Cobalt-based MOFs And Their Derivatives Are Used As Anode Materials For Potassium-ion Batteries

In today's society,lithium-ion batteries are one of the most widely studied and applied energy storage devices to solve energy problems.However,the relative scarcity and uneven distribution of lithium element resources have brought great obstacles to the large-scale use of lithium-ion batteries.Therefore,people began to focus on potassium,which has abundant reserves and similar chemical properties.The development of electrode materials with long cycle life and high rate capacity is particularly important for the application of potassium ion batteries.MOFs materials have a wide and highly uniform porous structure,simple preparation method,large specific surface area,and have great potential when used as electrode materials for potassium ion batteries.In addition,MOFs material contains abundant metal elements,carbon elements and other non-metal elements.At a suitable carbonization temperature,it can not only maintain the original solid structure,but also obtain in-situ doped,highly layered and porous carbon network,after further processing,a variety of nano materials can be obtained,such as metal oxide/carbon composite,metal sulfide/carbon composite,and carbon materials.This method can get rid of the inherent poor conductivity,poor thermal stability and other issues of MOFs materials.In this paper,based on two typical cobalt-based MOFs materials,reasonable electrode material structure design methods are used to obtain potassium ion batteries with excellent performance.The main research content of this article is divided into the following three parts:First,we synthesize Co₃[Co(CN)₆]₂ nanocubes by a simple co-precipitation method,and then uniformly mix them with graphene oxide,and finally reduce them in an H₂/Ar atmosphere and pass the resulting C?O?Co bonds.The bonds tightly couple the Co₃[Co(CN)₆]₂ nanocube and r GO,and finally achieves high-rate,long-cycle potassium storage.These chemical bonds shorten the distance between Co₃[Co(CN)₆]₂and r GO to the angstrom level,thereby significantly increasing the electronic conductivity of Co₃[Co(CN)₆]₂.In addition,r GO completely encapsulates the Co₃[Co(CN)₆]₂ nanocubes,making the structure of Co₃[Co(CN)₆]₂ highly stable,so it can withstand repeated insertion/extraction of potassium ions without noticeable morphological and structural changes.Secondly,the synthesis of ultrafine Co S₂ nanoparticles embedded in nitrogen-doped carbon nanocubes(Co S₂/CNCs)and their application in the anode materials of PIBs were demonstrated.The CNCs play the key role of conductive carbon matrix,which are transformed from the thermal decomposition of ZIF-67 nanocubes under a H₂/Ar atmosphere.Meanwhile,the cobalt ions are simultaneously reduced to tiny metallic cobalt nanoparticles and dispersed uniformly in CNCs due to a gentle pyrolysis process.After a subsequent sulfidation reaction with the aid of H₂S/Ar,the Co S₂/CNCs are formed and utilized as an anode material for PIBs.The resulting porous carbon matrix with a regular architecture provides a conductive path for electrons and enables ions to diffuse rapidly and easily enter the electrolyte through sufficient pores.In addition,the volume expansion of Co S₂ can be effectively accommodated during the electrochemical reaction.At the same time,the ultrasmall Co S₂ nanoparticles increase the activity of the reaction and provide more reaction sites.Benefiting from the above merits,the as-obtained Co S₂/CNCs nanocomposites exhibit greatly improved potassium storage properties.Finally,we used tannic acid(TA)to etch ZIF-67,and the obtained uniform hollow cubes were annealed in a H₂/Ar atmosphere and vulcanized in a H₂S/Ar atmosphere.Finally,the ultrafine Co S₂ particles were embedded in the wall of the hollow carbon nanocubes(u-Co S₂/WHCNC).This unique structure can not only alleviate the pulverization of the active material caused by the volume change,but also increase the contact range of the active material and the electrolyte.Using u-Co S₂/WHCNC for the negative electrode of potassium ion batteries,it was found that it achieved a high capacity of 440.1 m Ah g^-1 after 100 cycles at a current density of 0.1 A g^-1.