Modification Of Polypyrrole Coating And Preparation Of NCA Cathode Materials By Coprecipitation Method

With the development of rechargeable battery technology,lithium-ion batteries are widely used in life as a new generation of energy storage equipment due to their high energy density and environmental friendliness.As we all know,for every 50%increse in the capacity of the cathode material,the energy density of the battery will increase by 28%.Therefore,the key technology to meet the battery capacity requirement is to increase the discharge specific capacity and reversible capacity of the cathode material.NCA cathode materials are receiving international attention because they can deliver an exceptionally high rechargeable capacity of 190 mAh g-1 between 2.75-4.3V and low cost.However,NCA materials still have disadvantages such as poor rate performance and cycle instability.Thus,the NCA cathode material was prepared by coprecipitation method,and the surface modification of conductive polypyrrole was studied in this paper.The nucleation and growth mechanism of the crystal was studied by adjusting the objective conditions such as reaction temperature,pH and precipitant concentration via coprecipitation method,and the effects of different synthesis conditions on the morphology,phase structure and particle size distribution of oxalate precursors were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM)and laser particle size analyzer.It was found that adding 600 mL of bottom liquid to the reactor and controlling the pH to 6.0 of the whole process is more favorable for the precipitation of transition metal ions,and the reaction in a hot solution at 60? can increase the diffusion of the crystalline ions,forming regular spherical particles,which can reduce the adsorption of impurities and the loss of precipitation dissolution.Furthermore,the molar ratio of the precipitant to the transition metal is 1:1,and the feed rate is 12 mL/min and 3 mL/min,it is possible to obtain a radial spherical structure composed of radially grown rod-shaped particles,which can provide diffiision channel for Li+.According to the oxalate precursor prepared under different conditions,the NCA material was further synthesized by high temperature solid phase method,and its phase structure and electrochemical properties were characterized.According to the lattice parameters found that the materials have a good layered structure,and the lattice spacing of the material will also change with the reaction temperature,precipitant concentration and feed rate change,which will affect the diffiusion migration of Li+ during lithiation and delithiation cycling.Electrochemical tests further prove that the NCA material obtained from coprecipitation with 94%capacity retention after 50th cycle at 1C.In addition,the porous LiNi0.815Co0.15Al0.035O2(PLNCA)cathode materials was successfully synthesized via stepwise co-precipitation method in this work.The porous cathode microstructures achieve a high discharge specific capacity of 200.6 mAh g-1 at 0.1C with excellent rate capability(151.4 mAh g-1 at 5C),and the capacity retention of 91%after 100 cycles at 1C with the voltage of 2.75-4.3V versus Li/Li+.According to the lithium ion diffusion coefficient of porous material of 3.458×10-12 cm2 S-1,it is further confirmed that the porous structure can effectively shorten the diffusion path of lithium ions,and improves the reversible capacity and rate performance.The polypyrrole(PPy)with high electrical conductivity is designed to coat on the surface of NCA cathode material.PPy coating layer on the surface of NCA successfully is realized by means of liquid-phase chemical oxidation polymerization method,and which has been verified by the scanning electron microscopy(SEM),transmission electron microscope(TEM)and fourier transforn infrared spectroscopy(FTIR).It was found that PPy-coated NCA(PL-2)reaches a high electrochemical capacity of 206.6 mAh g-i at 0.1 C with coulombic efficiency of 91%at a current density of 1C with the voltage of 2.75-4.3V versus Li/Li+ at room temperature.It exhibits excellent rate performance and capacity retention at 1C,3C and 5C current densities(163.5 mAh g-1 at 5C).As a highly electrons conductive material,the PPy coating shows great potential in surface modification and is expected to improve performance for other types cathode materials of LIBs.