Designing Novel Carbon-Based Heterostructures Toward Advanced Potassium-Ion Storage

Potassium ion batteries have been widely studied in recent years due to the typical rocking-chair working mechanism,low production costs(including raw materials for electrode manufacturing,current collector and electrolyte)and high ionic mobility in organic electrolyte.Unfortunately,the diffusion process of large-size K+in electrode materials becomes difficult,and the volume change during the charge/discharge process is also large,which ultimately lead to the significant decline of the electrochemical performance.The rational selection and controlled preparation of anode material is the key to solve the above problems.Carbon-based material is considered as one of the most promising anode materials because of its merits including low cost,high safety,high conductivity,abundant defects and surface groups.Although great progress has been made in recent years,the electrochemical properties of carbon materials still need to be improved,and the actual role of carbon materials as well as the storage mechanism of potassium ions are not deeply explored.On the one hand,when a single type of carbon is used for potassium-ion storage,the volume of carbon material changes greatly during the potassiation/depotassiation process(the volume expansion of commercial graphite materials is 60%).On the other hand,when carbon-based material is used as the supporting skeleton of the active materials,the composite strategy with high-capacity active materials is limited.Therefore,it cannot solve the problems of volume expansion and sluggish reaction kinetics.In view of the above challenges,we successfully develop a series of novel carbon-based heterostructure and explore the mechanism of potassium ion storage:realizing the preparation of dual carbon heterostructure materials to build buffer spaces and introduce doping sites to alleviate volume changes;The electrochemical performance of the electrode material was significantly improved by developing effective composite strategies between carbon skeleton and high capacity active materials.The reaction kinetics of the carbon-based composite materials was promoted by constructing abundant interfacial active sites.The specific research contents are as follows:(1)Nitrogen-doped carbon core-shell heterostructure was prepared by salt sealing strategy to realize high reversible specific capacity and rate performance.By varying the pyrolysis temperature,N-doped dual-carbon composites(NC-700,NC-800 and NC-900)bearing different content ratios of edge-to graphitic-nitrogen can be constructed.A harmonized edge-to graphitic-nitrogen ratio in such dual-carbon materials enables outstanding rate capability(130 mAh g-1 at 10.0 A g-1)and cyclic performance in half-cell tests.As expected,thus-assembled potassium-ion hybrid capacitor full device with a working voltage of 4.2 V presents a high energy/power density(146 Wh kg-1/8000 W kg-1)and favorable stability.(2)Graphdiyne/graphene dual carbon heterostructure was prepared by a van der Waals(vdW)epitaxy strategy to realize long cycling stability.As a new type of carbon allotrope,GDY has large triangular pores and abundant alkyne bonds,which make it rich in active sites and rapid diffusion channels.In this case,GDY is considered as an ideal carbonbased anode material for storing K+.Nevertheless,the low surface area and disordered structure of bulk GDY typically lead to unsatisfied K+storage performance.Therefore,GDY film was readily grown on both sides of Gr owing to the vdW interaction and lattice match between GDY and Gr,resulting in GDY/Gr/GDY sandwich architecture.As tested in a halfcell configuration,the GDY/Gr/GDY electrode exhibits better capacity output,rate capability,and cyclic stability as compared to the bare GDY counterpart.A full-cell device comprising a GDY/Gr/GDY anode and a potassium Prussian blue cathode enables a high cycling stability,demonstrative of the promising potential of the GDY/Gr/GDY anode for potassium-ion batteries.(3)Selenide molybdenum/graphene heterostructure was prepared by a diatomitetemplated synthetic strategy to realize high reversible specific capacity.The selection of naturally abundant diatomite as the growth substrate is inspired by our foregoing investigations.The hierarchically versatile morphology of such biomass template would endow conformally grown 3D N-MoSe2/G with open frameworks and ample porosities,which is key to ultrafast electron/K+ transport and favorable structural stability.Benefiting from the unique biomorphic structure,high electron/K-ion conductivity,enriched active sites,and the conspicuous pseudocapacitive effect of N-MoSe2/G,thus-derived potassium-ion hybrid capacitors manifest high energy/power densities(maximum 119 Wh kg-1/7212 W kg-2),outperforming those of recently reported KIC counterparts.Furthermore,the potassium storage mechanism of N-MoSe2/G composite is systematically explored with the aid of first-principles calculations in combination of situ X-ray diffraction.(4)Nitrogen doped carbon/cobalt phosphide heterostructure was prepared by gradient annealing to realize high specific capacity and long cycling stability.MOF is a kind of three-dimensional porous materials constructed by metal ion centers and organic connectors,in which ZIF-8 and ZIF-67 have similar structural units.Therefore,this work obtains NC@CoP/NC core-shell structure through reasonable design.Among them,phosphating material has significantly higher theoretical specific capacity than carbon material,but the volume expansion problem is more serious.Therefore,the in-situ carbon coating obtained can greatly improve the volume change problem and improve the overall conductivity.Such composites enable an outstanding rate performance to harvest a capacity of 200 mAh g-1 at 2000 mA g-1.Furthermore,the K+storage mechanism of the NC@CoP/NC anode is systematically probed through theoretical simulations and experimental characterization.