Preparation And Study Of Lithium-rich Manganese-based Cathode Materials For Lithium-ion Batteries

In recent years,lithium-ion batteries quickly emerged in various chemical power sources because of their high volumetric energy density and weight energy density,and quickly occupied the electrochemical energy storage equipment market.However,due to the limited energy density of lithium ion battery cathode materials,it has limited its application in large-scale energy storage equipment and electric vehicles.Recently,lithium-rich manganese-based cathode materials have been extensively studied.Among them,xLi₂MnO₃·?1-x?LiMO₂?M=Ni,Co,Mn,etc.?solid solution composites retain the advantages of single transition metal ions,but also overcome their disadvantages,and are expected to become the next generation of commercial cathode materials for lithium-ion batteries.Based on the microstructure,this paper explored the doping of different proportions of transition metal ions in Li₂MnO₃,and studied its morphology,structure and electrochemical properties.The works include:?1?A simple in-situ template sacrifice strategy is used to synthesize xLi₂MnO₃·?1-x?LiNiO₂?x=0.3-0.7?of hollow spheres.With the increase of Li₂MnO₃ content,the superlattice peaks between 20°?23°become more and more obvious.It indicates that we have successfully synthesized monoclinic Li₂MnO₃ compounds and proved the orderly arrangement of Li⁺ions in the transition metal layer.In contrast,when the ratio of Li₂MnO₃/LiNiO₂ is 0.6/0.4,it is the optimum ratio of lithium-rich manganese-based cathode materials.After the first few cycles of activation,the discharge capacity of 0.6Li₂MnO₃·0.4LiNiO₂ increased significantly and remained at?210 mAh·g-1.Through the above discussion,if the Ni content is too high in x Li₂MnO₃·?1-x?LiNiO₂?x=0.3-0.7?,more transition metal ions occupy the Li⁺ions sites in the initial cycle to generate more irreversible capacity.In contrast,with low content of Ni leads to the oxidation of Ni²⁺/Ni³⁺to Ni4⁺cannot provide sufficient reversible capacity meanwhile Li₂MnO₃ cannot be activated effectively.Therefore a appropriate content Ni ions doping to Li₂MnO₃ is favorable to kinetically activate,which can not only stabilize the ordered layered structure while reduce the rearrangement of the transition metal ions to avoid destroy of the layered structure but also provide sufficient reversible capacity;?2?xLi₂MnO₃·?1-x?LiNi_0.8Co_0.2O₂?x=0.3-0.7?was prepared by simultaneously doping Ni and Co elements in a lithium-rich manganese-based layered oxide.By discussion,the ratio of Li₂MnO₃/LiNi_0.8Co_0.2O₂ is 0.6/0.4,which is the optimal ratio of synthetic lithium-rich manganese-based cathode materials.0.6Li₂MnO₃·0.4LiNi_0.8Co_0.2O₂ still has excellent cycle stability and high discharge capacity after 70 cycles,and the discharge capacity is reduced from 190 mAh·g^-1 to 176 mAh·g^-1,and the capacity retention rate is92.6%.Among them,Ni is the main electrochemical active element which is the main source of capacity in charge-discharge process;Mn has a positive effect on the lattice structure stability and thermal stability,while Co plays an irreplaceable role in reducing the electrochemical polarization effect and improving the rate performance;?3?Trace amounts of Na⁺doping Li_1.23[Ni_0.2464Mn_0.462Co_0.0616]O₂.From XRD analysis,it can be concluded that Li_1.23-xNa_x[Ni_0.2464Mn_0.462Co_0.0616]O₂?x=0,0.02,0.03 and 0.04?are based on hexagonal?-NaFeO₂ layered compounds?R-3m space group?with monoclinic structure?C2/m space group?.Meanwhile,the diffraction peaks move slightly to the lower angle after doping Na⁺,which proves that Na⁺are successfully doped into the interior of the crystal lattice and expanded the lattice space.Electrochemical tests reveal that doping of Na⁺can effective promote the discharge capacity.At a current density of 1 C,the discharge capacity of Li_1.2Na_0.03[Ni_0.2464Mn_0.462Co_0.0616]O₂ reduced from the initial 194.4 mAh·g^-1 to188.9 mAh·g^-1 after 110 cycles,while the working voltage attenuation only from 3.710 V to3.622 V.??V=0.088 V?.At the same time,the R_ct decreases after doping with Na⁺.It can be proved that a thinner SEI film can be formed after doping Na⁺,which accelerates the insertion and extraction of Li⁺during charge and discharge cycles,which helps the sample to have a more stable lattice structure and excellent electrochemical performance.