Synthesis And Electrochemical Performance Of Iron Based Fluoride Cathode Materials

Iron fluoride?FeF₃?stands out as a hot cathode material candidate for lithium-iron batteries because of the theoretically attractive capacity and high work potential.However,due to the large band gap and dense crystal structure,FeF₃ shows low electron/ion conductivity,which seriously affects its rate performance and conversion activity.It has been proved that structural expansion and anion substitution can lead to new mineral phases with high intrinsic electron/ionic conductivity.Among them,hexagonal tungsten bronze?HTB?type FeF₃·0.33H2 O and pyrochlore FeF₃·0.5H2 O featured with open channels perform well at high rate.Iron oxyfluoride?Fe OF?exhibits high conversion reaction activity.To develop safe,sample,low-cost and controllable synthesis method is a prerequisite to the practical application of these materials and also a big challenge in the field of fluoride research.In this thesis,we successfully synthesized these three materials using FeF₃·3H₂O powder via different reaction mechanisms,and designed different strategies to construct graphene-based conductive network for the corresponding fluoride.Synthesis processes,physical properties,and lithium storage properties of the related materials were also been explored and studied.A solvothermal strategy was adopted to achieve the conversion of FeF₃·3H₂O microcrystal to micro/nanostructured FeF₃·0.33H2 O in n-pentanol solvent.A reaction mechanism based on solid-solid topological phase transformation and self-templated morphology evolution was proposed after studying the effect of solvent and temperature on the morphology and structure of the products.Compared with the bulk FeF₃·0.33H2 O obtained through traditional heat treatment,the micro/nanostructured FeF₃·0.33H2 O showed high purity and crystalline,hierarchical structure can not only shorten both electronic and ionic transport pathways but also can buffer the structural stress during the cycling,resulting excellent rate capability: at 5 C,the discharge capacity of micro/nanostructured FeF₃·0.33H2 O was 113.5 m Ah g-1 higher than of FeF₃·0.33H₂O?24.8 m Ah g-1?.Adopt graphene oxide powders?GO?as precursors,we fabricated iron fluoride/ reduced graphene oxide?FeF₃·0.33H₂O/r GO?nanocomposite in one pot,wherein well-crystallined FeF₃·0.33H2 O nanoparticles?30-50 nm?uniformly anchored on r GO sheets,yielding a stable composite structure with high conductivity.FeF₃·0.33H₂O/r GO displayed good cycling stability?98% capacity retention over 100 cycles at 0.5 C?and impressive rate capability?122 m Ah g-1 at 10 C?,wherein surface-induced capacitive process plays an important role.A new synthetic strategy called anti-solvent precipitation method was developed to prepare pyrochlore FeF₃·0.5H2 O,using FeF₃·0.33H2 O as the precursor,methanol as the solvent and acetone as the anti-solvent.As prepared FeF₃·0.5H2 O was nanosphere with particle size of about 250 nm and specific surface area of 141.6 m2 g-1.The choice of solvent and anti-solvent,the influence of their proportion on the structure of the product were studied.A possible reaction mechanism was proposed.The whole reaction was carried out at room temperature,no expensive equipment was required and the product did not need any washing,which can be easily scaling-up.To enhance the conductivity of FeF₃·0.5H2 O,a FeF₃·0.5H2 O nanosphere/reduced graphene oxide composite?FeF₃·0.5H₂O/r GO?with sandwich structure was fabricated by vacuum filtration assisted sedimentation method.Compared with the FeF₃·0.5H₂O-r GO fabricated by direct mixing,the FeF₃·0.5H₂O/r GO has a more stable and complete conductive network.As a result,FeF₃·0.5H₂O/r GO performs much well as a cathode material: a discharge capacity of 140.5 m Ah g-1 with 88 % capacity retention is achieved after cycling 100 at 0.5 C,higher than FeF₃·0.5H₂O/r GO?110 m Ah g-1,77%?and FeF₃·0.5H₂O?86.5 m Ah g-1,71%?.Starfish-like Fe OF was fabricated by solvothermal treatment of commercial FeF₃·3H₂O in n-pentanol solvent.The possible formation mechanism of the hierarchical structure and its influence on lithium storage performance were also studied.Employing a electrostatic self-assembly strategy followed by a thermal reduction process fabricated a Fe OF/ reduced graphene oxide?Fe OF/r GO?composite.Graphene not only provides a good conductive network for Fe OF phase,but also avoids the product of conversion reaction?mainly metal Fe?in direct contact with the electrolyte.Fe OF/r GO displays a discharge capacity of 140.5 m Ah g-1 with 94.7 % capacity retention after cycling 100 at 0.5 C,higher than Fe OF?75.4 %?.