Construction And Electrochemical Performance Study Of Cobalt Disulfide Based Hybrid Magnesium-lithium Ion Battery Cathodes

Due to high theoretical volume energy density,no dendrites in deposition process,and low cost,magnesium rechargeable battery（MRB）is expected to a new generation of low-cost and high-efficiency energy storage battery system.However,the high charge density makes sluggish diffusion kinetics of Mg²⁺in host lattices,which limits the devolopment of MRB.Hybrid Mg²⁺/Li⁺battery（MLIB）simultaneously combines the Li⁺storage cathode,Mg metal anode and Mg²⁺/Li⁺hybrid electrolyte.This battery system has the advantages of MRB,while solving the problem of sluggish kinetics in storage Mg²⁺materials,thus exhibiting higher capacity,better rate performances,and longer cycle life than these of MRB.The present work focuses on the cathode materials of MLIBs,and CoS₂ is considered as one of the most appealing cathodes for high-performance MLIBs owing to its high theoretical capacity(871mAh g^-1)and remarkable electrical conductivity.However,in addition to the inevitable huge volumetric change of CoS₂ in cycling,the side reactions of nucleophilic MLIB electrolyte with polysulfide intermediates caused by CoS₂ conversion reaction during cycling seriously destroy the electrolyte stability.Furthermore,when these polysulfide intermediates shuttle to the surface of Mg anodes,they are liable to be reduced and form passivation layers on the Mg anode,which hinders the deposition of Mg²⁺and leads to an overpotential increase.Aiming at those shortcomings of CoS₂-based cathodes,three types of array electrodes with different micro-nano structures are constructed by optimizing the design of material structures and components in this thesis.The array structure provides accommodation space for the volume change of electrode material during cycling,and the dissolution of polysulfides is effectively inhibited by polar adsorption and isolation layers.These measures enable electrodes to exhibit excellent electrochemical performances in MLIBs.CoS₂-C nanotube array electrodes are prepared on carbon fibers（CC/CoS₂-C NTA）through a facile sulfidation process and heat treatment to ZIF-67 precursor,in which CoS₂ nanoparticles are uniformly embedded in N-doped amorphous carbonaceous nanotube array.N-doped carbon around CoS₂ nanoparticles limits the dissolution of solube polysulfide,protecting the electrolyte and Mg anodes.Hollow nanotube structure provides internal cavity area to buffer the volume expansion of the electrode material,improving the structural stability of electrode.In addition,compared with the voltage range of CoS₂ in Li ion battery（0.01～3.0 V vs.Li/Li⁺）,the voltage range of MLIB(0.01～2.0 V vs.Mg/Mg²⁺,corresponding to 0.7～2.7V vs.Li/Li⁺)avoids the agglomeration of reaction products and electrolyte decomposition at low voltage,which effectively improves the electrochemical reversibility of the electrode materials.CC/CoS₂-C NTA exhibits good cycling stability:at a current density of 1.0Ag^-1,the capacity maintains 225.4mAh g^-1 after 2000 cycles,corresponding to a capacity retention of 73.8%.The active adsorption sites in heteroatom-doped carbon materials are limited,and the adsorption effect on polysulfide generated by CoS₂ during cycling needs to be further enhanced.CC/CoS₂@TMS nanosheet array electrodes are constructed by depositing CoS₂ nanosheets on carbon cloth through an electrodeposition process and vulcanization treatment,followed by a 1T-MoS₂ coating process.The coating of 1T-MoS₂ has more active adsorption sites and strong polar adsorption effect on polysulfide,and isolates the contact between electrolyte and polysulfide,effectively avoiding the loss of active material and preventing damage to electrolyte and Mg anode.In addition,1T-MoS₂ improves the electrical conductivity of electrode,and promotes the diffusion of ion transport,as well as avoids agglomeration and shedding of the electrode material.Benefiting from the above advantages,CC/CoS₂@TMS exhibits excellent rate and cycle performances:at a current density of 5Ag^-1,a capacity of225.7mAh g^-1 is obtained,after 3000 cycles,the capacity maintains 179.6mAh g^-1,corresponding to a capacity retention of 79.6%.To further enhance the stability of CoS₂-based cathode materials,the active materials are anchored to the surface of the substrate material by constructing chemical bonds,thus suppressing the exfoliation of the active material from substrate and improving the cycle life of electrodes.TiO₂-_x/CoS₂@PPy nanorod array electrodes are constructed by preparing CoS₂ nanosheets on TiO₂-_x nanorod through an electrodeposition process and vulcanization treatment,followed by a PPy coating process.The existence of Ti–S bonding between TiO₂-_x substrate and CoS₂ is confirmed by XPS characterization,which avoids CoS₂ exfoliation from substrate.Theoreical calculation demonstrates that the introduction of oxygen defects enhances the chemical interaction between TiO₂-_x substrate and Li₂S₄,CoS₂,Li₂S,Co,improving the stability of the whole electrode.The outer PPy coating layer increases the electronic conductivity of the configuration,accommodates the volume expansion of CoS₂ as well as prevents the contact of APC electrode with polysufide.Benefiting from the above advantages,TiO₂-_x/CoS₂@PPy exhibits more excellent rate and cycle performances:at a current density of 5Ag^-1,a capacity of 245.5mAh g^-1 is obtained,after 3000 cycles,the capacity maintains 208.8mAh g^-1,corresponding to a capacity retention of 85.1%.