The design and synthesis of Prussian blue analogs as a sustainable cathode for sodium‐ion batteries

Sodium‐ion batteries (SIBs) present great appeal in various energy storage systems, especifically for stationary grid storage, on account of the abundance of sources and low cost. Unfortunately, the commercialization of SIBs is mainly limited by available electrode materials, especially for the cathodes. Prussian blue analogs (PBAs), emerge as a promising alternative for their structural feasibility in the application of SIBs. Decreasing the defects (vacancies and coordinated water) is an effective strategy to achieve superior electrochemical performance during the synthetic processes. Herein, we summarize crystal structures, synthetic methods, electrochemical mechanisms, and the influences of synthesis conditions of PBAs in detail. This comprehensive overview on the current research progresses of PBAs will give guides and directions to solve the existing problems for their application in SIBs.


INTRODUCTION
2][3][4][5][6][7] However, the intermittent properties of renewable green energy sources cause significant challenges for their integration into the electric grid.To eliminate the intermittency of renewable green energy sources, low-price and large-scale energy storage devices are necessary to enhance energy utilization efficiency.Electrochemical energy storage has received widespread attention among all energy storage technologies.][10][11][12] Lithium-ion batteries (LIBs) are widely applied because of their unique advantages, including high energy density, long cycle life, and so SusMat.2023;3:749-780.wileyonlinelibrary.com/journal/susmat749 F I G U R E 1 Four typical kinds of cathode materials as well as its strengths and weaknesses: TMOs, polyanionic compounds, PBAs, and organic materials.
forth. 1,13However, the practical application of LIBs is still a great challenge in the large-scale energy storage field, which is limited by the availability of lithium resources.9][20] Therefore, the pursuit of promising cathodes is crucial for SIBs.
To meet the scientific concept of sustainable development, it is of great significance to choose sustainable cathode materials that do not jeopardize the environment and resources.There are mainly four promising cathodes for industrial application, as shown in Figure 1, including transition metal oxides (TMOs), 10,[21][22][23][24] polyanionic compounds, 25 Prussian blue analogs (PBAs), [26][27][28][29][30][31][32][33] and organic materials. 34Among them, TMOs have received considerable attention due to their high theoretical capacity, high energy density, and voltage adjustable.Whereas, the oxygen evolution at high potentials might cause potential safety hazards.Polyanionic compounds with the sodium superionic conductor structure play an important role, because of their high working potential, thermal tolerance, and strong structural stability.However, the heavy molar weight of large polyanions limites the reversible capacity (<120 mA h g −1 ).As we know, organic materials demonstrate high energy density, high power density, and good structural designability, while the practical applications and popularization in SIBs are generally restricted by the intrinsic disadvantages of low conductivity, poor stability, and high solubility in electrolytes.PBAs, as a typical sustainable cathode material, display several advantages in the field of energy storage: (i) the abundant 3D diffusion channels in the tough open framework of PBAs, which are beneficial for the transport of alkali metal, and thus can provide excellent cycling stability; (ii) the lowcost and abundant raw materials, as well as the facile synthetic methods, can facilitate its large-scale energy storage applications; (iii) the rich redox couples and highly reversible phase transitions are in favor of the charge storage; and (iv) it is important to be recycled in an environmentally sustainable and cost-effective manner after large-scale application in SIBs.Therefore, PBAs have become extremely promising cathodes for their industrial application in SIBs.
For the development history of Prussian blue (PB), a book published by Georg Ernst Stahl mentioned that PB (Stahl writes "Caeruleum Berolinense") was first discovered in 1706 in the laboratory of Dippel in Berlin. 35,36And then, PB was first reported by Johann Leonhard Frisch in 1710.However, they did not mention the specific synthetic details. 37 It was not until 1724 that John Woodward and John Brown successively published the procedure for the production of this color and detailed experimental work on PB in Philosophical Transactions of the Royal Society. 38,39PB had been known to use as a pigment and a dye from the early 18th century until 1841.Subsequently, John Herschel invented the cyanotype or blueprint process in 1842, and it was successfully used in the commercial photocopying process until the 1940s. 40It was worth to mention that the detailed structural information of PB was precisely confirmed by Ludi et al. 41 in the 1970s through single-crystal electron and neutron diffraction F I G U R E 2 A unit cell with coordinated water, interstitial water, and migrating ions, as well as the challenges and optimizations of PBAs in energy storage/conversion applications.
measurements.Up to now, PB has been used in catalysis, 42 energy storage, 43 biosensing, 44 seawater desalination, 45 cancer therapy, and sewage treatment. 46,47Although PB has been widely studied in the energy storage fields, it still faces some problems for further development: (i) the presence of defects (vacancies and coordinated water) greatly reduces the crystallinity of the material, which in turn leads to the capacity loss and short cycle life; (ii) the irreversible phase transition in the course of the charge and discharge processes; and (iii) inherent low electrical conductivity of the material itself.
In the past decade, several reviews on PBAs have been published.9][50][51][52][53][54][55][56][57] In this review, based on the summary of crystal structures, synthetic methods, and electrochemical mechanisms of PBAs in detail, we summarize the influences of synthetic conditions on their crystal structures and electrochemical properties in SIBs.This work will present insight into the synthesis and future development of PBAs.

Crystal structure of PB and PBAs
Typically, as shown in Figure 2, for the basic PB, Fe 4 [Fe(CN) 6 ] 3 ⋅nH 2 O, where Fe 2+ and Fe 3+ ions connected with the −C≡N− appear alternate in the framework structure.Among them, divalent and trivalent Fe atoms are surrounded by carbon and nitrogen atoms, respectively, forming a regular octahedron structure.Moreover, there are 25% vacancies in the [Fe(CN) 6 ] 4− sites, as the stoichiometric ratio of divalent and trivalent Fe is 3:4 in the formula.The vacancies are occupied by up to 14 water molecules, including six coordinated water located at the empty nitrogen sites and coordinated to trivalent Fe atoms, and eight interstitial water occupied the body center of the unit cell. 41,58While PBAs are represented as A x M ′ 1 [M ′ 2 (CN) 6 ] y •□ 1-y •zH 2 O (0 ≤ x ≤ 2, 0 ≤ y ≤ 1), in which, A is alkali metals, and M′ is transition metals that bond with N and C in cyanide ions (C≡N − ).0][61] □ represents the vacancy due to the loss of [M′(CN) 6 ] 3−/4− , filled by the coordinated and interstitial (or zeolite) water.It is worth noting that the electrochemical reaction takes advantage of various valence states of M′ to store the metal ions.3][64][65][66][67] In 2012, Asakura et al. 62 explored the relationship between crystal phases and the content of K + ions (x), in which the sample displayed cubic phase (x < 1) and two-phase transformation process between the cubic and monoclinic phases (1 < x < 1.72), respectively.Wang et al. 63 also obtained the same results for Na x MnFe(CN) 6 by adjusting the amount of Na + ions.Song et al. 64 investigated the effect of interstitial water for the crystal phase of PBAs, in which the rhombohedral Na 2-δ MnFe(CN) 6 (dehydrated) was synthesized by removing the interstitial water from monoclinic Na 2-δ MnFe(CN) 6 (hydrated).In addition, Kosaka et al. 65 obtained PBAs with tetragonal structure according to the strong cooperativity between Jahn-Teller (JT) distortion and ferromagnetism.Peng et al. 67 reported the cerium hexacyanoferrate (HCF) with hexagonal structure synthesized via a convenient method, which served as a "zero strain" cathode material to store various alkali ions (Li + , Na + , K + ).It can be seen that the ability to control and manipulate the crystal structure of PBAs is of special significance in enhancing sodium storage properties.

Synthesis methods
A variety of methods are used for fabricating PBAs, including the co-precipitation method, the electrodeposition method, the hydrothermal method, the mechanical ball milling method, and the template method as shown in Figure 3.
F I G U R E 3 Synthetic methods for PBAs: co-precipitation method, electrodeposition method, hydrothermal method, mechanical ball milling method, and template method.

