【作者】 张翔；

【导师】 周飞；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2015， 硕士

【摘要】 富锂锰基正极材料xLi2MnO3·(1-x)LiMO2(0<x<1,M=Mn、Ni、Co、Fe、Cr等)因具有高比容量(200~300mAh/g)、高能量密度(~300Wh/kg)和新的充放电机制,近年来受到研究者们广泛关注,有望用于下一代动力锂电池中。本文使用共沉淀法合成富锂锰基正极材料0.6Li2MnO3·0.4Li[Ni0.5Co0.2Mn0.3]O2(Li1.2[Mn0.52Ni0.2Co0.08]O2),通过优化共沉淀工艺、进行掺杂改性,逐步提高其电化学性能;并使用第一性原理,对掺杂改善材料电化学性能的机理做出阐释。(1)采用碳酸盐共沉淀法合成前驱体。通过建立热力学模型和实验验证,选取了乳酸钠替代对人体有刺激性的氨水作为碳酸盐共沉淀的络合剂。当乳酸钠与过渡金属离子的摩尔比为0.75:1,共沉淀反应的pH值为8.0时,所得前驱体与正极材料形貌均匀,结晶性良好,正极材料电化学性能最优。该材料在2.0~4.8V电压区间内,0.5C(1C=200mAh/g)倍率下循环100次后,放电比容量仍有175.9mAh/g,容量保持率达到95.6%。(2)考察了不同掺杂方法、不同掺杂量和不同掺杂元素对正极材料的影响。实验结果表明:溶液法的掺杂效果优于球磨法。Ti掺杂量为2%时,材料具有最好的电化学性能。该材料在0.5C倍率下循环100次后,放电比容量仍有188.8mAh/g,容量保持率达到97.2%。Ti、Al掺杂均能提高材料放电比容量,相比较而言,Ti掺杂更能提高材料循环性能,而Al掺杂更能提高材料的倍率性能。电化学阻抗测试表明,Ti、Al掺杂均减小了电化学反应过程中的电荷转移阻抗。(3)使用密度泛函理论分析了掺杂前后正极材料电子结构的变化,计算认为:掺杂原子(Ti、Al)倾向于占据富锂锰基材料中Li2MnO3相的Mn位,Ti掺杂提高了充放电过程中材料晶格O的稳定性,因而材料具有更好的循环性能;而Al掺杂提高了Li2MnO3相的电子电导率,使材料拥有更好的倍率性能。更多还原

【Abstract】 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.更多还原