Sodium citrate-assisted prussian blue analogues as K-rich cathode materials for sodium ion batteries

Prussian blue analogue (PBA), as the cathode material of sodium ion batteries (SIBs), has attracted much attention because of its low price and open three-dimensional channel structure. However, the traditional methods for preparing PBA have high crystallization rate, high water content and many structural defects, which hinder the development of PBA for the application of in batteries. To solve the above problems, in this study, sodium citrate was used to improve the nucleation rate and crystallinity. Herein, PBM 1 synthesized by this method not only has good crystallinity and regular particles, but also shows excellent electrochemical performance when used as cathode in SIBs. Under the current density of 0.05A/g, it has a specic capacity of 109.6 mAh/g. At a current density of 0.1A/g, the reversible specic capacity could keep as high as 95.0 mAh/g and with a coulombic eciency of almost 100% after 300 cycles.


Introduction
The increasingly serious resource shortage and energy crisis have become the biggest obstacle to the rapid development of modern society. In order to meet the signi cantly increased energy consumption and reduce dependence on non-renewable resources, various types of energy storage systems have been developed, such as lithium-ion batteries (LIB), sodium-ion batteries (SIB) and lithium-sulfur batteries 1 .
Currently, lithium-ion batteries are the most versatile and considered the most promising energy storage equipment. However, the shortage and expensive price of lithium resources may restrict its further development. Considering the abundance of resources and cost, sodium ion batteries have received widespread attention from society and are considered to be the main energy storage equipment in the future [2] . Nevertheless, several challenges must be overcome to enable sodium-ion batteries to replace or replenish lithium-ion batteries. Especially, compared with lithium ion, the larger radius of sodium ion makes its diffusion speed in active materials slow, resulting in limited rate performance. In addition, during insertion and extraction of sodium ions, the large size radius of sodium ions will easily lead to structural collapse and lattice distortion of materials, resulting in the inhibition of cycling ability [3] . In order to solve the above problems, the preparation of prussian blue analogs (PBAs) electrode materials is considered to be an effective method due to its inherent open framework structure and large interstitial sites 4 .
As a typical cathode material for sodium ion batteries, the molecular formula of PBAs are A X M α [M β (CN) 6 ] y □ 1−y ·zH 2 O, where A are alkali metal ions, M α and M β are transition metal ions, □indicating the vacancy occupied by coordinated water [5] . In the structure of PBA, M α and M β transition metal cations are connected with cyanide ions through M α -C≡N-M β bonds, forming an open framework with a large number of interstitial sites [6,7]] . Due to the different types and contents of alkali metal ions, the types of transition metal ions and the number of vacancies, PBAs electrode materials have different properties. Moreover, the easy synthesis and low cost of PBA materials at room temperature are also very conducive to large-scale applications [8] . However, when PBAs were used as cathode materials of sodium ion batteries in the early stage, the target products were generally synthesized by traditional liquid phase precipitation method, which had high crystallization rate, high water content and many defects in the structure of the target products, resulting in poor cycle performance [9] . In order to solve the above problems, citric acid assisted method was adopted in this experiment. Compared with the traditional synthesis method [10] , the obtained material has more perfect crystal structure, fewer defects and lower water content [11] . As the cathode material of sodium ion battery, it shows high charge-discharge platform, high speci c capacity, high-rate performance and high cycle stability. At the same time, in order to effectively prevent the structure and lattice changes caused by sodium ion insertion/extraction, the prepared material must have a large enough gap and a rigid frame. In this experiment, PBA cathode material with rigid hollow nanobox structure was prepared, and alkali metal ion A was changed into potassium ion, which enlarged the gap [12] . Compared with the inserted/extracted sodium ions, the inner part has a larger gap and a stronger skeleton, which leads to the smallest material structure change and lattice modi cation during the insertion/extraction of sodium ions.
This work uses the traditional liquid phase method, adding potassium ferricyanide to provide potassium ions and different kinds of transition metal ions (Co 2+ , Mn 2+ ) salt compounds and adding sodium citrate as an auxiliary agent to synthesize the corresponding potassium-rich hexacyanoferrate salt compounds (PBC, PBZ, PBM) [13] . Performance tests on several synthesized materials show that potassium-rich manganese hexacyanoferrate has high speci c capacity, excellent performance rate and cycle life [14] . This may be due to PBM cathode material has perovskite structure and hollow nanobox structure, which leads to open three-dimensional network frame structure, large gap position, low water content and few structural defects compared with PBC. And compared with the PBC cathode material, the higher redox potential of PBM leads to the higher discharge voltage platform and the greater energy density [15] [16] . After standing at room temperature for 18 h, the precipitate was centrifuged and washed with deionized water and ethanol three times, respectively. Finally, the product was dried in a blast drying oven at 70 ℃ for 12 h to obtain the K-PBM 1 nanobox. Replace the Mn(CH 3 COO) 2 ·4H 2 O mentioned above with Co(NO 3 ) 2 ·6H 2 O, and the amount of other materials and experimental conditions remain unchanged, and K-PBC is synthesized respectively. At the same time, a control experiment was performed to synthesize K-PBM 2 with Mn(NO 3 ) 2 instead of Mn(CH 3 COO) 2 ·4H 2 O.