Co-precipitation method
The co-precipitation method has become the most popular synthetic method due to its facile synthesis at room temperature, low cost, and easy to obtain pure phases.In 1706, Diesbach added potash and HCF into a solution containing iron sulfate and achieved blue precipitates.This is the first time to obtain PB by the co-precipitation method. 37Generally, the reaction conditions, including feeding method, reactant concentration, reaction temperature, aging time, and so forth, are crucial for the control of crystal structure and electrochemical performance of the PBAs.If the conditions are not controlled very well, the as-obtained PBAs will exist vacancies and coordinated water due to the rapid crystallization rate between the ligand and the metal ions.
Based on the co-precipitation method, in terms of the reactions of multi-metal sources, high crystallinity and specific structure PBAs can be acquired by controlling the feeding modes.Mullaliu et al. 68 developed a sodiumrich manganese HCF (NMHCF) by dropping a solution containing Mn 2+ ions and [Fe(CN) 6 ] 4− ions into another solution at the same time.The as-obtained products featured high specific capacities (>130 mA h g −1 ) at high potentials due to the reversible electronic and structural modifications during the Na + ions de-intercalation process.Lu et al. 69 synthesized different PBAs consisting of a series of M′ by dropping a solution containing different metal ions into another solution containing [Fe(CN) 6 ] 3− ions.The as-obtained products as cathodes presented a flat discharge capacity and good reversibility for SIBs.In addition, Wang et al. 70 fabricated a unique gradient structure, in which the nickel-rich layer coated with the iron-rich layer was in a cube by utilizing a co-precipitation method.Nickel solution was continuously dropped into the iron solution to produce a gradient concentration, and then the [Fe(CN) 6 ] 4− solution and the gradient solution were dropped into another reactor.The gradient concentration structure could be achieved due to time-dependent evolution with the increased nickel concentration in the iron solution.The as-obtained gradient PBAs exhibited outstanding electrochemical property, and the gradient PBAs was a "zero-strain" material through the ex situ X-ray diffraction (ex situ XRD) patterns (Figure 4A-D).On the other hand, reactant concentration and aging time were illustrated by Li et al. 71 through the citrate-assisted coprecipitation method to obtain the high-quality (HQ) PBAs as cathodes in SIBs.All products exhibited similar crystal structures, while displayed different morphologies and electrochemical behaviors in the various preparation conditions.The lower reactant concentration was beneficial for forming large nanoparticles with low defects (vacancies and coordinated water).Whereas, the sample with small nanoparticles showed a relatively high specific capacity due to the benefited electrochemical kinetics.Moreover, the authors concluded that 24 h was the best aging time.With regard to reaction temperature, Tang et al. 72 reported cubic and monoclinic Na x MnFe(CN) 6 by adjusting the reaction temperature.It was observed that the Na content in the as-prepared samples was significantly improved with the increased reaction temperature from 25 to 80 • C, while the vacancy and the content of water in the crystal structure was kept at a low value.
For the reactions of single-metal sources, mainly [Fe(CN) 6 ] 3−/4− is utilized to slowly release part of Fe 2+ under acidic conditions, and then Fe 2+/3+ and undecomposed [Fe(CN) 6 ] 3−/4− form PBAs. In 2017, Rudola et al.C displayed the best excellent electrochemical performance of 113 mA h g −1 at 10 mA g −1 .On the other hand, the co-precipitation method, either multi-metal sources or single-metal sources, can be used to design PBAs composition materials, which can be divided into one-step and multistep methods.
For the one-step method, researchers always directly load PB on the substrate materials.Wang et al. 75 developed a reduced graphene oxide (RGO) anchored NMHCF by dropping a solution containing Mn 2+ ions into the Na 4 Fe(CN) 6 and RGO solution.Herein, the anchoring of RGO on NMHCF can suppress content of interstitial H 2 O and promote electron conduction.And the NMHCF/RGO cathode not only displayed highly reversible specific capacities of 121 mA h g −1 at 200 mA g −1 over 250 cycles in the halfcell but also showed better performance in the full cell (84 mA h g −1 after 50 cycles).Li et al. 76 developed a PB-RGO composition by heat treating the mixture of Na 4 Fe(CN) 6 •10H 2 O and GO in an acidic condition.The asgenerated Fe 2+ ions anchored on the GO layers through electrostatic interactions to reduce the GO to RGO.The asobtained NaFe 2 (CN) 6 @RGO composite displayed excellent rate capability at room temperature (Figure 4E-H).In addition, Morant-Giner et al. 77 reported a straightforward approach to prepare PB/MoS 2 -based composites by mixing potassium HCF(III), iron(III) chloride, and chemically exfoliated MoS 2 flakes.Among them, MoS 2 provided an active support role for the nucleation of PB nanocrystals, endowing large surface areas and compact PB shell coating the whole flake.Thus, the nanocomposite exhibited an outstanding performance for SIBs.
For the multistep synthesis method, the as-obtained PBAs were always mixed with other materials to prepare composite materials.Li et al. 78 prepared NMHFC powder and then mixed it with pyrrole, to synthesize NMHFC@PPy under ice bath conditions by polyreaction (PPy).PPy was used as the conductive and protecting layer to enhance the rate capability and reduce the dissolution of Mn during the charge/discharge processes (Figure 4I,J).Qiao et al. 79 reported a cubic PB coated with ZnO (PB@ZnO) through a low-temperature thermal treatment method.The as-obtained sample displayed a significantly enhanced rate performance and cycle stability.The ZnO layer could be adopted as a protection course to inhibit the decomposition of the lattice during the Na + ions de-intercalation process.Meanwhile, the removal of interstitial water in the framework could stabilize the structure and decrease the defective sites.

Hydrothermal method
The hydrothermal method has become one of the common synthetic methods due to its characteristics of short reaction time and uniform distribution of synthesized nanoparticles.Different from the co-precipitation method, the hydrothermal method provides an environment of high temperature and high pressure for the crystal nucleation and growth of PBAs.And it can also be divided into the reactions of single-metal sources and multi-metal sources.
It is facile to obtain PBAs materials with various structures by making use of the self-decomposition of single-metal sources hexacyanometalate via hydrothermal reaction.In 2015, Wang et al. 80 fabricated the rhombohedral Na 1.92 Fe 2 (CN) 6 utilizing the Na 4 Fe(CN) 6 as the single iron source solution (pH 6.5) at 140 • C.This cathode demonstrated high capacity and excellent rate capability, as well as long cycle life.Moreover, the electrochemical reaction was investigated by soft X-ray absorption spectroscopy (sXAS) (Figure 5A,B).Sun et al. 81 reported the effect of reaction temperatures on the crystal structure and the sodium storage mechanism.The iron HCFs prepared at 160 • C (NFFCN-4 M NaCl-160 • C) showed a monoclinic phase, and displayed higher potential plateaus and better electrochemical performance than that of NFFCN-4 M NaCl-80 • C with cubic phase.This was put down to the activated low-spin Fe LS (C)-induced intercalation pseudocapacitance for NFFCN-4 M NaCl-160 • C (Figure 5C-H).What is more, the hydrothermal method via utilizing the reaction of single-metal sources is used to fabricate composition materials.You et al. 82 reported a nucleated PB on carbon nanotube (CNT) conductive networks to construct a flexible and robust PB/CNT cathode via a one-step hydrothermal method at pH = 1, which demonstrated excellent sodium storage performance between −25 and 25 • C. Different from [Fe(CN) 6 ] 4− ions, [Co(CN) 6 ] 3− ions in hexacyanocobaltate-type PBAs can be decomposed not only under hydrothermal but also under solvothermal conditions.Cao et al. 83  For the reaction of multi-metal sources, the hydrothermal method provides a reaction environment for the nucleation and growth of PBAs.Duan et al. 84 proposed a two-phase stratified solution to fabricate PBAs via the hydrothermal method.The reaction that occurred in the oil-water interface could restrain the crystallization process.A relatively longer time for crystal nucleation and growth was needed and beneficial to regulate the crystal size of PBAs.The as-obtained PBAs materials showed a high specific capacity (152 mA h g −1 at 0.2 C), excellent rate performance (105 mA h g −1 at 20 C), and remarkable capacity retention (84% over 200 cycles).In addition, PBAs materials can be modified and post-treated by hydrothermal methods.Peng et al. 85 reported a defect-free PBAs framework structure with a highly ordered M 1 -CN-M 2 structure through the secondary crystallization by hydrothermal method.Quasi-solid-solution sodiation reaction mechanism and the effect of heterogeneous molecules were demonstrated by investigating the origin of the structureproperty relationships and the quasi-solid-solution behaviors in PBAs.As cathodes, the PBAs materials exhibited outstanding cycle and rate performances through Na + ions disordering.

Electrodeposition method
Compared with the traditional preparation method of nanocrystalline materials, the electrodeposition method possesses the following advantages: the size of the deposits can be conveniently adjusted; the uniformly dispersed, patterned, and faceted crystal growth of nanostructures can be easily controlled to achieve. 86In 1978, Neff first reported the electrochemical deposition of PB thin films in an acidic solution. 43

Mechanical ball milling method
Unlike the traditional co-precipitation and hydrothermal methods, mechanical ball milling is a process dominantly controlled by a shear force, which is created through the impact and attrition of grinding media (generally zirconia or steel balls), and rotating shell and the external mechanical energy can be utilized to break the intramolecular bonds of materials and further improve the reactivity and uniformity of spatial distribution of elements. 92Therefore, mechanical ball milling is adopted to grind the bulk materials to the nanoscale for synthesizing the inorganic materials.In 2022, He et al. 93

Template method
The template method is one important strategy for the morphology, structure, and size of nanomaterials.
It is often used to design nanomaterials with special morphology and structure.Depending on the nature of the templates, the methods can be summed up as soft template, hard template, and self-template methods. 98ifferent from other templates, the "template" in the self-template not only plays a role of supporting but also directly involves the reaction process.For PBAs, the self-template method can be classified into the following two categories according to the reaction mechanism: (i) The Ostwald ripening process is one of the important mechanisms, especially for the crystal growth for hollow structures with small particle sizes.In the solution, the high solubility of large particles surface, so it is easy to ripen and further form a shell layer on the particle surface, while the inner part of the particle ripens slowly.Accordingly, the dissolved small particles are also continuously absorbed into the shell layer until the hollow structure is formed.In 2017, Ren et al. 99 prepared a nanoflower structure through surface etching to enhance the active sites for the nickel HCF (NiHCF).The edges of nanocubes first dissolved owing to the decomposition of Fe-CN-Ni, and then the etched area extended to each face.Interestingly, with the prolongation of the reaction time, the nanosheets generated on the surface of nanocubes could be ascribed to the dissolution-recrystallization processes (Figure 7A).In addition, the unique structure exhibited high diffusion dynamics and ignorable volume evolution by in situ XRD and computational analysis.In 2019, Ren et al. 100 reported a self-template method to fabricate Na for CoHCF; Figure 7C).The hollow structures endowed CoHCF with high specific capacity and super higher rate capability, as well as excellent fullcell performance in aqueous SIBs.Huang et al. 102 also prepared hollow polyhedrons CoFe-PBAs, and the formed hollow structure was similar to the above work (Figure 7D).After coating with polydopamine (PDA), the CoFe-PBAs@PDA displayed better electrochemical performance than ordinary and hollow CoFe-PBAs.According to the above report, the ion diffusion process in the template could be summarized as the Kirkendall effect, and the subsequent formation of CoHCF and CoFe-PBAs belongs to Ostwald ripening.In total, it is found that the co-precipitation method has become the most likely one to be scaled up due to the advantages of easy synthesis of pure phase and low cost (Table 1). 49However, large water consumption and the supernatant liquid at the end of the reaction are urgent problems to be solved.In addition, the hydrothermal method is one of the common synthetic methods due to its characteristics of short reaction time and uniform distribution of nanoparticles.Whereas, the high-temperature, high-pressure environment, and low yield restrict its further application in a large scale.In these methods, the electrodeposition method possesses the following advantages: The size of the deposits can be conveniently adjusted; the uniformly dispersed and faceted crystal growth of nanostructures can be easily controlled, while the process conditions for mass production are still being explored.Recently, the mechanical ball milling was developed.The process is dominantly controlled by a shear force to improve the reactivity and uniformity of the spatial distribution of elements.Thus, the target product can be quickly obtained by controlling the rotation speed and reaction time.The relatively small particle sizes of the products could result in low energy density.For the template method, complex process conditions will hinder their further applications, although a variety of morphologies with excellent electrochemical properties can be obtained.

