Structural Characterizations And Electrochemical Investigations Of Lithium-Excess Cathode Materials In Lithium Ion Batteries

With the development of modern society,serious energy problems and environmental problems made efficient and clean energy utilization become more and more important. On one hand, most countries in the world are accelerating the adjustment of energy structure and increasing the use of renewable energy resources.On the other hand, more efficient energy storage and transportation technologies are also being developed quickly. Because of its high energy density, power density and cycle life, lithium-ion battery is widely used in mobile devices, electric vehicles and smart grids. In particular, with the popularization of electric vehicles worldwide and the elimination of fuel vehicles, the development of lithium-ion batteries is going through great challenge and opportunity. In all the component parts of lithium-ion battery,cathode material development is most important for cost reduction and performance improvement. It has been an urgent task to explore the structure-performance relationships and develop new type of high-performance cathode materials. In our work,we use various rock-salt type materials to study the electrochemical properties,microstructure transformations and the charge/discharge mechanisms in Li-excess cathodes. The main works are shown below:1. We use lithium enrichment and replacement of 4d precious metal ruthenium by cheap environmental 3d metal nickel to synthesize Li1+xNi1/2-3x/2Ru1/2+x/2O2 cation disordered rock-salt type lithium-excess cathodes. Among all the samples,Li1.23Ni0.155Ru0.615O2 shows a highest first discharge capacity -295.3 mAh/g, which is much higher than most of other cathodes, and contains a capacity retention of 198mAh/g after 50 cycles. Combining in-situ X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and other characterizations, its excellent electrochemical performance is demonstrated to come from the Ni2+/4+, Ru4+/5+, O2-/O2n- redoxes and 0-TM percolation networks.The research of the lithium nickel ruthenium oxides provide a feasible strategy for the design and construction of a novel high performance lithium cathode material,which is important for the development of lithium batteries.2. Combining NiO and Li3NbO4 two phase, we synthesize the LixNi2-4x/3Nbx/3O2 disordered rock-salt cathode series. The first discharge capacity of Li1.15Ni0.467Nb0.383O2 reaches 221.9 mAh/g. The electrochemical performance of all samples are decreasing sharply with the increase of lithium content. Through a combination of high resolution transmission electron microscopy, electron diffraction pattern and X-ray absorption spectrum characterization methods, we study the benefit from high-speed lithium-ion migration channels in Li1.15Ni0.467Nb0.383O2, the reason of electrochemical performance decline with increase lithium content and the higher capacity with more serious capacity fade when cycling at a lower potential window. This exploration expounds the influence of the lithium-excess phase selection on the structure and electrochemical properties,and has the guiding significance for the construction of novel lithium-excess cathode materials.3. We introduce nickel into Li4TeO5 lithium-excess base phase to synthesize Li1+xNi3/4-5x/4Te1/4+x/4O2 series in which lithium-excess content can reach 60%. Among them,Li1.33Ni0.33Te0.33O2 has both the highest discharge capacity ?150mAh/g and the maximum capacity retention of 127 mAh/g after 100 cycles. Then with X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and electrochemical testing characterization, its electrochemical properties while the changes of lithium content and redoxes during cycles are developed. The study of this system is of great significance to further expand the lithium content limit in Li-excess cathodes.4. By using the cheap environmental Fe and the Mo of multiple electron compensation,we synthesize Li1+xMo2xFei-3xO2 lithium-excess cathode material which is aiming at practical application. The obtained Li1.233Mo0.467Fe0.3O2 layered cathode has a maximum capacity of 305.4mAh/g with good cycling stability. The tests at different potential windows show us the origin of its cyclability and the improvement of electrochemical properties by the lattice activation process during the initial few cycles. The study of Fe based lithium-excess cathode has important practical significance for promoting the use of green battery and reducing battery cost.5. We replace Ru4+ by two Li+ and one Ni2+ ions in Li2RuO3 to obtain an over-lithiated series of Li2+2xNi,Rui1-xO3 lithium-excess cathode materials. Li2.1Ni0.05Ru0.95O3 has a discharge capacity of 148.1mAh/g after 100 cycles at a high current rate, and the capacity retention percentage is 75.3%, which is far exceeding Li2RuO3. Through electrochemical tests and XRD characterization, we proves that to break the stoichiometric ratio of cation and anion without lattice structure changes can be an effective way to further enhance the electrochemical performance of lithium-excess cathodes, which is expected to be widely applied to the design of other Li-excess materials.