Modulating the Li+/Ni2+ replacement and electrochemical performance optimizing of layered lithium-rich Li1.2Ni0.2Mn0.6O2 by minor Co dopant

The influences of the Li+/Ni2+ replacement modulated by minor Co dopant on cyclic capacity and rate performance of lithium-rich cathode material Li1.2Ni0.2-z/2Mn0.6-z/2CozO2 (z = 0, 0.02, 0.04, 0.10) were investigated from the microstructural point of view by comprehensive techniques of high-resolution transmission electron microscopy (HRTEM) imaging, atomic-resolution electron energy loss spectroscopy (EELS), selected-area electron diffraction (SAED), and X-ray diffraction (XRD). It is found that Co played a vital role in decreasing the Li+/Ni2+ replacement ratio in the hexagonal layered Li1 2Ni0.2-z/2Mn0.6-z/2CozO2 (R (3) over barm), which is closely related to the electrochemical performance. An evident cationic ordering in the transition metal layers and a stacking sequence vertical to the Li+ diffusion orientation were observed in the Li1.2Ni0.2-z/2Mn0.6-z/2CozO2 (z > 0) system rather than in the Li1.2Ni0.2Mn0.6O2 system. Compared with Li1.2Ni0 2Mn0.6O2, Li1.2Ni0.18Mn0.58Co0.04O2 showed excellent electrochemical performance with increase in discharge capacity to 288.3 mA h g(-1) from 166.3 mA h g(-1), improvement in capacity retention to 98.6% from 73.9% at a current density of 0.1 C after 40 cycles, and enhancement in capacity to 161.4 mA h g(-1) from 113 mA h g(-1) at a higher rate of 2 C. The largest interlayer spacing (0.218 nm of O-Li-O layer), highest proportion of Mn4+ ion component, and the most remarkable superstructure diffraction spots were found for Li1.2Ni0.18Mn0.58Co0.04O2 among all specimens, as confirmed by XRD refinement, EELS, HRTEM, and SAED. Three superstructure vectors modulated by 1/4 (q) over right arrow, 2/4 (q) over right arrow, 3/4 (q) over right arrow ((q) over right arrow = [0 (1) over bar1]) were simultaneously observed for Li1.2Ni0.18Mn0.58Co0.04O2, indicating a high degree of ordering. Our findings might shed new insights into the understanding of the Li+/Ni2+ replacement by doping minor amounts of Co for optimizing the electrochemical performance in Li-ion batteries cathode material from the microstructural point of view.