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Preparation of Prussian blue nanoparticles with pore structure by two-step optimization for Na-ion battery cathodes Renjie Chen, Yongxin Huang, Man Xie, Qianyun Zhang, Xiaoxiao Zhang, Li Li, and Feng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04151 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016

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ACS Applied Materials & Interfaces

Preparation of Prussian Blue Submicron Particles with Pore Structure by Two-Step Optimization for Na-ion Battery Cathodes

Renjie Chen,

†,‡,

* Yongxin Huang, † Man Xie, † Qianyun Zhang, † XiaoXiao Zhang, † Li Li, †,‡ and Feng Wu, †,‡

†

School of Materials Science & Engineering, Beijing Key Laboratory of Environmental Science

and Engineering, Beijing Institute of Technology, Beijing 100081, PR China. ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR China

KEYWORDS: Sodium ion batteries; Cathode materials; Prussian blue; Structural optimization; Porous submicron cubes

ABSTRACT: Traditional Prussian blue (Fe4[Fe(CN)6]3) synthesized by simple rapid precipitation shows poor electrochemical performance because of the presence of vacancies occupied by coordinated water. By reducing the precipitation rate and adding polyvinylpyrrolidone K-30 as a surface active agent, the as-prepared Prussian blue has fewer vacancies in the crystal structure than traditional Prussian blue. It has a well-defined face-centered-cubic structure, which can provide large channels for Na+ insertion/extraction. The material synthesized by slow precipitation has an initial discharge capacity of 113 mA h g−1 and maintains 93 mA h g−1 under a current density of 50 mA g−1 after 150 charge–discharge cycles. After further optimization by a chemical etching method, the complex nanoporous

structure of Prussian blue has a

high

Brunauer–Emmett–Teller surface area and stable structure to achieve high specific capacity and long cycle life. Surprisingly, the electrode shows an initial discharge capacity of 115 mA h g−1 and Coulombic efficiency of approximately 100%, with capacity retention of 96% after 150 cycles. Experimental results show that Prussian blue can also be used as a cathode for Na-ion batteries.

1. INTRODUCTION In the future, energy storage systems will be widely used in smart grids, which can solve the 1

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problem of resource fluctuation with time1-2. It is well-known that hydroelectric power and wind power generation with discontinuity are difficult to apply in practical production and life, but the development of secondary batteries will be a good solution to this problem. Storage devices for the grid must have a long cycle life, high storage efficiency, high output power, and very low cost3. Although Li-ion batteries have been widely studied and exploited, none of the single cells can meet all of the above requirements4-5. Sodium ion batteries could solve the problem because their cost is significantly lower than Li-ion batteries while their performance is similar. However, the radius of the sodium ion (0.98 Å) is larger than that of the lithium ion (0.69 Å), so it is a challenge to design a relatively wide insertion/extraction pathway for the sodium ion6-7. In recent years, many kinds of cathode and anode materials have been developed, including layered metal oxides8, phosphate and fluorophosphate9-10, hard carbon11, TiO212, and so on. Among the many choices, metal–organic frameworks (MOFs) are worthy of further study. MOFs are similar to sodium superionic conductor structures with open frameworks13-14. Considerable attention has been devoted to exploring the viability of Prussian blue (PB) and PB analogues (PBAs), which are a class of MOFs, for high-performance Na-ion batteries. ഥm) with Fe(II) It has been shown that PB and PBAs have a cubic framework (space group Fm૜ and transition metal elements on alternate corners of a cube of corner-shared octahedra bridged by linear (C≡N)− anions15-16. The electrochemical activity of PB layers obtained by chemical or electrochemical deposition onto an electrode surface was confirmed in 197817. Subsequently, the performance and characteristics of PB as electrodes in secondary batteries have been widely investigated18-19. Although PB electrodes have many advantages, such as high theoretical capacity, an open framework for Na+ ion insertion/extraction, low cost, and environmental friendliness, their actual capacity is still far below the theoretical value. The traditional synthetic method to produce PB (Fe4[Fe(CN)6]3) relies on the addition of a metal salt solution of FeCl3 to a solution of Na4Fe(CN)6 at 60 °C. Because of the fast nucleation rate of crystals in this method, the synthesized Fe4[Fe(CN)6]3 material usually has a lot of defects and thus shows poor cycling stability and low capacity20. Recently, the findings of Guo et al.21 suggest that high-quality PB (main component Prussian white) exhibits significantly higher cycling stability and capacity than PB. The presence of interstitial sites and vacancies occupied by coordinated water in PB and PBAs is the major reason for the poor cycling stability and low 2


