【摘要】 富锂锰基层状正极材料由于具有较低的成本和超过250 mAh·g^-1的放电比容量,成为很有前景的正极材料之一,但是其本身存在的首周库仑效率低和电压衰减等问题限制了产业化进程。高充电电压会导致富锂材料结构不稳定从而增加电池的安全隐患,因此在较高的充电电压下,材料的首周充放电结构演化有待进一步研究。本文首先通过碳酸盐共沉淀法合成前驱体,经过混锂后煅烧制备富锂锰基层状正极材料(Li_1.13Ni_0.18Co_0.09Mn_0.56O₂),并研究不同充电截止电压对其首周库仑效率和循环性能的影响。研究表明,高充电电压会带来容量增加,但其首周库仑效率将大幅降低。循环伏安表征发现,充电截止电压为5.0 V时,部分体相的晶格氧会发生可逆的氧化反应,这将有利于容量的提升。TEM、XRD和SEM表征结果显示,在首周充电后电极材料不仅发生了深入到体相的结构变化,同时出现了大片的层错和尖晶石相MnO_x和NiO_x等不可逆相变,还会与电解液发生反应,材料结构无法保持稳定。结合Mapping和XPS表征结果显示,充电电压较高时会有更多的体相晶格氧参与氧化还原反应,这将使更多具有强氧化性的过氧和超氧离子与电解液发生副反应,并伴随过渡金属的溶出,加速材料的结构坍塌,最终不利于电池的长循环性能。更多还原

【Abstract】 Lithium-rich manganese-based cathode materials have become one of promising cathode materials due to their low cost and large discharge specific capacity exceeding 250 mAh·g^-1.However,their problems such as low coulombic efficiency of first cycle and apparent voltage decay influence commercialization process.The high charging voltage will cause instability of structure and increase the hidden danger of the battery.Therefore,structural evolution of first cycle at higher voltage needs to be further studied.In this work,the precursor was synthesized by the co-precipitation method,and the lithium-rich manganese-based layered cathode materials were prepared by lithium-mixed and high-temperature sintering,and the effects of coulombic efficiency and cycle performance were studied at different charge cut-off voltages.Results have shown that high charging voltage would increase the capacity,but reduce the coulombic efficiency greatly in the first cycle,leading to the decayed specific capacity of long cycle.Cyclic voltammetric investigation proves that when the charge cut-off voltage was 5.0 V,part of the bulk lattice oxygen underwent a reversible oxidation reaction,which lead to the increase of capacity.TEM,XRD and SEM characterization results show that the electrode not only went deep into the bulk phase structural changes,including a large number of stacking faults and spinel phases MnO_x and NiO_x,and other irreversible phase changes,but also reacted with the electrolyte.Mapping and XPS results show that when the charging voltage became higher,more bulk lattice oxygen participated during redox reaction,which causes stronger oxidizing peroxygen and superoxide ions to undergo side reactions with the electrolyte and accelerates the structural collapse of the electrode,ultimately,becomes not conducive to long cycle performance of the battery accompanied by the dissolution of the transition metal.更多还原