Preparation And Potassium Storage Performance And Mechanism Of Carbon-based Electrode Materials For Potassium Ion Batteries

Similar to lithium metal,potassium and graphite can form KCs intercalation compound.Hence,the graphite can be used as an anode material for potassium ion batteries,with high theoretical capacity(279 mAh g-1)and stable potassium intercalation potential and platform.However,due to the 60%volume expansion during potassium insertion process,the cycle stability and rate performance of graphite electrode are poor.In order to overcome these problems,we adjust and control the structure of the carbon-based electrode materials for potassium ion batteries,so that the comprehensive electrochemical performance of the electrode materials can be significantly improved.The main contents are:The carbon nanofibers with exposure of abundant edge-plane active sites by adjusting the(002)orientation are developed by a molten-salt assisted route.Importantly,due to the radial(002)orientation with more active edge-plane sites to adsorpt K and shorten the K diffusion distance,the obtained carbon nanofibers harvest improved K adsorption/diffusion kinetics.Meanwhile,theoretical calculation indicates that the synchronically introduced N-doped defects could also lower the diffusion barrier and enhance the K adsorption kinetics.Ex-situ characterizations and electrochemical studies prove the improved kinetics that significantly enhance the K storage performance of the obtained carbon nanofibers.Hence,a high cycling capacity of 252.8 mAh g-1 at 100 mA g-1 after 500 cycles and rate capacity of 181.5 mAh g-1 at 1 A g-1 after 1200 cycles have been achieved.Remarkably,the as-developed PIHCs deliver an energy density of 170 Wh kg-1 in a cell voltage from 1 to 4 V,along with a capacity retention of 81.6%at 2 A g-1 after 10000 cycles.An amidation-dominated re-assembly strategy is developed to prepare uniform single atom Ni/S/C nanotubes.In this re-assembly process,a single-atom design and nano-structured engineering are realized simultaneously.Both the NiO5 single-atom active centers and nanotube framework endow the Ni/S/C ternary composite with accelerated reaction kinetics for potassium-ion storage.Theoretical calculations and electrochemical studies prove that the atomically dispersed Ni could enhance the convention kinetics and decrease the decomposition energy barrier of the chemically-absorbed small-molecule sulfur in Ni/S/C nanotubes,thus lowering the electrode reaction overpotential and resistance remarkably.The mechanically stable nanotube framework could well accommodate the volume variation during potassiation/depotassiation process.As a result,a high K-storage capacity of 608 mAh g-1 at 100 mA g-1 and stable cycling capacity of 330.6 mAh g-1 at 1000 mA g-1 after 500 cycles are achieved.Sb nanoparticles with size of 55 nm are fabricated via reduction of SbCl3 by metallic Al in the molten salt of SbClb at 80℃.In-situ XRD pattern and ex-situ Raman spectrum investigate that the potassium storage mechanism is alloying-type with formation of a cubic K3Sb phase in fully potassiation and an amorphous phase in fully depotassiation.As anode for potassium-ion batteries,the Sb nanoparticles mixed with 42.wt%of graphene could deliver a reversible capacity of 381 mAh g-1 at 100 mA g-1,and maintain a capacity of 210 mAh g-1 at 500 mA g-1 for 200 cycles.High-loading Sb single atom(23.3 wt%)anchored on few-layer graphene sheets(SbSA/C)was developed via a coordinative and spatial co-confinement methodology.The single-atom engineering is realized by quenching/freeze-drying the uniformly mixed Sb-thioglycollate chelates and suspended graphene oxides,followed by heating treatment.The GO substrate features open 2D framework and functionalized groups with high specific surface area,facilitating the chemical absorption and spatial dispersion of mononuclear Sb chelates.The heteroatoms(S,O)in Sb chelates can provide desirable micro-environment for coordinating and separating Sb ion species,suppressing the migration and agglomeration during the whole synthesis process.Hence,XAFS and HAADF-STEM reveals the Sb single atoms is obtained by coordinating with S,O and anchored on the carbon matrix.As host for K storage,in situ TEM reveals the uniform potassiation behavior without obvious volume change,ex-situ HAADF-STEM and XAFS suggest the reversible coordinately confined single-atom Sb during cycling.As a result,the SbSA/C electrode reaches a high reversible K-storage capacity of 450.3 mAh g-1 at 100 mA g-1 after 210 cycles and stable capacity of 331.3 mA h g-1 at 1000 mA g-1 after 1100 cycles.