MOFs-derived Cobalt-based, Iron-based Composites As Anode Materials For Potassium-ion Batteries

Lithium-ion batteries play an important role in large-scale energy storage systems.However,with rising costs and increasing demand,lithium-ion batteries have been unable to meet large-scale energy storage applications.Therefore,people focus on the potassium ion battery which has similar electrochemical performance and abundant resources as the lithium ion battery.Potassium ion batteries have become as one of the most promising alternatives to lithium-ion batteries.Potassium ion batteries have become one of the most potential alternatives to lithium ion batteries.However,due to the lack of suitable anode materials,the practical application of potassium ion batteries still have a long way to go.At present,the anode materials for potassium ion batteries mainly include the following:carbon-based materials,alloys,conversion materials,etc.Among them,cobalt-based materials and iron-based materials in transition metal materials are considered to be the negative electrode materials very suitable for potassium ion batteries because of their high theoretical capacity,low cost,abundant reserves,non-toxicity and rich redox reactions.However,with the process of insertion/extraction of potassium ions in these materials,due to the strong reorganization of the structure,these materials will expand and lose electrical contact,resulting in a sharp decline in capacity and reducing the potassium storage performance.Studies have shown that designing to reduce particle size,designing different forms of nanostructures and introducing carbon coating can effectively solve the above problems and enhance the electrochemical performance of materials.The nanostructured metal organic frameworks(MOFs)have adjustable morphology and composition,so they can be used as precursors to prepare porous carbon-based materials with hollow structures.In this paper,MOFs are used as precursors,and PAN is added as a carbon source.The cobalt-based and iron-based composite materials are obtained by heat treatment as the anode material for the potassium ion battery,and exhibit surprising potassium storage performance.The specific research conclusions are as follows:(1)First we prepare and synthesize ZIF-67 nanoparticles,then electrospinning,a mixture of ZIF-67 nanoparticles,polyacrylonitrile(PAN)and DMF to form a composite of PAN nanofibers containing ZIF-67,and then calcinate ZIF-67@PAN to get u-Co@HCFs.Afterwards,heat-treat u-Co@HCFs can obtain u-Co Mx@HCFs.In this process,PAN and ZIF-67 nanoparticles are converted into N-doped carbon nanofibers and Co nanoparticles,respectively.Through carbonization and subsequent heat treatment,theCo²⁺-2-methylimidazole-polyacrylonitrile(ZIF-67-PAN)inorganic/organic precursor can be successfully transformed into uniformly distributed Co Mx nanoparticles impregnated in hierarchical mesoporous carbon nanofibers.This space-limited reaction leads to the formation of ultra-small nanoparticles and more electrochemically active sites,which improves the storage performance of potassium.Due to its strong coupling effect and good structure,the prepared hierarchical porous composite can exhibit excellent performance.In addition,the high active surface area and unique porous structure can promote electrolyte penetration and can also alleviate the volume expansion during the cycle,thus performing excellent electrochemical performance.Facts have proved that when it is used as a potassium ion storage negative electrode,the synthesized Co S₂ ultrafine nanoparticles encapsulated in hierarchical porous carbon nanofibers(denoted as Co S₂@HCFs)exhibits excellent electrochemical performance.When the current density is 100 m A g^-1,reversible capacity is 428 m A h g^-1.When the current density increased to 5 A g^-1,reversible capacity is 148 m A h g^-1.It also has long-term cycling life of 268 m A h g^-1 at 0.5 A g^-1 after 1000 cycles.(2)First we prepare and synthesize ZIF-67 nanoparticles,then electrospinning,a mixture of ZIF-67 nanoparticles,polyacrylonitrile(PAN)and DMF to form a composite of PAN nanofibers containing ZIF-67,and then calcinate ZIF-67@PAN to get u-Co@HCFs.Afterwards,heat-treat u-Co@HCFs can be transferred obtain u-Co Mx@HCFs.Further we use ammonium dihydrogen phosphate and antimony powder as the source of phosphorus and antimony respectively,high temperature heat treatment can obtain Co M@CNFs(M=P,Sb)nanocomposites.It also shows excellent electrochemical performance in potassium ion batteries.At a high current density of100 m A g^-1,the capacity of the Co P@CNFs electrode can be maintained at 300 m A h g^-1 after 120 cycles.The material shows excellent performance because of he high initial theoretical capacity of Co P@CNFs,nanosized Co P@CNFs active particles,and hierarchical porous carbon nanofiber structure.These factors can promote electrolyte penetration and relieve the volume expansion during the cycle.(3)Firstly,we prepare and synthesize MIL-88B nanorods by hydrothermal method,then uniformly mix MIL-88B nanorods with PAN,afterwards heat-treat the mixture to prepare and synthesize small-particle Fe O@C nanocomposites for high-performance potassium ions.This synthesis strategy relies on the restrictive effect of the carbon material obtained by the heat treatment,which can ensure to a certain extent the iron-based MIL-88B nanorods in situ restricted pyrolysis to obtain nanocrystalline composites containing small-sized Fe O.In addition,the nitrogen-doped carbon matrix formed after carbonization of PAN can effectively prevent the formation of Fe O nanocrystals from gathering.Such a structure can alleviate the volume expansion of the electrode material during the cycle,and effectively prevent the aggregation of Fe O nanoparticles.When the prepared composite is used as anode material,it can show excellent performance.At the current of 100 m A g^-1,the capacity of the Fe O@C shows the capacity of 300 m A h g^-1 after 120 cycles.