Research On Doping And Coating Modification Of Spinel-type Lithium Nickel Manganate

With the development of renewable energy technology,new energy storage devices represented by lithium-ion batteries have attracted great attention.Spinel type Li Ni_0.5Mn_1.5O₄(LNMO)cathode material is one of the most promising cathode materials for lithium-ion batteries.It has many advantages,such as low cost,high operating voltage,high energy density and environmental friendness,etc.,and has been widely paid attention.However,during the preparation of Li Ni_0.5Mn_1.5O₄cathode material at high temperature,problems such as Li_xNi_1-XO heterophase,side reaction between high voltage charge and discharge and electrolyte and dissolution of Mn are easy to occur,leading to rapid capacity attenuation of spinel Li Ni_0.5Mn_1.5O₄cathode material,poor rate capability and cycling stability,and reduced its service life.Therefore,this paper mainly studied the modification of Li Ni_0.5Mn_1.5O₄from two perspectives.First,the influence of the regulation of bulk phase structure on the electrochemical properties of materials was studied.Second,the influence of interface structure regulation on the electrochemical properties of materials is studied.The bulk phase structure of Li Ni_0.5Mn_1.5O₄was controlled by hydrothermal method.The results show that the formation of Lix Ni_1-XO impurity phase can be effectively inhibited by regulating the bulk phase structure of Li Ni_0.5Mn_1.5O₄with Ru element,and the material presents Fd-3m space group structure.In addition,the primary particle structure of Li Ni_0.5Mn_1.5O₄material is refined with the bulk phase structure,which reduces the diffusion path of Li Ni_0.5Mn_1.5O₄material,thus improving the transport performance of Li Ni_0.5Mn_1.5O₄material.At the current density of 50m A g^-1,the initial discharge specific capacity of Li Ni_0.46Ru_0.04Mn_1.5O₄(Ru-4)is140.16m Ah g^-1,which is much higher than that of Li Ni_0.5Mn_1.5O₄(93.64m Ah g^-1).After 100 cycles at the current density of 1C,the capacity retention rates of Li Ni_0.46Ru_0.04Mn_1.5O₄(Ru-4)and Li Ni_1.5Ni_0.44Ru_0.06Mn_1.5O₄(Ru-6)are 99.86%and97.82%,respectively,which are both higher than those of the pure sample(95.39%),indicating that Ru ion doping has a significant effect on improving the discharge specific capacity of Li Ni_0.5Mn_1.5O₄cathode materials at high current density.The Mg element is adopted to Li Ni_0.5Mn_1.5O₄material phase structure regulation,the body in the crystal structure,lithium ions and electrons transfer performance aspects have been similarly improved,as a result of Mg element control cannot be effectively restrain side effects between the anode materials and electrolyte,in high magnification effect of electrochemical performance improvement is not obvious.The bulk phase structure of Li Ni_0.5Mn_1.5O₄material was controlled by solid phase method.The spherical Ni_0.5Mn_1.5CO₃precursors were used as templates to regulate the bulk phase structure of Li Ni_0.5Mn_1.5O₄cathode materials by Ru and La elements.It is found that Li Ni_0.5Mn_1.5O₄keeps the spherical morphology of the template,and many cavitates appear on the surface of the particles,forming a special porous structure.Ru ion doping increases the layer spacing of Li Ni_0.5Mn_1.5O₄,which is conducive to the embedding and deembedding of Li Ni_0.5Mn_1.5O₄,and reduces the charge transfer impedance(R_ct),thus improving the electrochemical performance of the material.The electrochemical performance test showed that the discharge capacity of the modified Li Ni_0.5Mn_1.5O₄cathode material was 123.1 m Ah g^-1(Ru-4)and125.3m Ah g^-1(Ru-6)at the current density of 0.2C,which was 16%higher than that of the pure phase Li Ni_0.5Mn_1.5O₄(107.9 m Ah g^-1),and the improvement effect was more obvious with the increase of the charge and discharge current density.However,when La element is used to adjust the bulk structure of Li Ni_0.5Mn_1.5O₄material,it has a great influence on the material body structure and is prone to generate heterophase,which leads to poor modification effect of the material.At the current density of 0.2C,the specific discharge capacity of the modified material is only 110m Ah g^-1.Theoretical study on phase structure of two-element cooperative regulator.Based on the primary principle of density functional calculations,the effects of single element Ru and Ru-Mg double element on the crystal structure of spinel Li Ni_0.5Mn_1.5O₄cathode material were discussed.The influence of the cathode materials research found that Ru-Mg double elements can control in different extent inhibited the Li Ni_0.5Mn_1.5O₄in the process of taking off the intercalated-Li cathode material volume change,predict Li Ni_0.5Mn_1.5O₄cycle performance of the cathode materials will be promoted.According to first-principles calculations,the volume change rate of Li Ni_0.5Mn_1.5O₄cathode material controlled by Ru-Mg double element is the least during the deintercalation process,and the volume change rate is only2.5%.When the ratio of Mg:Ru is 1:1,the Li Ni_0.5Mn_1.5O₄cathode material controlled by Ru-Mg double element exhibits the highest discharge specific capacity and better cycle stability performance.Study on interface modification of Li Ni_0.5Mn_1.5O₄cathode material.Firstly,the effects of different complexing agents and coating agents on the electrochemical properties of Li Ni_0.5Mn_1.5O₄cathode materials were studied.The results show that LNMO cathode materials prepared with citric acid as complexing agent have better discharge capacity(111 m Ah g^-1,0.2C)and rate performance(70 m Ah g^-1,5C)than malic acid and lactic acid.Then,Al F₃and Ce F₄were used to modify the interface of Li Ni_0.5Mn_1.5O₄cathode material,and the influence of the change of interface microstructure on the electrochemical performance was studied.When the amount of Al F₃is 1.5%,the discharge specific capacitance is 117.25m Ah g^-1at 0.2C and 108.79m Ah g^-1after 100 cycles at 1C,showing good cycling stability performance.When the interface of Li Ni_0.5Mn_1.5O₄cathode material was modified by Ce F₄,a similar modification effect was also achieved.When the amount of Ce F4 modification was3%,the specific discharge capacity was 120m Ah g^-1after 100 cycles at 1C.