Preparation And Mechanism Study Of High-performance Prussian Blue Analogue Materials For Sodium Storage

Large-scale energy storage technology,as a key part of the renewable energy system,is playing an important role.Sodium-ion batteries（SIBs）not only possess abundant and widely distributed sodium resources,but also have similar electrochemical behaviors to lithium-ion batteries,and are regarded as the most promising new-generation electrochemical energy storage technology in large-scale energy storage systems.And the key properties of sodium-ion batteries including capacity,stability,and safety mainly depend on their cathode materials.Prussian blue analogue（PBA）compounds,as a class of sodium storage cathode materials with open and stable three-dimensional framework,have received extensive attention.PBA crystal structure with large ion diffusion channels and interstitial space is highly conducive to the storage and diffusion of sodium ions,and the synthesis method of PBA materials is simple and easy to scale-up production.However,the presence of[Fe（CN）₆]^4-vacancies and water molecules in PBA lattice will deteriorate the sodium storage properties seriously.In addition,the high capacity usually can’t be compatible with long-cycle stability for sodium storage in monometallic PBAs.Consequently,it is imperative to explore a new approach to reduce the content of[Fe（CN）₆]^4-vacancies and water molecules in the PBA lattice and increase the sodium storage capacity of PBAs with good cycling stability as well.In this paper,we proposed chelating agent-assisted strategies to prepare high-performance PBA-based sodium storage materials.Firstly,we used sodium citrate as a chelating agent to modulate the growth kinetics of PBA crystals during the coprecipitation process,reducing the growth rate and obtaining high-quality monoclinic nickel hydroferricyanic（Ni HCF）materials with high sodium content,few[Fe（CN）₆]^4-vacancies and low water content,this material improved the capacity utilization of Ni HCF and exhibited ultra-long cycling stability.Then,the strong chelation between transition metal ions(e.g.Ni²⁺,Cu²⁺,etc.)and aminocarboxylic acid chelating agents（e.g.ethylenediaminetetraacetic acid disodium salt（Na₂EDTA）,diethylenetriaminepentaacetic acid（DTPA）,etc.）was utilized,it not only inhibited the crystal growth kinetics of PBA,but also provided a strong acidic environment for the decomposition of[Fe（CN）₆]^4-into Fe²⁺during the hydrothermal process.Finally,the bimetallic PBA cathode materials for SIBs with high specific capacity and long cycle stability were prepared by aminocarboxylic acid chelating agent-assisted one-step hydrothermal method.The main works are as follows:（1）Sodium-rich monoclinic Ni HCF（m-Ni HCF）material was successfully prepared by sodium citrate-assisted co-precipitation method.As a cathode material for aqueous SIBs,m-Ni HCF exhibited superior sodium storage performance including high specific capacity(70.1m Ah g^-1 at 100 m A g^-1),high rate performance(53.2 m Ah g^-1 at 2000 m A g^-1)and long-cycle stability(97.1%capacity retention over 8000 cycles,67.0 m Ah g^-1 at 500 m A g^-1).And the pseudocapacitance-controlled charge storage process dominated the electrochemical kinetic behavior of m-Ni HCF dring the sodium storage process,and m-Ni HCF achieved reversible phase transition between monoclinic and cubic phases with the reaction of N-coordinated Fe³⁺/Fe²⁺redox-active site during sodium storage process.In addition,the aqueous Na-ion full cells were assembled by matching m-Ni HCF cathode material and NTP@C anode material,exhibiting excellent sodium storage performance with 83.0%capacity retention over 600 cycles.（2）A low-defect nickel-iron bimetallic Prussian blue（H-PBA）SIB cathode material was successfully prepared by Na₂EDTA-assisted one-step hydrothermal method,which not only retained the low-strain advantage of the Ni HCF framework,but also created N-coordinated Fe³⁺/Fe²⁺sodium storage site,enableing compatibility of high specific capacity and long-term cycling stability during the sodium storage process.As a result,H-PBA exhibited high specific capacity(127.9 m Ah g^-1 at 10 m A g^-1,and 101.0 m Ah g^-1 at 2000 m A g^-1)and outstanding cycling stability(87.9%capacity retention over 900 cycles,109.0 m Ah g^-1 at 500 m A g^-1).Electrochemical kinetic analysis indicated that the pseudocapacitance-controlled charge storage process dominated the sodium storage process of H-PBA.The sodium storage mechanism of H-PBA included two processes of solid solution reaction（2.0-3.0 V）and reversible phase transition reaction（3.0-4.0 V）,in which Fe³⁺/Fe²⁺redox couple acted as reaction active site.Moreover,the full cells were assembled with H-PBA cathode material and NTP@C anode material,exhibiting outstanding cycling stability(76.0%capacity retention over 900 cycles,68.8.0 m Ah g^-1 at 1000 m A g^-1).（3）A high-quality copper-iron bimetallic Prussian blue（Cu HCF-90）was successfully prepared by DTPA-assisted one-step hydrothermal method.Fe²⁺generated by the decomposition of[Fe（CN）₆]^4-would gradually replace Cu²⁺in Cu HCF during the hydrothermal process,with the increase of reaction temperature,Cu²⁺would be completely replaced by Fe²⁺.Therein,Cu HCF-90 not only created N-coordinated Fe³⁺/Fe²⁺redox-active site,but also activated N-coordinated Cu²⁺/Cu⁺redox-active site,which further improved the storage capacity of bimetallic PBA materials with good cycling stability.As a result,Cu HCF-90exhibited high specific capacity(145.2 m Ah g^-1 at 10 m A g^-1,and 114.3 m Ah g^-1 at 2000 m A g^-1)and good cycling stability(76.1%capacity retention over 800 cycles,100.0 m Ah g^-1 at1000 m A g^-1).The pseudocapacitance-controlled charge storage process also dominated the sodium storage process of Cu HCF-90,and Cu HCF-90 achieved reversible phase transition between monoclinic and cubic phases with the reaction of N-coordinated Fe³⁺/Fe²⁺and Cu²⁺/Cu⁺redox-active sites during sodium storage process.Futhermore,the Na-ion full cells were assembled by matching Cu HCF-90 cathode material and NTP@C anode material,delivering an excellent cycling stability(90.2%capacity retention over 2000 cycles,87.8 m Ah g^-1 at 500 m A g^-1).