Synthesis And Modification Of Nickel Formate As Negative Material For Lithium Ion Battery

Metal-organic frameworks(MOFs)have been used as novel electrode materials for lithium ion batteries due to their tunable porous structures,high specific surface areas and high porosities.However,MOFs possess poor electronic conductivity,which hinders their practical application in lithium ion batteries(LIBs).In this thesis,Ni3(HCOO)6 material with a 3D diamond network was synthesized by a solvothermal method.Our work is devoted to investigate the crystal growth mechanism and adjust the reaction conditions to composite Ni3(HCOO)6 with CNTs materials effectively.Compared with the pristine Ni3(HCOO)6,the cycle performance,specific capacity and rate performance of the composites were remarkably improved.First,the growth process of Ni3(HCOO)6 material is investigated from the viewpoint of crystal engineering.It is found that the nucleation-growth process of Ni3(HCOO)6 follows the ‘aggregation-based nucleation-growth' mechanism.The morphology and size of the material are closely related to the reaction time and concentration.At low concentration,the nucleation rate is slower and the time required for crystal growth is longer.At high concentration,nucleation spots and nucleation rate speed up the growth of the crystal.The crystalline products can be formed in a short time,but they are not easily to control the morphology.However,the reaction concentration is controlled to medium concentration,the nucleation rate and crystallization rate are appropriate.Ni3(HCOO)6 material grows from loose nanoparticles to aggregated microsized ellipsoid,and then to the poly/single crystals.In this chapter,the exploration of crystal growth process is helpful for achieving the controllable morphology of Ni3(HCOO)6 and it also provides valuable information for the following modification experiments.Second,the carbon nanotubes(CNTs)are introduced into material to enhance the conductivity of the Ni3(HCOO)6 to improve the specific capacity and cycle performance of the material.Micro-ellipsoid Ni3(HCOO)6/CNTs composites are controllably synthesized by adjusting the reaction time and concentration which are important factors for the synthesis of Ni3(HCOO)6.The electrochemical properties of Ni3(HCOO)6/CNTs-n(n= 15,25,50,60)composites with different additive amount of CNTs are studied,which show better electrochemical performance than the pristine Ni3(HCOO)6 in the electrochemical capacity and rate performance.The specific capacity of the Ni3(HCOO)6/CNTs-n(n=15,25,50,60)is 230 mAh/g,400 mAh/g,560 mAh/g and 320 mAh/g,respectively,after cycling for 400 cycles at the current density of 300 mA/g.When the CNTs additive amount is 50%,Ni3(HCOO)6/CNTs-50 shows the best rate performance and the capacity retention is 288 mAh/g at the current density of 1200 mA/g,which is mainly due to appropriate amount of CNTs can be intertwined through the interior and outer surface of the ellipsoidal particles.The 3D conductive network is formed between the micro-ellipsoid and the particles to make a good conductive bridge.Thus,the electrochemical performance of the materials is improved effectively.When carbon nanotube loading in the sample is too low,it is difficult to form conductive networks between the particles.In addition,CNTs loading is too high to be dispersed,which is harmful for constructing an effective conductive network.Besides,if excessive CNTs replace Ni3(HCOO)6,the apparent capacity of electrode will decrease.