Electrochemical reaction mechanism
An understanding of the electrochemical reaction mechanism is fundamental and important to design highperformance PBAs cathodes for advanced SIBs.According to the molecular formula F I G U R E 7 (A) Schematic illustration of NiHCF at different etching time; reproduced with permission. 99Copyright 2017, American Chemical Society.(B) Synthetic process of PW-HN and PB-Bulk; reproduced with permission. 100Copyright 2019, Wiley-VCH.(C) The formation mechanism of CoHCF hollow cubes; reproduced with permission. 101Copyright 2019, Elsevier.(D) The formation mechanism for the CoFe-PBAs@PDA.Reproduced with permission. 102Copyright 2021, American Chemical Society.
and morphology of PBAs, we will elaborate its electrochemical reaction mechanism from the following several parts.Alkali metal, as the migrating ions, are reversibly inserted and extracted in the electrodes, and the content of migrating ions will affect the crystal structure and the electrochemical reaction mechanism of PBAs.Depending on the number of Na + ions, PBAs can be classified into three types: Na-free (x = 0), Na-poor (0 < x < 1), and Na-rich (1 ≤ x ≤ 2) phases, and the crystal structure can be further subdivided into cubic, monoclinic, and rhombohedral as the Na + ions content changes.In terms of Na x M ′ 1 [Fe(CN) 6 ] (M ′ 1 = Fe, Mn), the cubic will transform to monoclinic when the Na + ions content achieves a limit value of 1.6 and water molecule (z ≠ 0) still exists.And then, the rhombohedral PBAs can be obtained as the Na + ions content sustainably increases and the water molecule is further removed.The removal of water molecules will be discussed in the subsequent discussions.Moreover, PBAs with different Na + ions content show diverse ICEs.It is found that the increased contents of sodium are beneficial to enhance the ICE of half cells, as well as the energy density and reversible specific capacity of full cells.Wang et al. 103  Usually, the occupation sites and migration paths also affect the reaction mechanism of PBAs.For the occupation sites of Na + ions, Ling et al. 104 illustrated that the interstitial sites were related to the radius of alkali metal or alkaline earth metal by utilizing first-principles density functional theory (DFT) calculations and experimental observations.As shown in Figure 8A, the interstitial sites were denoted with Wyckoff notations.The 8c sites with the largest free space were more liable to be occupied by F I G U R E 8 (A) Crystal structure and interstitial sites; reproduced with permission. 104Copyright 2013, American Chemical Society.(B) CV curves with corresponding Na + ions interstitial sites; reproduced with permission. 106Copyright 2015, Elsevier.(C-N) Four possible Na + ions migration paths and corresponding migration energy.Reproduced with permission. 82Copyright 2016, Wiley-VCH.large K + , Rb + , and Cs + ions.Whereas, the Na + , Mg 2+ , Ca 2+ , and Sr 2+ ions easily tend to occupy 24d sites, while the occupation sites of Li + and Ba 2+ inclined to 48 g and 32f sites, respectively.Moreover, the same conclusions have been obtained for the Na + ions embedding Na 2 FeMn(CN) 6 by Xiao et al. 105 using the DFT calculation considering the Coulomb attraction (Na and N anions).Whereafter, Liu et al. 106 also investigated the intercalation sites of Na + ions via the DFT calculation for the cubic Na x FeFe(CN) 6 , however, the intercalation sites of Na + ions were different for Na-poor and Na-rich samples.The Na + ions preferentially occupied the 8c sites with large cavities in Na-poor.Nevertheless, part of Na + ions in Narich samples were pushed into the 24d metastable sites (Figure 8B).This viewpoint was proved subsequently by Sun et al. 81 For the mixed alkali metal ions, Zhang et al. 107 proposed a potassium-ions-assisted strategy to synthesize highly crystallized Fe-based PBAs by regulating the crystal structure and controlling the crystal phase orientation.
The as-obtained product displayed a preferred orientation on the (220) plane and a stable structure in the framework.Thus, the PBAs cathode exhibited a high redox potential, further leading to a high energy density (∼450 Wh kg −1 ), which was the same level as the LiFePO 4 cathode.Moreover, the Na + and K + ions would occupy 24d and 8c sites, respectively, due to the low adsorption energy according to the DFT results.Meanwhile, the redox potentials of the framework increased to a high value as the potassium content increased.For the migration paths of Na + ions, in 2016, You et al. 82 evaluated four migration paths to illustrate the Na + ions diffusion for Na x FeFe(CN) 6 via employing DFT calculation, as shown in Figure 8C-N.One single Na + via a W-shaped zig-zag path presented a lower energy barrier than that of the path through the body-center position.The cooperative Na + migration could remarkably decrease the energy barriers.Considering the electrochemical impedance spectroscopy (EIS) results, a step-by-step cooperative mode had been proposed with the collective migration of all Na + ions.Subsequently, Peng et al. 108 proposed S-shaped migration paths with a lower energy barrier.It could be concluded that Na + ions diffusion prefered to process in the way of S-shaped routes instead of the way through the body center along the axis.
0][111] From an industrialized perspective, the redox sites in PBAs are almost iron and manganese.On the one hand, it can be ascribed to the fact that these elements are abundant and cheap in reserves, which can greatly reduce production costs.On the other hand, iron and manganese can provide two electrons to participate in the electrochemical process and also increase the average voltage and specific capacity to achieve high energy density.Taking the monovalent Na + ion as the migration ion, the redox reaction in vacancy-free PBAs is shown as follows: ] .
During the intercalation of Na + ions, the M′ can synergistically interact with Na + ions to affect the structure framework of PBAs.The open frameworks of PBAs can be divided into rigid and flexible types based on whether a phase transition occurs. 112PBAs with rigid frameworks displayed satisfied cycling performance as cathodes, while flexible frameworks allowed PBAs to accept more Na + ions and thus increased the capacitance.As shown in Table 1, the multiple-phase transitions of PBAs have been concluded for flexible types, and the C, M, R, T, and O represent the cubic, monoclinic, rhombohedral, tetragonal, and orthorhombic phase, respectively, such as Ni- However, there is no necessary connection between migration ions, redox sites, crystal structure, different phase transition processes, and the initial synthesis method.In terms of the multifold M′ coordinated with N in cyanide ions (C≡N − ), Li et al. 113 developed an Fe/Mn-based binary HCF (NMFHCF) via an energy-efficient co-precipitation method.The as-prepared PBAs with sodium-rich cubic phase and the substitution of Fe could avoid the stress distortion.Meanwhile, the NMFHCF showed a negligible structural strain during the charge/discharge processes.Peng et al. 114 reported a high-entropy (HE) HCF inspired by the disordered Rubik's cube through the co-precipitation method.The HE-HCF sample showed outstanding cycling stability for over 50 000 cycles and fast-charging capability up to 75 C for SIBs.Moreover, multiple characterization techniques and theoretical calculations illustrated that the increased configuration entropy in HCF could enhance the structural stability and the thermal/air stability as well as provide a highly reversible zero-strain two-phase Na + ions storage mechanism.
On the other hand, the sufficient utilization of the redoxactive site of M′ is necessary for rationalizing high capacity and rate performance, especially for low-spin Fe LS (C).Therefore, many efforts are made to enhance the activity and utilization of low-spin Fe LS (C), including metal doping, carbon decoration, and surface activation.In 2015, Yu et al. 115 synthesized iron-NiHCF (FeNiHCF) through the substitution of Ni into high-spin Fe HS (N) in FeHCF.It might alter the electron atmosphere of low-spin Fe LS (C) and promote the de-intercalation properties of Na + ions during the electrochemical processes.The capacity contribution of low-spin Fe LS (C) and high-spin Fe HS (N) redox couples were nearly identical, indicating the equally intercalated Na + ions into the lattice of FeNiHCF at the two stages of the redox process.Benefiting from the sufficiently activated low-spin Fe LS (C), the relatively high discharge capacity and potential could be obtained.Moreover, it was also facile to alleviate the structural distortion and maintain the structural integrity upon Na + ions intercalation.Jiang et al. 116 reported the PB@C composite by utilizing Na 4 Fe(CN) 6 ⋅10H 2 O and Ketjen Black.DFT calculations confirmed that the activity of low-spin Fe LS (C) was enhanced by decreasing the concentration of Fe(CN) 6 vacancies with the help of Ketjen Black.Moreover, the high-spin Fe HS (N) showed superior reactive kinetics due to the elevated electron conduction.The as-obtained PB@C cathode displayed a high specific capacity.For surface activation, it is a recrystallization process, which could increase the amount of active sites and enhance the activity of reactive sites, such as the nanoflower structure through the surface etching approach. 99Nevertheless, there are few researches working on surface activation, as the synthesis of bulk PBAs usually is performed using the co-precipitation method.