Material characterization
The crystal structure of the powder material was characterized using X-ray diffraction (XRD), on Smart Lab Japan (40 kV, 200 mA, Cu-K α radiation). The microscopic morphology of the material was observed by using a eld-emission scanning electron microscope (SEM) (JSM-7200F, Japan) and a high-resolution transmission electron microscope (TEM, JEOL2100F). The material has been analyzed for surface element and chemical valence state by X-ray photoelectron spectrometer (XPS, ESCALAB250).

Electrochemical measurements
By mixing electroactive materials (PBA), conductive carbon black (acetylene black), and binder polyvinylidene uoride (PVDF) in a weight ratio of 7:2:1, add N-methylpyrrolidine in a mortar (NMP) and grind them evenly for about 30 minutes, until they are completely ground into a slurry. After that, the slurry was uniformly coated on the copper foil and dried at 80°C for 12 hours, and then assembled into a CR2025 coin battery. The separator used is glass ber (GF/D), and the electrolyte is a solution of 1.0M NaPF 6 in EC: DEC = 1: 1 Vol% with 5.0% FEC. At room temperature, the charging and discharging performance of the battery was tested in the voltage range of 2.7-4V through the LAND CT 2001A test system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on the battery on the CHI760E electrochemical workstation.

Results And Discussion
In this experiment, PBAs powder was synthesized by a simple precipitation method.
XRD and SEM were carried out to study the phase, composition and micromorphology of the prepared samples, and the results are shown in Fig 1. In the XRD images of all PBA products, the positions of the characteristic diffraction peaks were basically the same and the diffraction peaks were sharp and strong, indicating that these samples had high crystallinity. In addition, in the SEM image as shown in the gs1.c-e, all of them had regular structure of nanobox. It could be seen that by replacing Fe( ) in KFe( )Fe( ) (CN) 6 with similar transition metal ions such as Mn( ) or Co( ) the products still maintain a cubic framework structure and are arranged in an orderly manner. The XRD patterns of PBM in Fig. 1a were very consistent with the standard card of KMnFe(CN) 6 ·2H 2 O (JCPDS 51-1896) [17] . Moreover, the characteristic peaks of PBM 1 and PBM 2 at 17.5, 24.2, 24.9, 25.4 and 35.4° corresponded to (002), (202), (220), (202) and (040) crystal planes [18] . Comparing their characteristic peaks, the characteristic peak intensity of PBM 1 is higher and the adjacent impurity peaks are less, indicating that the purity of PBM 1 is purer. The SEM images in Figs. 1c and 1d showed that the product is a nanobox structure. PBM 1 was superior to PBM 2 in terms of regular morphology and uniform size of particles [19] . As shown in Fig. 1b 620) and (642) planes [17] , respectively. Fig. 1e shows the SEM image of PBC, which clearly shows the nanobox structure with a particle size of about 400 nm. Compared with the three samples, PBC is the smallest of the three, followed by PBM 1 . In addition, unobtrusive peaks can also be found in XRD patterns, indicating that the prepared samples have superior crystallinity, which is related to the slow nucleation process controlled with the assistance of sodium citrate.
Energy dispersive X-ray spectroscopy (EDX) was performed on the PBA sample to determine the proportion of each element. The analysis report in Fig. 2a (Figs. 3a, 3c and 3e) showed that all PBA successfully prepared hollow nanoboxes. These nanospheres have an advanced hollow box structure, which can provide more active sites and faster diffusion channels, thereby obtaining higher ion diffusion kinetics and excellent electrochemical performance [20] . As depicted in Figs. 3b, 3d and 3f, the distribution range of K, Mn(Co), Fe, C and N fell uniformly in the hollow nanoboxes [21] . Combined with XRD, SEM, EDX and TEM data, it can be con rmed that the nanoboxes with hollow structure has been successfully prepared. Unlike the smooth surface of PBC naobox, the rough surfaces of PBM 1 and PBM 2 nanobox can effectively provide a large number of active sites and diffusion pathways. Different from the irregular and uneven nanoboxes of PBM 2 , the box structure of PBM 1 is more perfect. It can't be ignored that the shell thickness of the sample is extremely important, because the thicker shell will introduce impurities, while the thinner shell may cause instability of the hollow box skeleton [22] . It can be clearly observed from the gure that compared with PBM 2 and PBC, the shell thickness of PBM 1 is between them, and the thickness is moderate.
