Preparation Of Ion Pre-intercalation MnO₂ Electrode Material For Magnesium Ion Supercapacitor

In recent years,flexible wearable electronic devices have gradually entered into daily life,which have been widely used in body movement detection,disease diagnosis,soft robots,and artificial intelligence devices.The demand for flexible energy storage devices is increasing rapidly.Among different kinds of flexible energy storage devices,flexible hydrogel supercapacitors have become a research hotspot due to their good mechanical properties,electrical conductivity and stability.However,flexible hydrogel supercapacitors still have many problems to be solved,which are mainly focused on electrode materials and hydrogel electrolyte:1)There is a large interface problem between traditional electrode materials and hydrogel electrolyte,resulting in low capacity and poor cycle stability;2)The type and concentration of ions in the hydrogel electrolyte not only determine ionic conductivity,but also affect its skeleton structure,and too high ion concentration will even affect the gelation process.Magnesium ion supercapacitors have attracted extensive attention due to their high working voltage and energy density.However,because of the limitation of strong electrostatic interaction between Mg²⁺and traditional electrode materials,it is difficult to improve the specific capacitance and cycle stability of magnesium ion supercapacitors.In this work,based on the traditional electrode material MnO₂,we modified the electrode material by the method of ion pre-intercalation,and systematically analyzed the effect of Co²⁺and K⁺ion pre-intercalation on the structure and properties of MnO₂.Meanwhile,the optimum ratios of Co²⁺and K⁺to MnO₂ were determined.The two electrode materials were combined with SA-AM,a hydrogel electrolyte with double network stability structure to construct a new flexible magnesium ion supercapacitor.The main research contents are as follows:Firstly,Co-MnO₂ electrodes with different Co²⁺pre-intercalation contents were prepared by controlling the proportion of Co²⁺in the synthesis process through simple hydrothermal post-calcination method.Trough the electrochemical performance test of the three-electrode and two-electrode systems,the influence of different Co²⁺pre-intercalation contents on the physical and chemical properties of MnO₂ was explored,and the Co-MnO₂ electrode with the Co²⁺:MnO₂ ratio of 2:10 has the best performance when applied in magnesium ion supercapacitor.The discharge capacity increases to510.25 F g^-1(0.5 A g^-1).Co-MnO₂ electrode was assembled with AC to construct Co-MnO₂//MgSO₄//AC supercapacitor,which exhibits an excellent performance with an energy density of 79.59 Wh kg^-1 at a power density of 332.82 W kg^-1,and the capacity retention rate still reaches 99.65%after 15,000 charge and discharge cycles(5A g^-1).Secondly,in order to compare the influence of pre-intercalation of different ions on the structure and electrochemical performance of MnO₂,K⁺with a smaller hydration ion radius was intercalated in MnO₂.Due to the excellent ionic conductivity of K⁺,the conductivity of MnO₂ can be significantly improved,which is conducive to the charge exchange between MnO₂ and electrolyte.Meanwhile,it is found that MnO₂ pre-intercalated with K⁺has a larger lattice spacing,which is conducive to the diffusion of Mg²⁺ions in MnO₂,and further improves its pseudocapacitance.When the ratio of K⁺to MnO₂ is 2:10,the maximum discharge capacity is 329.1 F g^-1(0.5 A g^-1).An asymmetric magnesium ion aqueous supercapacitor was assembled with K-MnO₂ as the positive electrode and AC as the negative electrode,achieving an energy density of79.6 Wh kg^-1 at a power density of 336.24 W kg^-1.Compared with Co-MnO₂,K-MnO₂ based supercapacitor displays a more stable cyclic performance and retains96.7%capacitance after 20000 cycles at 5 A g^-1.Thirdly,the flexible magnesium ion supercapacitor was assembled by SA-AM hydrogel as electrolyte,K-MnO₂ or Co-MnO₂as cathode,and AC as anode.The mechanical and electrochemical properties of the hydrogel were changed by adjusting the MgSO₄ concentration of in the hydrogel.The mechanical strength of SA-AM hydrogel increases with the increase of the MgSO₄ concentration.In the hydrogel with0.2 mol L^-1 MgSO₄,Co-MnO₂ achieves the best electrochemical performance with a discharge capacitance of 357 m F cm^-2 at a current density of 0.5 m A cm^-2.The energy density is 27.48 Wh cm^-2at a power density of 301.59 W cm^-2.Compared with Co-MnO₂,K-MnO₂ based SA-AM hydrogel supercapacitor manifests a better performance.In the hydrogel with 0.8 mol L^-1 MgSO₄,K-MnO₂ achieves the best electrochemical performance with a discharge capacitance of 406 m F cm^-2 at a current density of 0.5 m A cm^-2.The energy density reaches up to 123.86 Wh cm^-2at a power density of 370.48 W cm^-2.The electrochemical performance of flexible K-MnO₂ based hydrogel supercapacitor was tested.The capacitance loss is only 5%at the maximum bending angle（180°）,and the capacitance retention is 92%after bending at 90°for 20times,highlighting the flexible energy storage ability of K-MnO₂ based magnesium ion hydrogel supercapacitor.