With the fast co-precipitation reaction and aqueous reaction condition, crystal water (10%∼20%) inevitably exists in the PBAs structure, including adsorbed water, interstitial water (or zeolite water), and coordinated water, which were located on the crystal surface and interstitial sites by physically adsorbed as well as M′ sites connected by chemical bonds, respectively.The crystal water will extract and insert from/into the PBAs framework structure in the form of Na(OH 2 ) + ions, and the side reaction between crystal water and electrolyte will result in the swelling of pouch full cells and poor electrochemical performance.Xie et al. 117 revealed the crystal water evolution via the ex situ fourier transform infrared (FTIR) F I G U R E 9 (A-B) FTIR results at various charge states, and diagrammatic of two ions movement; reproduced with permission. 117opyright 2019, American Chemical Society.(C-D) 2D contour map and corresponding structures at different temperature; reproduced with permission. 118Copyright 2022, Wiley-VCH.(E-F) The charge/discharge voltage curves and capacity loss at different storage times and fitted to an exponential function, as well as the mechanism of Na loss; reproduced with permission. 119Copyright 2021, American Chemical Society.(G) The mechanism for removing coordinated water.Reproduced with permission. 120Copyright 2023, Royal Society of Chemistry.spectra during the Na + ions extraction/insertion processes (Figure 9A,B).The adsorption peaks of coordinated water (3548 and 3621 cm −1 ) disappeared after the full charging, demonstrating that the coordinated water was removed along with the extraction of Na(OH 2 ) + unit.Whereas, the vibration of interstitial water situated at 1618 cm −1 was weakened, illustrating that if partial interstitial water was removed, the residual interstitial water would occupy the 8c sites because of the large H 2 O molecule.At the full-discharged state, the adsorption peaks of coordinated water would reappear, and the peak of interstitial water was increased.Wang et al. 118 confirmed that the existence of water would result in swelling of pouch full cells.They further investigated the electrochemical reaction behaviors before and after removing water (Figure 9C,D).According to the CV curves, the lattice water in PBAs would strongly affect the redox potentials.The peak at ∼4.0 V emerged either before or after the removal of water because of the extraction of [NaH 2 O] + ions.Different from the pristine sample, the dehydrated sample displayed two strong redox couples.It was ascribed to the insert of Na + ions on other sites when the crystal field changed, Subsequently, the redox peaks shifted to lower voltage because of the different oxidation energies of Fe.In terms of phase transformation, in situ PXRD patterns demonstrated that the pristine sample underwent a significant phase transition, while the dehydrated sample only had volume variations.In addition to the crystal water in the structure, airborne moisture also can affect the morphology and electrochemical properties.Ojwang et al. 119 investigated the effect of airborne moisture on capacity loss for monoclinic hydrated and rhombohedral dehydrated PWs.First of all, the external sodium on the bulk PBAs reacted with moisture to generate sodium hydroxide.At the same time, part of Fe 2+ was oxidized to Fe 3+ .Subsequently, the PW disintegrated to [Fe(CN) 6 ] 4− and iron oxides under the basic environment loaded on the surface of PW particles.Although the capacity loss was existence in the first process, it was reversible.However, the decomposition in the second stage was irreversible along with the extension of time.All the processes would cause the production of a passivation surface layer, thereby preventing the loss of irreversible capacity (Figure 9E,F).
In order to eliminate the unfavorable effect of crystal water on electrochemical properties.At present, the most prevalent way to remove crystal water is adopting the heat treatment.Yang et al. 120 reported a heat-treatment method to remove coordinated water for GO/PB composite (GOPC).The coordinated water could be released from GOPC through electron exchange between PB and GO at a high temperature (Figure 9G).Besides the way of heat treatment, Xie et al. 121 utilized Lewis acid (AlCl 3 ) to capture coordinated water during the cycle process.The reaction mechanism could be summarized as 6Na(OH 2 ) + + 2Al 3+ = Al 2 O 3 •3H 2 O + 6Na + + 6H + .The as-formed Al 2 O 3 •3H 2 O could restrain the side reactions at high voltage and the dissolution of M′.For the moisture in the air, Yang et al. 122 proposed a new passivation strategy, forming a surface passivation to inhibit the reaction between airborne moisture and PBAs.Further exploration illustrated the physical interaction between surface passivation and PBAs could effectively avoid the re-adsorption of water.
Compared with crystal water, vacancy is regarded as another serious drawback for PBAs.Coordinated water can hinder the migration of Na + ions, while vacancies can affect the migration of electrons.The vacancies occur in the rapid co-precipitation process, which is the primary defect existing in the crystal lattice and occupied by sixfold coordinated water in each vacancy.The appearance of [M′(CN) 6 ] vacancies will reduce the numbers of Na + ions in PBAs in order to maintain electric neutrality.
In order to fabricate HQ PBAs, researchers have put forward some strategies.At present, most researchers concentrated on controlling the crystallization rate to reduce vacancy content.Guo and Goodenough prepared PBAs through the hydrothermal method adopting a single iron source. 80,82,123,124Cao and Huang reported PBAs with low defects via utilizing chelating agents. 106,125Chou systematically investigated synthetic conditions, such as metal-chelating agents and inert atmosphere protection. 126n addition, the vacancy can be repaired through solid reactions.Tang et al. 97 synthesized NaFeFe(CN) 6 with tiny vacancies by the solvent-free solid-state reaction using  128 proposed insitu vacancy repairing method to attain iron HCF (FeHCF) in a highly concentrated Na 4 Fe(CN) 6 ⋅10H 2 O solution.The vacancy defects significantly decreased, and the structure was obviously reinforced for the as-synthesized FeHCF.Therefore, the side reactions could be effectively suppressed and the capacity arising from low-spin Fe LS (C) can be activated.The relationship between the vacancy defects and the lowspin Fe LS (C) capacity contribution was investigated by DFT calculations.In a vacancy-poor FeHCF, the 24d and 8c were the most stable storage sites and the metastable sites, respectively.Whereas, 24d sites were destroyed arising from the deficiency of [Fe LS (CN) 6 ] 4− in a vacancy-rich FeHCF, leading to the low capacity contribution of lowspin Fe LS (C).
In addition to the crucial factors mentioned above, the microscopic morphology structure of PBAs also has an important effect on the reaction mechanism.For the majority of PBAs, which have a common nanocube morphology.Whereas, the longer Na + ions diffusion distance make the active sites cannot be availably utilized.Tang et al. 129 successfully synthesized hierarchical hollow nanospheres (Na 1.58 Fe[Fe(CN) 6 ] 0.92 ) by a facile template method.The hollow nanospheres consisted of numerous nanoparticles, which performed a larger surface area as well as faster mass transportation.Therefore, the sample with a hollow structure exhibited faster diffusion kinetic between the interfaces of electrolyte and electrode materials, and delivered excellent electrochemical performances.Wan et al. 130 reported a binder-free cathode consisting of PBAs nanoframes/CNTs.The sample displayed an increased contact area, more active sites, and shorter Na + ions diffusion path than that of nanocubic PB.Meanwhile, the stepwise hollow structures were distributed in CNTs conductive networks, which could increase electronic conductivity.Chen et al. 131 prepared a beads-on-a-string structure, and among them, the nanoframes inherited the size of nanocubes and exhibited the cavities structure.In synergy with CNT, the nanoframe/CNT composite showed vast active sites and fast ions diffusion.On the other hand, the electrochemical behavior is sensitive to various crystallitesize materials due to the ion diffusion dynamics.In 2017, He et al. 132 developed various crystallites by utilizing controlled chemistry routes and discovered a non-negligible influence of crystallite size on the electrochemical property.The crystallites with a diameter of 20 nm showed an optimal capacity, which was close to 140 mA h g −1 and two plateaus located at 3.2 and 4.0 V, while the crystallites with a diameter of 160∼200 nm delivered slightly inferior electrochemical behavior.Unfortunately, the micron crystals displayed limited capacity.

