XAFS Study Of LiNi_1-YCo_YO₂ Cathode Materials For Lithium-Ion Batteries

Li-ion batteries are extensively used in electronic equipment, electric vehicles, medicinal devices and navigation products due to their special properties, such as high energy density, small size and long cycle life. LiCoO2, the most widely used cathode material in commercial field, has some shortcomings, including low specific capacity and high cost, etc. Compared with LiCoO2, LiNiO2 has higher energy capacity and less expensive. However, the poor cycleability, the low reversible capacity and the difficulty to synthesize stoichiometric phase, are the most factors limiting the application of LiNiO2, and recently, researchers try to improve electrochemical properties of LiNiO2. In order to enhance the reversible capacity and prolong the life time, the charge-discharge mechanism of cathode has been studied by many groups.In the thesis, LiNi0.9M0.1O2 (M=Al, Co and Mn) and the gradient LiNi0.8Co0.2O2 are studied; these materials are characterized by XRD and XAFS; the structural change of LiNi0.8Co0.2O2 in charging process is investigated with ex situ XRD and in situ XAFS. LiNi0.9Al0.1O2, LiNi0.9Co0.1O2, LiNi0.9Al0.05Co0.05O2 and LiNi0.9Mn0.1O2 are synthesized at a calcinations temperature of 700℃for 15 h after mixed excessive lithium hydroxide and Ni0.9M0.1(OH)2(M = Al, Co and Mn) which are prepared with co-precipitation, respectively. It is shown that each of Al, Co and Mn doping improves the cycleability and discharge capacity of LiNiO2. For the first time, effects of Al, Co or Mn doping on the LiNiO2 structure is investigated by the mean of XRD and EXAFS. Results show that Al, Co or Mn doping didn't induces any phase change but can reduces a and c value of lattice, and has higher c/a value. In addition, Al, Co or Mn doping enhances the local order due to the weakening of the deformation of NiO6 octahedron in LiNiO2-based cathode.The gradient LiNi0.8Co0.2O2 is, for the first time, synthesized by coating the spherical grain of nickel hydroxide with Li- and Co- containing sol-gel. The so-prepared precursor is heated at 700℃for 15 h to gain the active materials with higher reversible capacity. The first discharge capacity of gradient LiNi0.8Co0.2O2 is 187.3 mAh/g. The gradient LiNi0.8Co0.2O2 is characterized by XRD,SEM,EDS,XPS and EXAFS. Results show that gradient LiNi0.8Co0.2O2 is layeredα-NaFeO2 type structure, and appears spherical grain with Co content gradually decreasing from the surface to the inner. The homogeneous LiNi0.8Co0.2O2 is also studied for comparison purpose. The lattice constant of the gradient LiNi0.8Co0.2O2 is larger than that of the homogeneous one. The local structure of transition metal in both of LiNi0.8Co0.2O2 are hardly different from each other but the order of the gradient LiNi0.8Co0.2O2 is poorer than that of the homogeneous materials based on the Debye-Waller effects of the first and the second shells.During charging the structural changes of LiNi0.8Co0.2O2 are investigated with ex situ XRD and in situ XAFS. Results show that during charging the value of a, one lattice constant, decreases, while the value of c increases at the beginning and then decreases. And the length of M-O (M=Ni, Co) bond contracts and the Debye-Waller effect decreases in the first shell. The length of M-M (M=Ni, Co) bond and the Debye-Waller effect also decrease in the second shell.There are obvious changes in Ni-K XANES and Co-K XANES of gradient LiNi0.8Co0.2O2 during charging. For Ni-K XANES, the B, C and D peaks and the absorption edge continuously shift to higher energy; for Co-K XANES, the B, C and D peaks shift to higher energy but the absorption edge keeps constant during charging. For the first time, Ni-K XANES and Co-K XANES of LiNi0.8Co0.2O2 during charging are studied with First Principle. The shifts of Ni-K XANES and Co-K XANES during charging are mainly attributed to three factors: vacancy number after Li+ ions remove from the host; the change of lattice constants, and O atom position. The first factor induces the D peak in XANES to shift to lower energy. The latter two factors induce the bond of M-O (M=Ni,Co) to contract and the D peak in XANES to shift to higher energy.The behavior of Ni atoms is significantly different from Co atoms during charging. Co takes part in the reaction at 3.9V while Ni responses to the Li+ removal at the beginning. Co reacts at higher voltage and Co-O bond dose not shorten largely, which might stabilize the structure of LiNi0.8Co0.2O2, thus resulting in a better cycleability of such cathode.