New insight into structural transformation in Li-rich layered oxide during the initial charging

Abstract

The formation of a spinel cubic framework is identified as a main trigger for the decreasing of the voltage profile for the high-voltage Li-rich manganese-based layered oxide, known as “voltage fade”. This dropping of the operating potential could cause massive loss in the energy density in lithium ion battery systems. To apply a profound understanding into the formation of a spinel cubic phase in Li_1.2Ni_0.12Co_0.15Mn_0.53O₂ oxide, high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD) and galvanostatic intermittent titration technique (GITT) are employed. At different states of the initial electrochemical charge process, we found that the spinel cubic region mostly appears in the crystal defective area. The proportion of the spinel phase corresponds well to the Williamson–Hall type microstress value. At 4.70 V charged state, the Inverse Fast Fourier Transform (IFFT) graph shows structural transformation from an O3-type rhombohedral phase to a spinel cubic framework forced by stressful squeezing. The variations in occupation of lithium and manganese ions in the tetrahedral sites (Li_tetra and Mn_tetra), and the kinetic performances indicate two forms in the O3-spinel transition: “Li–Mn dumbbell” and spinel nucleus. These experimental results clarify the correlation between the formation of a spinel phase and microstress in Li-rich layered oxides and provide a possible way to depress voltage fade through compromising the crystal defects of the layered oxide.