THE INFLUENCE OF SYNTHESIS CONDITIONS OF PBAs
As far as PBAs are concerned, designing PBAs with perfect crystal structure and excellent electrochemical performance is significant to further promote the industrialization of PBAs in SIBs.However, there are some inevitable problems occurring during the synthetic process of PBAs.PBAs are generally prepared by directly mixing the solutions containing metal ions and ligands (ferricyanide, etc.) and always have defects in their crystal structures, such as interstitial and coordination water and vacancy as well as fewer guest ions.In this section, the important influence of the preparation process on PBAs is summarized.

The influence of auxiliary reagents
To acquire desired and satisfactory crystal structures with extraordinary electrochemical performance, adjusting the categories and amounts of the auxiliary agents in various synthetic methods can be adopted during the preparation process, such as chelating agents, surfactants, metal salts, reducing agents, and inert atmospheres.

Chelating agents
Chelating agents can be summed up into two categories: organic chelating agents and inorganic chelating agents.At present, organic chelating agents mainly involve citrates, citric acid, and ethylenediaminetetraacetic acid salts.
Citrates have received widespread attention as a chelating agent.In 2013, Hu et al. 133 used trisodium citrate to chelate with multiple metal ions and observed the subsequent reaction process was controllable.The reaction between metal ions and citrate ions was detected by ultraviolet-visible (UV-Vis) and 1 H Nuclear magnetic resonance (NMR) spectra, demonstrating that the production rate of PBAs was drastically slowed, and thus the crystal growth was in control.Moreover, the morphological transitions were also investigated by adjusting the reaction times, in which the particles with small sizes were formed during the initial reaction stage and acted as nuclei for further crystal growth.In addition, the existence of trisodium citrate could inhibit the generation of a good deal of nuclei in the crystallization process.In this case, the nucleation and growth processes could be separated and finally lead to the formation of monodispersed colloidal particles.What is more, the sodium citrate exhibited unequal binding strength for different metal ions.Gebert et al. 134 synthesized PBAs with the assistance of sodium citrate by the one-pot procedure.The stability constants and free energy of formation favored the achievement of [Ni 2+ -citrate].
The tendentiousness for Ni 2+ seemed so pronounced that only when the Mn 2+ ions were fully consumed, Ni 2+ ions would be released from the [Ni 2+ -citrate], which led to the NiPB growing on the surface of the inner MnPB.The inner layer was subjected to anisotropic strain from the epitaxial outer layer, which could inhibit the JT distortions during the Na + ions extraction process (Figure 10A).Nevertheless, there are relatively few reports on citric acid as well as ethylenediaminetetraacetic acid (EDTA) salts.In 2020, Hu et al. 135 successfully synthesized concentration-gradient Na x Ni y Mn 1-y Fe(CN) 6 ⋅nH 2 O (g-NiMnHCF) through a facile co-precipitation method with citric acid as a chelating agent.The as-obtained cathode possessed Ni-rich shell and Mn-rich core structure.The phase-field modeling demonstrated that the g-NiMnHCF suffered from low von Mises stresses and hydrostatic stress levels as well as negligible damage.The unique and robust structure can effectively remit the stresses and damage during the Na + ions extraction/insertion processes (Figure 10B).In which, EDTA salts not only show strong chelating ability for metal ions but also play an important role in the construction of special morphologies.Peng et al. 136 utilized the strong complexation ability of EDTA 4− to Mn 2+ ions, and the Mn 2+ ions could be slowly released from [EDTA-Mn] 2+ under appropriate acid concentration to prepare the highly crystalline Na x Mn[Fe(CN) 6 ] y ⋅nH 2 O (H-PBM) micro-cube crystals.Therefore, the monoclinic H-PBM showed promising electrochemical performance due to its enhanced microstructural and electrical properties.Shang et al. 137 developed Mn vacancies (V Mn ) in hexapod-like Mn-Fe PBAs through ethylenediaminetetraacetic acid disodium (Na 2 EDTA).Mn 2+ chelated with EDTA to form a six-coordinated octahedron, which showed a larger stability constant than that of Mn-Citrate (Cit) to slow down the formation of EDTA-Na 2 Mn[Fe(CN) 6 ] (NMF).At the same time, the unreacted EDTA would snatch the unstable Fe 2+ /Mn 2+ at edges and corners, leading to hexapod structures (etching process).Benefiting from the suppressed JT distortion as well as the structural integrality of NMF on the surface, the as-produced samples exhibited record-breaking cycling stability and capacity retention (Figure 10C).In addition, Li et al. 138 synthesized a K 2 Mn[Fe(CN) 6 ] (KMF) octahedra by utilizing the Mn 3 [Fe(CN) 6 ] 2 as the precursor, and octahedral superstructure could be further obtained by adjusting the kinetics of topotactic transformation.The special structure was achieved by the K 2 EDTA, which was utilized as a chelating agent and structure-directing agent and played an important role in chelating Mn 2+ ions and inducing the formation and growth of {111} facets (Figure 10D).
The inorganic metal ions chelating agents are mainly including sodium pyrophosphate (Na 4 P 2 O 7 ) and sodium oxalate (Na 2 C 2 O 4 ).In 2019, Xu et al. 139 134 Copyright 2021, Wiley-VCH.(B) The simulation results of stress generation and damage evolution; reproduced with permission. 135Copyright 2020, American Chemical Society.(C) The Mn−N bond evolution as the Na + ions extraction; reproduced with permission. 137Copyright 2020, Elsevier.(D) The evolution mechanism of KMF-octahedral superstructure.Reproduced with permission. 138Copyright 2022, American Chemical Society.
Na content and fewer vacancies when the ratio of Na 4 P 2 O 7 and Ni 2+ ions was 2:1.Benefiting from the Na 4 P 2 O 7 as the chelating agent, both NiHCF-3 and NiHCF-2 displayed an obvious peak splitting, which belongs to the characteristic peaks of the monoclinic phase.Whereas, the NiHCF-1 displayed a traditional cubic phase.The pair distribution function analysis was utilized to make sure the structural difference, and it presented a longrange consistency, while there was an obvious structural difference in a short range (Figure 11A).Therefore, NiHCF-3 demonstrate better electrochemical performance than that of NiHCF-2 and NiHCF-1.Afterward, in 2020, Xu et al. 140  (C-G) All panels are reproduced with permission. 141Copyright 2019, American Chemical Society.
to the N and Ni shared electrons to form covalent bonds when Ni 2+ ions coordinated with Fe(CN) 6 4− groups, further resulting in the improvement of C≡N strength and the higher shifting values of v(CN) vibrations.On the other hand, the whole reaction process was related to Fe-CN-Ni at 2088 cm −1 .At first, there was a burst nucleation process occurring in precipitation solutions.Subsequently, the relative intensity after the nucleation process remained almost unchanged under the control of C 2 O 4 2− ions.That is to say, less Fe-CN-Ni skeletons were formed over time under the assistance of C 2 O 4 2− ions than those without C 2 O 4 2− ions.The chelating agents play a tremendous role in coordinating with metal ions and slowing down the rate of nucleation.For diverse metal ions, various chelating agents show different chelating abilities and special morphologies can be obtained by adjusting the categories and amounts of the chelating agents.Thus, the selection of suitable chelating agents is momentous for the construction of PBAs.

Surfactants
The commonly used surfactants include nonionic surfactants and ionic surfactants (cationic and anionic surfactants) for the synthesis of PBAs.Poly(vinylpyrrolidone) (PVP) is one common nonionic surfactant, which plays a nonnegligible role in morphology and structure control during the preparation of PBAs.In 2011, Hu et al. 142 investigated the role of PVP in terms of the nucleation process and crystal growth as well as the homogeneity of products.During the synthetic process of Mn 3 [Co(CN) 6 ] 2 ⋅nH 2 O, PVP would absorb on the specific crystal nuclei surfaces to achieve the purpose of capping, further lower the surface energy of the particle.The amide moiety in PVP would weakly coordinate to the Mn 2+ ion, and hence the steric stabilization was beneficial to construct the PVP-protected and uniform nanocubes.If PVP was removed, only the irregular particles and nanocubes could be obtained, further confirming the function of PVP as a capping agent during the reaction system.Different morphologies can be obtained with PVP as an additive.Wu et al. 143 developed a sphere-like Zn-PBAs via co-precipitate method with the assistance of PVP.The nuclei would generate at first and gradually grow to intermediates, and then the nanocrystallites aggregated to produce mosaic intermediates.When PVP participated in the synthetic processes, the nuclei would be capped and then agglomerated because of the low surface energy.Consequently, the sphere-like Zn-PBAs sample would be acquired due to the similar agglomeration at different directions.Nai et al. 144 prepared nanoboxes, nanocages, and nanoframes by utilizing PVP and citrate in the The illustration of the formation of different structure; reproduced with permission. 144Copyright 2018, Elsevier.(B) The synthetic process of OPB2.Reproduced with permission. 147Copyright 2016, American Chemical Society.(C-D) Sketch map of volume change, Fe K-edge XANES and Fourier-Transformed of K 0.33 FeFe(CN) 6 /RGO electrode; reproduced with permission. 150Copyright 2017, Royal Society of Chemistry.(E) The growth mechanism of nanocubes and octahedrons.Reproduced with permission. 151Copyright 2012, American Chemical Society.
reaction.As shown in Figure 12A, the PVP could effectually control the growth kinetics and stabilize the {100} facets, while the removed cubic cores could be attributed to a thermodynamic factor.On the other hand, suitable concentration of surfactants can selectively adsorb on specific crystal faces and alter the growth rate of the crystal faces.In consequence, the morphology and size of products would be changed.Zhang et al. 145 reported the capping role of PVP for the shape tunable in the synthesis of Mn 2 Mo(CN) 8 ⋅xH 2 O crystals.At the low concentration of PVP (0.1 M), its influence on the morphology and size of the samples was limited.The micro/nano-crystals were obtained, which possessed the same shape as the bulk crystal.With the PVP concentration increased to 0.5 M, the PVP molecules could selectively bond on specific crystal planes and induce the formation of one-dimensional particles.When the PVP concentration reached 0.75 M, the morphology was obviously changed, and the size of the products displayed a continuous decrease.The phenomenon was related to the conventional route for the preparation of size-tunable crystals, and the high-concentration additive could effectively suppress the crystal growth.
In addition, PVP also exhibits a protective effect and weakly reducing property.Hu et al. 146 illustrated the protecting role of PVP in the synthesis of hollow interior via controlled chemical etching.The chemical etching was only related to the etching time at a specific concentration of PVP, and the hollow interior of PB nanoparticles would be acquired under an appropriate reaction time.PVP could exhibit the protecting role by adjusting the concentration of PVP.When none of or too much of PVP was dissolved into the solution, the hollow interior of PB nanoparticles cannot be observed.The PVP layers could protect the shells and decrease the etching rate, as its amide unit could bond with iron ions and further adsorb easily on the nanoparticle surface at a specific concentration.Chen et al. 147 prepared porous submicron cubes (OPB2) via a chemical corrosion reaction (Figure 12B).Among these, PVP could control the micromorphology to display smaller sizes and smoother edges than that without PVP because of its surfactant effect.In addition, PVP as a protecting agent could restrain serious etching due to its outstanding binding ability with Fe ions.Zhang et al. 148 found that the rapid formation of Co 3 [Fe III (CN) 6 ] 2 nanocubes would react with the produced Fe II species from [Fe(CN) 6 ] 3− using PVP as a reducing reagent.The Fe II would selectively etch the face-center of nanocubes to form core-shell cubes as the PVP selectively accumulated to edge/corner sites.
Ionic surfactants can be divided into two types, cationic and anionic surfactants.Cetyltrimethylammonium bromide (CTAB) belongs to a king of cationic surfactants.Hu et al. 149 reported an elongated cubic PB crystal via selective etching.It was well known that the surfactants could adsorb on the crystal surfaces and induce the selective growth of crystal faces, while selective adsorption was able to induce selective etching.It was worth to mention that HCl was responsible for etching, and CTAB might have undertaken the role of selective etching, as only the irregular crystals that were randomly destroyed by HCl could occur in the absence of CTAB.Whereas, an originally intact nanocube would be obtained through the selective etching at the corner.The faces were protected by CTAB, and then the elongated cubic PB was formed as the etching extended along the edge.Wang et al. 150 developed K 0.33 FeFe(CN) 6 /RGO samples, which was achieved using CTAB as surfactant and RGO as the template by a facile synthesis method, and demonstrated CTAB preferred to pin the {100} plane via DFT.The asobtained products manifested superior electrochemical performances, benefiting from their high specific surface area, robust framework, conductive RGO coating, and greatly reduced lattice water defects.Besides, the lattice structure maintained un-deformed, and the state evolution of Fe was reversible during the charge/discharge processes (Figure 12C,D).
Whereas, there were relatively few reports about the effects of sodium dodecylbenzenesulfonate (SDBS, as a type of anionic surfactant).Hu et al. 151 fabricated nanocubes and octahedrons PBAs with different surfactants (Figure 12E).The cubes and octahedrons were determined by the competition of ( 111) and (100) crystal faces.During the processes of crystal growth, SDBS would first adsorb on specific crystal faces, and then obviously decrease the surface energy as well as induce crystals epitaxy and assemble into an octahedron.In addition, SDBS could provide steric stabilization for the surfactantprotected nanoparticles to prevent aggregations due to the weak coordination between surfactants and Cd 2+ ions.
The surfactants display enormous roles in the formation of PBAs: (i) Selectively adsorb on specific crystal surfaces to prevent the random agglomeration of particles and achieve various structures with uniform morphology; (ii) protect the structure from being damaged during the chemical etching process; and (iii) keep metal ions from oxidization.
Therefore, it is necessary to introduce surfactants into the synthetic process.

