Synthesis And Lithium Storage Properties Of High-performance Cathode Materials In Lithium-selenium/Sulfur Batteries

With the fast development of a variety of portable devices,electric vehicle and renewable energy,it’s an urgent need to develop the new battery system with high energy/power density.The theoretical capacity of sulfur（S）is 1675 mAh g^-1 and the theoretical energy density of lithium-sulfur（Li-S）battery is 2600 Wh kg^-1.In addition,sulfur is abundant,low-cost and environmentally friendly.Thus,Li-S batteries are considered as one of the most promising next-generation rechargeable batteries.Se is the same group（VIA）with S in the periodic table,which has the similar chemical properties.The theoretical capacity of selenium（Se）is 675 mAh g^-1,which is lower than the capacity of sulfur.However,Li-Se batteries have the similar high volumetric capacity(3253 Ah L^-1)with Li-S(3467 Ah L^-1).Moreover,the conductivity of Se is much higher than that of S,rendering higher active material utilization.So the Li-Se batteries attracted much attention.This thesis focused on the rational design and fabrication of the high-performance selenium and selenium/sulfur composite cathodes and investigate their lithium storage properties.The main contents in this work are summarized as follows:（1）We report the rational design and fabrication of a novel cathode material in which Se is fully encapsulated and attached to the inner shell of hollow-core nitrogen-doped carbon（CN_x）nanobelts forming hollow double-shell Se@CN_x nanobelts.The Se@CN_x nanobelts can easily fabricate the self-supporting electrode film with a large Se mass loading,producing large capacity,long cycling stability and an excellent rate capability.The inorganic-organic hybrid ZnSe[DETA]_0.5.5 nanobelts were firstly prepared by a hydrothermal method.The as-prepared ZnSe[DETA]_0.5 nanobelts were subsequently coated with a thin polydopamine（PDA）by dopamine self-polymerization in a Tris buffer（pH=8.5）solution containing dopamine.After thermal calcination at 700 ^oC,the PDA coating was carbonized into the CN_x shell to produce the core-shell ZnSe@CN_x nanobelts.The double-shell hollow Se@CN_x nanobelts were obtained by immersing the core-shell ZnSe@CN_x nanobelts in aqueous solution of FeCl₃.The inner core of ZnSe is oxidized to Se and Zn²⁺,while the Fe³⁺is reducted to Fe²⁺,with free Zn²⁺and Fe²⁺leached out into the solution during subsequent washing.The hollow double-shell Se@CNx nanobelts can readily form a self-supporting film without the need of a binder or conducting additives by the simple vacuum-filtration method.The paper-like Se@CNx cathode has a large Se mass loading of 62.5 wt%.With a large areal loading of 3 mg cm^-2,the freestanding cathode of Se@CN_x nanobelts delivered a high capacity of 608.8 mAh g^-1 after 100 cycles with only 0.03%capacity decay per cycle and maintained 453.2 mAh g^-1 for over 400 cycles.When the current density is increased from 80 to 1600 mA g^-1,70%reversible capacity is observed,indicating the high rate capability.The excellent electrochemical performance of Se@CN_x nanobelts arise from the unique structure of well-defined hollow double-shell morphology.（1）First of all,our technique can fully encapsulate Se inside the highly conductive CN_x shell with a controlled void volume which can accommodate the large volume expansion of Se and effectively trap the Se and polyselenides intermediates during charging/discharging.Secondly,with an ultrahigh aspect ratio,the double-shell Se@CN_x nanobelts can readily form a self-supporting film with a 3D interconnected conductive network.The binder-free freestanding Se@CN_x electrode film comprising intertwining and interpenetrating hollow double-shell Se@CN_x nanobelts is adopted as the cathode in a Li-Se battery,which increases the Se content in the cathode and the 3D structure facilitates electron and ion transfer during charging/discharging.（2）We designed and synthesized the double-shell carbon shell（CN_x）coated the sulfur and selenium(S_nSe_1-n@CN_x)nanosphere in which the S and Se atoms are encapsulated into the nitrogen doped carbon shell（CN_x）.ZnS nanospheres were firstly prepared by a facile solution-phase thermal decomposition route.The ZnS nanospheres are comprised of many smaller ZnS nanocrystals coated by poly（N-vinyl-2-pyrrolidone）（PVP）.The ZnS nanospheres were subsequently coated with a thin PDA.After thermal calcination under Ar at 800 ^oC,the PDA and PVP coating were carbonized into the CN_x shell to produce the double-shell ZnS@CN_x nanoparticles.The double-shell S_nSe_1-n@CN_x were prepared via immersed ZnS@CN_x into H₂SeO₃ solution,in which the ZnS reacted with H₂SeO₃ to produce S and Se.The unreacted ZnS is further oxided by FeCl₃ and ZnS is oxided to S and Zn²⁺,while the Fe³⁺is reducted to Fe²⁺.Afte washing to remove the free Zn²⁺and Fe²⁺,the double-shell S_n Se_1-n@CN_x were produced.The S/Se ratio could be controlled via adjusting H₂SeO₃ content.The S_nSe_1-n@CN_x nanoparticles delivered a large capacity of 823.8 mAh g^-1 during the first lithiation process and maintained a high capacity of 495.9 mAh g^-1 after 100 cycles at 635 mA g^-1.The good electrochemical performance of S_nSe_1-n@CN_x is attributed to the unqiue structure of the composite.（1）Se and S are fully encapsulated into the highly conductive CN_x shell with a controlled void volume which can accommodate the large volume expansion of Se（Se）;（2）the double CN_x shell can also effectively trap the S（Se）and polysulfides（polyselenides）intermediates during charging/discharging;（3）the inner CN_x shell from carbonnized PVP can effectively prevent the agglomeration of S and Se,improving the utilization of S and Se and the outer CN_x shell from carbonnized PDA improve the conductivity of electrode materials andf effectively reduce the resistance between interfaces of the nanospheres,thus leading to high rate capability and large capacity.