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Coulombic efficiency22-23. Nevertheless, the reaction between hydrochloric acid and sodium ferrocyanide can effectively decrease the number of vacancies through a slow precipitation process. In addition to the adverse effects of defects, volume expansion and the contact area with the electrolyte also affect application of PB24. Specially designed structures may be a way to solve these problems25. Mesocrystalline PB submicron particles obtained with the assistance of polyallylamine hydrochloride have a special polyhedron structure with high amounts of boundaries, which are beneficial for fine contact between the active electrode and the electrolyte26. Porous submicron structures not only provide a high specific surface area similar to hollow structures, but they also have a high volume energy density. These structures can be obtained by adjusting the amounts of the corrosion inhibitor and protective agent. In this study, optimized Fe4[Fe(CN)6]3 (OPB1) was obtained by slow growth of submicron cubes. Compared with the traditional synthetic steps, this process changes the stoichiometric ratio of the raw materials. By removing [Fe(CN)6] vacancies in the compound, the cycling performance and capacity improve. Porous Fe4[Fe(CN)6]3 (OPB2) submicron cubes were then successfully synthesized by a chemical corrosion reaction. Polyvinylpyrrolidone (PVP, K-30) was used as a surface active agent and protecting agent for chemical etching, which is important for OPB2 synthesis. Because of the high Brunauer–Emmett–Teller (BET) surface area and stable structure, the OPB2 electrode exhibits better cycling stability and rate capability than the OPB1 electrode.

2. EXPERIMENTALSECTION 2.1 Materials synthesis OPB1 was prepared according to a previously published method21. In brief, PVP (K30, MW ≈ 40000, 3.0 g) and Na4Fe(CN)6·10H2O (1.008 g) were added to HCl solution (0.1 M, 200 mL) under magnetic stirring for 30 min, and a clear yellow solution was obtained. The glass beaker was then placed in an electric oven and heated at 80 °C for 24 h. The obtained blue product was aged for another 48 h to obtain uniform particle sizes. The precipitate was collected by high-speed centrifugation and washed several times with deionized water and absolute ethanol. After vacuum drying at 60 °C for 20 h, OPB1 submicron cubes with approximate particle sizes of 400–500 nm were obtained. To obtain porous PB, OPB1 (40.0 mg) and PVP (200 mg) were added to 1.0 M HCl (40 mL) 3



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in a Teflon vessel under magnetic stirring for 2 h. The vessel was then transferred to a stainless steel autoclave and heated at 140 °C for 4 h in an electric oven. After aging, the precipitate was collected by centrifugation and washed several times with distilled water and ethanol. After vacuum drying at 60 °C for 20 h, OPB2 submicron cubes were obtained. The synthetic method and experimental results of the TPB material are based on a previous report20. According to a certain stoichiometry, Na4Fe(CN)6·10H2O and FeCl3·10H2O were dissolved in deionized water to form homogeneous solutions. These two solutions were slowly mixed to obtain TPB as a blue precipitate.

2.2 Structural characterisations Powder X-ray diffraction was carried out using a Rigaku D/Max-2550 PC diffractometer with monochromatized Cu-Kα radiation. The morphologies of the as-prepared materials were examined by SEM and EDX spectroscopy using a Hitachi S-4800 scanning electron microscope. TEM images were taken on a JEM-2100F transmission electron microscope. FT-IR spectra were obtained to detect cyanide ligands and hydroxyl groups using a Bruker Alpha FT-IR spectrometer (ATR-Ge, 400–4000 cm−1). The chemical compositions of the samples were examined using inductively coupled plasma (ICP) mass spectroscopy for Na and Fe elements with a Vista-MPX ICP atomic emission spectrometer. XPS was carried out on a PHI Quantera-II scanning XPS microprobe. TG and DSC analyses were conducted on a Netzsch STA449F3 simultaneous thermal analyzer. Raman spectra were obtained on a Raman spectrometer (JY Labram HR 800). Nitrogen adsorption–desorption isotherms were obtained and the BET surface areas of the OPB2 particles were simultaneously measured using an Autosorb-IQ-MP micropore analyzer.