Metal salts
In PBAs, one of the urgent problems is the low Na + ions content.In this regard, a variety of strategies have been proposed, one of which is to optimize the Na + ions concentration in solution by adding metal salts.The NaCl concentration will affect the electrochemical properties and morphology of PBAs.Li et al. 152 prepared multifarious Na 1+x FeFe(CN) 6 in various concentrations of NaCl.The XRD Rietveld refinement results demonstrated that more Na + ions could insert the crystal lattice of the framework, resulting in the increase of lattice parameter a and Na-Fe distance, corresponding to the PB-1 to PB-5.Based on the abundant Na + ions, the PB-5 electrode exhibited better electrochemical properties than the other samples.In addition, the structural evolution was illustrated through in situ PXRD (Figure 13A-C).Afterward, Chen et al. 153 developed a nanosized PB particles and explored the role of Na + ions content for the morphologies of PBAs products.The PBAs products showed four morphology structures with different Na + ions content.With the addition of Na + ions into the reaction system, the vacancies would be occupied by Na + ions, further inhibiting the growth of PBAs by breaking down the Fe(CN) 6 4− , finally obtaining the irregular morphology (PB-1 M).PB-2 M displayed a multirod-like structure, owing to the certain growth orientations of PB that were suspended when the NaCl concentration was 2 M. With the NaCl solution increased to nearly saturation, the suspension in all directions became more severe, further leading to the synthesis of nanosized particles (PB-4 M).In addition, the structural stability would be improved with the increased concentration of Na + ions.Moreover, Na 2 SO 4 can also act as a sodium source, which is mostly used in the electrodeposition method.Wang et al. 154 synthesized a 3D hybrid cathode in an aqueous solution containing Na 2 SO 4 as Na + ions source, which showed a surprisingly high specific capacity upon the initial discharge (194 mA h g −1 at 0.5 C), good cycling performance (121.6 mA h g −1 over 500 cycles), and excellent rate performance (97 mA h g −1 even at 15 C).

Reducing agents and inert atmospheres
It is well known that there is an existing inevitable metal oxidization due to its instability under air atmosphere during the preparation processes of PBAs materials, which can lead to a low content of Na + ions in PBAs materials.In  152 Copyright 2015, American Chemical Society.(D) In situ XRD and Raman spectra of electrode; reproduced with permission. 155Copyright 2015, Springer Nature.(E) UV-visible absorbance of PB crystals prepared with VC(AAPB) and regular PB (control); reproduced with permission. 156Copyright 2022, Wiley-VCH.(F-G) Reaction scheme for the synthesis of PW, as well as the representative galvanostatic charge-discharge plots.Reproduced with permission. 158Copyright 2021, American Chemical Society.
order to solve the problem, the reducing agents and inert atmospheres are usually introduced during the synthesis of PBAs materials.
Currently, the commonly used reductants and protection atmosphere are VC as well as N 2 and Ar.You et al. 155 developed Na 1.63 Fe 1.89 (CN) 6 by employing VC and N 2 as the reducing agent and protecting atmosphere, respectively.Based on the elemental analysis, the Na + ions content in various PBAs fabricated in different conditions were detected.It could be clearly seen that the Na + ions increased to 1.63 in PBAs, as the addition of VC and N 2 atmosphere could effectively prevent Fe 2+ /[Fe(CN) 6 ] 4− from oxidizing to Fe 3+ /[Fe(CN) 6 ] 3− .Moreover, PBAs with different content of Na + ions all exhibited similar morphologies.In terms of electrochemical performance, the initial charge-specific capacity was 21, 82, 122, and 153 mA h g −1 , corresponding to the increasing content of Na + ions in PBAs.In addition, the Na + ions storage mechanism and the valence state change of Fe were clarified, and these results suggested the Na 1.63 Fe 1.89 (CN) 6 was highly reversible (Figure 13D).Wang et al. 126 synthesized rhombohedral NaFeHCF under Ar atmosphere.The samples displayed a stacked cubes structure and outstanding electrochemical performance.
Moreover, VC can not only act as a reductant but also as a chelating agent.Lim et al. 156 fabricated sodium-PB cathode via employing excess VC in the preparation, with a followed 200 • C heat-vacuum drying process.It was found that VC chelated with Fe 2+ would disrupt the crystal structure, further leading to nano-porous structure (Figure 13E).The as-formed nano-cracks allowed Na + ions to deeply percolate into the PBAs and unlock the inner volumes.Camacho et al. 157  Furthermore, researchers have also reported other kinds of reducing agents.Lim et al. 158 presented a mild, low-temperature, and facile method to synthesize PW by using common sodium borohydride (NaBH 4 ) as a reducing agent.The as-produced PB was suspended in a mixing solution involving ethanol and water, and then PW was synthesized after adding NaBH 4 into the mixing solution.Different from the other synthetic conditions, this work employed a convenient post-synthesis treatment, so that the Fe 2+ oxidation during the synthesis of PB could be essentially ignored.Moreover, PW demonstrated a remarkably higher specific discharge capacity than that of PB (Figure 13F,G).Wu et al. 159 reported a low-defect Na 1.33 Fe[Fe(CN) 6 ] 0.82 nanoparticles by utilizing NaI solution.With the assistance of low defects as well as nanocubic structure, the as-prepared electrode showed outstanding electrochemical performance.Brant et al. 160 prepared three samples by adjusting the drying temperature and using saturated NaI in inert conditions to control the contents of sodium, vacancy, and water.Ojwang et al. 161  .Meanwhile, similar results were acquired when using sodium thiosulfate (Na 2 S 2 O 3 ).In which, the Fe III was reduced to Fe II upon increasing x, and at the same time, accompanied by the decreased a-axis.

3.2
The influence of solvents properties

The influence of different solvents
As far as the synthetic environments of PBAs are concerned, they can be divided into solid-phase reactions and liquid-phase reactions.At present, PBAs materials generally are fabricated by liquid-phase reaction with water as a solvent.Consequently, the rapid precipitation processes will cause inevitably considerable defects in PBAs.In this regard, the researchers try to solve these problems by introducing solvents other than water, as different solvents with different physical and chemical properties (Table 2) might play an important role in controlling the morphology and sizes of the products.It is worth mentioning that ethanol as the solvent can not only slow down the growth rate of PBAs but also induce the formation of special morphologies.Hu et al. 142 probed into the effect of introducing ethanol into the water on Mn 3 [Co(CN) 6 ] 2 ⋅nH 2 O.The nanocubes showed large scaled and uniform shape and size (a mean particle diameter was 240 nm), and the surfaces were extremely smooth (ethanol/water = 3:1).And then the nanocubes would increase to 2 μm, and there were some irregular structures that emerged (ethanol/water = 1:2).When there was no addition of ethanol, the morphology of sample transformed to microframes and the corresponding size increased.The crystallite sizes demonstrated that the viscosity of ethanol was stronger, and the crystal nucleation and isotropic growth would slow down to form uniform nanocubes.Zuo et al. 162 also evaluated the role of hydroxyl groups (−OH) in ethanol.As shown in Figure 14A, the {100} plane could control the formation of nanocubes, and −OH mainly adsorbed on the {100} plane.Moreover, the surface energy of −OH adsorption on Fe-CN was the lowest, and the crystalline edges and vertices were adsorbed at first sites.Thus, −OH that adsorbed on Fe-CN could inhibit the growth of Fe-CN sites on the crystalline edges, further leading to generate flower structures.Other irregular particles were obtained without ethanol, further proving the importance of the −OH in regulating crystal morphology.
As we know, ethylene glycol exhibits both coordination and reduction effects.Ma et al. 163 fabricated a monodisperse copper HCF (PBCu) nanoflake in a mixing solution (40 mL water and 20 mL ethylene glycol).Due to the coordination effects of ethylene glycol, Cu 2+ ions were slowly released from chelate and then coprecipitated with [Fe(CN) 6 ] 4− ions to form tiny crystals, in which ethylene glycol could absorb on the surface of these tiny crystals to suppress the PBCu crystal growth.Su et al. 164 prepared uniform K 2 Fe[Fe(CN) 6 ] nanocubes with an open-framework structure in a large scale.In which ethylene glycol was used as the solvent, and a low-temperature solvothermal method was adopted.The as-produced nanocubes showed a much smaller particle size with an edge length of 200 nm than that of the sample synthesized in an aqueous solution.Fe was retained in the divalent state, due to its mild reducing property.
Moreover, other solvents are also adopted.Zhang et al. 165 synthesized truncated CoHCF nanocubes threaded by CNT (CoHCF-Cit/CNT) by a simple co-precipitation process.The ratio of glycerol/water was crucial for the synthesis of truncated nanocubes.The samples with cubic shapes could be produced without the participation of glycerol.Bu et al. 166 adopted non-classical crystallization and an externally oriented attachment strategy to synthesize hierarchical PB composed of ultrathin nanosheets (HPBNs).It was fabricated in an acidic mixture of water and N, N-dimethylformamide (DMF; 5 and 25 mL) by the solvothermal method (Figure 14B).Besides the reaction time, the species and the content of solvent were also The illustration of formation mechanism for different morphologies; reproduced with permission. 162Copyright 2020, American Chemical Society.(B) The illustration of the formation of hierarchical PB composed of ultrathin nanosheets; reproduced with permission. 166 The sample performed a mean side length of ∼750 nm and an average thickness of ∼160 nm, as well as relatively smooth surfaces of the microplates.When the volume of isopropanol and water was 35 and 5 mL, respectively, the microplates disappeared, and irregular particles and particle aggregations were observed.When isopropanol was not added, the sample was amorphous based on its XRD pattern.These results clarified that isopropanol had a crucial influence on both the morphology and crystallinity of products.Duan et al. 84 proposed a two-phase stratified solution to synthesize PBAs.The oil-water (butanol and water) interface could restrain the crystallization process to produce PBAs with low defects and high crystallinity.Li

