Preparation Of 3D Graphene Composite And Its Application In Lithium-Sulfur Battery

With the increasing energy density requirements of lithium-ion secondary batteries for electric vehicles and portable electronic devices,conventional lithium-ion batteries are difficult to satisfy with the high capacity requirements of today's power batteries due to low energy density.Among them,lithium-sulfur?Li-S?battery using elemental sulfur as cathode materials have attracted extensive attention due to its high theoretical specific capacity(1675 mAh g^-1)and theoretical energy density(2600 Wh kg^-1).Therefore,Li-S battery is considered as one of the most promising high energy density secondary batteries.However,the development of Li-S battery is still struggled by many challenges,mainly due to the electronic insulation of sulfur,the volume change of sulfur during charging and discharging,and the shuttle effect of polysulfide,resulting in low actua l capacity and poor cycle stability for Li-S battery.Therefore,in order to improve the above problems,this study constructs sulfur cathode based on graphene composite material,and polysulfides are immobilized by both physical confinement and chemical adsorption to improve the cycle performance and rate performance of Li-S battery.The main research contents of this paper are divided into the fol owing aspects:First,the initial discharge capacity of graphene decorated by cobalt-nickel metal oxide as sulfur host material exhibits a discharge specific capacity of 746 mAh g^-1 at1 C,and the capacity decreases at 489 mAh g^-1 after 300 cycles.However,static absorption experiments shows that cobalt-nickel metal oxides decorated by graphene can only adsorb a portion of polysulfides.Second,cobalt-nickel sulfide and N co-doped graphene?CNS/N-rGO?is synthesized by adding thiourea as nitrogen and sulfur source to graphene oxide with cobalt and nickel salts.The size of the formed cobalt-nickel sulfide?CNS?particles is approximately 4-7 nm.The initial discharge capacity of the CNS/N-rGO/S cathode is766 mAh g^-1 at a rate of 1 C.After 500 charge/discharge cycles,the CNS/N-rGO/S cathode still has a capacity of 526 mAh g^-1,and the capacity decay rate is only 0.063%per cycle.The stable cycling performance of CNS/N-rGO/S is attributed to the fact that N-doping improves the conductivity of graphene,and N and CNS nanoparticles can serve as effective active sites for anchoring polysulfides.Third,in order to further improve the electrochemical performance of Li-S battery,CNS/N-rGO composite material is used as the interlayer,and it is found that the addition of the interlayer can improve the cycle performance and rate performance of the battery at high rate.The battery with CNS/N-rGO interlayer maintains a capacity of 610 mAh g^-1 at high rate of 8 C.At the same time,the battery still has a reversible capacity of 600 mAh g^-1 after 350 cycles at a high rate of 2 C.This is attributed to the double adsorption of polysulfides by CNS/N-rGO interlayer,the physical confinement of carbon skeleton and the strong chemisorption of N and CNS particles to anchor polysulfides,and the synergistic effect of the two alleviates the shuttle of polysulfides.