Study On Preparation And Doping Of Cation-Disordered Li-Ni-Nb-O Cathodes

The rapid development of modern portable electronic devices and high-power vehicles has increased the demand for mobile energy.Lithium-ion batteries are considered to be one of the most promising energy storage and conversion technologies due to their volume control,high energy density and long cycle life.Cathode materials account for the largest proporion（more than 33%）of the component cost of lithium-ion batteries,so it is particularly important to find cathode materials with low cost and high performance in the development of lithium-ion batteries.Cation-disorder metal oxide was unconsidered because of no electrochemical activity previously,but in recent years there are experiments and theories to prove that when the molar ratio of lithium to transition metal ions is greater than 10%,tmaterial can form 0-TM lithium ion migration channels,which activates electrochemical properties of the material.Moreover,the cation-disordered cathode has advantages of high capacity and high voltage.This discovery opens up a new field of lithium ion batteries cathode materials research.For such a promising cathode material,this paper explores its preparation method,internal mechanism and performance improvement.In this paper,a new preparation method,sol gel+short time annealing,was proposed to prepare cation-disordered cathode.Li-N i-Nb-O precursor powder prepared by sol gel method was mixed with citric acid powder and then calcined at 800℃for several minutes.By the electrochemical test at the current density of 50 mA g^-1 between 1.5 and 4.8 V,the sample calcined at 800℃for 5 min has the best performance.The first discharge specific capacity can reach 151.5 mAh g^-1 and capacity retention is 57.4%after 100 cycles.Then,the proprecursor powder mixed with citric acid was annealed at 600℃,700℃,800℃and900℃for 5 min to explore the influence of different annealing temperature on the material.By the electrochemical test,it was found that the sample annealed at 800℃still had the best electrochemical performance.At the same time,the influence of citric acid on the material was also investigated.It was found that the addition of citric acid was more conducive to the formation of sample phase and small uniform particles which also cause a better electrochemical performance.Then,a series of Li_1.3+4x/3Ni_0.27-x/2Nb_0.43-x/15O₂ samples were prepared by the optimal process to investigate the structure and morphology of Li-Ni-Nb-O materials with different lithium-excess content,as well as their electrochemical properties and lithium ion diffusion behavior.XRD results showed that the change of lithium content within a small range did not affect the crystal structure of cation-disordered and rock-salt structure and SEM results showed that the particle size gradually increased with the increase of lithium content.Electrochemical test results showed that with the increase of lithium content,the discharge capacity of the materials in the first cycle increased,but their cycling performance became worse and worse.To summarize,when x equaled to 0.05,the sample（labeled as LNNbO+0.05）had the best cycling performance and rate capacity.The initial discharge capacity of LNNbO+0.05 was 174.2 mAh g^-1 at the current density of 50 mA g^-1 and the discharge specific capacity was 103.7 mAh g^-1 after 100 cycles with 59.5%.retention rate.Even at the high rate of 300 mA g^-1,a discharge capacity of 72.8 mAh g^-1 was obtained.The results of electrochemical impedance and cyclic voltammetry analysis showed that LNNbO+0.05 also had the largest lithium ion diffusion coefficient(D=4.42×10^-13 cm² s^-1)and electronic conductivity(σ_e=2.298×10^-6 s cm^-1)than others.Finally,to improve the poor cyclic stability of the materials,the same process was adopted to carry out F doping treatment on LNNbO+0.1（when x=0.1）.F atom can replace lattice oxygen,reducing oxygen loss and improving the structural stability of the material and thus improving the cycling performance.The results showed that this process can successfully dope F into the structure,and the sample lattice constant decreased and the particle size increased after F doping.The results of electrochemical performance test showed that the cycle stability and capacity recovery of F-LNNbO+0.1 were improved by 38%and39%compared with that of undoped sample LNNbO+01.Unfortunately,the initial disc harge capacity of F-LNNb O+0.1 was greatly reduced accompanied by poor rate performance.This may be due to the excessive doping ratio of F,which inhibited the oxygen loss as well as the capacity provided by the oxygen redox reaction of O^2-/O^-.