Designed Synthesis And Modification Of Layered LiNi_1/3Co_1/3Mn_1/3O₂ Nanomaterials

Due to the high specific capacity,good cycling stability,thermostability,and low cost,layered LiNi_1/3Co_1/3Mn_1/3O₂ has been regarded as one of the most potential cathode materials to replace the expensive LiCoO₂ for lithium ion batteries.We aim to improve the electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂.The precursor materials were firstly prepared by the liquid co-precipitation method.Then,the target products can be obtained from the high temperature solid phase sintering after mixing the pre-sintering oxides with LiOH.The morphologies of the as-prepared samples change with the different co-precipitation systems.The electrochemical performance is improved via the ions doping and surface modification.The main research results of this dissertation are as follows:?1?Based on the purpose of reducing the mixing of Li⁺/Ni²⁺,the optimal synthesis temperature system of LiNi_1/3Co_1/3Mn_1/3O₂ is determined to be 800oC/16 h by the experiments.LiNi_1/3Co_1/3Mn_1/3O₂ nanoparticles are synthesized by using a mixture of Na₂CO₃ and NH₄HCO₃ solution as the precipitant,LiOH as a source of lithium.The particle size is distributed in 100⁵00 nm.NH4HCO3 can be used as a chelating agent to control the metal ions concentration in the solution and as a pH-buffer to stabilize the alkalinity of the system,which is beneficial to the uniform settlement completely.Using PEG-400 as the main dispersant solution,a mixture of Na₂CO₃ and NH₄HCO₃solution as the precipitant,twin-spherical LiNi_1/3Co_1/3Mn_1/3O₂ is synthesized with the size distribution between 0.5¹.0?m.The twin-sphere is composed of nanoparticles.This new structure is advantageous to the release of structural stress during the processes of electrode reaction and beneficial to the improvement of the tap density.Otherwise,when NaOH solution is used as the precipitant,the micro-morphology of prepared LiNi_1/3Co_1/3Mn_1/3O₂ is also nanoparticle with the size distribution between100⁵00 nm.Many methods of analysis and testing are employed to research the performance of as-prepared samples,such as XRD,SEM,XPS,CV and EIS etc.?2?In order to improve the properties of LiNi_1/3Co_1/3Mn_1/3O₂,V₂O₅ is selected asthe dopant and the optimal doping amount is confirmed to be 3%.The vanadium-doping cathode materials show outstanding electrochemical performance.The initial discharge capacity of vanadium-doping LiNi_1/3Co_1/3Mn₁/3O₂ nanoparticles synthesized by carbonate co-precipitation is 168 mAh g^-1 at 1 C,after 1000 cycles,a capacity of 119 mAh g^-1 is delivered,corresponding to a capacity retention ratio of70.83%.When cycling at a high rate of 20 C,the initial discharge capacity remains138 mAh g^-1,after 500 cycles,the capacity is 74 mAh g^-1.The capacity retention ratio of vanadium-doping twin-spherical LiNi_1/3Co_1/3Mn_1/3O₂ is 71.21%after 1000 cycles at 1 C.The initial discharge capacity is 139 mAh g^-1 at 20 C,a capacity of 68 mAh g^-1is delivered after 500 cycles.Compared with the pure LiNi_1/3Co_1/3Mn_1/3O₂,vanadium-doping cathode materials possess better long cycle performance and high-rate capability.The mechanism of vanadium doping modification is that the vanadium element enters into the lattice to occupy the size of transition metal,which results in the reducing of the mixing of Li⁺/Ni²⁺,stables the crystal structure,and enhances the transport ability of ions and electron.?3?The surface modification of carbon nanotubes is also an effective method to enhance the electronic conductivity and reduce the polarization of LiNi_1/3Co_1/3Mn_1/3O₂.The carbon nanotubes can form a network to wrap in the surface of nanoparticles,which is beneficial for the conductivity of electrons and the improvement of the high-rate performance and cycling stability.Comparing with the vanadium-doping material,the improvement of performance is limited via carbon nanotubes modification.The addition of carbon nanotubes reduces the mass of active material and the simple physical contact makes it weaker in maintaining the structure stability than vanadium-doping.