Study On The Synthesis And Doping Of LiNi_0.5Mn_1.5O₄ For 5V Cathode Material Of Lithium-ion Battery

The main content of this thesis includes: synthesis by solid state method and Aluminum ion doping in LiNi_0.5Mn_1.5O₄were studied;preparation and doping of chromium, fluoride properties for 5Vcathode materials by sol-gel,and The Cr-F co-substitution on the electrochemical properties and thermal stability of LiNi0.5Mn1.5O4 was firstly studied in detail.The samples were characterized by XRD,ESEM and laser particle test to examine the structure,surface morphology, particle size. Constant current charge-discharge, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) were applied to test the electrochemical properties of the material.LiNi0.5Mn1.5O4 synthesized by solid state was characterized with XRD, suggested that the material adopted spinel structure,and appeared LixNi1-xO impurity peaks.The charge-discharge curves of LiNi_0.5Mn_1.5O₄showed two voltage plateaus in the 4V region and 5V region.The cycle performance of LiAl-(0.1)Ni_0.45Mn_1.45O₄ and LiAl0.1Ni0.4Mn1.5O4 was identical at 0.2C rate,however, LiAl-(0.1)Ni_0.45Mn_1.45O₄ had more excellent electrochemical cycling performance at 1C rate.XRD results indicated that the sample synthesized by sol-gel was pure phase, and belonged to cubic crystal spinels structure.The charge-discharge curves of LiNi0.5Mn1.5O4 had 4.7V platform,almost no 4.0V platform,and the result was consistent with the CV.With an aim to improve the cyclability of the material,chromium-doped LiNi0.5Mn1.5O4 was prepared by sol-gel method. LiCr0.10Ni0.45Mn1.45O4 had the best electrochemical performance. The doping of chromium not only increased discharge capacity,but also increased high rate cycling performance of the material. CV curve indicated that redox peak potential differenceΔφ=0.179V was smaller around 4.7V, compared with undoped samplesΔφ= 0.209V. So the doping of chromium decreased the polarization,and increased reversible. LiNi0.5Mn1.5O4-xFx prepared by sol-gel method belonged to cubic, spinel structure without impurity phase. LiNi_0.5Mn_1.5O_3.9F_0.1 had the best electrochemical performance,especially high rate charge-discharge cycle performance. The galvanostatic charge–discharge test indicated that the initial discharge capacities were 116.8 mAh/g and 118.6 mAh/g at 1C and 2C rates,respectively. After 100 cycles, their discharge capacities were still 100.7 mAh/g and 94.9 mAh/g, respectively,with capacity retention rate 86.2% and 80.0%.XRD showed that all the samples LiCryNi_0.5-0.5yMn_1.5-0.5yO_4-xF_x had high phase purity,and the powders were well crystallized,belonged to Fd3m space group.The peaks in the CV curve corresponded to the oxidation of Mn3+,Ni2+ around 4.0V,4.7V, respectively,indicating that small amounts of Mn3+ present in the cathode material. LiCryNi_0.5-0.5yMn_1.5-0.5yO_4-xF_x ( x=0.05,y=0.05 ) had the best electrochemical performance with the initial discharge capacity 137.0 mAh/g in 3.5⁵.0V,and the volume retention was 99.7% after 20 cylces at 0.2C. The cathode had a larger initial discharge capacity 123.1 mAh/g at 1C,after 100 cycles, their discharge capacities was still 112.8 mAh/g, and capacity retention rate was 91.6%. The galvanostatic charge–discharge test indicated that the initial discharge capacities were 120.4mAh/g and 109.5 mAh/g at 2C and 3C rates,respectively. After 150 cycles, their discharge capacities were still 104.0 mAh/g and 91.1 mAh/g, respectively,with capacity retention rate 86.4%% and 83.2%.5V material is a promising cathode material for lithium ion battery...