Research On Synthesis And Electrochemical Properties Of LiF/Fe As Conversion Cathode Material

Electrochemical conversion reaction mechanism is a new mechanism which isentirely different from the traditional intercalation reaction mechanism. The traditionalanode materials which are based on the intercalation reaction allow no more than oneelectron transfer in the reaction, consequently, the electrode materials based on theconversion reaction have a very high capacity. The main electrode materials based onthe conversion reaction include oxides, sulfides and fluorides of transition metals,among which metal fluorides are more suitable to be the anode material for lithium-ionbattery, due to the relatively high working voltage. FeF₃becomes the research focuswith the advantages of high-capacity（a high theory capacity of712mAh/g）, low-cost,non-toxic,and environmental friendliness. The discharged production of FeF₃is thehighly-dispersed LiF/Fe nano-composite, the charging reaction of LiF/Fe is achievedowing to the particular characteristics of nano materials. However, the lithium-poorFeF₃material can’t match the lithium-poor cathode material to complete thedischarge/charge reaction, thus astrict the practical application of FeF₃. To surmountthis barrier, the lithium-rich LiF/Fe composite is synthetized.FeF₃was prepared by evaporating the hydrofluoric acid solution of FeF₃, and thenelectrochemical performance was researched, proving that FeF₃, especially the FeF₃/Ccomposite displayed a very good performance, but the application may be limited bythe weakness of lithium-defcient. Therefore, LiF/Fe was prepared as a lithium-richmaterial for lithium batteries. There are several methods to prepare LiF/Fe composite,such as cosputtering, ball-milling, pyrolysis,et al. But these methods are to complex toapply to industrial production. In this passage, a brief and applicative method ofpreparing LiF/Fe composite in the liquid phase was used. The electrochemicalperformance of LiF/Fe and the process conditions were mainly researched to perfectthe method. In addition, LiF/Fe/C composite were prepared by ball-milling and soidphase reduction to measure the effect of carbon-coating to the composite.The results of the research show that the big size of the composite reduced thecapacity of LiF/Fe electrode prepared in liquid phase, thus surfactant was added to thereaction system to adjust the size of the composite, and the temperature was alsocontrolled. After a series of experiments, the best process conditions for synthesizingthe LiF/Fe composite were confirmed by the following details: FeCl2provides the Fesource, LiOH provides the Li source, NH4HF2provides the Li source, HHA（hydrazinehydrate） acts as the reductant, the temperature keeps in100℃, and LAS is added to thereaction system as the surfactant with a concentration of0.06mol/L. Although the optimized LiF/Fe composite shows an bvious progress compared with the unoptimizedLiF/Fe composite, it still can’t reach the level as reported in the literatures. Furthermore, ball-milling and soid phase reduction were combined to synthetize LiF/Fe/Ccomposite. High capacity of LiF/Fe/C composite showed a very high capacity of320mAh/g with the current of20mA/g,the capacity still keep in180mAh/g even whenthe current reached100mA/g.