Reaction Mechanism And Modification Of Spinel LiMn₂O₄ Cathode Materials Synthesized By Solid State Sintering

Rechargeable lithium ion battery is one of the most important electrochemical energy storage devices.It is widely used in electric vehicles,hybrid vehicles,energy storage facilities and consumer electronics.Spinel LiMn₂O₄ has attracted much attentionfor researchers because of its advantages such as low cost,high voltage platform,good safety,non-toxic and pollution-free.Solid-phase sintering method has the advantages of simple synthesis process,high production efficiency and favorable product consistency.So it is one of the most suitable methods for industrial preparation of LiMn₂O₄.However,LiMn₂O₄ suffers some problems,such as the solid phase sintering process,the unknown synthesis mechanism,the volume strain during charging and discharging,and the dissolution of manganese ions at high temperature.These problems have seriously restricted the further application of LiMn₂O₄ cathode materials for lithium ion batteries.In this work,the optimum reaction temperature of LiMn₂O₄ sintering reaction is predicted by thermodynamic theory.LiMn₂O₄ is modified by high pressure spray drying method for Al.The first principles DFT+U algorithm is used to analyze the change of electronic properties before and after doping.The surface modification of La₂O₃ is carried out by the auxiliary combustion method of LiMn₂O₄.Al₂O₃ surface cladding modification of LiMn₂O₄ is also carried out using two-step method,Furthermore,the extensive tests are also performed.In this paper,the decomposition mechanism of Li₂CO₃ is elucidated by the thermodynamic theory.The thermodynamic database of standard molar Gibbs free energy for Li₄Mn₅O₁₂,LiMnO₂,LiMn₂O₄ and Li₂MnO₃ is supplemented.The generation order of the reaction products is given.The optimun reaction temperature range of LiMn₂O₄ sintering reaction is predicted to be 775～850℃.The effects of temperature on the phase composition and microstructure of the products during sintering are studied by using MnO₂ and Li₂CO₃ as raw materials.The experimental results are in good agreement with the thermodynamic calculations.Moreover,according to prolonging the sintering time in the optimum reaction temperature range,the LiMn₂O₄ cathode material of lithium ion battery with good crystallinity and stable cycle performance is obtained.Among them,through the insulation at 825℃LiMn₂O₄ for 16 h,the degree of cation mixing is lowest.The initial discharge capacity of 0.2 C is 116.9 mAh/g,the discharge capacity is 105.7 mAh/g after 20 cycles,and the capacity retention rate is 90.4%.Al doped spherical LiMn₂O₄ is prepared by high pressure spray drying method,and its solid density and structural stability are improved.The solid density of the sample is 2.15 g/cm3at the inlet temperature of 300℃.After 100 cycles,the capacity retention rate is 79.8%at room temperature of 0.5 C magnifications.Al doping can reduce the mixing degree of LiMn₂O₄cations,improve the structural stability and the cycle performance,and inhibit the erosion of electrolyte to spinel LiMn₂O₄.The capacity retention rate of LiAl0.075Mni.925O4 is 76.1%after 100 cycles at 60℃ and 1C rate.Furthermore,the lattice structure and electronic properties of LiAl_0.075Mn_1.925O₄ with the best electrochemical performance are calculated by the first principles DFT+U algorithm.The total energy and band gap of LiAl_0.075Mn_1.925O₄ are decreased significantly,the conductivity of the sample is improved.A novel double combustion combustion and coplexation synthesis method is proposed by using glycerol and citric acid as combustion supporting agent.The proposed method of preparing of nano-La₂O₃ and modifying the LiMn₂O₄ surface forms a uniform and highly crystalline La₂O₃ layer with a thickness of 20～30 um on the surface of spinel material.Compared with the pristine LiMn₂O₄,the LiMn₂O₄ coated with 3 wt%La₂O₃ exhibits a higher rate capability and better reversibility,exhibiting 103.5 mAh g-1 and 90.6 mAh g-1 at 5 C and 10C,respectively.After 100 cycles at 60℃ and 1C,the 3 wt%nano-La₂O₃-coated sample still exhibits a high capacity retention of 91.68%.Nano-Al₂O₃ coating layers on spinel LiMn₂O₄ is introduced by using a simple two step coating method.A uniform coating layer with favorable ionic conductivity is formed on the surface of LiMn₂O₄,the manganese ion dissolution under the same condition is also effectively suppressed.The sampe coated by two step method exhibits excellent capcity retentions of nearly 90%after 300 cycles at the elevated temperature.