Synthesis And Modification Of LiNi_0.5Mn_1.5O₄ Cathode Material For Lithium-ion Batteries

Electric vehicles and hybrid electric vehicles have attracted researchers' attentions because of the air pollution brought by traditional cars running on fossil fuel. However, currently commercialized lithium ion batteries, which possess low operating voltage and energy density, are not able to meet the demand of new energy vehicles. Spinel LiNi0.5Mn1.5O4 possesses a high theoretical discharge capacity of 146.7 mAh?g-1, an enhanced operating voltage of 4.7 V, and thus an ideal theory energy density of 690 Wh?kg-1, making it an excellent candidate for lithium ion batteries.Ball-milling solid state method was employed to prepare LiNi0.5Mn1.5O4 with Li2CO3, NiCO3?2Ni(OH)2?4H2O or NiO, and MnCO3 as raw materials. Influences of different Ni sources, calcination temperature and calcination time on the electrochemical performance of LiNi0.5Mn1.5O4 were studied. Electrochemical tests indicated that NiCO3?2Ni(OH)2?4H2O was a better Ni source and the sample preheated at 500 ? for 5 h and then calcinated at 800 ? for 12 h possessed the best electrochemical behaviors. Under the charge and discharge rate of 0.2 C, 1 C and 3 C, discharge capacities of the LiNi0.5Mn1.5O4 prepared by the optimal process conditions could reach 137.0, 124.8 and 119.6 mAh?g-1, respectively. After 100 cycles, capacity retentions of 81.9% and 84.6% were obtained for 1 C and 3 C. Discharge capacities as high as 123.8, 119.0 and 102.4 mAh?g-1 could be obtained with a charge rate of 1 C and discharge rate of 3 C, 5C and 10 C, respectively.Co-precipitation method was employed to synthesize LiNi0.5Mn1.5O4 with Ni(CH3COO)2?4H2O, Mn(CH3COO)2?4H2O, H2C2O4?H2O and Li2CO3 as raw materials. Effects of different pre-heating temperature, calcination temperature and calcination time on the electrochemical performance of LiNi0.5Mn1.5O4 were studied. Electrochemical tests indicated that the sample preheated at 450? for 5 h and then calcinated at 800? for 12 h possessed the best electrochemical behaviors. Under the current density of 1 C, discharge capacities of the LiNi0.5Mn1.5O4 prepared by the optimal process conditions could reach 113.4 mAh?g-1, and a capacity retention of 95.1% could be reached after 50 cycles. When being cycled at 3 C and 5 C, the best sample's discharge capacity could reach 104.5 mAh?g-1 and 87.0 mAh?g-1 respectively and capacity retentions of 93.1% and 80.9% could be obtained after 100 cycles.Lithium boron oxide(LBO) was employed to modify the surface of LiNi0.5Mn1.5O4 through a simple liquid method. Transmission electron microscope analysis indicate that LBO was coated on the surface of LiNi0.5Mn1.5O4. Electrochemical tests showed the sample 1 wt.% LBO coated LiNi0.5Mn1.5O4 possessed a discharge capacity of 111.0 mAh?g-1, which decreased to 101.5 mAh?g-1 after 100 cycles. The capacity retention is 91.4%, higher than that of pristine LiNi0.5Mn1.5O4 which is 85.9% compared with the highest discharge capacity. When the cell was charged at 1 C, discharged at 10 C, the discharge capacity was as high as 106.3 mAh?g-1. Electrochemical impedance spectroscopy tests indicated that LBO enhanced the diffusion speed of lithium ions and reduced the cells' resistance.