2.3 Electrochemical measurements The electrochemical performance of the samples was tested in 2032-type cells assembled in an argon-filled glove box with water/oxygen content less than 1 ppm. The prepared working electrode, a Na metal anode, and a glass fiber separator from Whatman constituted the Na-ion battery. A slurry of the sample (OPB1 and OPB2), acetylene black, and poly(vinyl difluoride) at a weight ratio of 7:2:1 was coated onto aluminum foil to prepare the working electrode. The active material loading amount of pole piece is approximately to 2 mg cm-2. The electrolyte was 1.0 M 4


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NaPF6 in ethylene carbonate/diethyl carbonate (1:1 volume ratio). The galvanostatic charge–discharge tests were performed on a LAND cycler (Wuhan Kingnuo Electronic Co., China) at room temperature. Cyclic voltammetry and electrochemical impedance spectroscopy were carried out on a CHI 660e electrochemical workstation (ChenHua Instruments Co.) at a scan rate of 0.1 mV s−1 within the potential range of 2.0–4.2 V (vs. Na+/Na).

3. RESULTS AND DISCUSSION 3.1 Chemical composition and crystal structure To confirm that OPB1 submicron cubes with low [Fe(CN)6]4− vacancies were synthesized by the slow precipitation process, the elemental composition of the sample was analyzed (see Table S1). The precise composition of OPB1 was determined to be Na0.4Fe[Fe(CN)6]0.82⋅□0.18⋅2.75H2O (□ is a [Fe(CN)6]4− vacancy), which suggests a lower amount of Fe[(CN)6]4− vacancies (18%) than previously reported using a rapid precipitation synthetic process (32%)27. The formula indicates that the ratio of Fe3+:Fe2+ ion in the compound is 3.8:3.1, which is similar to the expected ratio of 4:3. Only lower amounts of sodium were detected in the OPB1 sample, indicating that the major component of final products is the TPB. Because the process of making porous particles does not change the chemical composition of the material, OPB2 has the similar formula (Na0.39Fe[Fe(CN)6]0.82⋅□0.18⋅2.35H2O) as OPB1. However, the content of interstitial water in framework slightly decreased due to the effect of chemical etching. Although the water content in the materials is a litter higher, they are crystal water in framework, which are very stable during the electrochemical reactions. Energy dispersive X-ray (EDX) analysis (Fig. S1) of the as-prepared OPB1 and OPB2 materials shows the presence of Fe, C, N, and O elements with the absence of any discernible Na, further proving that pure OPB1 and OPB2 were successfully synthesized. Figure 1(a) shows the powder X-ray refinement pattern of the OPB1 material, which can be ഥm space group. The lattice well indexed to a pure face-centered-cubic (fcc) structure with the Fm૜ parameter of OPB1 is calculated to be a = 6.2403 Å (V = 243.01 Å3 and Rwp = 4.34%), which agrees well with the numerical value of the Fe4[Fe(CN)6]3 lattice28. The sharp characteristic peaks illustrate the well-defined crystal structure of OPB1, resulting from the slow synthetic process of OPB1 submicron cubes. From local magnification of the characteristic peaks at 2ࣂ = 15.1°, 29.2°, 5



