The Potassium-Ion Storage Performance And Mechanism Of Zn/Fe-Based Prussian Blue Analogue Cathode Materials In Aqueous Batteries

This paper focuses on the Zn/Fe-based Prussian blue analogue（PBA）electrode material,based on the correlations between the material structure/chemical reaction environment and electrochemical performance.The controllable preparation and related structure/composition characterization were conducted.In addition,comprehensive tests and systematical analysis of the effects of different components and electrolyte concentration/component ratio on their electrochemical performances were carried out.At the same time,we revealed the energy storage mechanism and structural degradation mechanism of the electrode materials using advanced in-situ X-ray diffraction technology（XRD）and in-situ infrared in-depth exploration（ATR-FTIR）.Finally,high-rate and long-cycle-life K-Zn hybrid-ion aqueous batteries were established based on the advantages of dual-ion storage chemistries.This paper could provide some scientific bases for the researches and applications of aqueous electrochemical energy storage devices with high energy/power density and long cycle life.The specific research results are as follows:（1）Zinc hexacyanoferrate（Zn HCF）cathode materials were synthisized based on a simple co-precipitation method.The morphology,structure and material composition of the cathode material were characterized in detail.The best electrolyte was chosen through systematic research and analysis for electrochemical K⁺storage of the electrode materials,which ensured the excellent high-rate performance of the half-cells.Moreover,comparative analysis of the structural evolution of Zn HCF cathode materials through advanced in-situ XRD characterization,combined with electrochemical behavior analysis in different electrolytes,revealed that Zn HCF cathode materials preferentially select K⁺insertion/extraction in terms of kinetics and thermodynamics,thereby resulting in excellent rate performance of the electrode in K-Zn hybrid-ion electrolyte（0.5 M Zn SO₄+0.25 M K₂SO₄）.It was also found that high concentration of Zn²⁺can improve the cycling stability of Zn HCF electrode materials.Specifically,the Zn HCF cathode material exhibits a long discharge plateau at 0.928 V（vs.SCE）at a rate of 2 C(=172 m A g^-1),with a discharge capacity of 78.7m Ah g^-1;ranging from 2 C to 300 C,the rate capacity retention is up to 60%.（2）To sovle the unstable issue of Zn/Fe-based PBAs in aquous solutions,the synthesis of bimetallic Prussian blue derivative,i.e.Ni_xZn_yHCF（x+y=3,x=1,1.5or 2）was proposed.Subsequently,the morphology,structure and material composition of the Ni_xZn_yHCF electrode materials were characterized in detail.Through systematical performance testing and comparison,it was found that Ni₂Zn₁HCF electrode materials have the most excellent cycling stability.Through structural characterization and electrochemical analysis,it was believed that Ni₂Zn₁HCF electrode materials involve a synergistic effect:the high content of Ni ensures the structural stability,and Zn improves the overall discharge potential and capacity of the electrode material.Combining electrochemical methodes with advanced in-situ XRD analysis,it was supposed that Ni₂Zn₁HCF electrode materials have an ultra-fast near-pseudocapacitive ion insertion/extraction energy storage mechanism via solid-solution phase,and thus could exhibit excellent cyclability and rate performance.When cycling at a high rate of 500 C,the electrode materials show a high capacity retention of 89.6%after 30000 cycles.In addition,in the rate testing from 5 C to 1000 C,and then back to 5 C,the rate capacity retention is up to 66%,and the rate capacity recovery is as high as 95.3%.（3）The potassium zinc hexacyanoferrate（KZn HCF）cathode material was synthesized based on a simple co-precipitation method,and the morphology,structure and composition were characterized in detail.Through systematic electrochemical analysis of the electrochemical behavior of KZn HCF cathode material in different concentrated K⁺electrolyte and K⁺/Zn²⁺hybird-ion electrolyte,it could be seen that with the increase of K⁺concentration,side reactions were inhibited and the cycle stability of the electrode material was enhanced.Adding Zn²⁺had a better effect on enhancing the cyclability.In addition,through advanced in-situ XRD and in-situ ATR-FTIR,comparative analysis of the structural evolution in electrolytes with different concentrations and compositions,combined with electrochemical behavior analysis,were carried out.The concrete results showed that KZn HCF cathode material udergos solid-solution phase and two-phase transition ion storage mechanisms simultaneously in low-concentrated K⁺electrolytes,accompanied by a large irreversible change in chemical structure,that is,the gradual degradation of-C≡N(Fe²⁺);while only solid-solution phase reaction mechanism occurs in a higher concentrated K⁺or K⁺/Zn²⁺hybrid-ion electrolyte,with the chemical structure highly reversible.This liquid-phase induced solid-solution phase energy storage mechanism ensured the KZn HCF cathode material a high cyclability and rate performance.The capacity retention of KZn HCF after 10000 cycles at a current density of 20 A g^-1is as high as 93.7%,and the rate capacity retention is up to 66.6%(from 1 A g^-1to 20 A g^-1).（4）Based on dual-ion electrochemistry,the advantages of ions with different electrochemical properties could be combined.And the thermodynamics and kinetics of the insertion/extraction or deposition/dissolution of the anode and cathode materials can be well regulated to achieve an energy storage system with high voltage,high energy/power density and long cycle life.The Zn/Fe based PBAs were chosen as cathode materials,and the zinc sheet/foil or monoclinic vanadium oxide（VO₂（B））nanorods were chosen as anode materials to construct the K-Zn hybrid-ion battery.In this battery,the selective insertion/extraction of K⁺occurs in the Zn/Fe-based PBA cathode,and Zn²⁺deposition/dissolution or Zn²⁺insertion/extraction occurs in the anode.The assembled Zn HCF//Zn battery shows a discharge plateau of 1.94 V and a capacity of 69.1 m Ah g^-1at 2 C rate;while at a 60 C rate discharge,the plateau still exhibits at 1.80 V and a capacity of 46.7 m Ah g^-1is obtained.The highest energy density is 67 Wh kg^-1,and the highest power density can reach 4.76 k W kg^-1.It was proved that the Zn HCF//Zn full battery not only has a high discharge plateau,but also shows fast reaction kinetics and thermodynamics.The average discharge plateau of the constructed KZn HCF//VO₂could reach 1.20 V,and the energy density reached50.0 Wh kg^-1.The constructed KZn HCF//Zn full battery shows a long cycle-life of2000 times and a capacity retention of 89.0%.The rate capacity retention from 1.0 A g^-1to 10 A g^-1is as high as 85.5%,and the rate capacity recovery reachs up to 97.1%.Thus,it’s believed that the KZn HCF//Zn full battery has high rate performance and cyclability.The highest energy density and power density of the full battery is 79.6Wh kg^-1and 11.5 k W kg^-1,respectively.The open circuit voltage of the assembled pouch cell reaches 1.892 V,which can also provide a high capacity of more than 3.60m Ah(0.207 m Ah cm^-2).This paper could set some scientific and theoretical foundations for the development and application of the follow-up K⁺/Zn²⁺hybrid-ion energy storage system.