| As one of the modern energy storage devices, Lithium-ion battery (LIB) has been drawn hot interests in recent years because of its high output voltage, high energy density, good safety performance, low self-discharge rate, no memory effect and soThe traditional cathode materials are limited to be widely used nowadays due to their many disadvantages, i.e. the a low capacity and high cost of LiCoO2, strict synthsis condition and poor reversibility of LiNiO2, a poor ionic conductivity and low discharge capacity though the relatively low cost of LiFePO4. There is a great need to develop better cathode materials to meet the urgent needs for high-capacity and high energy density lithium-ion batteries due to the rapid development of modern information technology and mobile devices.Lithium rich cathode material xLi2MO3· (1-x) LiM’O2 (M=Mn, Zr......; M’= Ni, Mn, Co, Al, Fe, Mg......one or any combination) shows great prospect being applied in the field of electric vehicles, hybrid vehicles, portable electronic devices due to the high capacity (>250mAhg-1), excellent cycling capability, security and stability at high working temperature.In this paper, Li2MnO3-based lithium rich Li1.2Ni0.2Mn0.6O2 was synthesized through a facile, low cost coprecipitation/one-step sintering method. The obtained submicron polyhydron particles were characterized by comprehensive techniques of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) imaging, selected-area electron diffraction (SAED) and electron energy loss spectroscopy (EELS). The stacking and dynamic microstructures which have great effects on the electrochemical properties were investigated in depth and the Li+/Ni2+ replacement and cationic ordering were optimized by minor Co doping. Conclusions can be obtained as following:1) In the preparation of Li1.2Ni0.2Mn0.6O2 cathode material, reaction conditions such as the stirring time in the preparation of carbonate precursors, the sintering temperature and holding time in the preparation of final products have a certain influence on the crystallinity of the final products:the morphologies and molar ratios of Mn, Ni elements of the spherical carbonate precursor synthesized by co-precipitation are slightly different based on the stiring time. A dense spherical surface and a molar ratio of Mn, Ni elements which is very close to the feed ratio can be got with a stirring time of 12 h; low sintering temperature and short holding time lead to poor crystallinity and quite small particle size, however, undesirable phases may form under the extra-high temperature and holding time. A pure, high crystallized and excellent hexagonal layered cathode material has been obtained when sintered at 850℃ for 10 h; The average particle size of materials synthesized using different lithium salts in the step of sintering differed greatly:387 nm (LiAc),151 nm (Li2CO3), and 181 nm (LiNO3). When using Li2CO3 as lithium salt, the as prepared particles have good hexagonal ordering and pure pahse. The sintering process also has an impact on the material electrochemical performance. Though the materials synthesized with a pre-sintering process exhibited high coulombic efficiency, the fast cyclic capacity fading could not be ignored. The material obtained by one-step sintering method showed better cyclic performance with a discharge capacity of 143.3 mAhg-1 and capacity rention ratio of 74.6% at a current density of 1.0 C after 100 cycles, which is 16% higher than that of material synthesized with pre-sintering process. The rate capacities were 310.5 mAhg-1 (0.05 C),285.2 mAhg-1 (0.1 C),237.8 mAhg-1 (0.2 C),186.5 mAhg-1 (0.5 C),150.9 mAhg-1 (1.0 C) and 113.8 mAhg-1 (2.0 C) at various charge and discharge rate.2) Through the microsctructure analysis of the fresh and cycled Li1.2Ni0.2Mno.6O2 cathode after 100 cycles, a small increase of the particle size as well as deterioration of the hexagonal ordering of the cycled sample have been proved by XRD; the formation of Fd3m structural spinel in the cycled cathode material has been verified by HRTEM and SAED images; EELS data exhibited the decrese of the oxidation state of Mn to be+3 in average.3) By doping a minor amount of Co to modulate the Li+/Ni2+replacement, the crystal defects were optimized and enhanced electrochemical performance has been achieved due to the improvement of the diffusion property of Li+. The moderate Li+/Ni2+replacement ratio, fine cationic ordering in the transition metal layers and stacking sequence along the [003] direction, and larger O-Li-O interlayer spacing (0.2181 nm) guaranteed high Li+ diffusion coefficient and small charge transfer resistence, the higher content of Mn4+ offered a more stable structure during cycling. Among all the Co doped lithium-rich Li1.2Ni0.2-z/2Mn0.6-z/2CozO2 (0≤z≤0.1), Li1.2Ni0.18Mn0.58Co0.04O2 has the most excellent electrochemical performance:a discharge capacity of 288.3 mAhg-1 has been obtained after 40 cycles at a charge-discharge capacity of 0.1 C, and the capacity decreased only 1.2% compared to the capacity of the first cycle. At a higher rate of 2.0 C, its discharge capacity is as high as 166.3 mAhg-1, which was improved by 42.8% than the non-doped Li1.2Ni0.2Mn0.6O2. Besides, the appropriate reduction of the Li+/Ni2+ replacement ratio can improve the electrochemical performance significantly, however, not the less, the better. |
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