Construction And Electrochemical Study Of High-performance Conversion-alloy Anode Material

Sodium ion batteries have become one of the promising systems in the next generation of low-cost and high-efficiency energy storage battery systems,due to natural abundance,wide distribution and low cost of sodium resources.Among various anode materials for sodium ion batteries,conversion-alloy type anode materials show great potential because of its high theoretical capacity.However,the intrinsic huge volume change during Na⁺insertion/extraction process servely deteriorates the cycle stability of such conversion-alloy type anode.Meanwhile,the larger radius of sodium ion results in sluggish electrochemical reaction kinetics,leading to poor rate performance.Aiming at those shortcomings of conversion-alloy anodes,three types of multi-component electrode systems with special micro-nano structures are synthesized through micro-nano structure design and component optimization in this paper.Combining physicochemical characterization techniques and electrochemical research methods,the physicochemical properties and Na⁺insertion/extraction behavior of the prepared materials are systematically studied.Flower-like bismuth sulfide/graphene aerogels composite is successfully fabricated through a facile hydrothermal reaction and freeze-drying process,in which the stack of nanorods composes the flower-like bismuth sulfide.Nanostructured bismuth sulfide can shorten the transfer path of electrons and ions,impropving its transfer rate and accelerating the reaction kinetics.Graphene aerogels can not only improve the conductivity of the electrode but also serve as a buffer medium to ease the volume change of bismuth sulfide during charging/discharging process.Besides,the rich pore structure in graphene aerogels ensures fast mass transportation inside the electrode.Typically,the bismuth sulfide/graphene aerogels composite exhibits a reversible specific capacity of587 m Ah g^-1 at current density of 0.1 A g^-1,and maintains at 397 m A h g^-1 after 50 cycles.Even at high current density of 2 A g^-1,a reversible specific capacity of 336 m Ah g^-1 is achieved.Carbon coated amorphous cobalt-tin bimetallic sulfides hollow nanoboxes is prepared by sulfidation process and facile carbon-coating strategy based on a self-template method.Hollow structure in the composite material can provide void space to accommodate the volume expansion of the electrode material;And the discharge products of the additionally introduced cobalt sulfides also can buffer the huge volume expansion of alloy reaction between tin and Na⁺;While the coated carbon component not only improves the conductivity of the electrode,but also avoids agglomeration of nanomaterials.Additionally,the amorphous feature of this composite can provide more transfer channels for sodium ions,improving the transfer rate of Na⁺in electrode.Benefiting from the structure design and multicomponent features,the carbon coated amorphous bimetallic sulfides hollow nanoboxes achieve a stable reversible capacity of607 m Ah g^-1 at 0.1?A?g^-1 after 100 cycles and a reversible capacity of 478 m Ah g^-1 is obtained even at a high current density of 10?A?g^-1.Considering the advantages of hollow structure and graphene component in improving the electrochemical performance,heterostructures and three-dimensional carbon networks are additional introduced in electrode via high-temperature gas-solid reaction and carbon coating method.And finally,three-dimensional carbon networks encapsulated heterostructured Zn S-Sn S hollow nanocubes are constructed.The interconnected conductive networks from the graphene skeleton and outer coating carbon can improve the integral conductivity of the electrode and relief the volume change,as well as separate the nanocubes effectively,avoiding aggregation,pulverization and inactivation of active materials caused by volume expansion.Specifically,at current density of 2 A g^-1,such composite exhibit a reversible specific capacity of 247 m Ah g^-1after 1000 cycles,and the corresponding capacity decay rate is only 0.04%per cycle.At high rate of 20 A g^-1(charging completed in 35 s),the reversible specific capacity corresponds to 267 m Ah g^-1.