Low Cobalt Of Lini <sub> X </ Sub> Co <sub> 0.2 </ Sub> Of Mn <sub> 0.8-x </ Sub> The O <sub> 2 </ Sub> (x = 0.4,0.5,0.6) Synthesis And Properties Of The Cathode Material

Intensive research and development work is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode material. LiCoO₂ has been widely used as a cathode material in commercial lithium ion battery production. But due to its high cost and toxicity, many efforts has been made to replace it. LiNi_1/3Co_1/3Mn_1/3O₂ cathode material is one of the focus researches recently. This dissertation emphasizes on layered LiNi_xCO_0.2Mn_0.8-xO₂ (x=0.4, 0.5, 0.6) cathode materials which have lower content of Co than LiNi_1/3Co_1/3Mn_1/3O₂ for reducing cost, but still have good performance. Make use of the advantage of transition metal Ni,Co,Mn in the materials and restrain the side-effect. The synthesis, electrochemical behavior and kinetics of lithium insertion-extraction process of layered LiNi_xCO_0.2Mn_0.8-xO₂ (x=0.4, 0.5, 0.6 ) cathode materials were studied in details.The reaction conditions in sol-gel process, pre-calcinations process and calcinations process were analyzed and the optimized flowsheet of LiNi_0.4CO_0.2Mn_0.4O₂ was obtained: the gel was prepared by control the pH value 6～7, layered LiNi_0.4CO_0.2Mn_0.4O₂ was synthesized with 5% excess lithium under 450℃for 5h, 850℃for 12h and quenched in the environment temperature, then ballmilled with carbon for 4h. The material shows a high initial discharge capacity of 164mAh·g^-1,146.4 mAh·g^-1 at the rate of 0.2C and 1C during the voltage range of 2.75～4.3V and good capacity retention of 93.3%,94.2%. Cycle voltammetry (CV) test showed that the major oxidation peak at 3.98V and reduction peak at 3.69V corresponded to the redox process of Ni²⁺/Ni⁴⁺ couple. The kinetics behaviors upon the deintercalation/intercalation of lithium ion for layered LiNi_0.4CO_0.2Mn_0.4O₂ were studied by means of electrochemical impedance spectroscopy (EIS) measurements. It was found that the LiNi_0.4Co_0.2Mn_0.4O₂ cathode has lower electrochemical impedance at full charge, compared with it before charge.Using the optimized sol-gel method, the cathode materials LiNi_xCO_0.2Mn_0.8-xO₂ (x=0.4, 0.5, 0.6) were synthesized. Influence of different synthesis gas environment was studied. Compared the differences of structures and performances between the materials LiNi_xCo_0.2Mn_0.8-xO₂ (x=0.4, 0.5, 0.6 ) LiNi_0.4CO_0.2Mn_0.4O₂,LiNi_0.5Co_0.2Mn_0.3O₂, LiNi_0.6Co_0.2Mn_0.2O₂ calcined in air show discharge capacity of 157.1 mAh·g^-1, 161.2 mAh·g^-1, 138.7 mAh·gl atthe rate of 0.2C during the voltage range of 2.75～4.3V, the capacity retention were 94.0%,91.8%,81.2% after 30 cycles. LiNi_0.4Co_0.2Mn_0.4O₂,LiNi_0.6Co_0.2Mn_0.2O₂ calcined in oxygen show discharge capacity of 154.2 mAh·g^-1,174.2 mAh·g^-1 and capacity retetion of 90.6%,89.8% after 30 cycles. The result indicated that the discharge capacity increased when the content of Ni augment, but the synthesis condition became more rigorous, the capacity retention descended as the content of manganese decreased.