Experiment And Theory Research On Synthesis And Doping Modification Of Li-rich Mn-based Cathode Material

Nowadays, lithium-rich manganese-based oxide x Li2MnO3·(1-x)LiMO2(0<x<1, M=Mn, Ni, Co, Fe, Cr, etc.) has attracted more and more researchers’ attention because of high capacity(200~300mAh/g), high energy density(~300Wh/kg) and new charge-discharge mechanism, which makes it possible to become the cathode material of next-generation lithium-ion power battery. In this paper, 0.6Li2MnO3·0.4Li[Ni0.5Co0.2Mn0.3]O2(Li1.2[Mn0.52Ni0.2Co0.08]O2) cathode materials were synthesized by co-precipitation and solid-phase sintering method, then process optimization and doping modification were carried out to gradually improve their electrochemical performance. Furthermore, first-principle calculations were performed to unravel mechanism of improvement on electrochemical performance after doping.(1) Sodium carbonate was used as the precipitant in co-precipitation reaction. Sodium lactate instead of ammonia which is irritating to eyes and skin was selected as the chelating agent according to thermodynamic analysis and experiment. The optimal conditions for preparing precursor confirmed by process optimization are list as follow: First, the molar ratio of chelating agent and metal ion(Ni2++Co2++Mn2+) is 0.75: 1. Second, the pH value of co-precipitation reaction is 8.0. Both precursors and cathode materials synthesized under optimal conditions had uniform morphologies and fine crystallinities. The cathode material delivered a discharge capacity of 175.9mAh/g with a retention of 95.6% at a current density of 0.5C(1C=200mAh/g) between 2.0~4.8V after 100 cycles.(2) The effects of doping method, doping content and doping element on properties of Li1.2[Mn0.52Ni0.2Co0.08]O2 were systematically investigated. Results showed that electrochemical performance of Ti-doped material prepared by wet chemical method was better than that prepared by ball mill method. Capacities of all Ti-doped samples with different doping content(0.5%~5%) were higher than that of pristine sample, and the optimal content was 2%. After 100 cycles at 0.5C, the discharge capacity of Li1.2[Mn0.52Ni0.20Co0.08](1-x)Ti0.8xO2(x=2%) sample maintained 188.8mAh/g, while its capacity retention ratio was 97.2%. Compared with Ti-doped sample, Al-doped sample showed better rate performance and a little worse cycle performance. EIS analysis revealed that both Ti-doping and Al-doping reduced the charge transfer resistances.(3) The electronic structures of doped and pristine lithium-rich manganese-based oxide were studied by density functional theory. Calculation results indicated that doping atoms Ti and Al were preferred to take place of Mn site in Li2MnO3 phase. Ti-doping suppressed O2 release reaction, thus improving the structural stability of Li2MnO3 during cycling. However, Al-doping improved the electronic conductivity of Li2MnO3, which may result in good rate performance.