Lithium Storage Properties Of Graphitized Carbon Combined With Sulfur And Carbon Coated FeF₃ Cathode Materials

Nowadays,the specific capacities of the cathode materials of lithium-ion batteries are relatively low,which restrict improvement of the energy density,making it difficult to satisfy the increasing requirements for high energy density.Sulfur and iron fluoride are both promising cathode materials for lithium-ion batteries,because of their high specific capacity and low price.However,the low electrical conductivity of sulfur and the shuttle effect of soluble polysulfide generated after lithiation result in poor cyclic performance of lithium-sulfur batteries.The low electrical conductivity and complicated preparation process of Fe F₃ also restrict its application.To address the above-mentioned questions,in this thesis,spray pyrolysis was adopted to prepare Fe₂O₃/carbon spheres as precursors.Then,pomegranate-type nitrogen-doped graphitized carbon spheres were fabricated through Fe-catalyzed graphitization,which were loaded with sulfur to form cathode material for lighium-sulfur batteries.In addition,Fe F₃/carbon spheres were fabricated as cathode material for lithium-ion batteries by fluorinating Fe₂O₃/carbon precursor.The microstructure and the electrochemical lithium ion-storage performance of these cathode materials have been studied using various analysis methods.The results are as follows:1.The pomegranate-type nitrogen-doped graphitized carbon spheres with a graphitization degree of 67%were obtained through iron-catalyzed graphitization of the nitrogen-doped Fe₂O₃/carbon precursor at 750?and etching off iron using diluted HCl.The electrical conductivity reaches 5.6 S·cm^-1,which is an order of magnitude higher than that of the porous nitrogen-doped carbon spheres obtained by carbonizing at 550?and etching off Fe₂O₃.The S@N-GC cathode material obtained by loading sulfur into the pomegranate nitrogen-doped graphitized carbon spheres delivers an initial discharge capacity of 857 m Ah·g^-1 at 1C,and maintains 600m Ah·g^-1 after 150 cycles.The discharge capacities are higher than those of the S@N-C cathode material obtained by loading sulfur into the porous nitrogen-doped carbon spheres.2.The Fe₂O₃/carbon sphere precursor was carbonized at 500?and then converted to Fe F₃/C composite material under action of HF stream.Subsequently,Fe F₃/C was heated at different temperature to remove the crystal water.The obtained Fe F₃/C cathode materials show a big difference in structure and lithium ion-storage performance.As the heating temperature increasing,Fe F ₃·3H₂O gradually changes to Fe F₃·H₂O.When the temperature exceeds 260°C,Fe F3 gradually decomposes and transform to Fe F₂.Fe F₃/C-260 obtained with heating temperature of 260°C exhibits the best cyclic performance.Its first-cycle discharge capacity reaches 222 m Ah·g^-1 at a current density of 1 A g^-1(about 4.3 C),and the capacity decay rate of each cycle in500 cycles is only 0.074%.