Preparation And Modification Of Layered LiMnO₂ As Cathode Material For Lithium Ion Batteries

Layered LiMnO₂ as cathode material for rechargeable lithium-ion batteries has many merits, such as the safety, low cost, high theoretical capacity, and low toxicity of manganese-based materials. However, the layered structure readily transforms to spinel structure during charge-discharge cycle, as a result of the electrochemical behavior was not satisfactory. Researches focus on doping LiMnO₂ by substituting part of Mn with other metal ions such as Cr³⁺, Al³⁺, Ga³⁺, Co³⁺, Ni²⁺, Li⁺ and so on, have been shown to stabilize the layered structure and improve cycling stability during charge-discharge process. But part doping compound convert to spinel structure during charge-discharge process, which lead to the degradation of capacity and bad cycling stability. So, the reversible capacity and cycling stability have to be improved, especially in high temperature.Layered LiMnO₂ are not thermodynamically stable, hence, they can only be synthesized by soft chemical routes. In this paper, the solid state method is employed to synthesize the NaMn_1-xCo_xO₂ (0.1 ≤ x ≤ 0.5) precursors by controlling the atomic ratio Mn/Co and calcining temperature. Then 02 structure LiMn_1-xCo_xO₂ was synthesized through ion-exchange method. The particles prepared by salt melting ion-exchange method have the best morphologies and well-crystallized. With the increase of the Co substitution amount, the more ordered layered structure was synthesized. The electrochemical results show that the reversible capacity of the layered LiMn_1-xCo_xO₂ increases with increasing Co content of the products, so does the irreversible capacity. LiMn_0.5Co_0.5O₂ delivered the highest discharge capacity of 175 mAh g^-1 and the capacity retention rate of more than 90% after 50 cycles.Anion and cation co-doping can improve the electrochemical performance of LiMn_1-xCo_xO₂. Through LiMn_0.5Co_0.5O_1.9F_0.1 has lower capacity than LiMn_0.5Co_0.5O₂ in the first cycle, it has better cycle properties, its discharge capacity after 40 cycles is higher than LiMn_0.5Co_0.5O₂ and has 136 mAh g^-1 after 50 cycles. This indicates that F-doping can improve the stability of the layered structure.In this dissertation, the surface of as-prepared LiMn_1-xCo_xO₂ was modified successfully by a melting impregnation method, which can improve the cycleability greatly at both room temperature and 55℃. Among all samples, ZnO-coated LiMn_0.5Co_0.5O₂ exhibits the lowest capacity fading rate. The improvement of electrochemical performance on the surface modified samples by a melting impregnation method is due to suppression of the surfaceJahn-Teller distortion and a slow-down of manganese dissolution by the existence of oxide coating on the surface.