Synthesis And Characterizations Of New Cathode Materials Na₂FePO₄F For Lithium Ion Batteries

Energy storage technology plays an important role in the production and use of energy. Amongthem, lithium ion batteries have become the most important and potential energy storage modedue to its advantages, such as high working potential, high specific energy, high cycle stability,no memory effect, low rate of self-discharge, environmental friendliness and so on. However, thetraditional cathode materials and electrolytes have their own drawbacks, which can not meet theneeds of the development. Therefore, people are paying more and more attention to the newcathode materials and electrolytes for lithium ion batteries.Researches showed that some sodium-based fluorophosphate compounds used as the cathodematerials of lithium ion battery could get perfect electrochemical performances. On the otherhand, the cycle stability of lithium ion batteries could be effectively improved by adjusting thetype of the solvent or using some additives which could form thin film on the surface of thecathode materials. In this thesis, we synthesised the sodium-based fluorophosphate Na₂FePO₄Fvia solid-state method and investigated its electrochemical performances. On the other hand, wealso explored the high-voltage electrolyte of lithium ion battery by using different solvents andadditives, such as nitrile, fluoro-carbonate, isocyanate and so on. The main results achieved inthis paper were listed as follows:（1） The influences of various carbon sources and different raw materials on the performancesof Na₂FePO₄F materials were compared. A series of Na₂FePO₄F materials carbon-coated withdifferent carbon sources were prepared using NaF, FeC₂O₄2H2_O, NH₄H₂PO₄as raw materials, inwhich sucrose, cyclodextrin, citric acid, ascorbic acid acted as the carbon source, respectively.Besides, a sample of carbon-free Na₂FePO₄F was also synthesized as compared. In addition, twosamples were prepared using2NaF+FeC₂O₄2H₂O+NH₄H₂PO₄+sucrose and2NaF+FePO42H2_O+sucrose as raw materials, respectively. According to the test results, carboncoating did not change the structure of Na₂FePO₄F, but can improve its electrochemicalperformances remarkably. Moreover, different carbon sources affected the morphology andcarbon contents of the samples. In this thesis, the sample using sucrose as the carbon sourceshowed relatively well-distributed morphology, highest carbon content and most excellentelectrochemical performance. Compared the two samples synthesized by different raw materials, the sample obtained by2NaF+FeC₂O₄2H₂O+NH₄H₂PO₄+sucrose showed more uniform particlesize, higher carbon content and more excellent electrochemical performance.（2） The optimal temperature of synthesizing Na₂FePO₄F, the mechanism of solid-state reactionto form Na₂FePO₄F and the possibility of utilizing the second Na⁺in Na₂FePO₄F were exploredin this thesis. The tests indicated that the phase of Na₂FePO₄F could not be totally formed below400°C and began to decompose when the temperature was above650°C. According to the tests,the sample prepared at600°C showed the most excellent electrochemical performances. Thewhole process of solid-state reaction could be divided into three main steps: The first step wasthe pre-reaction to form interphases. The second step was the formation of Na₂FePO₄F phase.The third step was the decomposition of Na₂FePO₄F, which occurred at650°C or above.Notably, the first and second steps were actually interwoven together. In addition, theexperiments indicated that the second Na⁺could not be extracted even when Na₂FePO₄F/C wascharged to5.2V vs. Li/Li⁺.（3） Two approaches were proposed to study the high-voltage electrolytes. The mixed solventswere prepared by mixing PC with QB, FEC and GLN, respectively. It was demonstrated thatperformance of the electrolyte prepared by mixing QB and PC was the best, compared withother electrolytes prepared by pure PC, PC+FEC and PC+GLN according to the test of cyclicvoltammetry. What is more, when using HMDI or PTSI as additives in PC solvent, the testresults showed that there was no significant improvement on the performance by using PTSI asan additive. Differently, the properties could be obviously improved with HMDI as an additive,compared with pure PC, when the charging voltage was below6V, however the oxidationreaction was obvious when the charging voltage was higher than6V.