Research And Modification Of NaTi₂（PO₄）₃ A Kind Of Anode For Rechargeable Sodium-ion Batteries

Nowadays the first three big problems of the world are environmental pollution, population explosion and resource limit. Traditional fossil energy resource is still important. But this kind of non-renewable energy resource was abused and over exploitation in the past decades of years, which made it unavailable to support the world in the future. Fortunately, new kinds of renewable energy resource are rapidly discovered and the related technology is quickly developed. Among them, the second battery especially the lithium-ion battery is an excellent kind for its great electrochemical performance in electric vehicles and portable electric divices. But there is still a problem of whether the Li resource on the earth is enough. In this case, sodium-ion battery is a suitable alternate for lithium-ion battery because the two elements are in the same group in the periodic table of the elements. In fact, the research of sodium-ion batteries is started at the same time with lithium-ion batteries. What’s more, sodium resource is more abundant than that of lithium on the earth. Above all, sodium-ion battery will get a bright future especially in electric vehicles, family energy storage and large scale energy storage.Recently, cathode materials of sodium-ion batteries are widely study such as transition metal oxide type and MOFs, and the electrochemical performance is great. On the other hand, anode materials are not that good. In this paper, we successfully synthesized NaTi₂（PO₄）₃ with two different methods. For modification, we synthesize the Al-doped materials as Na1+xTi2-xAlx（PO4）3 to increase the cycle performance. In addition, samples with modified by carbon are also synthesized. Through the study, we get the following conclusions:（1） NaTi₂（PO₄）₃ samples and the doped samples are successfully synthesize with two method. On the one hand, we use sol-gel method to synthesize the precursor. On the other hand a rheological phase method was used. We get the conclusion below: NaTi₂（PO₄）₃ synthesized via a sol-gel method shows great morphology and the electrochemical performance is better. The first discharge specific capacity is 132.2 mAh g-1, but that falls to 45 mAh g-1 after 50 cycles recharge.（2） In addition, we do some research on the temperature during calcining. We get the conclusion that the sample which was prepared by first heating to 500℃ for 3 h in the air and then 800℃ in air for 12 h keeps the best morphology and crystal structure in particular and exhibits the best electrochemical performance. The first discharge specific capacity is 122.2 mAh g-1, and after 50 cycles recharge, the capacity is 82.7 mAh g-1.（3） For the Al-doped samples. We have tried different x（from 0 to 0.5 with a 10% increase for each sample） in different Na1+xTi2-xAlx（PO4）3 sample. The electrical conductivity will increase and the resistance will decrease with Al-doping. So that the cycling performance and capacity retentions will both developed. The result shows that when x=0.3,the sample exhibits the best electrochemical performance with an initial discharge specific capacity of 117.9 mAh g-1, and after 50 cycles that is still 105.4 mAh g-1, which shows a capacity retention of 89.4%. The rate discharge ability is also very good.