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and 38.7° shown in Fig. 1(b) and (c), the (111), (311), and (331) peaks of the cubic crystal system are very weak or even absent, indicating that using PVP as a crystal growth inhibitor gives the preferred crystal orientation29. The inset in Fig. 1(a) shows the crystal structure of Fe4[Fe(CN)6]3, which consists of a three-dimensional skeleton. Fe3+ ions are connected to nitrogen atoms and Fe2+ ions are surrounded by carbon atoms of cyanide ligands, forming a large storage space for insertion/extraction of alkaline cations. As shown in the scanning electron microscopy (SEM) images in Fig. 2(a) and (b), the OPB1 submicron cubes have a typical cubic shape with sizes of 400–500 nm, and the distribution of submicron cubes is very uniform without any aggregation of submicron cubes. The SEM image of a single submicron cube and its elemental mapping results shown in Fig. 2(c) and (d) show the uniform distribution of Fe within the submicron cube of OPB1. It is also indirect evidence of regular and smooth particle surfaces. To further describe the micromorphology of OPB1, OPB1 submicron cubes were observed by transmission electron microscopy (TEM), as shown in Fig. 2(e). A solid cube with 500 nm sides is well crystallized with defined edges and appears as a typical cubic structure. From the high-resolution TEM image in Fig. 2(f), clear lattice fringes were obtained at the edge of the particle, which shows a lattice spacing of 0.502 nm. The electron diffraction pattern of a single particle indicates that the sample is polycrystalline in nature. According to previous reports30, the crystalline character is consistent with the (200) lattice distance of fcc PB. By chemical etching, the regular OPB1 submicron cubes transformed into openwork submicron cubes with channels extending deep into the interior of the particles. In the SEM image of OPB2 particles in Fig. 3(a), most of the particles are uniformly etched by hydrochloric acid. In the magnified SEM images of OPB2 particles in Fig. 3(b)–(d), the particles show well-distributed corrosion holes on the surface of the particles. Signs of corrosion can also be seen in the elemental mapping results (Fig. 3(e) and (f)), such as an uneven distribution of Fe on the particle surface. These phenomena demonstrate that a porous sample was successfully synthesized. To further investigate its morphology, Fig. 3(g) and (h) shows TEM images of the OPB2 particles. Some pieces around the particles formed during the chemical etching process. A cubic particle with thick edges and a thin lumen can be observed in the images. Because of the protective effect of PVP, the shells of the particles are very strong and a porous structure forms inside the shell. 6


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3.2 Other physical and chemical properties During the corrosion process, some properties of materials are unchanged while other properties may change. Therefore, we conducted a series of measurements to determine the qualitative changes of OPB2 particles. Because the frequency of the cyanide vibration stretching mode (ν(CN)) is sensitive to the oxidation state of iron, the valance state of iron can be determined by Raman spectroscopy, as shown in Fig. S231. Characteristic peaks of OPB1 and OPB2 only appear at 2157 cm−1, indicating that both Fe3+ and Fe2+ ions exist in the materials. It also indicates less sodium ions in the materials compared with Prussian white. Qualitative and quantitative analysis of the iron ion valence states in OPB1 and OPB2 is possible by X-ray photoelectron spectroscopy (XPS). The curve fittings are shown in Fig. S3, where the peak clusters around 711.6 and 720.1 eV are attributed to Fe 2p3/2 and Fe 2p1/2, respectively. The peaks at 707.1 and 720.1 eV correspond to FeII 2p3/2 and FeII 2p1/2 in [FeII(CN)6]4−, while the peaks at 711.6 and 724.9 eV (723.6 eV) correspond to FeIII 2p3/2 and FeIII 2p1/2 in FeIII4[FeII(CN)6]3. Actually, there are some FeII ions connected with nitrogen atoms due to the existence of a little amount of sodium ions. These FeII ions may come from the dissociation of ferrocyanide without further oxidation. The similar positions and areas of FeII and FeIII in the XPS spectra of the two samples illustrate that the structure and composition of OPB2 is similar to that of OPB1. Both of the above phenomena indicate that except for morphology change the structural characteristics of OPB1 did not change during the chemical corrosion process in the hydrothermal reaction. According to the test results, the reaction equation can be speculated to be FeIII4[FeII(CN)6]3 + 4Na+ + 4e− ↔ Na4FeII4[FeII(CN)6]3. Owing to one-electron transfer per formula unit, the Fe4[Fe(CN)6]3 electrodes can deliver a theoretical capacity of about 120 mA h g−1, which can meet the requirements for application as sodium-ion batteries in large-scale energy storage. However, the safety, cyclic lifetime, synthesis, and environment friendliness of this material are more important. Fourier transform infrared spectroscopy (FT-IR) spectra confirm that the compositions of OPB1 and OPB2 are consistent with the design (Fig. S4)32. In the 400–4000 cm−1 range, the FT-IR spectra of OPB1 and OPB2 show a strong absorption peak at around 2058 and 2052 cm−1, respectively, which can be attributed to the cyanide functional group (–C≡N–). The Fe–CN 7