In order to further determine the elemental composition, chemical valence and electronic state of PBA materials, the samples were characterized by XPS. XPS spectra of PBM 1 are shown in Figs. 4(a-c). Full spectrum (Fig. 4a) contains Mn2p, Fe2p, C1s, K2p and N1s peaks. As shown in Fig. 4b, in the high resolution XPS spectrum of Mn 2p, the peaks at 641.8 and 653.7 eV are designated as Mn 2p 3/2 and Mn 2p 1/2 , which are directly divided into two corresponding sub-peaks, and a satellite peak appears at 645.6 eV in the middle, indicating that the valence state of Mn in the sample is only positive divalent.
Accordingly, in Fig. 4c, the spectrum shows that the peaks of Fe2p 3/2 and 2p 1/2 appear at 708.6 eV and 721.6 eV, and the corresponding sub-peaks have no satellite peaks, which belong to Fe 2+ in [Fe(CN)6] 4+ and Fe 3+ in [Fe(CN)6] 3+ , respectively [23] . It is worth noting that the orbital peaks and binding energies in the tting curves of Fe2p in the three samples are in the same position, which con rms the existence of iron cyanide ions. The elements displayed in the scanning spectrum (Fig. 4d) of PBM 2 correspond to PBM 1 one by one, and the tting curve of its Fig. 4e Mn2p is basically consistent with that of Fig. 4b, except that the position where binding energy appears changes slightly. The above proves the successful preparation of PBM materials. The scanning spectrum of PBC (Fig. 4g) includes Mn2p, Fe2p, C1s, K2p and N1s peaks. As shown in Fig. 4f, it can be observed that the binding energies of Co2p 1/2 and Co2p 3/2 appear at 779.7 and 782.4eV, and there is a satellite peak at 786.7eV in the middle, which reveals the existence of Co 2+ , combined with the spectrum of Fe2p (Fig. 4g) can be proved the successful synthesis of PBC materials [24] .
In order to understand the electrochemical properties of the prepared materials, cyclic voltammetry and rate tests were carried out. Fig. 5a shows the rst three cyclic voltammetry curves of PBM 1  was well maintained suggested the highly reactive reversibility and good structural stability 25 . Fig. 5b shows the rate curve at a current density of 0.05 to 1 A/g. At current densities of 0.05, 0.1, 0.2, 0.5 and 1 A/g, the reversible discharge speci c capacities of the PBM1 cathode are 109.6, 95.0, 71.1, 38, and 19.1 mAh/g, respectively. At a large current density of 0.5 A/g, 64% of the initial speci c capacity can still be maintained. When the current density goes back to 0.05 A/g again, the reversible discharge speci c capacity is 114.1 mAh/g, indicating that PBM 1 has superior rate performance. The reversible discharge speci c capacities of PBM 2 at current densities of 0.05, 0.1, 0.2, 0.5 and 1 A g −1 are 65.4, 51.6, 33.5, 17.8 and 11.1 mAh/g, respectively. At a large current density of 0.5 A/g, the speci c capacity is only 27% of the initial value. When the current density goes back to 0.05 A/g again, the reversible discharge speci c capacity is 69.1 mAh/g, and the overall performance is inferior to PBM 1 . As for PBC, there is almost no reversible speci c capacity at a current density of 0.05-1A/g without further discussion. The reason for the extremely low energy storage performance of the PBC cathode may be that the content of K + in the PBC material is too small, which is only 0.41, far less than 1.43 of PBM 1 and 1.51 of PBM 2 . There is no large amount of free K + movement inside, so that the number and size of the material gap points are not up to the standard. At the same time, the smooth surface of its microscopic nanobox cannot provide su cient active gap points and diffusion pathways [26] . The last point is that its thinner shell thickness causes the hollow