Preparation And Performance Of Bismuth-based Anode Materials For Potassium Ion Batteries

Bismuth-based materials（Bi、Bi₂O₃、Bi₂S₃ and so on）can store potassium based on the alloying and/or conversion reaction,and have received attention in the anode materials for potassium ion batteries（PIBs）due to their high theoretical capacity and low platform potential.However,bismuth-based materials suffer from poor reaction kinetics and severe volume expansion during potassium storage.These severe issues make the bismuth-based materials with unsatisfactory rate and cycling performance,limiting their practical application for PIBs.In this dissertation,various strategies were proposed to improve the potassium ion storage performance of bismuth-based materials,such as modulating the structure,recombining with high-conductive materials,and stabilizing the electrode/electrolyte interface.Three kinds of bismuth-based anode materials for potassium ion batteries including 0D-2D Bi/r GO,Bi/Bi₂O₃-C,and Bi₂S₃/IG were prepared through ball milling-thermal reduction method,hydrothermal method,etc.（1）With Bi₂O₃ nanosheets as precursor,the Bi nanoparticles were assembled with r GO nanosheets through ball milling and thermal reduction process.The r GO with high conductivity can not only inhibit the agglomeration of Bi nanoparticles and ensure them to contact well with electrolyte,but also alleviate the huge volume variation of Bi particles during the potassiation/de-potassiation to guarantee the structural stability of electrode.Besides,the r GO can facilitate the fast electron transfer and improve the conductivity of the composite.When the proportion of graphene in the composite reaches 40%,the Bi/r GO composite shows the best electrochemical potassium storage performances,which can deliver a reversible specific capacity of 384.8 m Ah g^-1 at a current density of 50 m A g^-1,while still maintains 147.3 m Ah g^-1 at 1 A g^-1 even after 1000 potassiation/de-potassiation cycles.（2）Bi/Bi₂O₃-C composites were in situ prepared through high temperature carbonization using accordion-like Bi MOF synthesized by room temperature liquid phase reaction as the precursor.In this composite,Bi/Bi₂O₃ nanoparticles are uniformly loaded on the carbon substrate,where the carbon can improve the electron transfer of the composite as well as the electrode stability during potassium storage,avoiding the loss of active materials.In addition,a built-in electric field can be formed in the hetero-interface of Bi and Bi₂O₃,which further promotes the fast electron transfer at the hetero-interface,thus the potassium storage performances of the Bi/Bi₂O₃-C are optimized.The Bi/Bi₂O₃-C composite obtained under 900℃shows an initial reversible capacity of 339.1m Ah g^-1 at 100 m A g^-1,and still maintains 259.6 m Ah g^-1 after 200 cycles.（3）A unique Bi₂S₃/iodine-doped graphene（IG）composite was prepared by in situ growing the Bi₂S₃ nanorods on the iodine-doped graphene through the hydrothermal-sulfurization-thermal reduction process.In the hydrothermal process,iodine was in situ doped within the graphene in polyanionic salt form of I₃^-and I₅^-,which is beneficial for accelerating the electron transfer rate and surface chemical activity of graphene,and making the stable combination of iodine-doped graphene and Bi₂S₃ nanorods.The graphene can improve the conductivity of the composite,avoid the agglomeration of Bi₂S₃ nanorods for more exposed surface potassium storage sites,and buffer the volume change of Bi₂S₃ during potassiation/de-potassiation.The Bi₂S₃ nanorods can also prevent the layer-restacking of graphene,ensuring the composite to have sufficient contact with the electrolyte.It is revealed that the high concentration KFSI-based ether electrolyte（5 M KFSI-DME）can form a stable SEI layer with inorganic component and thus ensure the stability of the Bi₂S₃/IG electrode.As a result,the Bi₂S₃/IG composite exhibits excellent potassium storage performance,which delivers a reversible capacity of 525.4 m Ah g^-1 at 50 m A g^-1,and maintains a capacity of 411.0 m Ah g^-1 after 100 cycles at 100 m A g^-1.