| Lithium-ion batteries (LIBs) have been attracted much attention because of their higherenergy density, better stability, and better environmental benignancy relative to otherrechargeable electrochemical energy conversion devices. They have been widely used inhigh-energy and high-power applications such as electric vehicles (EVs), large-scale energystorage, and military power instruments. However, lack of high performance cathodematerials has already become a technological bottleneck to realize advanced LIBs. Lowenergy density of the conventional cathode materials, such as LiCoO2, LiMn2O4,LiMn1/3Ni1/3Co1/3O2, and LiFePO4, makes them unable to satisfy high power demands innew portable applications. Recently, layered lithium-rich cathode materials,xLi2MnO3(1-x)LiMO2(M=transition metal), have been considered as one of the mostpromising cathode materials due to their high specific capacity (ca.250mAhg-1) and highoperating voltage.This research aimed at synthesis of high-performance lithium-rich cathode material,Li1.2Mn0.54Ni0.13Co0.13O2. X-ray diffraction (XRD), scanning electron microscopy (SEM),inductively coupled plasma (ICP), energy dispersive X-ray detector (EDX), transmissionelectron microscopy (TEM), electrochemical impedance spectroscopy (EIS), galvanostaticintermittent titration technique (GITT) and electrochemical tests have been employed toinvestigate the crystal structure, morphology, kinetics of electrode process andelectrochemical performance of the as-synthesized materials. A thorough study on synthesisprocess, coating modification, composite modified materials, nanomaterials and design ofspecial morphology was carried out, which focused on the relationship betweenmorphology, electrochemical performance and kinetics of electrode process of theas-prepared cathode materials. These research achievements establish solid theoretical andpractical basis for the preparation and development of high-performance lithium-richcathode material Li1.2Mn0.54Ni0.13Co0.13O2for LIBs. The obtained main achievements andprogress are as follows:(1) A series of lithium-rich cathode materials, xLi2MnO3(1-x)Li[Ni1/3Mn1/3Co1/3]O2,were successfully synthesized by a sol-gel method followed by a calcination treatment in this paper, using metal acetate and citric acid as raw materials. The synthetic conditionswere optimized. The results showed that, Li1.2Mn0.54Ni0.13Co0.13O2(x=0.5) achieved goodcrystal structure and best electrochemical performance when treated at900℃for12h. Thismaterial showed an initial discharge capacity of260mAh g-1at20mA g-1, and maintained244.7mAh g-1after40cycles with capacity retention of94.12%.(2) MoO3was successfully mixed with Li1.2Mn0.54Ni0.13Co0.13O2through high-energyball-milling, or coated on the pristine material by liquid-phase coating modification. Theinfluence of these two methods on structure, morphology and electrochemical performancewas investigated. The Li1.2Mn0.54Ni0.13Co0.13O2mixed with an optimum MoO3content of20wt.%exhibited a high discharge capacity of273.9mAh g-1, and a little irreversiblecapacity loss of1.2mAh g-1. The best cycling ability was achieved with a MoO3content of5wt.%. After50cycles, the discharge capacity was242.5mAh g-1. Both amorphous andcrystallographic MoO3existed in the coating layer for the pristine material, thus decreasingthe electrolyte/electrode surface area and further inhibiting undesirable reactions. Also theerosion of the electrolytes and dissolution of transition-metal ions at high potential cyclingwas alleviated by the coating layer, thus achieving an excellent cycling performance.Moreover, the layered structure formed from the crystallographic MoO3could affordunimpeded paths for fast Li+diffusion, which is benefit from the delithiated LixMoO3witha higher electron/Li+diffusion coefficient. This advantage of the coating layer brought asuperior rate performance, especially when the content of MoO3was3%, the coatedmaterial yields high specific discharge capacities of251.6mAhg-1,230.4mAhg-1,205.1mAhg-1,189.8mAhg-1,141.5mAhg-1100.7mAh g-1and57.7mAh g-1at0.1C,0.2C,0.5C,1.0C,2.0C,5.0C and10.0C rate, respectively after50cycles, the correspondingcapacity retention was76.4%,93.9%,91.6%,95.2%,87.6%,76.3%and65.9%.(3) Li1.2Ni0.13Co0.13Mn0.54O2was successfully composited with carbon nano tube(CNTs) and graphene. The CNTs/graphene wrapping outside the Li1.2Ni0.13Co0.13Mn0.54O2particles built up a conductive network, which could not only provide a3D electrontransport, but also make the cathode material accommodate the structural stress fromcharge/discharge process. Therefore, a higher discharge capacity, superior cycling stabilityand rate performance was obtained. When the content of CNTs was4%, the composite material showed a high initial discharge capacity of277.8mAh g-1, and maintained253.5mAhg-1after50cycles; the discharge capacity at1.0C,2.0C,5.0C and10.0C was225.1mAh g-1,192.7mAh g-1,150.7mAh g-1, and110.9mAh g-1, respectively. Whencomposited with4wt%graphene, the discharge capacity at0.1C after30cycles was245.5mAh g-1, with a capacity retention of88.2%; and the discharge capacity at1.0C,2.0C,5.0C and10.0C was184.2mAh g-1,156.5mAh g-1,120.6mAh g-1, and73.9mAh g-1,respectively. Results of GITT test confirmed that, during the initial charging progress,electrochemically activation of the Li2MnO3component over4.5V accompanied by releaseof Li2O occurred after the process that Li+extracted from the Ni1/3Co1/3Mn1/3O2component.(4) Nano-sized Li1.2Ni0.13Co0.13Mn0.54O2was successfully synthesized by PVP-assistantsol-gel method and ethanol system co-precipitation. The nano dimension provided moreactive surface sites for lithium storage, and also shortened the lithium-diffusion pathways.The nanomaterials prepared by PVP-assistant sol-gel method presented excellent cyclingand rate performance. The discharge capacity after100cycles was214.2mAh g-1with acapacity retention of81.6%; and the discharge capacity at5.0C and10.0C was159.2mAh g-1and125.3mAh g-1. On the other hand, the nanomaterials prepared ethanol systemco-precipitation, which is a one-step process, also showed a good electrochemistryperformance. The initial discharge capacity was280.8mAh g-1at0.1C. The dischargecapacity could still maintain187.7mAh g-1,166.9mAh g-1,104.8mAh g-1and62.2mAhg-1, respectively at1.0C,2.0C,5.0C and10.0C after100cycles.(5) Hollow spherical and3-Dimensional pores structured Li1.2Mn0.54Ni0.13Co0.13O2wasprepared through a hydrothermal assistant homogeneous precipitation method. The hollowspherical material assembled by nanoplates exhibited a good cycling performance themaintained a discharge capacity of170.5mAh g-1after112cycles at1.0C; and thedischarge capacity at0.5C,1.0C and2.0C was232.1mAh g-1,205.8mAh g-1and183.9mAh g-1, respectively. The3-Dimensional pores structured Li1.2Mn0.54Ni0.13Co0.13O2exhibited an initial discharge capacity of270.8mAh g-1at0.1C, and maintained242.6mAhg-1after100cycles with a capacity retention of89.6%; the discharge capacity was150.8mAh g-1at1.0C after112cycles with a capacity retention of70.9%; and184.3mAh g-1,148.2mAh g-1,84.7mAh g-1at2.0C,5.0C,10.0C after120cycles with capacity retention of89.6%,74.5%and55.5%. |
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