Study On Modification Of Cathode Materials For Power Battery

Lithium-ion batteries have been considered as a promising candidate for electric vehicles,hybrid electric vehicles and plug-in hybrid vehicle due to high power density,high energy density,long cycling life,low cost and outstanding safety.At present,the commercial cathode materials are lithium transition metal oxides and their derivatives,such as LiCoO₂,LiNiO₂,LiMn₂O₄,LiFePO₄ and Li(Ni_1-x-yCo_xMn_y)O₂.The typical cathode materials for power batteries,such as LiFePO₄ and Li(Ni_1-x-yCo_xMn_y)O₂,are still insufficient to meet the demands of aforementioned vehicles markets because of its own shortcomings: poor conductivity and low rate performance of LiFePO₄;dissolution of transition metal ions for Li(Ni_1-x-yCo_xMn_y)O₂ in the charge-discharge process,which results in the decrese of cycle performance.Moreover,under the high cut-off voltage or high temperature,the side reaction between cathode materials and electrolyte can easy to produce,reduce the service life of the battery.In view of the shortcongmings and deficiencies of above materials,the common improvent methods include ion doping and surface coating.Metal oxides,phosphates,carbon and polymer have been investigated as coating mateials,which can effectively prevent the contact between active material and electrolyte and suppressed dissolution of transition metals.This work selects LiNi_0.5Co_0.2Mn_0.3O₂ and LiFePO₄ as the research object,using metal oxide coated LiNi_0.5Co_0.2Mn_0.3O₂,polymer and carbon coated LiFePO₄ to improve electrochemical performance.The specific content as follows:1.We use H₂O,Al?NO₃?₃·9H₂O as solvent and aluminum source,respectively.We have successfully synthesized uniform Al₂O₃ coating layer on LiNi_0.5Co_0.2Mn_0.3O₂ by a simple,facile and economical solution-based method.By controlled solution crystallization,the control of pH is realized by adjusting Al?NO₃?₃ concentration and the ratio of Al?NO₃?₃ to LiNi_0.5Co_0.2Mn_0.3O₂ at room temperature?theoretical calculation of Al₂O₃?,homogeneous nano Al₂O₃ coating layer is obtained.When the mass ratio is 0.75%,the coated material exhibits the best electrochemical performance.The Al₂O₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ cathode delivers an initial capacity of 177.4 mA h g-1,and a discharge capacity of 124.9 mA h g-1 with capacity retention of 70.4% at 1 C?1 C=183 mA g-1?after 200 cycles.LiMn₂O₄ and LiNi_0.8Co_0.15Al_0.05O₂ were coated with Al₂O₃ using the same procedure.LiNi_0.8Co_0.15Mn_0.05O₂ at high cut-off voltage and LiMn₂O₄ at high temperature also show good cycling performance.2.PVK and LiFePO₄ as raw materials,NMP as solvent.PVK coated LiFePO₄ has been synthesized by a simple solution method.By optimizing the amount of PVK,homogeneous nano coating layer is obtained to improve the rate performance and cycle stability of low conductivity materials.The results show that when the amount of PVK is 2.5%,the coated LiFePO₄ exhibits the best electrochemical performance,the specific capacity can reach to 68 mA h g-1 at 10 C?1 C=170 mA g-1?,which shows excellent rate performance.3.Polyvinylpyrrolidone?PVP?,nickel nitrate nitrate,phosphoric acid and lithium hydroxide as raw materials.The fibrous structure of LiFePO₄/PVP precursor has been fabricated by using an electrospinning apparatus.After the sintering,an integrated electrode material that has been obtained,no current collector,no conductive agent and no binder.The results show that the crystal structure of LiFePO₄/C is oriented along the?010?plane.The specific capacity of Li FePO4 can reach 160 mA h g-1 at 0.1 C at the ratio of 1: 2 of PVP and LiFePO₄.It is almost attenuated after 50 cycles.It shows high discharge capacity and excellent cycle performance.