Structural Evolution In Electrochemical Process And Modification Of LiNi_0.85-x Co_0.15Al_xO₂?x=0,0.03,0.05,0.07? Cathode Materials For Lithium Ion Batteries

The research work of this thesis is mainly based on the high-nickel layer cathode material LiNi_0.85Co_0.15O₂ for lithium ion batteries.Al-doped Li Ni_0.85Co_0.15O₂ cathode materials were synthesized by a high-temperature solid-state reaction.The effect of Al on the crystal structure,electrochemical property and thermal stability of LiNi_0.85Co_0.15O₂ were systematically investigated by XRD combined Rietveld refinement,SEM,EDS,DSC,EIS and electrochemical tests.Moreover,SiO₂ modified Al-doped cathode materials were prepared via a one-step high-temperature calcination.The main results are as follows:?1?High temperature solid state method is an effective way of doping Al into LiNi_0.85Co_0.15O₂ cathode material.When the amount of aluminum is increased from 0to 0.07 for LiNi_0.85-xCo_0.15Al_xO₂ cathode materials,the following results can be obtained.?i?Lattice parameter a is decreased from 2.873nm to 2.867nm and cell parameter c is increased from 14.183nm to 14.196nm.The cation mixing is increased first and then decreased.LNCA-5 sample shows the minimum degree of cation mixing while compared with another three samples.The theoretical density of unit cells shows a decreasing trend.The morphology and size of LiNi_0.85Co_0.15O₂ particles are not significantly changed while the doping amount of aluminum is within the current range.?ii?The charging voltage platform is increased while discharging platform decreased,which results in the increased electrode polarization.Besides,the polarization phenomenon is more serious when the upper charge voltage is 4.6V.?iii?Though the discharge specific capacities are decreased due to the increased amount of electrochemical-inactivity aluminum,the increase in aluminum content is benefit to improve the magnitude of discharge specific capacities when the upper limit of charge voltage is increased from 4.3V to 4.6V.?iv?The capacity retention and thermal stability of LiNi_0.85Co_0.15O₂ are increased first and then decreased,respectively.LNCA-5 sample shows the best comprehensive performance,which could be closely related to the lowest degree of cation mixing.?2?Aluminum does not change the irreversible two-phase transition of LiNi_0.85Co_0.15O₂ from H1 to H2 during the first charging process.And Al does not affect the subsequent reversible solid reaction generated by the de/intercalation of Li ions in H2 phase.In the first charging process,the lattice parameter c of Al-doped materials is larger than that of pristine,which demonstrates that aluminum is conducive to the migration of Li⁺in H2 phase.And aluminum is benefit to stabilize the structure of H2 phase during the first discharging process.The structural parameters(a?c?V?I_LiO2?d_Li-O?S_MeO2?d_Me-O)of LiNi_0.8Co_0.15Al_0.05O₂ exhibit a smaller change range than that of LiNi_0.85Co_0.15O₂,revealing that aluminum is benefit to improve the structural stability of H2 phase during the cycle process.Furthermore,the improved effect of structural stability is more significant while the batteries are cycled at a large current density.?3?The mixture of silane coupling agent and LiNi_0.8Co_0.15Al_0.05O₂ cathode material calcined at high temperature can be used as a simple method for LiNi_0.8Co_0.15Al_0.05O₂ coated by SiO₂.The results show that?i?the cycle performance and thermal property of LiNi_0.8Co_0.15Al_0.05O₂ are obviously improved by the coating of SiO₂.?ii?The high temperature?60??rate performance of the SiO₂-coated material is also enhanced.The SiO₂-coated LiNi_0.8Co_0.15Al_0.05O₂ compounds show the specific capacities of 176.4mAh/g and 158.9mAh/g at the current density of 0.2C and1C,respectively,which are larger than 174.2mAh/g and 153.8mAh/g for the pristine,respectively.It can be found that the coated SiO₂ is benefit to decrease the resistance of the surface film(R_f)and stabilize the structure of LiNi_0.8Co_0.15Al_0.05O₂ during cycles,which significantly improve the rate capability,cycle performance and thermal property of LiNi_0.8Co_0.15Al_0.05O₂ cathode material by studing on the structure and performance of the materials.