skeleton to be not strong enough. Comparing the charge/discharge curves of PBM 1 (Fig. 5c) and PBM 2 (Fig. 5d), noticed that although both are charged/ discharged with constant current at a voltage of 2.0-4.0 V. The initial charging voltage of PBM 1 is around 2.5 V and the voltage of PBM 2 is 2.75 V, and the initial discharge voltage is the same. The lower initial charging voltage may be the reason why the rate performance of PBM 1 surpass PBM 2 .In order to have a deeper understanding of the electrochemical performance of the PBM cathode, the cycling and impedance performance of PBM 1 and PBM 2 were tested. Fig. 6a is the cycle performance curve of the PBM cathode material at a current density of 0.1A/g. The initial reversible speci c capacity of PBM 1 is 75.1 mAh/g, and the highest speci c capacity is 91.1 mAh/g. After 300 cycles of charging and discharging, the speci c capacity is 75.4 mAh/g, which has no attenuation compared with the initial speci c capacity and can still maintain the highest speci c capacity of 82%, which proves that PBM 1 has excellent cycle performance [27] . The initial speci c capacity of PBM 2 is 60 mAh/g, and the highest is 69.0 mAh/g. The speci c capacity after 300 cycles is 42.2 mAh/g, which is 30% attenuated compared with the initial speci c capacity, which is equivalent to the highest speci c capacity of 61%. Comparing the cycle charge and discharge curves of PBM 1 (Fig. 6b) and PBM 2 (Fig. 6c), the initial discharge voltage of PBM 1 during each cycle of 300 cycles is about 2.5V, which is lower than the 2.75V of PBM 2 . This phenomenon also appears in the rate performance, which is more certain that this is why the electrochemical performance of PBM 1 is superior.
In addition, since the energy storage capacity of the electrode material is directly related to its impedance, in order to compare the reaction kinetics of the PBM 1 and PBM 2 electrodes, electrochemical impedance spectroscopy analysis was carried out. The frequency spectrum is shown in Figure 6(b). A concave semicircle is formed in the high frequency area and the middle frequency area, and a straight line is formed in the low frequency area. The semicircle in the high-frequency region and the long diagonal line in the low-frequency region correspond to the charge transfer process and the Warburg diffusion process, respectively. Obviously, the charge transfer resistance (Rct) of PBM 1 is lower, which is more conducive to Na + and electron migration. The reason why the electrochemical performance of PBM 1 is better than that of PBM 2 is that the electrode material of PBM 1 has higher purity, more regular particles and more suitable shell thickness. After characterizing the physical and electrochemical properties of the three electrode materials, the results show that Mn 2+ is easier to combine with potassium ions to form a rough surface, and acetate ions can form uniform particle and shell thickness [28] .

Conclusion
In summary, the PBM (KMnFe(CN) 6 ) electrode material with high crystallinity rich in K ions and the PBC (Mn 2 [Fe(CN) 6 ] 2 )electrode material with a small amount of K+ were prepared by a simple and convenient liquid phase precipitation method assisted by sodium citrate. The prepared electrode material well retains the hollow nanobox structure, in which a large amount of K + inside makes the box rough and provides more and larger active sites and diffusion channels to form a more open framework. In addition, the control experiments show that acetate and Mn + are more likely to form electrode materials high purity, regular particles, rough surface and moderate shell thickness. Open structure, rough surface, high purity, uniform particles and proper shell thickness make PBM 1 have remarkable electrochemical performance, showing a speci c capacity of 109.6 mAh/g at a current density of 0.05A/g and long-cycle stability at 0.1 A/g. We believe that the strategy of increasing the nucleation rate and crystallinity with the assistance of sodium citrate and increasing the K + content will make PBA a promising cathode material for SIBs.

Declarations
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