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bending mode is verified by the peak at 601 cm−1 in both spectra. The similarity of these two spectra is attributed to maintenance of the –FeII–C–N–FeIII– skeleton. In addition, except for the weak absorption peak at about 1605 cm−1 ascribed to the stretching mode of OH, no other OH group peaks are observed in the spectra. The OH groups mainly come from coordinating water in the materials. The vacancy and coordinating-water contents may affect the performance of materials. Therefore, the water contents of OPB1 and OPB2 were determined by thermogravimetric (TG) and differential scanning calorimetric (DSC) analysis (Fig. S5). Because of the simultaneous presence of an endothermic reaction (as shown in the DSC curves) and weight loss (as shown in the TG curves), it can be speculated that the coordinating water in the OPB1 (OPB2) framework is released in the range 130 °C (115 °C) to 230 °C (190 °C). The TG curves show that the total percentage weight loss of OPB1 (OPB2) is 9.3 wt% (8.8 wt%). This proves that more coordinating water remains in the formwork of OPB1 than in that of OPB2. [Fe(CN)6]4− vacancies occupied by coordinating water cause lattice distortion and structural collapse during Na+ insertion/extraction, resulting in poor electrochemical performance22. In this work, PVP was used to control the speed of the precipitation reaction to slow down synthesis of OPB1. Furthermore, the reaction between hydrochloric acid and sodium ferrous cyanide also extends the process of nucleus formation. These measures effectively remove vacancies in the material. During the corrosion process, coordinating water is further removed by weak oxidation of HCl33. Excluding the error of the test, (C≡N)2 was released from the skeleton of –FeII–C–N-FeIII–, corresponding to the weight loss at around 300 °C.

3.3 Optimization process To reduce volume expansion of the submicron particles and increase the contact area of submicron cubes with the electrolyte during the working process of the cell34, OPB2 submicron cubes with a complex porous structure were produced by the chemical etching reaction. The process of chemical etching can be divided into two steps, as shown in Fig. 4. First, an appropriate amount of protecting agent (PVP) was coated on the surface of the OPB1 particles with well-defined cubic structure, which were prepared by a coprecipitation reaction with a slow precipitation rate. Second, addition of hydrochloric acid solution made the solution acidic. During 8


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this step, OPB1 particles can be etched to obtain a porous structure with migration of hydrochloric acid molecules in the particles, illustrating the instability of the particles in acidic solution. Because of the stable structure, hydrochloric acid molecules can easily diffuse into the center of the particle. After aging and repeated washing, OPB2 particles were obtained. In this article, there are three reasons for adding PVP into the solution. The first reason is that PVP can control the micromorphology of the particles because of its surfactant effect. In contrast to PB submicron particles without PVP, the sizes and edges of submicron cubes become smaller and smoother with PVP in the solution, which can be validated by the OPB1 particles. The second reason is that it can effectively remove vacancies and interstitial water formed during the crystallization process30. Owing to the weakly reducing property of molecular PVP, the slow reduction of Fe3+ to Fe2+ can reduce the rate of nucleation. The third reason is that PVP acts as a preservative. Because of its good binding ability with iron ions of PB compounds, PVP can protect the surface of particles from serious etching. Therefore, OPB2 particles can retain their original morphology.

3.4 Electrochemical properties The discharge capacity of traditional Fe4Fe(CN)3 (TPB) is approximately 75% of its initial value after 50 charge–discharge cycles35. In this work, OPB1 exhibits significantly improved cycling performance compared with previous research, as shown in Fig. 5(a). OPB1 shows an initial discharge capacity of 113.4 mA h g−1. After 150 cycles, the discharge capacity of the OPB1 electrode is still 93.0 mA h g−1 (capacity retention of 82%). Because of the removal of vacancies and coordinated water in PB particles by the slow precipitation process, Na+ storage becomes more effective. The main decrease in capacity occurs during the first 50 cycles, which is mainly caused by deformation of the structure during Na+ insertion/extraction. Galvanostatic charge–discharge voltage profiles of OPB1 electrodes for different cycling were obtained at a current density of 50 mA g−1 between 2.0 and 4.2 V (Fig. 5(b)). The double-plateau discharge profiles of OPB1 indicate that Na+ insertion involves two steps at 2.78 and 3.64 V, corresponding to charge platforms at 3.11 and 3.78 V, respectively. Because the two iron ions have electrochemical activity (low-spin FeII and high-spin FeIII), the end product under full charge or discharge contains two Fe3+ or Fe2+ sites, respectively, resulting in two redox peaks. Figure 5(c) shows the rate capability of the OPB1 electrode between 2.0 and 4.2 V. The 9



