Investigation On Synthesis And Electrochemical Properties Of Iron-based Fluorides

In order to build a sustainable energy future,electrochemical energy storage equipment has made great progress in the past two decades and is now widely used in portable electronic equipment and energy-saving industrial tools.However,due to the rapid development of science and technology,people's demand for safer and more efficient next-generation energy storage equipment continues to increase.With the rapid emergence and application of advanced anode materials,cathode materials have become an important factor hindering the increase in battery energy density.The capacity of traditional cathode materials is limited,and ferric fluoride is attracting more and more people's attention.Because of its high theoretical capacity,low cost,low toxicity and great working potential.Unfortunately,the commercial application of ferric fluoride is still hampered by numerous challenges.One of the obstacles is poor conductivity,which is due to the large band gap caused by the strong ionic nature of the Fe-F bond.Another major obstacle is the rapid decay of capacity during cycling.Moreover,traditional liquid batteries use organic electrolytes,which have the disadvantages of low capacity density and flammability.In order to overcome these shortcomings,this article focuses on improving safety,increasing energy density,and improving the electrochemical performance of iron-based fluorides.The results are summarized as follows:(1)Nanosized Fe F₃·0.33H₂O was prepared by a low-cost and safe hydrothermal method.Micron particles of Fe F₃·3H₂O with irregular shapes were converted to nanoscale Fe F₃·0.33H₂O by the solvent method using n-propanol as the solvent at different temperatures.Meanwhile,the particle size and morphology of iron fluoride could be controlled by adjusting the synthesis temperature.Using relaxation time distribution(DRT)analysis of electrochemical impedance spectra(EIS),Fe F₃·0.33H₂O synthesized at 180°C showed a high capacity of 200 m Ah g^-1 after 160 cycles,with excellent rate performance.(2)In order to improve the electrical conductivity of the cathode material and to improve the safety of conventional lithium-ion batteries with fluoride as the cathode,multi-walled carbon nanotubes were compounded with nano-sized Fe F₃·0.33H₂O using a solvothermal method,and fluoride was applied to the sulfur-based solid-state electrolyte for the first time.The ionic conductivity was electrochemically tested and calculated to be 1.1×10^-3 S/cm at 25°C and the activation energy was 26.5 k J mol^-1 at25°C.The nanostructure of Fe F₃·0.33H₂O reduced the ion transport distance and the multi-walled carbon nanotubes.The use of multi-walled carbon nanotubes formed a high speed ion transport network and the use of solid electrolyte Li₇P₃S₁₁ improved the safety of the battery.At low current density,the Fe F₃·0.33H₂O/MWCNTs/Li₇P₃S₁₁/Li all-solid-state battery had a first-cycle embedded lithium capacity of 701.4 m Ah g^-1 and a calculated coulomb efficiency of 46.2%,after the first 10 weeks of constant current charging and discharging,the coulombic efficiency has remained at 99%.However,the cycle performance of the all-solid-state battery was not as good as that of the liquid battery.After 60 charge/discharge cycles,the capacity of the all-solid-state battery was only 171.3 m Ah g^-1.(3)In order to improve the cycle performance of fluoride all-solid-state batteries,r GO replaced MWCNTs.The reason why we chose graphene was that graphene itself had excellent electrical conductivity.The larger interlayer spacing was conducive to the rapid diffusion of lithium ions and was conducive to alleviation the volume expansion caused by the conversion reaction of fluoride.The ionic conductivity of Li₁₀Ge P₂S₁₂(LGPS)solid electrolyte was higher than that of Li₇P₃S₁₁,and its activation energy was lower than that of Li₇P₃S₁₁.It can provide more lithium ions during the cycle to optimize cycle performance,so Li₁₀Ge P₂S₁₂ was used instead of Li₇P₃S₁₁.At a small rate,the Fe F₃·0.33H₂O/r GO/LGPS/Li all-solid-state battery had a first-lap lithium insertion capacity of 676.3 m Ah g^-1and a calculated coulombic efficiency of 60%.After the first5 weeks of constant current charge and discharge,the coulombic efficiency had remained at 99%.After cycling the all-solid-state battery for 60 weeks,the capacity remains at 274.6 m Ah g^-1.In this paper,there are 62 figures,7 tables and 141 reference articles.