The Preparation Of Layered Double Hydroxides And Their Performance For Room-temperature Fluoride Ion Batteries

In recent years,lithium-ion batteries（LIBs）with high operating voltage,high capacity and long cycle life have been applied in electric vehicles and aerospace.However,the low and unevenly distributed lithium resources and the existence of safety issues limit their further development.As a novel type of battery,fluorine ion batteries（FIBs）are very promising alternatives for lithium-ion batteries.Compared with lithium-ion batteries,fluoride ion batteries have many advantages,such as high theoretical volume/mass energy density,low cost and higher safety.However,the development of fluoride ion batteries is still very slow and there are still some challenges.Firstly,the capacity and cycling performance of the cathode materials,which is the dominant factor of fluoride ion batteries,are still unsatisfactory;Secondly,the most fluoride ion batteries can only work at high temperatures above 150°C,which limits their practical applications.Layered double hydroxides（LDH）is a class of two-dimensional layered material with unique two-dimensional anion channels and topological transition properties,which is a very promising anion storage material.In this paper,LDH intercalated by fluoride ions were applied as cathode materials for fluoride ion batteries,which displayed remarkable performance at room temperature.（1）Co Fe-NO₃^--LDH nanosheets were prepared by co-precipitation method and Co Fe-F^--LDH nanosheets were obtained by following ion exchange method.After the replacement of NO₃^-by F^-,Co Fe-F^--LDH still maintains the original size of 90-120 nm,while the layer spacing is reduced to7.50（?）.When it was first used as a cathode material for fluoride ion batteries,it exhibited a reversible capacity of about 50 m Ah g-1 after 100 cycles at room temperature,which is much higher than the reported cathode materials for fluoride ion batteries.By the means of ex-situ XRD,EDS,and XAFS,it is showed that the energy storage mechanism of Co Fe-F^--LDH is an intercalation-type mechanism.The layer spacing of Co Fe-F^--LDH shifts reversibly during the charging and discharging process,accompanied by the valence changes of Co²⁺/Co³⁺.The excellent cycling performance of Co Fe-F^--LDH are attributed to its unique topological chemical transformation properties,and small volume change（～0.82%）.The two-dimensional diffusion paths in the interlayers and the weak interaction between F^-and the host layer contribute to the reversible intercalation and de-intercaltion of F^-at room temperature.First principles calculations（DFT）also further reveal that the diffusion energy barrier of F^-is very low during charging and discharging.（2）Co Ni-F^--LDH nanosheets were prepared by in situ topological transformation and ion exchange method of Co₂Ni（OH）₆ nanosheets.The polypyrrole homogeneously was coated on the surface of Co Ni-F^--LDH with large size of 2-4μm and high crystallinity by in situ polymerization reaction.It was found that when the ratio of Co Ni-F^--LDH and pyrrole monomer is 0.1g:30μL,the most stable electrochemical performance was achieved.A capacity of 60 m Ah g^-1 after 100 cycles at a current density of 10 m A g^-1 was exhibited at room temperature,with a capacity retention rate of 86%.When the current density was increased to 100 m A g^-1,the capacity was still 50.5m Ah g^-1,showing excellent cycling performance and multiplicative performance.The polypyrrole layer can improve the overall conductivity of Co Ni-F^--LDH and promote the rapid diffusion of anions while maintaining the unique layer structure of Co Ni-F^--LDH,improving the electrochemical performance.