Study On Preparation And Doping Of Li-Excess Cation-Disordered Rock-salt Li-Ni-Ti-O Cathodes

This work is mainly based on the lithium-rich cation-disordered rock-salt cathode materials for Li-ion batteries.The influence of synthetic process on material structure,morphology and electrochemical properties has been intensively investigated.In addition,electeochemical behavior and lithium diffusion have also been studied between different vltage regions.Due to the poor rate capability and cycle stability,doping is performed to enhance and improve its electrochemical properties.The main research contents and results are as follows:Lithium-excess disordered cathode material LiNi_0.5Ti_0.5O₂ with nano-scale was syhthesized via sol-gel method.The relationships between the calcination temperature,dispersing agent,ball milling method and structure,morphology and electrochemical properties were investigated.When the ratio of ethanediol and metal ions is 1.4:1,and the xerogel is processed with dry milling before sintering at 600?.LiNi_0.5Ti_0.5O₂has the best electrochemical performance.An initial discharge capacity of93.2 mAh g^-1 is obtained at 20 mA g^-1,and the capacity retenion is 69.7%after 55cycles.The comparatively higher capacity is attributed to the good crystallinity and well-distributed particle size with 100 nm.A nominal composition of Li_1+x/3Ni_1/2-x/2Ti_1/2+x/6O₂?x=0,0.1,0.2,0.3,0.4,0.5?was also synthesized via sol-gel method.The characterization results of XRD and TEM show that disordered rock-salt compounds with cubic structure have been obtained,and the lattice parametres increase with the increasing content of Li.The electrochemical test results show that Li_1.1Ni_0.35Ti_0.55O₂ and Li_1.17Ni_0.25Ti_0.58O₂ have the best performance between 2.5-4.5 V and 1.5-4.5 V,repectively.Between 2.5-4.5 V,the first discharge capacity of Li_1.1Ni_0.35Ti_0.55O₂ is 116.5 mAh g^-1,and the capacity remains at 100 after 55 cycles.However,between 1.5-4.5 V,the initial discharge capacity of Li_1.17Ni_0.25Ti_0.58O₂ is up to 209.2 mAh g^-1.When the current density is increased to 200 mA g^-1,its discharge capacity can still keep at 150 mAh g^-1.Both Ni²⁺/Ni⁴⁺and O^2-/O^1-can be inferred from the cyclic voltammetry?CV?tests.Electrochemical impedance spectroscopy?EIS?shows that the charge transfer resistance of the samples decreases after increasing the level of Li excess and diffusion coffecients of Li are enhanced.The calculated Li diffusin coffecients of Li_1.1Ni_0.35Ti_0.55O₂ and Li_1.17Ni_0.25Ti_0.58O₂ are 4.08×10^-14 and 6.26×10^-14 cm² s^-1,which means that 0-TM channels will form after Li excess and provide fast diffusion channels for Li⁺.In addition,XRD characterization of electrodes after cycle shows that the series can still keep the cubic disordered rock-salt structure.In order to improve the properties of Li_1+x/3Ni_1/2-x/2Ti_1/2+x/6O₂,Nb doping was adapted and Li_1+x/100Ni_1/2-x/100Ti_1/2-x/100Nb_x/100O₂ was synthesized via sol-gel method.The effect of Nb doping on the material structure,morphology and electrochemical performance was investigated,and EIS was performed to analyse the mechansim of Nb doping on electrochemical behavior.The results of XPS show that the valence of Nb,Ni and Ti is+5,+2 and+4,respectively.The electrochemical test results show that Nb doping can improve rate ability and cycle stability.For Li_1.2Ni_0.3Ti_0.3Nb_0.2O₂,an initial discharge capacity of 220 mAh g^-1 is obtained at 20 mA g^-1,and the discharge capacity can remain at 175 mAh g^-1 after 50 cycles,whose capacity retention is 79.1%.Notably,a stable discharge capacity of 120 mAh g^-1 can be obtained when the current density is increased to 400 mA g^-1.In addition,a detailed study of Li⁺is carried out to further understand the Nb-doped sample.The calculated Li diffusion coffecient of Li_1.2Ni_0.3Ti_0.3Nb_0.2O₂ is 9.35×10^-14 cm² s^-1,which is higher than the original sample.