Synthesis And Doping Modification Of High-Voltage LiNi_0.5Mn_1.5O₄ Cathode Material Using MOFs As Precursors

High-voltage spinel LiNi_0.5Mn_1.5O₄（LNMO）is considered as one of the promising cathode materials for lithium-ion batteries because of its high energy density and low cost.However,the interface problems and the internal structural stability of the material during charging and discharging seriously affect the battery rate and cycling performance,limiting its large-scale commercial application.Ion doping is considered as one of the most effective modification methods at present.Therefore,in this dissertation,based on the advantages of stronger complexation of organic ligands to metal ions in metal-organic frameworks（MOFs）and designable topology,the modification of LNMO by ion doping is investigated in the following aspects:（1）The effects of three carboxylic acid organic ligands,terephthalic acid,3,3,4,4-biphenyldicarboxylicacidand3,3,4,4-biphenyltetracarboxylic acid,on the structure,microscopic morphology and water stability of MOFs as well as on the structure,morphology and electrochemical properties of LNMO were investigated.It is concluded that the organic ligands with a higher content of carboxylic acid functional groups（3,3,4,4-biphenyltetracarboxylic acid）,the synthesized MOFs can resist water attack,easily form spherical particles with larger specific surface area(112.38 m² g^-1).And the impurity content of the BCA-LNMO generated after high temperature calcination is only 0.76%.In addition,the discharge specific capacity is 116 mAh g^-1 at 0.1 C.The discharge specific capacity is still 92 mAh g^-1 at 10 C.（2）The tetrabutyl titanate was added to the MOFs mixed with lithium,and the titanium-doped combined in situ Li₂CO₃-coated LiNi_0.5-xTi_4xMn_1.5-3xO₄（x=0.02、0.03、0.04、0.06）was prepared after one-step calcination.With the increase of titanium doping content,the impurity phase in the material decreased,the particle size became larger,and there was a proper increase of Mn³⁺content.The best electrochemical performance was achieved at a doping amount of 0.03.The discharge specific capacities of LNMO and 003Ti-LNMO were 71 mAh g^-1 and 91 mAh g^-1 at 15 C,respectively,and the capacity retention was 91.7%and 98.8%after 300cycles at 10 C,respectively.（3）Si-doped LiNi_0.5-xSi_4xMn_1.5-3xO₄（x=0.03）was prepared using a mixture of water and ethylene glycol in a volume ratio of 1:1 as the solvent.The obtained materials have good electrochemical properties with discharge specific capacities of 129 mAh g-1 and 96 mAh g-1 at 0.1 C and15 C,respectively.And a capacity retention of 99.7%after 200 cycles at 2C.