Improving The Rate Performance Of Layered Lithium-Rich Cathode Materials For Lithium-ion Batteries

With the deterioration of the environment,the shortage of energy,electric vehicles are widely used worldwide.The fast charge-discharge capacity of lithium-ion battery has become an important indicator of daily use.In recent years,with the advantages of high capacity,environment-friendly,the lithium-rich manganese-based cathode material is developing rapidly.However,its poor rate performance results in limited practical application.In this paper,Li_1.2Mn_0.6Ni_0.2O₂ was chosen as the research object.Through the surface coating of amorphous structure and amorphous/crystal composite structure,to improve the discharge capacity and cycle stability at high rate.Specific research carried out as follows:lithium-rich manganese-based cathode material Li_1.2Mn_0.6Ni_0.2O₂ and 1wt%,3wt%and 5wt%LiMnPO₄ coated Li_1.2Mn_0.6Ni_0.2O₂?LMP-1?LMP-3?LMP-5?,were prepared by sol-gel method respectively.This material is very friendly to the environment.XRD patterns of bulk materials and modified materials show that there is no obvious difference between the XRD patterns of these materials.The different quality of coating layer does not change the lamellar structure of the material,and it still has good crystallinity.It can be seen from the SEM diagram that the coating material and the bulk material do not change much in the particle size of the material.The edges of the cladding material are significantly more blurred,more rounded,and the bulk material margin is clearer.EDS test can prove that the experimental composition of materials and theoretical target material composition is basically the same.As can be seen from the TEM image,the cladding layer is indeed an amorphous layer.With the increase of the coating amount,the amorphous cladding layer on the material surface becomes thicker.Electrochemical tests showed that the LMP-3 sample with a 4.95 nm thick coating presented the highest capacity of 143.8 mAh·g^-1 after 150 cycles.The stability of LMP-3 sample is best under high magnification,and the discharge capacity reaches about 126 mAh·g^-1 at 10C.Indicating that the appropriate thickness of the coating layer for the material to enhance the performance of a great help.The electrochemical impedance spectroscopy?EIS?shows that the amorphous coating layer can effectively reduce the transfer resistance of lithium ions from the surface of the material to the electrolyte at high rate.This type of polyanionic phosphate compound,its amorphous coating can effectively improve the material rate performance,for other types of layered materials to improve the performance of reference value.The lithium-rich manganese-based cathode material Li_1.2Mn_0.6Ni_0.2O₂ with the theoretical capacity of ?250 mAh·g^-1 was prepared by sol-gel method.The surface of the lithium-rich cathode was modified by 3wt%?5wt%?7wt%of zirconium sodium phosphate compounds?NZP-3,NZP-5,NZP-7?.After comparing the XRD patterns of the bare and the modified materials,it was found that the main structures of the materials were almost the same and still showed a layered structure.However,there are two weak peaks at 2?=29?32oand 2?=43?44o,which correspond to NaZr₂?PO₄?₃and spinel phase,respectively.The TEM plot shows a clear crystal-like inclusion and an amorphous layered coating on the surface of the NZP-5 sample,and a spinel phase is observed in the bulk phase.The presence of elemental Na,Li,P and spinel phase Mn in the surface region can also be seen from the XPS spectra.The electrochemical results show that the amorphous layer can inhibit the contact of the bulk material with the electrolyte,and the crystal layer Li_xNa_?1-x?Zr₂?PO₄?₃?0?x<1?is a fast ion conductor.Together with the amorphous layer together for the lithium ions to provide more of the lithium diffusion channel.The spinel phase in the bulk phase also provides a three-dimensional channel.All of this accelerates the diffusion of lithium ions.Electrochemical tests showed that the amorphous/crystalline composite coating layer on the NZP-5 sample surface was the most stable at 1C,reaching 186.3 mAh·g^-1.The initial discharge capacity can reach 276.6 mAh·g^-1 at 0.1C and 98.5 mAh·g^-1 at 10C when the charge-discharge test is carried out under the step rate performance.NZP-5 material takes into account a high-capacity of the low and high rates.The electrochemical impedance spectroscopy?EIS?also showed that the composite coating layer effectively reduced the charge transfer resistance at high rate cycles and made outstanding contributions to the improvement of the rate performance.