The Preparation And Applications Of Ni-doped FeSe₂/Fe₃Se₄ And Carbon-Sphere@NiS₂/Cu_7.2S₄ Composite For Sodium/Potassium Batteries

Lithium ion batteries（LIBs）have been increasingly challenged by the growing energy needs of human beings,due to uneven distribution of lithium resources,the low reserves in the earth’s crust and limited performances.Therefore,it is necessary to develop new-type energy storage devices as alternatives of lithium ion batteries.In virtue of the features of favorable standard redox potential,high reserves and low cost of K/Na,potassium ion batteries（PIBs）and sodium ion batteries（SIBs）are identified as appealing candidates.Nevertheless,their practical applications are still subjected to the large ionic radius of K+/Na+,which leads to huge volume effect and sluggish reaction kinetics of anode materials and thus poor rate performance and cycling stability upon potassiation/depotassiation processes.In order to solve the above problems involved in the iron-group metal selenides/sulfides,an in-situ carbon coated nickel doped iron selenide porous structured composite was designed and employed for potassium/sodium storage and carbon sphere loaded with nickel sulfide/copper sulfide composite was also designed as anode materials for PIBs.The storage mechanisms of the above materials were further investigated.The main research contents of the thesis are provided as follows:（1）A porous heterostructure of nickel doped iron selenide coated by carbon framework was successfully synthesized via facile in-situ selenization and carbonization of nickel doped iron based MOFs precursors in Ar/H2 mixture.Nickel ion doping is designed to improve the electrical conductivity of the composite and carbon coating can not only relieve volume effect,maintaining the integrity of the material structure,but also can avoid the direct contact of the selenide with the electrolyte,effectively inhibiting the side reactions occurred between the selenide and the electrolyte.Moreover,the porous structure could accelerate immersion of electrolyte and offer favorable channels for electron/ion transport.Benefiting from its merits mentioned above,the composite exhibites superior long-cycling stability,where a reversible capacity of 177.3 mAh g^-1 is achieved for more than 3050 cycles at current density of 2.0 A g1 in PIBs,with a low attenuation rate of 0.006%.When it was applied as anode for SIBs,a capacity of 281.2 mAh g^-1 could be maintained and stably delivered for 2000 cycles at high current density of 8.0 A g^-1.（2）NiS₂/Cu_7.2S₄ nano-particles loaded on carbon sphere composite was synthesized using SiO₂ as template and PDA as carbon source.The hollow structure and nano-sized feature can relieve volume expansion upon insertion of K+ion,ion migration could be facilitated based on the specific channels in the wall of carbon sphere,boosting the reaction kinetics.Additionally,the carbon spheres can maintain structural integrity of the NCS and increase the overall conductivity of the material,improving the ion transport rate.When tested as the anode material for PIBs,it exhibits outstanding rate performance,and capacity of 320 mA h g^-1 could still be retained even at current density of 5.0 A g^-1.In order to explore the practical potential of the material,a full-cell was assembled,and good performance has been achieved.