| Energy is very important for economy develepment.Lithium-ion batteries as the representative of the "green energy" have moved into a period of rapid development.Lithium-ion batteries have gradually occupied the market of batteries due to their high energy and power density,high safety,long cycle life and low pollution.Electrode materials are key factors determing the performance of lithium-ion batteries.Spinel and zero-strain Li4Ti5O12 has become one of the most promising anode materials of lithium-ion batteries due to its long cycle life and satisfactory safety.However,the application of Li4Ti5O12 has been limited due to its low electronic conductivity,which results in fading of capacity dusing cycling and severely polarization at high charge and discharge rate.This dissertation focuses on the improvement of high rate performance and cycling stablility of Li4Ti5O12 by reducing its particle size and increasing its electronic conductivity.First,we prepare the Li4Ti5O12/CMK-3 composite using the ordered mesoporous carbon CMK-3 as a template.Nanosized Li4Ti5O12 with short Li+ transfer path and a CMK-3 conductive network was obtained,which shows excellent rate performance.Second,we studied the doping of Li4Ti5O12 by Sr and Zr and found the doped Li4Ti5O12 has reduced particle size and increased lattice parameter,which resulted in significantly improved rate performance.Finally,we investigated the co-coating of carbon and nitrogen on the Li4Ti5O12 surface with hexamethylenetetramine(C6H12N4),chitosan(C6H1,N2O4),and p-phenylenediamine(C6H4(NH2)2)as carbon and nitrogen sources,and found that a conductive network was formed on Li4Ti5O12 surface and improved the electronic conductivity of Li4Ti5O12,which lead to excellent electrochemical performance.The preparation of Li4Ti5O12/CMK-3 composite by a wet impregnation/calcination procedure was studied.It was found that Li4Ti5O12 distributed inside and outside of the mesopore channels of CMK-3,which have diameters less than?50 nm.The specific capacity of Li4Ti5O12 in the composite reached 171 mAh·g-1 at 0.5 C,which is close to the theoretical value of 175 mAh·g-1.The high rates performance of Li4Ti5O12 in the composite was compared with that of commercial Li4Ti5O12(BTR battery Materials Co.,Ltd).It was found that,at 5 C and 10C charge-discharge rates,the specific capacity of Li4Ti5O12 in the composite is 131 and 99 mAh·g-1,respectively,which is significantly higher than that of commercial Li4Ti5O12,which is 77 and 48 mAh·g-1,respectively.Li4Ti5O12 obtained by removing CMK-3 shows a specific capacity of 117.8 mAh·g-1 at 1 C,which is lower than that of Li4Ti5O12/CMK-3 composite(143 mAh·g-1),indicating that CMK-3 not only play a role of template,but also form a conductive network.The rate capability for Li+deintercalation in Li4Ti5O12 is higher than that for Li+ intercalation,showing that there is an asymmetric behavior between the charge and discharge of Li4Ti5O12.The Li4Ti5O12/CMK-3 composite has good cycling stability and the coulombic efficiency maintained at almost 100%at all the cycling rates.In the first 100 cycles at 0.5 C,the specific capacity of Li4Ti5O12 in the composite for Li+ intercalation decreased from 174 mAh·g-1 to 1+0 mAh·g-1,corresponding to a capacity loss of around 8%.While in the subsequent cycles at 1 C,2 C,5 C,and 10C each for 100 cycles,the corresponding capacity loss is 0.39%,0.07%,0.62%,and 4%,respectively.Sr-doped Li4Ti5O12 was prepared by a solid-state reaction method.It is found that Sr-doping can effectively improve the high rate performance of Li4Ti5O12,which can be attributed to the reduction of particle size,the decrease of charge transfer resistance and the enlargement of