode because of their high specific surface area,large pore volume,small nanopore structure,and high electric conductivity.Herein,we investigated the electrochemical performance of activated carbon materials with ultrahigh specific surface area,activated graphene materials with tailored pore structure parameters,in situ sulfur reduction graphene oxide-coated activated graphene materials,and 3D honeycomb-like activated carbon materials for encapsulation of sulfur as cathode for Li–S batteries.Activated carbon materials with ultrahigh specific surface areas(3164 m2 g-1 and 3334 m2 g-1)were prepared from waste litchi shells and waste mango stones via KOH activation method.The macroporous structure of raw materials is believed to be conducive to distribute the activation agent,which enables the sufficient activation.The as-prepared activated carbons were developed as conducting frameworks for lithium–sulfur battery cathode materials.The resulting activated carbon/sulfur composite cathode possesses high specific capacity,good rate capability,and long-term cycling performance.The ultrahigh specific surface area and large pore volume not only enhance the sulfur content(> 70 wt%)but also ensure dispersion of elemental sulfur in the conducting framework,thereby improving sulfur utilization.The small nanopores of the activated carbons can effectively inhibit the diffusion of polysulfides during charge/discharge process.Activated graphenes with tailored pore structure parameters were fabricated,and the impact of physical structural parameters(specific surface area and pore volume)and various sulfur loading levels on the electrochemical performance of Li–S battery were systematically investigated.The high specific surface area and pore volume of the activated graphene matrices can achieve high sulfur loading and boost sulfur utilization in electrochemical reactions.Specifically,the activated graphene matrix(AG3)with a high specific surface area(3064 m2 g-1)and pore volume(2.18 cm3 g-1)introduces a more reaction sites,which can improve electrical contact to insulating sulfur and discharge products(Li2S2/Li2S)and reduce pore blocking during charging/discharging.Hence,when the long-term stability of the AG3/S composite electrode with a high sulfur loading of 72 wt% was tested up to 1000 cycles,the cells exhibit excellent capacity retention of 50% at 800 mA g-1.A reduced graphene oxide coated activated graphene/sulfur multicomposites were synthesized via an in situ sulfur reduction method.The in situ sulfur reduction graphene coating layer can dramatically promote the specific capacity and long-term cycling performance in electrochemical reactions.Such a graphene coating layer facilitates electronic transport at high current rate and effectively prevents the polysulfides from dissolving in the electrolyte.Moreover,the obtained saccule-like structure has enough space to accommodate volumetric expansion of sulfur during charge–discharge process.Hence,when the long-term stability of the multicomposite electrode was tested up to 2000 cycles,the cells exhibit excellent capacity retention of 39% at a high current rate of 1600 mA g-1.3D honeycomb-like hierarchical activated carbon materials were prepared from rapeseed shells via KOH activation method.The unique 3D honeycomb-like hierarchical structure is consisted of numerous interconnected macropores with a diameter changing from several hundred of nanometres to several microns;and the porous wall is composed of interconnected free-standing nanosheets with the thickness of dozens of nanometers.The as-prepared activated carbons were developed as conducting frameworks for lithium–sulfur battery cathode materials.The resulting activated carbon/sulfur composite cathodes possess excellent capacity retention of 49% at a high current rate of 1600 mA g-1 over 1000 cycles.Note these macropore networks facilitate the diffusion process of the electrolyte ions into the inner micropores at high rate.Meanwhile,the thin porous wall is great propitious to fast electron transfer throughout the whole electrode matrix.