Synthesis, Modification And Electrochemical Properties Of High-nickel Cathode Materials For Lithium-ion Batteries

With the rapid development of science and technology,lithium-ion batteries(LIBS)are no longer limited to small-scale information appliance industry,but also expanded in large-scale industrial production such as rail transit,aerospace and artificial intelligence.The storage system with high energy density,high power density,high security performance and long cycle life has become the main focus of energy storage research.The"high nickel ternary"cathode materials with high theoretical specific capacity(280m Ah g^-1),such as LiNi_xCo_yAl_1-x-yO₂(NCA)and LiNi_xCo_yMn_1-x-yO₂(NCM)(x?0.8),are the materials of choice for high energy density batteries.However,its poor cycle stability,low coulomb efficiency and slow lithium-ion diffusion rate have always been the main reasons limiting its promotion.In this thesis,the improvement of the electrochemical performance of the high nickel ternary cathode is studied from two aspects of material preparation and modification.Ni-rich materials Ni_0.9Co_xAl_0.1-x(OH)₂(x=0,0.02,0.05,0.07,0.1)with different Co-Al ratios were prepared by co-precipitation method,and the influence of different Co-Al ratios on their structure and electrochemical performance was explored.The results show that the materials with different Co-Al ratios are all layered structures.With the increase of Co content,the degree of crystallization of the cathode material increases,and the degree of Li⁺/Ni²⁺mixing decreases.When the Co content is 0.02,the material has better electrochemical performance,and the charge-discharge specific capacity at 10C is 125m Ah g^-1 Based on the above research,polyvinylpyrrolidone(PVP)was used as a dispersant and protective agent to assist the synthesis of Ni_0.9Co_0.02Al_0.08(OH)₂ material.Adding PVP and prolonging the synthesis time can promote the increase of the primary particles of the precursor and further improve the electrochemical performance of the material.The LiNi_0.9Co_0.02Al_0.08O₂ material obtained under the optimal process conditions of pH=10,adding 1%PVP and stirring for 15 h,has a charge and discharge capacities of 141 and 129 m Ah g^-1 at 5C and 10C,respectively.After 50 cycles at 0.2C,the capacity retention rate is 87%;after high-rate cycles,the discharge specific capacity of the material accounts for 93%of the first discharge specific capacity.without changing the physical and chemical properties of the initial material,through simple mechanical grinding,three different transition metal high-valent oxides of Cr₂O₅,V₂O₅ and MoO₃ were blended with commercial Ni-rich NCA/NCM material to obtain composite material.The effects of the materials before and after blending on the morphology,crystal structure and electrochemical performance of the two materials were compared and analyzed,and the influence mechanism of the blending ratio on the physical properties of the composite electrode and the Li⁺de-intercalation kinetics was explored.The results show that Cr₂O₅ modified NCA with high capacity and good cycle stability increases the Li⁺transmission channel,which significantly improves the first coulombic efficiency and cycle stability of NCA;Although high-capacity V₂O₅ has excellent electrical conductivity,the resulting phase transition makes the battery cycle performance not significantly improved;and the higher-capacity MoO₃ is chemically active and easily reacts with the electrolyte,which inhibits the electrochemical performance of NCA.The coordination between the components of the electrode can affect the performance of the battery.Considering this aspect,the conductive agent,an important component in the cathode electrode,has been studied.The influence of the morphology,crystallinity and graphitization degree of three commercial carbon conductive agents on the electrochemical performance is systematically studied.The binary composite conductive agents SP/VGCF and SP/KS-6 with different proportions were prepared by mixing dot-shaped conductive carbon black(SP)with linear vapor-grown carbon fiber(VGCF)and flake graphite(KS-6)respectively.It was mixed with NCA to prepare electrode materials,and the mechanism of action between different conductive agents is discussed.The study found that the SP electrode can better retain the electrolyte;a small amount of VGCF forms a stable supporting carbon skeleton in the electrode,which ensures and shortens the Li⁺migration channel,avoids the formation of"islands"of particles,inhibits polarization,and improves the stability performance of the electrode.The sub-oxidized phase Ti₄O₇ was prepared by rapid thermal reduction of the tetragonal anatase and rutile mixture of raw material TiO₂(P25).The structure,morphology and electrical conductivity of Ti₄O₇ were studied in detail.It is added to the conductive carbon black SP in different proportions,and the cyclic voltammetry and impedance test results show that due to the poor lithium intercalation performance of Ti₄O₇,a large amount of Ti₄O₇ makes the cycle performance worse.However,an appropriate amount of Ti₄O₇(0.5%)greatly improves the rate characteristics of NCA.At 10C,its first discharge specific capacity reaching 158.9 m Ah g^-1.