Structural Modulation Of Carbon-based Electrode Materials For Potassium/Zinc Ion Hybrid Capacitors And Their Electrochemical Performance

Ionic hybrid capacitors（IHCs）combine the advantages of the high energy density of secondary batteries with the high power density of supercapacitors,making them a valuable energy storage device for a wide range of applications.Lithium ion hybrid capacitors（LIHCs）have been well developed since their introduction in 2001 and are now commercially available.However,LIHCs currently face two problems:（1）the scarcity of lithium resources and uneven geographical distribution make LIHCs face high costs;（2）the electrolyte for LIHCs uses lithium salts dissolved in organic solvents,which are not only more costly but more importantly,highly flammable during use.Potassium resources are abundant in nature and the redox potential of K⁺/K（-2.93 V）is close to the redox potential of lithium.Aqueous zinc ion hybrid capacitors（AZIHCs）use zinc salts dissolved in aqueous solvents as the electrolyte to avoid combustion of the devices during use.Potassium ion hybrid capacitors（PIHCs）and AZIHCs are considered promising alternatives to LIHCs for large scale energy storage devices due to factors such as resource reserves,capacity and safety of energy storage devices.In this paper,carbon based electrode materials were synthesised and used as capacitive electrodes to assemble half-cells and to investigate their electrochemical properties.It is shown that the electrochemical properties are good.The carbon-based electrode materials were used as capacitive electrodes and assembled with capacitive electrodes（N-HPC-A）to form IHCs devices.The results show that the devices have high energy/power densities and capacity retention over very long cycles,with the following main conclusions:（1）Nitrogen-doped graded porous carbon（N-HPC）was prepared by chemical blowing.n-HPC has a honeycomb-like three-dimensional graded porous structure and large carbon layer spacing,which provides sufficient space for volume expansion caused by the embedding of K⁺.Furthermore,the high nitrogen atom doping provides more active sites for potassium storage in carbon materials.As a result,the porous carbon anode（N-HPC-1.5）has a reversible specific capacity of 105.0 m Ah g^-1 at 1.0 A g^-1 and a good cycling stability with 800 cycles.The constructed N-HPC-1.5//N-HPC-A PIHCs achieved a high energy density of 103.5 Wh kg^-1 at a power density of 1000.0 W kg^-1 and a capacity retention of 70.2%for 10,000 cycles(current density of 1.0 A g^-1).（2）Carbon composite calcium manganate（CaMn₂O₄/C）was synthesised by the solid phase method using PVP as the carbon source and Ca（NO₃）₂·4H₂O and Mn（NO₃）₂·4H₂O as the calcium and manganese sources,respectively.The carbon composite structure in CaMn₂O₄/C was able to increase the electrical conductivity of the material.The introduction of calcium ions into the manganese based material can reduce the structural collapse of the manganese based oxide and promote electrochemical stability.As a result,the CaMn₂O₄/C cathode exhibited an ultra-long cycle life of 5000 cycles at a current density of 2.0 A g^-1 and a reversible specific capacity of 195.6 m Ah g^-1.Using CaMn₂O₄/C as the cathode and N-HPC-A as the cathode,the AZIHC was assembled at a current density of 1.0 A g^-1,with a capacity retention rate of 84.9%after 8000 cycles.（2）Carbon composite calcium manganate（CaMn₂O₄/C）was synthesized by solid phase method using PVP as the carbon source and Ca（NO₃）₂·4H₂O and Mn（NO₃）₂·4H₂O as the calcium and manganese sources,respectively.The carbon composite structure in CaMn₂O₄/C can improve the electrical conductivity of the material.The introduction of calcium ions can effectively reduce the structural collapse of manganese-based materials and promote the stability of their electrochemical properties.As a result,the CaMn₂O₄/C cathode exhibited an ultra-long cycle life of 5000 cycles at a current density of 2.0 A g^-1 and a reversible specific capacity of 195.6 m Ah g^-1.Using N-HPC-A as the negative electrode and CaMn₂O₄/C as the positive electrode,the assembly of N-HPC-A//The capacity retention of CaMn₂O₄/C AZIHCs was 84.9%（8000 cycles）at 1.0 A g^-1.（3）Zinc vanadate/nitrogen doped porous carbon（N-HPC/ZVO）composites were synthesised by annealing process using NH₄VO₃ as the vanadium source,PVP as the carbon source and Zn（NO₃）₂·6H₂O as the zinc source.The porous carbon structure of N-HPC/ZVO not only facilitates the diffusion of ions but also significantly increases its electrical conductivity.At the same time,the in situ nitrogen doping facilitates the increase of active sites.As a result,the N-HPC/ZVO-0.6 cathode exhibits an ultra-long lifetime of nearly 6000 cycles at a high current density of 5.0 A g^-1 and a reversible specific capacity of 221.4 m Ah g^-1,corresponding to an ultra-low decay rate of only 0.0029%per cycle.n-HPC-A//N-HPC/ZVO-0.6 AZIHCs have excellent cycling stability with a capacity retention of 72.8%after 10,000 cycles at 1.0 A g^-1.