Surface Structure And Electrochemical Properties Of Li-rich Manganese-based Cathode Materials

Lithium-ion battery,a kind of secondary battery,has been widely used in smart phones,notebook computers and other portable devices because of its high output voltage,high specific energy,long cycle life and other quite unique characteristics.It has even become the preferred power source in new energy vehicles,large-scale energy storage and other new fields.Li-rich manganese-based layer oxides（LRMOs）are regarded as the cathode materials for the next generation of lithium-ion batteries due to their advantages of high specific capacity and low cost.However,the irreversible anion redox and phase structure transformation during the initial charging process lead to the low initial coulomb efficiency,capacity attenuation,and poor rate performance and low cycling performance.To sum up,in this paper,Li-rich lithium manganese-based cathode materials Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ with different morphologies and surficial structure were synthesized by different synthesis methods and their characteristics and electrochemical properties were studied in detail.Especially,we create oxygen defects on the surface of the LRMOs by means of molten salt pre-activation,extracting the Li source on the surface to form the concentration gradient material with Li-poor on the surface and Li-rich in the core,along with the spinel phase coating layer on the surface of materials.This strategy greatly reduces the irreversible capacity loss and voltage attenuation of materials.In the first part of work,different synthetic methods,including solid phase method,sol-gel method and solvothermal method,were used to synthesize different morphology and particle size of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂.This part of the work explores the influence of different morphologies on the structure and properties of LRMOs materials.The results show that nano-sized LRMOs particles obtained by sol-gel method exhibit the highest initial Coulombic efficiency and the higher first-discharge specific capacity,compared with the solid-phase method.This is attributed to the fact that the particles of the material obtained by sol-gel method are smaller（100 nm-300 nm）,the size dispersion is more uniform,and the material has a more complete layered structure.The electrochemical properties of the materials with rod-like or spherical shape synthesized by solvothermal method are much higher than those synthesized by solid-phase.Due to their nanometer/micron hierarchical structure and highly exposed^[1]crystal planes,the rod-like or spherical LRMOs have excellent rate performance(a discharge capacity of40 m Ah·g^-1at 10 C).Moreover,the capacity retention of spherical LRMOs materials after 100 cycles is about 100%,and their initial Coulombic efficiencies are over 70%.This is because the structure obtained by solvothermal method has the dual advantages of nano and micron structure,short diffusion path of lithium ions and good structural stability,which gives the material excellent cycling performance,structural stability and rate performance.In the second part,the nanoscale Li_1.2Mn_0.54Co_0.13Ni_0.13O₂（p-LRMO）synthesize by sol gel method was treated by the surface modifying agent of NH₄VO₃ to extract Li source from the surface of p-LRMO under the high temperature calcination.The modification strategy cis simpler and more controllable compared with Li⁺/H⁺exchange method.Firstly,we created Li₃VO₄-coating on the surface of p-LRMO by the reaction of NH₄VO₃ with p-LRMO at 350℃and then removed the excess oxygen on the surface of p-LRMO by washing out Li₃VO₄.The strategy can easily remove a certain amount of the Li₂O and form a Li-poor layer on the surface of p-LRMO by controlling the amount of vanadium salts.In the process of high temperature treatment,due to the defects and vacancies on the surface of the material,cations rearrange to form spinel phase coating layer and Li poor layer on the surface of the material.The modified material can significantly inhibit the generation of O₂ and CO₂ during the first charging process,and keep the structure of LRMOs stable during the charging and discharging process.Therefore,the fused vanadate-treated p-LRMO material exhibits a high discharge capacity of 276 m Ah·g^-1at 0.1 C and a high first-cycle coulombic efficiency of 95.1%during the first cycle,in which the attenuation of voltage is also reduced to0.03 V（after 100 cycles）.In the third part,we used acetate as raw materials to synthesis nano/micro grade rod-Li_1.2Mn_0.54Co_0.13Ni_0.13O₂（LRMO）by solvothermal method.In this work,we applied（NH₄）₂Si F₆ to react with LRMO at 700?°C to form Li F and Li₂Si O₃ on the surface of LRMO.The washing out of Li₂O from LRMO leads to the formation of Li and O vacancy on the surface of the material.Meanwhile,the lattice dislocation and cation rearrangement resulted in the establishment of spinel layer on the LRMO surface.In the end,Li-gradient and surface spinel-coating phase are introduced into LRMO spontaneously,boosting the electrochemical performance.The initial Coulombic efficiency of the modified LRMO is up to 87%.The capacity retention of is up to 90.4%of initial capacity after100 cycles,and the discharge capacity can reach 76 m Ah·g^-1 at10 C.