| Potassium ion batteries and potassium ion hybrid capacitors are promising devices in the field of energy storage.However,due to the limitation of the large ionic radius of potassium ions,traditional conversion and alloy anode materials often suffer from large volume strain,resulting in poor cycling stability.As the most common intercalation-type carbonaceous material,graphite can react with potassium ions to form a stable intercalation compound KC8,which provides a high theoretical capacity of 279 mA h g-1 and a low potassium storage potential.However,due to the small interlayer spacing of graphite,it also produces a volume expansion of up to 60%during potassium storage,resulting in poor cycling stability and rate performance.Reducing the graphitization degree of carbonaceous materials,pore-forming of carbon matrix and designing micro-nano structure are effective methods to improve the electrochemical performance of potassium storage of carbonaceous materials.Based on this situation,this paper takes pressure as the means to optimize the microstructure of carbonaceous materials for the purpose of controlling the orientation of crystal surface,pore structure and surface chemical configuration,so as to obtain potassium-storage carbonaceous anode materials with high capacity,long cycling life and excellent rate performance.The details are as follows:(1)Pressure-assisted preparation of carbonaceous materials with(001)orientation to improve potassium storage kinetics.Nano-spring-like mesoporous graphitic carbon(OGCS)with high(001)orientation was fabricated by hydrothermal treatment of epoxy resin at 500 ℃ and followed by annealing at 1400 ℃,and the graphitic layer is arranged perpendicular to its axial direction.The results show that the high-temperature hydrothermal treatment can not only induce the(001)orientation of the carbonaceous material,thus exposing a large number of edge active sites,but also promote its transformation into a nano-spring structure with rich mesopores in the subsequent high-temperature calcination,which contributes to promote the rapid migration of potassium ions.A series of in-situ and ex-situ tests revealed that the OGCS electrode would first adsorb potassium ions in the mesopores,and then inserts potassium ions at low potentials to form KC8.In addition,the nano-spring structure can stretch along the one-dimensional direction,which can alleviate the huge volume strain during potassium storage,thus exhibiting excellent structural stability.As a result,the synthesized OGCS electrode shows a much better K-storage performance than that of unoriented graphitic carbon,such as a stable capacity of 361.7 mA h g-1 at 50 mA g-1 over 100 cycles,and a capacity of 170 mA h g-1 even at high current density of 2000 mA g-1 with long-term cycling stability over 10000 cycles.(2)Pressure-assisted preparation of carbonaceous materials with different pore structures to improve potassium storage capacity and explore the mechanism of mesoporous potassium storage.A series of porous carbonaceous materials(MPC-Ts)with different pore structures(including specific surface area,pore size and pore volume)were fabricated in a closed autoclave by adjusting the pressure medium and calcination temperature during pyrolysis of 1-dodecanol.Combined with various electrochemical characterizations and density functional theory(DFT)calculations,it is demonstrated that the intercalation capacity is positively correlated with IG/ID ratio rather than interlayer distance,and adsorption capacity is positively correlated with mesopore volume rather than surface area or pore size.Besides,mesopores can not only serve as active sites for potassium ion storage,but also promote ions migration and accommodate the huge volume expansion during(de)potassiation process.As a result,the optimized sample(AHC-700)with the largest mesopore volume and appropriate surface area exhibits a high capacity of 633.2 mA h g-1 at 50 mA g-1,and maintains a reversible capacity of 100.3 mA h g-1 at 5000 mA g-1 after 10000 cycles.(3)Pressure regulates the pore structure and surface chemical configuration of carbonaceous materials to improve the energy density and cycling life of potassium ion hybrid capacitors.Pressure-dependent porous carbonaceous materials(MPCs)were prepared by controlling the pressure environment during pyrolysis with magnesium citrate as carbon precursor.The results demonstrate that the negative pyrolysis pressure(ΔP=-0.1 MPa)can not only limit the growth of the template in the carbon matrix to form a rich microporous structure with a specific surface area of 2383.6 m2 g-1,but also promote the removal of oxygen-containing functional groups on the surface of the carbon material to obtain a low surface oxygen content.In this way,the obtained carbon material(NPC)has high adsorption capacity and exhibits good compatibility with the electrolyte at high voltage as a capacitive cathode.Relatively,a positive pyrolysis pressure(ΔP=10.0 MPa)can inhibit the escape of pyrolysis gas and promote its continuous reaction with the self-generated template,thus forming a hierarchical macroporous carbon materials(PPC)composed of interconnected carbon nanosheets and nanoparticles with excellent ion transport capacity.As a result,the NPC cathode exhibits a high capacity of 63.7 mA h g-1 after 8000 cycles at 2.0 A g-1,while PPC anodes deliver high capacity of 258.8 mA h g-1 with capacity retention of 93.5%after 10000 cycles at 5.0 A g-1.The assembled PPC//NPC potassium ion hybrid capacitors also exhibit high energy density of 172.8 Wh kg-1 at 223.1 W kg-1,with long-term cycling stability over 10000 cycles at 2.0 A g-1.(4)Pressure-assisted pyrolysis of biomass to prepare high-rate battery-type carbonaceous anode materials and combined with chemical pre-potassiation strategy to promote the energy density and cycling life of potassium ion hybrid capacitors.Glucose was converted into micron-sized hard carbon materials(GDC)assembled by interconnected carbon nanoparticles by pressure-assisted pyrolysis,and then chemically pre-potassiated with commercial activated carbon(CAC)cathode in potassium-naphthalene-tetrahydrofuran solution.Combined with various in-situ and ex-situ characterizations,a radical reaction between pre-potassiation regent and carbon electrodes is confirmed,which not only deactivates electrochemical irreversible defects and heteroatomic oxygen,but also promotes to pre-form a uniform and dense KF-rich electrolyte film on the electrodes.As a result,the pre-potassiation treatment presents multiple advantages:(Ⅰ)at 0.2 A g-1,the initial Coulombic efficiency of the GDC anode increases from 45.4%to 84.0%and the reversible capacity increases from 329.8 mA h g-1 to 444.8 mA h g-1 with higher cycle Coulombic efficiency(CE)and rate capability;(Ⅱ)the CAC cathode exhibits the improved cycling CE and stability due to the enhanced resistance to electrolyte oxidation at 4.2 V(even up to 4.5 V);(Ⅲ)the assembled potassium ion hybrid capacitor after pre-potassiation achieves a high energy density 172.5 Wh kg-1 at 593.8 W kg-1 with a long-term cycling life over 10000 cycles at 1.0 A g-1. |
PDF Full Text Request