Miao Shu, Kocher Michael, Rez Peter, Fultz Brent, Ozawa Yasunori, Yazami Rachid, Ahn Channing C
California Institute of Technology, Pasadena, California 91125, USA.
J Phys Chem B. 2005 Dec 15;109(49):23473-9. doi: 10.1021/jp0542266.
Samples of Li(x)Ni0.5Mn0.5O2 and Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2 were prepared as active materials in electrochemical half-cells and were cycled electrochemically to obtain different values of Li concentration, x. Absorption edges of Ni, Mn, Co, and O in these materials of differing x were measured by electron energy loss spectrometry (EELS) in a transmission electron microscope to determine the changes in local electronic structure caused by delithiation. The work was supported by electronic structure calculations with the VASP pseudopotential package, the full-potential linear augmented plane wave code WIEN2K, and atomic multiplet calculations that took account of the electronic effects from local octahedral symmetry. A valence change from Ni2+ to Ni4+ with delithiation would have caused a 3 eV shift in energy of the intense white line at the Ni L3 edge, but the measured shift was less than 1.2 eV. The intensities of the "white lines" at the Ni L-edges did not change enough to account for a substantial change of Ni valence. No changes were detectable at the Mn and Co L-edges after delithiation either. Both EELS and the computational efforts showed that most of the charge compensation for Li+ takes place at hybridized O 2p states, not at Ni atoms.
制备了Li(x)Ni0.5Mn0.5O2和Li(x)Ni(1/3)Mn(1/3)Co(1/3)O2样品作为电化学半电池中的活性材料，并进行电化学循环以获得不同的锂浓度x值。通过透射电子显微镜中的电子能量损失谱（EELS）测量这些不同x值的材料中Ni、Mn、Co和O的吸收边，以确定脱锂引起的局部电子结构变化。这项工作得到了使用VASP赝势包、全势线性缀加平面波代码WIEN2K进行的电子结构计算以及考虑了局部八面体对称性电子效应的原子多重态计算的支持。随着脱锂，Ni从Ni2+到Ni4+的价态变化会导致Ni L3边处强烈白线的能量发生3 eV的位移，但测量到的位移小于1.2 eV。Ni L边处“白线”的强度变化不足以解释Ni价态的实质性变化。脱锂后，Mn和Co L边也未检测到变化。EELS和计算结果均表明，Li+的大部分电荷补偿发生在杂化的O 2p态，而非Ni原子处。