The influence of acidic and alkaline environments
In liquid-phase reaction systems, the acidic and alkaline environments besides the solvents will also have different effects on the reaction of single metal source and multi-metal source, and the optimum structure and properties can be obtained by choosing the appropriate reaction environment.
In terms of reactions involving only a single metal source, such as [Fe(CN) 6 ] 3−/4− , PBAs can be formed only under acidic conditions.Shen et al. 169 investigated the influence of acidic and alkaline environments on PB.In an alkaline environment, the powder could be obtained, which was consistent with the report by Cao et al. 170 The Fe 3+ ions would be hydrolyzed to obtain FeOOH or Fe(OH) 3 in an aqueous solution, and then were decomposed into α-Fe 2 O 3 .In acidic conditions, the hydrolyzation process of Fe 3+ ions was restrained (pH < 3.0).At last, the final product was PB (pH 2.0) rather than α-Fe 2 O 3 .In 2014, You et al. 124 synthesized HQ PBAs (HQ-NaFe) via a simple process using a single iron source in acidic conditions (1 mL hydrochloric acid).The sizes of HQ-NaFe nanocubes gradually increased to 300∼600 nm with the prolonged reaction time.HQ-NaFe nanocubes endowed an excellent ion-storage capability and abundant transportation pathways as well as robust crystal structure upon cycling (Figure 15A).Therefore, the HQ-NaFe electrode exhibited impressive electrochemical performance.In addition, Wang et al. 171 explored PBAs' growth and etching mechanism in acidic conditions.The PBAs products synthesized in different pH values, labeled as 1.78, 1.38, 1.08, and 0.92.When the pH values were 1.78 and 1.38, both the samples showed cubic morphology.Whereas, the size distribution of the sample synthesized at pH = 1.38 was narrower and exhibited rougher surfaces than that of the sample obtained at pH = 1.78, indicating the etching role from the PBAs surfaces.When the pH value increased to 1.08, the morphologies changed to spheres.While the pH value was 0.92, PBAs exhibited an irregular sphere (Figure 15B).
In terms of electrochemical performance, the as-obtained samples displayed a good rate performance and cycle stability when the pH value was 1.38.As the slight etching was beneficial to the reaction kinetics of Na + ions, excessive etching would take a large numerous defects in PBAs, leading to a poor sodium storage performance.
The synthesis of PBAs from multi-metal sources can be obtained for both acidic and alkaline environments.Wu et al. 143 demonstrated the influence of acidity on the morphology of PBAs materials.When more H + ions participate in the reaction system, the crystal nuclei could be rapidly assembled on the high-energy crystal planes to form a cubic structure.When H + ions were lower, the assemble rate was slower, and the corresponding crystal planes were retained.The polyhedral particles were obtained in this condition (Figure 15C).Zuo et al. 172 prepared various Na 2 MnFe(CN) 6 cathode materials (PBM-X) by controlling the pH value with ethylic acid (CH 3 COOH) or ammonia (NH 3 ⋅H 2 O) during the co-precipitation reaction.The XRD patterns in Figure 15D,E demonstrated that products were the face-centered cubic structure.While the PBM-11 showed some impurity and the intensity of all peaks was relatively weak, the infrared spectra of PBM-11 showed a weak absorption peak at 636.92 cm −1 , corresponding to the Mn-O bond, which could be attributed to some manganese oxides or hydroxide.Moreover, all the products delivered irregular spherical shapes, and the minimum mean particle size was calculated to be 64.3 nm for PBM-3.When the PBM-X was applied as the cathode, the redox peaks of PBM-3 were more conspicuous than the other samples, while PBM-9 showed a relatively good reversibility.

CONCLUSION AND OUTLOOK
PBAs have been widely applied in electrochemical energy storage due to their low price, easy preparation, stable open framework, and nontoxicity.In this paper, we have provided an insightful review of the crystal structures, synthetic methods, and electrochemical mechanisms of PBAs, along with a discussion of the influences of the synthetic conditions of PBAs on morphology and electrochemical performance.At first, the crystal structures of PB and PBAs are analyzed, and the synthetic methods of PBAs are summarized in detail, including the co-precipitation method, electrodeposition method, hydrothermal method, mechanical ball milling method, and template method.Second, we carry out an in-depth analysis and discussion on the electrochemical reaction mechanism of PBAs from five aspects: migration ions, various transition metals, water molecules, [Fe(CN) 6 ] vacancies, and morphology of PBAs.Finally, the influences of the synthetic conditions of PBAs, such as chelating agents, surfactants, metal salts, reducing agents, and inert atmosphere, as well as a variety of solvents and their pH values, and so forth, on the morphology and size of PBAs as well as the electrochemical performance are analyzed.For the future development of PBAs, some prospects have been summarized as follows: (i) The electrochemical properties of PBAs materials are influenced by the defects that are occupied by the structural water content, thus resulting in lower initial charge capacity and worse cycling stability.In response to this question, the crystallinity is crucial for constructing high-performance PBAs materials.
In our opinion, suitable chelating agents selected and supplemented with corresponding surfactants or metal salts are the most feasible way to construct sodium-rich PBAs with less defects and special morphology.Moreover, there are relatively few categories of additives applied in the synthesis of PBAs, and the development and investigation of new additives and their corresponding mechanism are essential for PBAs materials.It is worth to note that the introduction of neutral ligands, such as CH 3 CN, NH 3 , CO, C 5 H 5 N, and so on, into the PBAs framework can achieve partial or complete replacement of crystal water.It is inspired that less crystal water exists in PBAs containing K + ions, as the large K + ions will occupy the 8c sites with large free spaces due to their low adsorption energy.Therefore, this is an effective way for the partial or complete replacement of crystal water in PBAs by bringing in K + ions.(ii) As far as the most widely studied HCF-type PBAs are concerned, the structural evolution caused by migration ions (de)intercalation, involving lattice distortion and volume change, as well as the dissolution of transition metals, are important and urgent problems.
Various optimization strategies can be adopted to solve this problem, including transition metal doping, carbon decoration or polymer coating, concentration gradient, core-shell structure, and so forth.Besides, in situ observation and analysis techniques should be introduced to evaluate the complex behaviors of the migration ions, water molecules, and the structural evolution in different cycling periods.(iii) The charge storage of bulk PBAs materials synthesized by the co-precipitation method is limited by ion diffusion and further leading to sluggish kinetics.Thus, enhancing the utilization of active sites and improving ions diffusion kinetics are crucial in promoting the electrochemical performance.
The improvement strategies are shown as follows: (a) skillful selection of organic solvents with different polarity and viscosity as well as adopting suitable methods can fabricate hollow or porous structures 73 fabricated a monoclinic Na 2 Fe 2 (CN) 6 •2H 2 O by heating the solution containing Na 4 Fe(CN) 6 and ascorbic acid (VC) in a flask at 140 • C. The full cells using Na 2 Fe 2 (CN) 6 •2H 2 O as cathode and graphite or Na 2 Ti 3 O 7 as anode were assembled to obtain the cell with an energy density of 70-90 Wh kg −1 .These results indicated that air-stable and water-insoluble monoclinic Na 2 Fe 2 (CN) 6 •2H 2 O was very attractive for grid-storage applications.In addition, it is crucial to select appreciative synthetic temperatures.Fu et al. 74 investigated the effect of synthetic temperatures on the morphology and electrochemical performance of PBAs.PBAs were obtained using Na 4 Fe(CN) 6 and VC as the precursors under different temperatures of 60, 80, and 100 • C in the oil bath.It was worth to mention that the size distribution became wide with the enhanced reaction temperature.The sample obtained at

F I G U R E 5
(A-B) Fe L-edge as well as C and N K-edge sXAS spectra at different charge states; reproduced with permission.80Copyright 2015, American Chemical Society.(C-D) Cycle voltammetry (CV) curves, (E-F) plotting of ln (current) versus ln (scan rate) curves, (G) the capacitance at different scan rates, (H) the variation of peaks voltage at different scan rates.Reproduced with permission.81Copyright 2021, Elsevier.chemical composition of the film.He et al.91 synthesized K 2 Zn 3 [Fe(CN) 6 ] 2 ⋅9H 2 O cube arrays on carbon cloth (CC@KZHCF) by electrodeposition and subsequent water bath treatment.The as-obtained CC@KZHCF served as a self-standing cathode in aqueous rechargeable SIBs (ARSIBs) and delivered outstanding electrochemical performances and could power multiple light emitting diodes (LEDs) under various bending angles (Figure6A-D).

