富锂正极材料xLi3NbO4·(1-x)LiMO2(M=Mn,Co;0＜x＜1)的制备及电化学性能研究

摘要

采用高温固相法合成了含铌富锂材料xLi3NbO4·(1-x)LiMnO2 (0＜x＜1),研究了Li3NbO4含量对其结构和性能的影响.X射线衍射分析结果表明当x取值在0.25～0.67之间时可得到纯相固溶体材料,属立方晶系,为Fm-3m空间群.x=0.25和x=0.43的材料具有较好的电化学性能,首次放电容量可达216 mAh·g-1,但是两者都表现出明显的电压衰退,x=0.43的材料由于含有更多的Li3NbO4组分,电压衰减的情况得到了抑制.x=0.43材料的非原位XPS和XAFS研究表明,其充电过程分为两个阶段,4.3 V以下的阶段发生的电化学过程是脱出Li+同时Mn3被氧化到Mn4+,4.3 V以上的阶段则由O2-的氧化来提供电荷补偿.通过对x=0.43的样品进行钴掺杂,考察了钴掺杂对0.43Li3NbO4·0.57LiMn1-yCoyO2(y=0.25,0.5)结构和性能的影响.研究表明钻掺杂改善了材料电导,降低了电极传荷阻抗,从而提高了材料倍率性能,同时保持了较好的循环稳定性.%Lithium rich material xLi3NbO4·(1-x)LiMnO2 (0＜x＜ 1) was successfully synthesized by solid state method.Stoichiometric amounts of Li2CO3,Mn2O3 and Nb2O5 were mixed by ball milling,and the mixture was calcinated at 900 ℃ for 5 h under Ar atmosphere.X-ray diffraction (XRD) results indicate that the samples with 0.25＜x＜0.67 can be indexed as a cubic structure with Fm-3m space group.Electrochemical results show that the samples of x=0.25 and 0.43 have better electrochemical performance,both delivering 216 mAh·g-1 in the initial cycle between 2 V and 4.8 V.Although voltage decay is an intrinsic drawback of lithium excess materials,the sample of x=0.43 decays slower.We speculate that Li3NbO4 helps stabilizing the crystal structure.Ex-situ XPS and XAS studies show that the charging process can be divided into two stages.In the first stage,below 4.3 V,Mn3+ is oxidized to Mn4+,in the second stage,O2-is oxidized.The reversible oxidation of O2-is the origin of the achievement of large reversible capacity.Co3 + doped material 0.43Li3NbO4· 0.57LiMn1-vCovO2 (y=0.25;0.5) was also synthesized by the same procedure.The structure of the doped material maintains the cubic structure with smaller lattice constant and the variation of lattice constant is in proportion to the amount of Co3+.Galvanostatic charge and discharge tests show that 0.43Li3NbO4·0.57LiMn0.75Co0.25O2 also delivers a large capacity of 215 mAh·g-1 in the first cycle between 2 V and 4.8 V,but the voltage plateau in the charging process decreased from 4.3 V to 4.1 V,it can be attributed to the weak dissociation energy of Mn-O bond and the overlap of Co3+/4+ 3d and O2-2p energy band.The electrochemical impedance spectroscopy results show that a moderate amount of Co3+ doped into the material decreases the charge transfer resistance.After doped with Co3+,the rate capability is improved.