Controllable Design And Synthesis Of Conducting Polymers-Sulfur Composites For Lithium-Sulfur Battery

Environmental pollution,exhaustion of fossil fuels,and increasing demands for consumer devices cause great urgent to develop the next-generation of safe,low cost,and high energy density rechargeable batteries.Among the most promising candidates,such as Lithium-air batteries,Alloy batteries and Sodium batteries,Lithium-Sulfur（Li-S）batteries meet aforementioned criteria and attract intense scrutiny for over two decades.Indeed,based on an overall electrochemical reaction denoted as+16Li⁺?8 with an average voltage～2.15 V,Li-S batteries providing a high theoretical capacity(1673 mAh g^-1)and a theoretical energy density(2500 Wh kg^-1).Both of them are much higher than that of conventional cathode materials.In addition,sulfur is also inexpensive,abundant on earth,and environmentally benign,hence readily available and fairly cheap for commercialization.Despite those advantages,there are several main challenges unfortunately impeding the practical applications of Li-S batteries base on pure sulfur as cathode.Primarily,the low electrical conductivity of elemental sulfur and the discharge products（Li₂S₂/Li₂S）inevitably cause poor rate capability and low utilization of active materials.Another serious issue is the dissolution of intermediate lithium polysulfides（Li₂S_n,4≤n<8）in the organic electrolytes along the repeated charge-discharge process.The dissolved polysulfides can shuttle between the cathode and the anode,resulting in the deposition of Li₂S₂/Li₂S on anode surface and loss of active materials.Moreover,the significant volume variation of sulfur in the conversion reaction between sulfur(2.07 g cm^-3)and Li₂S(1.66 g cm^-3)during lithiation can leads to a loss of electrical contact and structural instability.Conducting polymers,such as polyaniline（PANI）,polydopamine（PDA）,poly（3,4-ethylenedioxythiophene）（PEDOT）,and polypyrrole（PPY）,have aroused much attention to improve the performance of Li-S batteries due to their flexibility,good electrochemical stability,simple preparation and favorable morphologies.Therefore,in this thesis,the utilization of conducting polymers in sulfur-derived composites as cathode material was investigated.Several new types of lithium-sulfur battery cathode were designed and fabricated,and its unique electrochemical performance and mechanism were studied.The main research contents are summarized as follows:（1）Synthesis of manganese dioxide nanosheets functionalizing PEDOT@S core-shell nanospheres for Li-S batteries.MnO₂/PEDOT@S composites are synthesized through in-situ polymerization of 3,4-ethylenedioxythiophene on sulfur nanoparticles and manganese dioxide modification.The as-synthesized products are characterized using TEM,SEM,XRD,EDS and XPS measurements.A series of electrochemical test confirm that the PEDOT layer effectively enhance the electrical conductivity of composite and acts as protective layer to prevent dissolution of polysulfides.The MnO₂ nanosheets functionalized on PEDOT further provide high active contact area to enhance the wettability of the electrode materials by electrolyte and further interlink the polymer chains to improve the conductivity and stability of the composite.The S@PEDOT/MnO₂ exhibits an initial capacity of 1150 mAh g^-1with active materials utilization of 68.7%and decrease to 827 mAh g^-1 with capacity retention of 71.9%after 200 cycles at 0.2 C.（2）Synthesis of nickel-based hydroxides coated sulfur-polyaniline composite（Ni（OH）2@P@S）for Li-S batteries.Ni（OH）2@P@S composites are synthesized through in-situ polymerization of PANI and nickel hydroxide encapsulation.The as-prepared samples are characterized using TEM,SEM,XRD and EDS measurements.A series of electrochemical test confirm except for being a durable physical barrier,such hydroxide thin films can irreversibly react with lithium to generate protective layers that combine good ionic permeability and abundant functional polar/hydrophilic groups,leading to drastic improvements in cell behaviors.After a high initial capacity of 1220 mAh g^-1,Ni（OH）₂@P@S electrode can stabilize the capacity of 828 mAh g^-1 with capacity retention of 67.9%at 0.2 C after 200cycles.（3）Synthesis of hollow sulfur sphere with double polyaniline（PANI）encapsulation（P@S@P）for Li-S batteries.Hollow PANI nanospheres are first synthesized through polymerization of aniline at 140°C.Then sulfur layer is grown on PANI surface through in-situ reaction between sodium thiosulfate and hydrochloric acid in the presence of PVP at room temperature.The as-prepared materials are characterized using TEM,SEM,XRD,EDS and XPS measurements.The electrochemical characterizations confirm that the sandwich-like P@S@P composite has the unique double chemical bonding,soft-hollow PANI matrix,and high sulfur utilization,exhibiting excellent electrochemical performance in long-term cyclic stability.After a high initial capacity of 1142 mAh g^-1,P@S@P electrode exhibits the capacity of 865 mAh g^-1 with capacity retention of at 0.2 C after 500cycles.（4）Synthesis of nest-like 3D PANI coated sulfur composite（P@E-CNT/S）for Li-S batteries.P@E-CNT/S composites are synthesized through the normal temperature liquid phase reaction and in-situ polymerization of PANI.The as-prepared products are characterized using TEM,SEM,XRD and EDS measurements.A series of electrochemical test confirm covalent stabilization of sulfur and its discharge products on E-CNT,and its great contribution to high reversible capacity,excellent rate capability and exceptionally long lifespan of Li-S cells.Moreover,E-CNT consisted nest-like architecture effectively improve e^-/Li⁺transportation,stabilize electrode microstructure,and confine polysulfides within cathode itself.After a high initial capacity of 1273 mAh g^-1,P@E-CNT/S electrode demonstrates the capacity of 763 mAh g^-1 with capacity retention of 59.9%at 0.2 C after 200 cycles.