Synthesis Of Li₂FeSiS₄ Material By High-temperature Solid-state Method

As a novel polyanion compound,lithium iron silicate has recently received much attention as an alternative cathode active material for LIBs because of its excellent chemical stability,low cost and environmental benignancy.However,as other polyanion cathode materials,Li2FeSiO4 has slow lithium ion diffusion rate and its inherent electronic conductivity is not sufficiently high.To attain a high performance,S and O belong to the same group of elements,it is often employed to substitute O to improve the performance of materials.In this article,we perform first principles calculations of the virtual Li2FeSiS4 compound to investigate the effects of S substitution for O on the cystal structure,deinsertion voltage,rate capacity and structural stability of Li2FeSiO4.The synthesis and synthesis conditions of Li2FeSiS4 were studied and the actual electrochemical properties of Li2FeSiS4 were tested.First principle calculations have been performed to evaluate the effect of S substitution for O on the electrochemical performance of Li2FeSiO4.Basing on the Li2FeSiO4 structure,the hypothetical models of Li2FeSiS4 have been analyzed.It has been found that;Li2FeSiS4 is energetically stable in a β-Li2CoSiO4 type structure,in which half of the Li ions would exchange their positions with Fe.The first(Fe2+/Fe3+)and the second(Fe3+/Fe4+)deinsertion voltages for Li2FeSiS4 are 3.07 V and 3.14 V.The full deinsertion of two Li ions per formula unit can be achieved in Fe-based silicates within the stability window of conventional electrolytes.Density of Li2-xFeSiS4(x=0、1、2)suggest that the electronic conduction would be favored at the lithiated-delithiated junction resulting in high charge-discharge current rates.The calculation of Li2FeSiS4 in elastic properties indicated that The improved ductility ensures structural stability and excellent cycle performance of Li2FeSiS4 during charge-discharge cycles.Using high-temperature solid-phase method,the two different sulfur sources of H2S and S were selected to explore the synthesis conditions of Fe2SiS4.The composition of the materials and elements of the materials were investigated by XRD and XRF.The synthesis condition of Fe2SiS4 using hydrogen sulfide is optimized to be 700℃ with the reaction time of 12 h.The synthesis condition of Fe2SiS4 using elemental sulfur is optimized to be 800℃ with the reaction time of 72 h.Two different sulfur sources of H2S and S were selected to explore the feasibility of Li2FeSiS4 in high-temperature solid-phase method.experimental result demonstrates that Li2FeSiS4 has been synthesized by high-temperature solid-phase method using the reactants lithium sulfide,ferric sulfide,elemental silicon and elemental sulfur.And the synthesis condition of Li2FeSiS4 using elemental sulfur is optimized to be 800℃ with the reaction time of 72 h.SEM analyses indicate that the sample emerges multilayer of clumps.The two deinsertion voltages of Li2FeSiS4 obtained by solid phase synthesis are 3.4 V and 4.1 V respectively.The initial charge/discharge capacities of Li2FeSiS4 cycled at 0.1 C in the voltage range from 1.5 V to 4.8 V are 110.4 mAh·g-1 and 88.7 mAh·g-1 respectively,the coulomb efficiency is 80.4%.But Li2FeSiS4 cannot keep a stabilized structure during repeated lithium insertion/extraction processes.