Fabrication And Modification Of LiNi_0.8Co_0.15Al_0.05O₂ Cathode Materials For Lithium-ion Battery

LiNi_0.8Co_0.15Al_0.05O₂ cathode material has the advantages of high specific capacity?278 mAh/g?,excellent thermal stability,low cost and environmental protection.Compared with the traditional commercial LiCoO2 cathode material,LiNi_0.8Co_0.15Al_0.05O₂ is a promising cathode material for advanced lithium-ion battery cathode material.However,LiNi_0.8Co_0.15Al_0.05O₂ cathode material still has many problem to be solved urgently,such as serious nickel-lithium mixing,low charge-discharge efficiency,poor cycle stability and so on.In order to solve the above problems,in this thesis,the co-precipitation method has been used to prepare LiNi_0.8Co_0.15Al_0.05O₂ cathode material.The effects of precursor concentrations and pH values on the phase,morphology microstructure and electrochemical properties of LiNi_0.8Co_0.15Al_0.05O₂ have been studied simultaneously.Meanwhile,LiNi_0.8Co_0.15Al_0.05O₂@Li[Li0.2Mn0.54Co0.13Ni0.13]O2 has been prepared by recombining LiNi_0.8Co_0.15Al_0.05O₂ with Li[Li0.2Mn0.54Co0.13Ni0.13]O2.The relationship between the phase morphology,microstructure and electrochemical properties of the composites has been studied in detail,and a new type of high performance cathode material for lithium ion battery was obtained.The main contents are as follows:LiNi_0.8Co_0.15Al_0.05O₂ was prepared by co-precipitation method at different precursor concentrations and pH values of the reaction system.The phase morphology microstructure and electrochemical properties of all samples were made a comparison systematically.In the co-precipitation route,NaOH solution was chosen as the base solution and precipitator.And ammonia solution was used as the complexing agent.In more detail,ammonia solution and the mixed solution containing Ni?NO3?2·6H2O,Co?NO3?2·6H2O and Al2?NO3?3·9H2O were dropwise into NaOH solution to prepare precursor simultaneously.The temperature of mixed solution was kept at 50 oC.Then the precursor was mixed with a stoichiometric amount of Li2CO3 by ball milling.Finally,the LiNi_0.8Co_0.15Al_0.05O₂ sample was obtained by calcining theaforementioned mixture.The experimental results are as follows: LiNi_0.8Co_0.15Al_0.05O₂ samples obtained at different pH valves had hexagonal ?-NaFeO2 structure?space group R-3m?and had good crystallinity.The particle size of primary particles of LiNi_0.8Co_0.15Al_0.05O₂ decreased with increasing pH values.At the same time,the particle size of the secondary particles is increasing which is resulting from the agglomeration of primary particles.Electrochemical tests show that the sample prepared at reaction pH=11 had the best electrochemical performance.The first discharge specific capacity was 238.8 mAh/g,and the capacity retention after 50 cycles was 79.2%.The results of XRD showed that the samples synthesized at different precursor concentrations have hexagonal ?-Na Fe O2 structure?space group R-3m?,and have similar crystal size and layered structure.The results of SEM showed that the samples prepared at precursor concentration of 0.5 mol/L were completely irregular polygons.With the increase of the precursor concentration,the sample had a tendency to be spherical.The electrochemical tests showed that the samples with best electrochemical performance were obtained at the precursor concentration of 2 mol/L.The first discharge specific capacity was 236.7 mAh/g,and the capacity retention rate was higher than 80.1% after 50 cycles.The aqueous solution of Ni?Ac?2·4H2O,Co?Ac?2·4H2O and Mn?Ac?2·4H2O was prepared as the precursor solution.NaOH solution was chosen as the precipitator.And the ammonia solution was used as the complexing agent.In the co-precipitation route,the ammoin solution and the NaOH solution were dropwise into the precursor solution and.obtained the mixture Subsequently,the aqueous solution of Ni?NO3?2·6H2O,Co?NO3?2·6H2O and Al2?NO3?3·9H2O was dropwise into the mixture mentioned above.Then the precursor was mixed with a stoichiometric amount of Li2CO3 by ball milling.Finally,the composite material sample was obtained by calcining the aforementioned mixture.In order to obtain a high performance cathode material for lithium ion battery,LiNi_0.8Co_0.15Al_0.05O₂@Li[Li0.2Mn0.54Co0.13Ni0.13]O2 composites were synthesized by co-precipitation method.The relationship between the phase morphology,microstructure and electrochemical properties was obtained by SEM,TEM and XRD.The results showed that LiNi_0.8Co_0.15Al_0.05O₂@Li[Li0.2Mn0.54Co0.13Ni0.13]O2composites were synthesized successfully.Meanwhile,LiNi_0.8Co_0.15Al_0.05O₂ nanoparticles and Li[Li0.2Mn0.54Co0.13Ni0.13]O2 nanoparticles were mixed very well.Compared with the particles of Li Ni0.8Co0.15Al0.05O2 sample?300nm?and Li[Li0.2Mn0.54Co0.13Ni0.13]O2 sample?300nm?,the composite sample had smaller particles,and the composites had more spherical particles.Electrochemical tests show that cycle performance and rate performance of LiNi_0.8Co_0.15Al_0.05O₂ sample and Li[Li0.2Mn0.54Co0.13Ni0.13]O2 sample were both lower than that of the composites.Thus the results indicate that Li Ni0.8Co0.15Al0.05O2 phase and Li[Li0.2Mn0.54Co0.13Ni0.13]O2phase produce the synergistic effect,improving the electrochemical performance of composites of the cathode material.