Study On Bulk Doping Modification Of Cobalt-free Lithium Nickel Oxide Cathode Materials

Long life,safe and stable pure electric vehicles are the development trend of future vehicles,but the energy density of lithium-ion battery used to provide power limits its sustainable and stable development.However,the increase in energy density and long-cycle stability of lithium-ion batteries are limited by the cathode materials that occupy the highest cost of batteries.High nickel and cobalt-free cathode materials are the future trend of cathode materials for lithium ion batteries.As nickel can provide higher specific capacity,in order to pursue higher specific capacity of positive materials,in recent years,the content of nickel is higher and higher,but it brings increasingly problems.The main problems of Li Ni O₂materials are as follows:（1）It is difficult to prepare Li Ni O₂materials with standardized chemometric ratio;（2）Even in synthetic materials,the radius of Li⁺and Ni²⁺is not very different,which results in serious mixing of them,which prevents Li⁺from being removed/embedded in the materials;（3）During the synthesis process,too high heat treatment temperature will cause Li Ni O₂to decompose into heterophase without electrochemical activity.At present,bulk doping is one of the most effective means to improve the electrochemical performance and safety of the cathode materials for lithium ion batteries.The work of this paper is to improve the above mentioned problems with Li Ni O₂cathode materials by replacing the Ni position with trace other metal elements.Firstly,four layered cathode materials Li Ni_1-xIn_xO₂（x=0,0.01,0.02,0.03）with different amounts of indium doping were synthesized by sol-gel method.When doped with1%In,the cell volume of Li Ni O₂increases,but its phase structure is stable.In the wide operating voltage range of 2.5 V to 4.5 V,the discharge capacity of 1%In doped Li Ni O₂is222.6 m Ah g^-1,which is higher than that of undoped Li Ni O₂207.6 m Ah g^-1.At the same time,the rate performance and cycle stability are improved significantly.At a high current density of 2000 m Ah g^-1,the discharge specific capacity reaches 88.5 m Ah g^-1,which is much higher than 34.4 m Ah g^-1of undoped Li Ni O₂.After 100 cycles,the capacity retention was 77.0%and 67.8%for the doped and undoped materials,respectively.The improvement of these properties is related to the increase of the doped material unit cell volume and its phase structure stabilization.Secondly,four Li Ni_1-xFe_xO₂（x=0,0.01,0.03,and 0.05）cobalt free layered positive electrode materials with different iron doping amounts were synthesized by the same material preparation as In doping.Li⁺/Ni²⁺mixing degree decreases due to the increase of cell volume after doping,so Li⁺can be better de-embedded in materials;The results of XRD and SEM show that the doped materials have better crystallinity,which is helpful to improve the electrochemical properties of the materials.At 2.5 V-4.5 V wide working voltage,the constant current charge-discharge test results show that the material doped with 1%Fe and 3%Fe has better multiplier performance.The material doped with 1%Fe has a discharge specific capacity of 116.5 m Ah g^-1at a high current density of 4000 m A g^-1,which is much higher than the discharge specific capacity of 67.7 m Ah^-1without doping.After 100 cycles of charging and discharging at 100 m A g^-1current density,the discharge specific capacities of materials doped with 1%Fe and 3%Fe are 125.5 m A g^-1and 118.6m A g^-1.The capacity retention rates are 74.0%and 72.0%,while those without doping are only 53.4%.Differential capacitance analysis confirms that doping with appropriate amount of Fe can slow down the discharge plateau decay,restrain the phase transition of the structure,and stabilize the crystal structure during the charging and discharging cycle.Finally,three layered materials Li Ni O₂、Li Ni_0.99W_0.01O₂、and Li Ni_0.97Fe_0.02W_0.01O₂were prepared by co-doping tungsten with iron doping.The influence of Fe and W co-doping on the properties of the materials was studied.The results show that the co-doping improves the high-power performance and long-cycle stability significantly.At2.5 V-4.5 V wide operating voltage,Li Ni_0.97Fe_0.02W_0.01O₂material has a discharge specific capacity of 121.9 m Ah g^-1at 4000 m Ah g^-1,and only 65.2 m Ah g^-1is undoped.At 200m Ah g^-1current density,the capacity retention rate of Li Ni_0.97Fe_0.02W_0.01O₂reaches 88.1%after 100 cycles.The d Q d V^-1curves show that the doping inhibits the harmful phase transition of material H2（?）H3 and makes the crystal structure of the material more stable during the cycle.At the same time,the charge transfer impedance R_ctat the solid-liquid interface increases less after the cycle,and Li⁺diffusion coefficient is greater.