Preparation And Performance Of Long Cycles Life And High Power Spinel LiMn₂O₄-based Cathode Materials

Spinel LiMn₂O₄ has received special attention as a very prospective cathode material for widespread large-scale application in plug-in hybrid electric vehicles（PHEVs）and pure electric vehicles（EVs）because of its high theoretical capacity,low toxicity,low cost,high safety,environmental friendness and good thermal stability.However,its practical application in lithium ion batteries in large scale has been hampered due to the problem of capacity decay upon extended cycles,especially at high temperature（> 55 ℃）,originated from many factors such as structural transformation that is induced by Jahn-Teller distortion of Mn（III）,manganese dissolution and electrolytes decomposition.In order to improve the cycle performance of the LiMn₂O₄-based cathode materials at elevated temperature,in this thesis,two strategies,including doping by other elements and off-stoichiometric composition,were used to prepare novel LMO-based cathode materials with high electrochemical performances.The main research content of this thesis is divided into the following sections:1)The effects of Co-doped,Al-doped,F-doped,Co,F-co-doped,Al,F-co-doped and Co,Al,F-tri-doped on the crystal structrue,particle morphology and the electrochemical performances of the as-obtained LiMn₂O₄-based cathode materials were investigated.2)The influences of calcination temperatures on the crystal structrue,particle morphology and the electrochemical performances of new cathode materials with off-stoichiometric composition were checked.3)The relationship between the electrochemical performances and the crystal structure,particle morphology and surface element distribution of the as-prepared LiMn₂O₄-based was analyzed and discussed.Based on the above-mentioned works,some results and conclusions are as follows:1)Spinel LiMn₂O₄-based with controllable stoichiometry ratio,truncated octahedron morphology and high crystallinity can be prepared easily by a sol-gel method.2)The cathode material with a chemical composition of Li1.05Mn1.85Co0.1O3.9F0.1 exhibits superior electrochemical performance（rate and cycle performances）than that of other LiMn₂O₄-based materials such as Li1.05Mn1.9Co0.05O4,Li1.05Mn2O3.9F0.1 and Li1.05Mn₂O₄.3)The cathode material with a chemical composition of Li1.05Mn1.85Al0.1O3.9F0.1 shows superior electrochemical performance（rate and cycle performances）than that of other LMO-based materials such as Li1.05Mn1.9Al0.05O4,Li1.05Mn2O3.9F0.1 and Li1.05Mn₂O₄.4)In contrast to Li1.05Mn1.85Co0.1O3.9F0.1 and Li1.05Mn1.85Al0.1O3.9F0.1,the as-synthesized Li1.05Al0.05Mn1.85Co0.05O3.9F0.1 exhibits higher specific capacity,superior rate capability and much more stable cyclability as cathode materials for LIBs at high temeprature（55 ℃）,with specific discharge capacities of 115.9 and 111.6 mAh g-1 and corresponding capacity retention of 72% and 73% for up to 800 cycles at 2 and 5 C rates,respectively.The high specific capacity,excellent cyclability and good rate performance are believed to be resulted from two main reasons:（1）the stable crystal structure of the Li1.05Al0.05Mn1.85Co0.05O3.9F0.1 derived from the synergistic roles of of multi-doping by Li+,Al3+,Co3+ and F-,which decrease the Jahn-Teller distortion and Mn dissolution;and（2）the stable interface of the active material/electrolyte results from the high content of Mn4+ in the surface which reduces the transfer resistances and favor fast Li+ intercalation/deintercalation kinetics.5)A truncated octahedral structure spinel cathode material with an off-stoichiometric of Li1.08Al0.08Mn1.85Co0.05O3.9F0.1 and an Al-rich surface layer is successfully prepared by a sol-gel route at 800 ℃.Concentration gradient of Mn4+ ions is found to decrease through the surface to the interior of the as-prepared sample.As the consequence,the Li1.08Al0.08Mn1.85Co0.05O3.9F0.1 sample delivers high rate capability and excellent cycling stability at elevated temperature,showing an initial discharge capacity of 111.1 mAh g-1 and capacity retention of 70.5% over 850 cycles at 1 C rate under 55 ℃.Even at 5 C rate under 55 ℃,it also displays a high capacity of 102.5 mAh g-1 with capacity retention of 80.0% over 850 cycles.These results reveal the importance of an off-stoichiometric strategy and a temperature-controlled heat-treatment process for the preparation of Li Mn₂O₄-based spinel cathode material with unique crystal orientation and Mn4+-rich surface layer,which both decrease the Mn dissolution and provide crystal and interfacial stability while preserving fast Li+ diffusion,resulting in improvement of cycle performance and rate capability of the as-prepared material.