lattice parameter caused by the introduction of Sr to Li4Ti5O12 lattice.The Sr content has a significant effect on the performance of Sr-doped Li4Ti5O12 and the optimum Sr content was found to be at the atomic ratio of Sr to Ti of 0.02.At 5 C,the discharge specific capacity of 0.02Sr-LTO is 104.1 mAh·g-1,which is about 1.62 time of that of LTO(64.3 mAh·g-1).A small amount of SrLi2Ti6O14 was formed in the Sr-doped Li4Ti5O12,which however has a positive effect on the charge/discharge specific capacity of the doped Li4Ti5O12.Zr-doped Li4Ti5O12 was prepared by a solid-state reaction method.It is found that Zr-doping can effectively improve the low rate performance of Li4Ti5O12,which can be attributed to the reduction of particle size,the highly dispersion of particle and the enlargement of lattice parameter caused by the introduction of Zr to Li4Ti5O12 lattice.The Zr content has a significant effect on the performance of Zr-doped Li4Ti5O12 and the optimum Zr content was found to be at the atomic ratio of Zr to Ti of 0.03.At 0.5 C,1 C,2 C,and 5 C,the discharge specific capacity of 0.03Zr-LTO is 165.7,136.1,98.6,and 70.8 mAh·g-1,which is higher than that of LTO(145.3,110.2,86.6,and 64.3 mAh·g-1).Co-coating of Li4Ti5O12 with C and N using hexamethylenetetramine(C6H12N4)as carbon and nitrogen source was syudied.C and N were found uniformally distributed on Li4Ti5O12 surface.When the mass ratio of hexamethylenetetramine to Li4Ti5O12 is 1:40,the reaction temperature is 800 ? and the reaction time is 30 min,the coated Li4Ti5O12 shows the best electrochemical performance.Its specific capacity at 1 C,2 C,5 C,10 C,15 C,and 20 C is 169.3,161.2,133.3,109.1,80.7,and 55.5 mAh·g-1,respectively.The capacity retention after 100 cycles at 10 C is 99.8%.This high performance can be attributed to the formation of highly conductive TiN like species and the firm C coating layer.Co-coating of Li4Ti5O12 with C and N using chitosan(C6H11N2O4)as carbon and nitrogen source was syudied.C and N co-exist on the Li4Ti5O12 surface with a uniform distribution.The coating did not change the structure of Li4Ti55O12 The best coated Li4Ti5O12 was obtained at the 1:40 mass ratio of chitosan to Li4Ti5O12 The specific capacity of the best coated Li4Ti5O12 at 1 C,2 C,and 5 C is 160.8,146.9,and 101.5 mAh·g-1,respectively,which is higher than that of pristine Li4Ti5O12,which is 147.4,129.8,and 77 mAh·g-1,respectively.The charge and discharge platform of the coated Li4Ti5O12 are more flat than that of pristine Li4Ti5O12 at all of the charge/discharge rates.After 100 cycles at 2 C and 5 C,the capacity retention of the best coated Li4Ti5O12 is 98%and 99%,respectively.So,C and N co-coating to Li4Ti5O12 with chitosan(C6H11N2O4)as the raw material significantly ehanced the performance of Li4Ti5O12.Co-coating of Li4Ti5O12 with C and N using p-phenylenediamine(C6H4(NH2)2)as carbon and nitrogen source was syudied.C and N were found to be co-existiance on the Li4Ti5O12 surface with a uniform distribution.The coating of C and N leads to no change in the structure of Li4Ti5O12 When chitosan and Li4Ti5O12 was in a mass ratio of 1:35,the coated Li4Ti5O12 displayed the best electrochemical performance.The specific capacity of the best coated Li4Ti5012 at 0.2 C,0.5 C,1 C,2 C,5 C,and 10 C is 172.7,170.2,165.7,145.8,133.5,and 114.8 mAh·g-1 respectively.The coulombic efficiency maintained at almost 100%at all the cycling rates.The specific capacity loss after cycled at 2 C,5 C,and 10 C each for 100 times is 6.7%,4.2%,6.5%and 8.8%,respectively.So,high rate performance Li4Ti5O12 can be obtained by surface coating using p-phenylenediamine as the C and N source. |
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