Design And Preparation Of Fe-Based Chalcogenide Composites And Their Performance For Li/Na Ion Batteries

With the rapid development of the global economy,the continuous consumption of fossil fuels have exacerbated the energy crisis and environmental pollution,propelling people to develop new energy sources.Lithium ion batteries?LIBs?have attracted significant attention because of their high energy density,high output voltage,high charge-discharge efficiency,long cycling life,and without memory effect.However,with the large consumption of lithium resources,the residual lithium resources are becoming rapidly decreased.Sodium and lithium are in the?A group.Therefore,the chemical properties of sodium are similar to lithium.In addition,sodium resources are more abundant than lithium resources in the earth's crust.Therefore,sodium ion batteries?SIBs?are considered as the most potential system for replacing LIBs.For lithium/sodium-ion batteries,their electrochemical properties mainly depend on the properties of anode and cathode materials.The development of cathode materials gradually tends to stabilize.However,the development of anode materials is still challenging.Therefore,the development of high-performance anode materials is crucial to promote the wider applications of LIBs/SIBs.Iron-based materials stand out as anode materials due to their high theoretical capacity,originating from stepwise intercalation-conversion reaction process,high abundance,low cost,and environmental-friendly nature.Nevertheless,the unsatisfactory intrinsic electronic conductivity,sluggish ion diffusion kinetics,and large volume variations of iron-based anodes have undermined their effectiveness to present excellent performance,especially in low-temperature environment.On the basis of the above discussion,we have successfully prepared a series of iron-based anode materials with different structures.The iron-based materials take full advantage of their well-developed structure,enabling the rapid transmission of electrons/ions,which is benefical to achieve fast dynamics of reaction.On the other hand,the high conductivity of carbon materials can effectively improve the cycling stability.In addition,the well-designed structure endows the electrode with a high tap density,which is in favour of future applications.As a result,these synergetic merits culminate in superior rate performance and stable cycle performance at wide temperatures.The details are shown below:?1?MOFs have the merits of easily tailored structure.Therefore,employing the layered Fe-MOF as the template is a promising strategy to achieve two-dimensional?2D?bi-continuous porous Fe₂O₃?bp-Fe₂O₃?.The hierarchical mesoporous structure can not only expose more active sites but also increase electrolyte diffusion paths,thus promoting better transfer of lithium ions.Furthermore,the 2D structure can effectively shorten the transport distance of ions/electrons,and endow the electrode with appropriate faradaic charge-transfer reactions to improve the power density of the elecrode.The results show that the bp-Fe₂O₃ electrode delivers a high reversible capability of 818.7 mAh g^-1 after 350 cycles at a current density of1.5 A g^-1.Even at 0°C and-25°C,the 2D bp-Fe₂O₃/Li still shows high capacities of 840.3 and635.6 mAh g^-1 at 0.5 A g^-1,respectively.?2?Iron sulfides process higher electronic conductivity than that of iron oxides.Herein,a hierarchical marcasite?m-FeS2?and carbon nanofibers composite?m-FeS2/CNFs?was prepared to improve its lithium storage properties.The m-FeS2/CNFs exhibit excellent cycling performance with a high capacity of 573.4 mAh g^-1 up to 5 A g^-1 after 1000 cycles.The composite displays superior Li-storage performance compared with previously reported iron sulphide-based anode materials.Interestingly,the hierarchical m-FeS₂ microspheres assembled by small FeS₂ nanoparticles in the m-FeS₂/CNFs composite is converted into a mimosa with leaf-like shape during Li⁺insertion process and vice versa.Accordingly,a“CNFs accelerated decrystallization-recrystallization”mechanism is proposed to explain such morphological variations and the decent electrochemical performance of m-FeS₂/CNFs.?3?On the basis of the previous system,Fe_1-xS nanosheets wrapped in nitrogen-doped carbon layers(Fe_1-xS@NC)were synthesized through thermal annealing of the Fe-MOF,followed by a subsequent high-temperature sulfidation.The as-prepared Fe_1-xS@NC composite delivers high performance,due to efficient contact with electrolyte ions and highly accessible active sites.This method not only remove the addition of external carbon material but also reduce the cost of raw materials.Additionally,the nitrogen-doped carbon layer not only enhance the conductivity but also serves as a robust buffer layer to enhance the structural stability.The electrochemical measurements indicate that the Fe_1-xS@NC composite displays excellent Na-storage performance in both half and full cells.The Fe_1-xS@NC electrode manifests a remarkable rate capability of 510.2 mAh g^-1 at a high rate of 8000 mA g^-1.The kinetics analysis clearly illustrates that the pseudocapacitive-dominated behavior is responsible for the excellent rate performance of the Fe_1-xS@NC electrode.?4?In view of the complicated synthetic methods of Fe_1-xS@NC,we combine the in-situ carbonization and structural engineering strategy to achieve a micro-nano structured FeS spheres coated with ultrathin graphitic carbon layer?FeS@g-C?via a straightforward solvothermal method followed by a low-temperature heat treatment.The hierarchical FeS microspheres?1.3?m?were assembled by uniform nanoparticles?⁴4 nm?with graphitic carbon layers.Such a pomegranate-like skeleton design endows the composite with multiple advantages.Firstly,the in-situ grown graphitic carbon offers an effective buffer layer to relax the large volume changes upon cycling,thus maintaining the structural integrity.Secondly,the intimate electrical contact between the graphitic carbon and FeS nanoparticles not only enhance the electrical conductivity of the material but also accelerate the kinetics of redox reactions.Moreover,the hierarchical structure can shorten the diffusion paths for ions/electrons and offer a high tap density(1.55 g cm^-3).As a result,these synergetic merits culminate in superior electrochemical performance at both low temperature and room temperature.The FeS@g-C/Na battery delivers a high capacity of 742.9 mAh g^-1 at 0.05 A g^-1 after 180 cycles.Particularly,when operate at-25°C,the Na₃V₂?PO₄?₂O₂F//FeS@g-C full batteries can achieve capacities of221.9 mAh g^-1 and 147.6 mAh g^-1 at 50 mA g^-1 and 100 mA g^-1,respectively.Thus,the proposed FeS@g-C composite has great potential for applications in rechargeable batteries at both low-temperature and room-temperature environment.?5?Iron selenides exhibit higher conductivity than that of iron sulfides,which are better choice for anode materials.The obtained Fe₇Se₈ nanoparticles are confined in carbon layers,leading to a coral-like morphology.The hierarchical channels can realize rapid transfer of electrolyte ions,meanwhile,the grown conductive carbon effectively improves the electric conductivity of the electrode and buffers the structural stress.More importantly,Na₃V₂?PO₄?₂O₂F//cl-Fe₇Se₈@C maintains a high capacity of 232.1 mAh g^-1 at 1 A g^-1 after 200cycles.Even at-25°C,the capacity of the cl-Fe₇Se₈@C//NVPOF full-cell still remains at 165.6mAh g^-1 at 0.05 A g^-1 after 440 cycles.