Cathode Material Li_1.25Ni_0.2Fe_0.13Co_0.33Mn_0.33O₂ Polyvinyl Alcohol-assisted Synthesis And Ammonium Fluoride Pretreatment

LiNi_1/3Co_1/3Mn_1/3O₂?NCM111?is the ancestor of ternary cathode materials,and its development process is to first synthesize LiNi_1/2Co_1/2O₂ by doping nickel element in LiCoO₂,and then synthesizing LiNi_1/3Co_1/3Mn_1/3O₂ by doping manganese element in LiNi_1/2Co_1/2O₂.In terms of cost,the lower price of nickel and manganese doping instead of cobalt reduces the overall price of ternary materials.The ternary material belongs to the layer structure of?-NaFeO₂,and the two-dimensional channel provided by this structure can improve the deintercalation rate of lithium ions.LiNi_1/3Co_1/3Mn_1/3O₂ has the advantages of high reversible capacity,stable structure,and high operating voltage.However,this material has low charge-discharge efficiency for the first time,serious cation mixing in the lithium layer,and low electronic conductivity and lithium ion diffusion coefficient.Another problem is that as the number of charge and discharge cycles increases,side reactions occur between the electrode and the electrolyte,causing some metal ions to dissolve and the battery performance to decrease significantly.Solving the above problems is generally done by means of element doping,construction of special morphology,surface coating,etc.In the early stage,we have used hollow spherical manganese dioxide as a self-template to synthesize LiNi_1/3-xFe_x Co_1/3Mn_1/3O₂?spherical shell morphology?with LiNi_1/3Co_1/3Mn_1/3O₂ doped with quantitative iron element instead of nickel element?0.000?x?0.267?,and found that x=0.133 is the best doping ratio,and this material has the best electrochemical performance.In this paper,Li_1.25Ni_0.2Co_0.33Fe_0.13Mn_0.33O₂ will be further studied.The first part is to add different contents of polyvinyl alcohol?PVA?to explore its impact on battery performance;the second part uses different molar ratios of fluorination Ammonium pretreatment Li_1.25Ni_0.2Co_0.33Fe_0.13Mn_0.33O₂,to explore the difference between different molar ratios of NH₄F pretreatment.The tests involved in this thesis include XRD test,SEM test,EDS test,TEM test and battery related electrochemical performance test,and use the RIETAN-FP software to process the XRD test results to obtain the corresponding lattice parameters and refined patterns.The first part uses the polyvinyl alcohol sol-gel method to synthesize Li_1.25Ni_0.2Co_0.33Fe_0.13Mn_0.33O₂ to explore the effect of different polyvinyl alcohol dosages?PVA=0.0,5.0wt%,10.0wt%,15.0 wt%?on battery performance.After comparing the test results,it is found that the sample PVA-10%has the best battery performance,and the capacity retention rate can reach 85.8%after 50 charge and discharge cycles?rate of 0.1C?.The second part is the pretreatment of Li_1.25Ni_0.2Co_0.33Fe_0.13Mn_0.33O₂ with ammonium fluoride to explore different molar ratios?positive electrode material:ammonium fluoride=100:0,95:5,90:10,85:15,80:20?The effect of ammonium fluoride pretreatment on battery performance.Comparative test results found that sample NFCM 90 has good cycle performance and rate performance.After 50 cycles?rate 0.1C?,its capacity retention rate was 85.2%.