| Because of the massive consumption of fossil energy and the critical issue ofenvironmental pollution, the application of sustainable energy and energy-efficiencytechnology is being pursued, and lithium-ion batteries are indispensable as highly effecientenergy storage and conversion devices. However, lack of high-performance cathodematerials has become a technological bottleneck for the commercial development ofadvanced lithium-ion batteries, thus the development of advanced cathode materials isfocused on. In this thesis, with the purpose of exploiting high-energy, high-power and highthermally stable cathode materials, the high-capacity layered lithium-rich cathode isselected as the objective, by reviewing and analyzing the latest development of cathodematerials for lithium ion batteries. The material design and synthetic strategy ofhigh-performance layered lithium-rich cathode has been systematatcially studied. The mainobtained achievements and progress are listed below:Super-large aliovalent yttrium (Y3+) is selected as a dopant to substitute Mn4+inlayered lithium-rich materials, Li1.2Mn0.6Ni0.2O2, and an oxalate co-precipitation methodhas been adopted to successfully synthesize a systematic series of Y3+doped samples. Thedoping Y3+ions with a “super-large” radius (r Y3+=0.090nm) vs. Mn4+ions (r Mn4+=0.053nm) expands Li+pathways in the layered structure, and the stronger bond energy ofY–O than Mn–O or Ni–O benefits the stability of the bulk layered structure. All theelectrochemical results have clearly demonstrated that a suitable amount of Y3+substitutionfor Mn4+in layered Li1.2Mn0.6-xNi0.2YxO2results in significant improvement in capacity,cycling ability, rate capability as well as initial coulombic efficiency. This uniquesuper-large aliovalent cation doping has been never reported before, and it is quite suitablefor industrial fabrication.Electrochemical active delithiated manganese oxides MnOx, as a coating agent, havebeen thickly decorated on the layered lithium-rich cathode. We have broadened the conceptof surface modification method for both the usage of electrochemical active coating agent,and the thickness of coating layer. Because the delithiated MnOxcoating layer can keepoxygen vacancies to maintain a part of lithium vacancies, and itself can provide plenty ofLi+intercalating sites, the thick coating with electrochemic alactive MnOxcan elevateinitial coulombic efficiency (from71.6%to over100%) and discharge capacity of layered lithium-rich cathode more effectively in comparison with conventional surface modification.Meanwhile, this thick coating layer can not only protect the bulk layered from the erosionand dissolution of the electrolytes, but also reduce the charge transfer resistance, and thenenhance the cycle-ability (265.0mAh g-1after30cycles) and rate capability of layeredlithium-rich cathode. Our work probably gives a new insight for designing and modifyinghigh-performance materials for the next generation lithium ion batteries.A reasonable design and synthesis of the spinel/layered heterostructured material isproposed, by encapsulating a high-capacity layered lithium-rich material with a high Li+conductive nanospinel mixture LiNixMn2–xO4. The ions diffusion at post heat-treatmentleads to the good Li+penetrability between the layered core and the outer spinel. The3DLi+diffusion channel of the spinel ensures rapid Li+ion exchange between the layered coreand the electrolytes. Thus, this heterostructured material can maximize the inherentadvantages of the high Li+storage capacity of layered structure and high Li+conductivityof the outer spinel structure. It yields high maximal discharge capacities of274.6mAh g1,268.8mAh g1,218.9mAh g1and189.5mAh g1at1C,2C,5C and10C rate, respectively.This high capacity is among the highest values reported for the power performance oflayered or spinel cathode materials. We anticipate this novel insight into the design andsynthesis of cathode materials should inspire the development of a wide range of otherstable, high-rate, and high-capacity intercalation materials, and promote the development ofhigh-energy and high-power lithium ion batteries.We propose a biomimetic design and novel versatile nano-coating strategy of ultrathinspinel membrane-encapsulated layered lithium-rich cathode material. It is well-known that,in most higher eukaryotes, the thin plasma membrane is not only essential for the cell to bestable in the environment, but the membrane protein can also play as “alkali ion pump” toactive transport alkali ions across the membranes by consuming the ATP. Hence, with anultrathin nanolayer of4V spinel Li1+xMn2O4as a “membrane” encapsulating on a layeredlithium-rich cathode, the spinel membrane can be expected to maintain the layered bulkstable during high-voltage cycling; and most importantly, this high Li+conductivemembrane can rapidly transport Li+between the electrolytes and the layered bulk as a “Li+pump”. The easy and versatile nanocoating strategy has been adopted: the polymerdispersant, polyvinylpyrrolidone (PVP), was initially dispersed on thepristine layeredlithium-rich materials to assist the subsequent homogeneous decoration of the manganesesalt, and eventually, the ultrathin spinel membrane could be yielded on the surface after heat treatment, due to the ion diffusion from the bulk. This cathode yields maximaldischarge capacities of247.9mAh g1,223.8mA h g1and200.1mA h g1at1C,2C and5C rate, respectively, which are among the highest values in previous report. Meanwhile,the stubborn illnesses of the layered lithium-rich cathode, such as voltage decay andthermal instability, are found to be relieved as well. This bifunctional cathode material mayprovide a bridge to the future advanced Li-ion batteries for application in electric vehiclesand renewable energy storage. |
PDF Full Text Request