Study On High-voltage Electrochemical Performance Of LiCoO₂enhanced Viaalien Element Doping And Its Mechanisms

As a representative and important cathode material for lithium ion batteries,Lithium cobalt oxide（LiCoO₂,LCO）normally shows relatively low capacity in practical commercial applications as a result of the cutoff voltages being usually lower than 4.6 V.When the cutoff voltages reach 4.6 V a practical capacity as high as 220 m Ah g^-1can be achieved,which may be accompanied by the presence of a safety issue for LCO and thus jeopardize its practical applications.This is because the high cutoff voltages may trigger and accelerate oxygen evolution,electrolyte decomposition and irreversible phase transitions which all can lead to rapid failure of a cathode or battery.Therefore,the stability at high voltages is very important for the achievement of high capacity for LCO which thus makes it possible to be widely applied with higher energy density and better safety.In this thesis,we employed several alien elements to co-dope and modify LCO for significant improvement of its electrochemical performance via a conventional solid-state reaction approach with low cost and mass productability,and explored the underlying mechanisms for performance improvement.The main contents of this thesis are as follows:We co-doped Mg cations and F anions into the lattice of LCO by one-pot calcination at 950℃for 10 h via a conventional solid state synthesis approach.The contents of both Mg and F were optimized to form Li_0.985Mg_0.0075Co O_1.9975F_0.0025（LCO-MF）cathode with the optimum electrochemical performance.For half cells,the capacity retention rate is 80.8%after 500 cycles within the range of 2.8-4.6 V at 1 C,in sharp contrast with 33%for the pristine LCO.For full cells of LCO-MF/C,a capacity retention of 92.3%was achieved after 2200 cycles at 1 C within the voltage range of 2.8-4.5 V.The underlying mechanisms of performance improvement were explored in detail in which Mg and F play different roles in stabilizing the structure of LCO.This superior high-voltage performance can be ascibed to substitution of Li⁺by Mg²⁺doping can produce the"Pillaring"effect,inhibiting the particle breakage during the high voltage cycle,while the substitution of a small amount of O^2-by F^-doping can stabilize the surface structure and inhibit the surface metal dissolution.Therefore,the strategy of Mg and F co-doping opens a low-cost and up-scalable avenue for mass production of LCO with high performance.By combining heavy rare earth element of Eu and Mg as well as Al,their co-doping was successfully realized for LCO with much improved performance to form LCO-MAE using a similar one-pot solid-state synthesis approach at 950℃for 10 h.For half cells,a capacity retention as high as 72.0%was achieved after 2000 cycles at 1 C within the range of 3.0-4.6 V.This excellent high-voltage performance could be understood in terms of a new concept of so-called high entropy complex layers formed at the surface of LiCoO₂proposed.The near-surface high-entropy complex layers are composed of a thin disordered rock salt shell and a doped concentrated zone which can afford to suppress oxygen evolution and Co ion dissolution,and thus prevent the formation of CEI against the deconstruction of near-surface structures and facilitate the reversible transitions between O3 and H1-3.This novel co-doping concept of stabilizing the surface and near-surface structures can be lent for commercial mass production of LCO with low cost and high performance as well.