F I G U R E 6
(A-D) Schematic illustration and CV curves of the flexible aqueous rechargeable SIBs (FARSIBs), normalized capacities and LEDs powered; reproduced with permission.91Copyright 2019, Wiley-VCH.(E-F) In situ Raman and in situ PXRD 3D patterns.Reproduced with permission.94Copyright 2022, Wiley-VCH.stability with 84% retention in the full cell, and outstanding rate performance (127 mA h g −1 at 2000 mA g −1 ).Peng et al.94 introduced trace amounts of deionized water in the mechanical ball milling process to synthesize PBAs.The "water-in-salt" nanoreactor strategy was proposed to fabricate PBAs in a restricted reaction zone without additives.The obtained Mn-based PBAs (MnHCF-S-170) showed stable and fast property during −10 to 50 • C and excellent air stability.As shown in Figure6E,F, the Na + ions storage mechanism was revealed through in situ powder X-ray diffraction (PXRD) and Raman, illustrating that the structure of MnHCF-S-170 was highly reversible.Xu et al.95 fabricated PBAs with hollow structures via an antioxidant and chelating agent co-assisted non-aqueous (ethanol) ball milling method.The as-prepared PBAs with low vacancies and interstitial water content delivered a high initial Coulombic efficiency (ICE), excellent cycling stability, and rate performance via a highly reversible two-phase transition reaction confirmed by in situ XRD.Furthermore, mechanical ball milling can be used to modify the PBAs for designing composites.In 2018, Shen et al.96 prepared NMHCF/graphene (NMHCF/G) nanocomposites by a facile two-step method.NMHCF was first synthesized via a co-precipitation process in an aqueous solution and then combined with commercial graphene with the help of the vibration milling method to obtain NMHCF/G nanocomposites.Compared to the pure NMHCF, the NMHCF/G nanocomposites exhibited excellent electrochemical performance.It might be ascribed to the increased electronic conductivity from graphene.Moreover, graphene could restrain the corrosion of NMHCF particles in electrolytes to achieve long-term cycle life.Tang et al.97 synthesized NaFeFe(CN) 6 to solve the Na + ions deficiency of Fe 4 [Fe(CN) 6 ] 3 through ball milling the Fe 4 [Fe(CN) 6 ] 3 and Na 4 Fe(CN) 6 mixtures.The as-obtained sample displayed a cubic phase indexed to Fm3m and large lattice parameters owing to Na + ions insertion.As a cathode, the electrode presented excellent cycling stability with capacities of 96.8 mA h g −1 at 1 C.

3 . 1
Fe 2 [Fe(CN) 6 ] with a hierarchical nanotube (HN) structure (Prussian white [PW]-HN).The formation mechanism of special morphological structures could be put down to the Ostwald ripening process.In which, the inner part partly dissolved and regenerated on the surface, as shown in Figure7B.For PW-HN electrode, intercalation pseudocapacitive dominated the charge storage process.Moreover, in situ Raman and XRD illustrated the reaction of N-Fe III /Fe II sites accompanied by ignorable volume variation (<2.1%) in PW-HN enabled by intercalation pseudocapacitance.This cathode showed an ultrahigh rate of ∼83 mA h g −1 at 50 C, as well as unprecedented cycle life (65% over 10 000 cycles).Full cells exhibited high energy density (225 Wh kg −1 ) and capacity retention (84% over 100 cycles).(ii) The combination of the Kirkendall effect and the Ostwald ripening process: The Kirkendall effect is originally a concept in metallurgy, which refers to the phenomenon of the defects formation because of diverse diffusion rates of metal atoms.Afterward, the researchers propose to prepare hollow nanostructures by utilizing the Kirkendall effect.However, there are relatively few reports on the synthesis of hollow structures by only utilizing the Kirkendall effect in solution, especially for PBAs, although various hollow nanostructures have been achieved by adopting the Kirkendall effect under high-temperature conditions.Moreover, it is debatable for some reports to use the Kirkendall effect to explain the formation of hierarchical hollow structures.In 2019, Ren et al.101 developed a metal-organic framework-templated method to construct hollow structures.Cobalt-containing zeolitic imidazolate (ZIF-67) was composed of Co 2+ ions and imidazolate, which could easily release Co 2+ ions in an aqueous solution.After that, Co 2+ ions could react with [Fe(CN) 6 ] 4− to form cobalt HCF (CoHCF) on the surface of ZIF-67 due to the low solubility product constant (K sp = 6.7 × 10 −22 synthesized PBAs materials containing different Na contents by co-precipitation method.Among them, Na 1.53 Fe[Fe(CN) 6 ]•4.2H 2 O (PB-S1) and Na 1.73 Fe[Fe(CN) 6 ]•3.8H 2 O (PB-S3) displayed similar initial discharge capacities, while the initial charge capacity of PB-S1 was lower.In addition, the pouch full cell assembled with rhombohedral PB-S3 cathode demonstrated a high cycle stability (78% over 1000 cycles).

Fe 4 [
Fe(CN) 6 ] 3 and Na 4 Fe(CN) 6 as precursors.Cattermull et al. 127 further illustrated the vacancy filling by consuming the K 3 Co(CN) 6 and chemical modification of the Mn[Co(CN) 6 ] 2/3 .□. 1/3 ⋅xH 2 O to obtain low-vacancy PBAs.During the mechanochemical and post-synthetic modification processes, [Co(CN) 6 ] 3− ions are incorporated onto the initially vacant sites, accompanied by the intercalation of K + ions to maintain charge balance in the cavities of the PBAs framework.Inspired by this strategy, Wan et al.
fabricated a monoclinic NiHCF by introducing Na 4 P 2 O 7 as the chelating agent.The chemical structure and electrochemical properties of the NiHCF were interpretated under the various concentrations of Na 4 P 2 O 7 .The NiHCF-3 possessed higher F I G U R E 1 0 (A) The proposed stabilization mechanism of PBAs; reproduced with permission.
reported a monoclinic copper HCF nanosheet (CuHCF-P) with fine cycle stability based on the Na 4 P 2 O 7 as the chelating agent.It was ascribed to the formation of CuP 2 O 7 2− chelates between P 2 O 7 4− ions and Cu 2+ ions, and then the Cu 2+ ions would be dissociated slowly from CuP 2 O 7 2− chelate.Herein, the P 2 O 7 4− ions not only decreased the crystallization rate of CuHCF-P but formed a multilayer sheet structure.The thinner nanosheets could be obtained with the increased concentration of Na 4 P 2 O 7 .Moreover, the inactive mechanisms of PBAs were simulated by the DFT, which is owing to the coordinated H 2 O molecules (Figure 11B).In 2019, Xu et al. 141 clarified the synthetic mechanism of the NiHCF using Na 2 C 2 O 4 as the chelating agent by the operando detection with in situ FTIR.The varied coordination compounds showed different stability constants (in the range of 4∼9) when adopting different ratios of C 2 O 4 2− ions to Ni 2+ ions.As shown in Figure 11C-G, the FTIR spectra illustrated that the intensity of the peak with C 2 O 4 2− ions was distinctly low, further indicating a low reactive rate in the co-precipitation processes.During the reaction process, the absorption peak located at 2034 cm −1 significantly decreased after adding the Ni 2+ ions.What is more, a new peak in Fe-CN-Ni emerged at 2088 cm −1 .It could be owing F I G U R E 1 1 (A) The pair distribution function of NiHCF-1 and NiHCF-3; reproduced with permission. 139Copyright 2018, Wiley-VCH.(B) Charge density difference section and the CV curves of CuHCF; reproduced with permission. 140Copyright 2020, Elsevier.(C-D) Insitu infrared spectroscopy without/with C 2 O 4 2− assistant, (E-F) FTIR spectra of initial/final state, and (G) the illustration of the action of C 2 O 4 2− .

F
I G U R E 1 3 (A-C) Lattice parameters changes and cycling performance of Na 1+x Fe[Fe(CN) 6 ] electrodes as well as in situ PXRD patterns; reproduced with permission.
evaluated the synthetic conditions for PBAs.In terms of the sample synthesized by co-precipitation route at 60 • C (Na-FeHCF-Cop-60 • C), the rapid precipitation and nucleation induced irregular small-size particles, as the low concentration of VC cannot slow down the nucleation and growth, while the sample fabricated by hydrothermal route at 60 • C (Na-FeHCF-hyd-60 • C) exhibited micrometric well-shaped cubic morphology because of excess VC as a chelating agent.

F I G U R E 1 5
(A) Ex situ XRD patterns of HQ-NaFe; reproduced with permission.124Copyright 2013, Royal Society of Chemistry.(B) The acid etching mechanism; reproduced with permission.171Copyright 2020, Elsevier.(C) The illustration of formation mechanism for different morphologies; reproduced with permission.143Copyright 2016, Elsevier.(D-E) XRD and FTIR spectra of PBM-X samples.Reproduced with permission.172Copyright 2019, Elsevier.et al.168 developed the Na-enriched PBAs by utilizing acetic acid as the iron defect inducer.During the synthetic process, acetic acid could coordinate with Fe 2+ and induce the formation of iron defects in the PBAs lattice, in which acetone could control the number of iron defects by adjusting the pH value.In order to maintain the charge balance, Na + ions would insert into PBAs, resulting in less [Fe(CN) 6 ] vacancies and H 2 O (Figure14C).
Crystal water existing in raw materials was crucial for preparing the NaMHCF, and without crystal water, it could not obtain NaMHCF.The defects could be adjusted by the drying temperature.The NaMHCF electrode exhibited a high specific capacity of 168.8 mA h g −1 , long cycling Na 2.2 Ni[Fe(CN) 6 ] 0.8 ⋅□ 0.2 ⋅2.5H 2 O TA B L E 1 Summary of morphology, synthesis method, and phase transition of PBAs.Na 1.64 Ni[Fe(CN) 6 ] 0.92 •1.83H 2 O Particles Hydrothermal M ↔ C Na 1.45 Ni[Fe(CN) 6 ] 0.87 ⋅3.02H 2 O Nanocubes Co-precipitation M ↔ C Note: C, M, R, T, and O, represent the cubic, monoclinic, rhombohedral, tetragonal, and orthorhombic phases, respectively.

Relative polarity Viscosity (20 • C) Boiling point ( • C) Relative density (g mL −1 )
Physicochemical properties of commonly used solvents for the synthesis of PBAs.
TA B L E 2