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OPB1 electrode delivers reversible capacities of 110.8, 97.2, and 86.9 mA h g−1 at current densities of 50, 100, and 200 mA g−1, respectively. However, when the current densities are 400, 800, and 1600 mA g−1, the OPB1 electrode shows capacities of only 30.7, 7.1, and 3.6 mA h g−1, corresponding to 27.7%, 6.4%, and 3.3% of the maximum capacity at 50 mA g−1, respectively. Finally, when the current density returns to 50 mA g−1, the capacity of the OPB1 electrode only recovers to about 98.3 mA h g−1. This indicates that the structure of OPB1 has been somewhat destroyed, which may be caused by volume expansion. Hence, the coulombic efficiency was unstable under the high current density. Clearly, the rate capability of the OPB1 is not good. The synthetic processes to produce TPB and OPB1 are shown schematically in Fig. 5(d). The raw materials used to synthesize TPB contained two types of iron sources, resulting in a direct reaction between Fe3+ ions and [Fe(CN)6]4− ions. In contrast, the raw materials used to synthesize OPB1 only contained a single iron source from [Fe(CN)6]4−, and Fe3+ ions are obtained from oxidation of Fe2+ ions by [Fe(CN)6]4− ion decomposition. Therefore, the nucleation rate of OPB1 was slower than that of TPB. The amounts of vacancies and coordinating water in OPB1 were reduced to a low value. Owing to the above advantages, the long-term cycling performance of OPB2 electrodes has been further enhanced (as shown in Fig. 6(a)). The initial discharge capacity of OPB2 (115.2 mA h g−1) is close to its theoretical capacity (120 mA h g−1), corresponding to full utilization of the OPB2 material with insertion of one Na+ ion per molecule. This can be attributed to the removal of vacancies, which leads to more unoccupied active sites for sodium ion storage. Moreover, the OPB2 electrode shows a stable capacity of about 105.9 mA h g−1, and about 92% of the capacity remains after 150 charge-discharge cycles. The reversible capacity of OPB2 decreases much slower than that of OPB1 because its porous structure can inhibit expansion. The Coulombic efficiency of OPB2 can also reach more than 99.5% in complete cycles, indicating efficient Na-ion insertion/extraction along with large channels. From the inset in Fig. 6(a), the capacity of the OPB2 electrode slowly decreases with a certain rate during the first 100 cycles. The large decrease in the capacity is mainly in cycles 100–130. After 130 cycles, the OPB2 electrode will remain at a reversible capacity in subsequent cycles. From the CV profiles (inset in Fig. 6(b)), the charge–discharge curves exhibit two couples of redox peaks between 2.0 and 4.0 V during charge and discharge (Fig. 6(b)). According to the 10


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two-plateau charge/discharge profiles of the OPB2 electrode, it is speculated that Na+ insertion/extraction is a two-step process, as with the OPB1 electrode. Although the OPB2 electrode is used as a cathode for the Na-ion battery, it should be discharged in the first cycle because of the absence of Na ions. In the first cycle, redox peaks are observed at 2.78 V/3.11 V, corresponding to electrochemical reduction/oxidation of the low-spin FeII/FeIII ions connected to carbon atoms. During the second cycle, in addition to the above well-defined symmetric redox peaks, redox peaks at 3.64 V/3.74 V are assigned to reduction/oxidation of high-spin FeII/FeIII bonded to nitrogen atoms. These couples of redox peaks are relatively symmetric in shape. In subsequent cycles, the cathodic peak of low-spin FeII/FeIII reduction shifts to a lower potential and the charge–discharge curves gradually separate. The changes of the CV profiles and charge–discharge curves prove that slight polarization exists in the electrode, indicating high redox reversibility. Compared with the OPB1 electrode, the rate performance of the OPB2 electrode is enhanced, as shown in Fig. 6(c). The OPB2 electrode shows reversible capacities of 107.6, 99.1, and 77.2 mA h g−1 at high current densities of 100, 200, and 400 mA g−1, respectively. Even at a current density of 800 mA g−1, the reversible capacity is still 33.9 mA h g−1. Finally, it recovers to around 109.1 mA h g−1 at 50 mA g−1, which is 95% of the initial capacity, illustrating the structure of OPB2 is not destroyed. There are three reasons for this phenomenon. First, vacancies are removed from the material, ensuring effective transportation pathways for Na+ ions and electrons. Second, the porous structure shortens the insertion/extraction path length of Na+. Third, the high BET surface area of OPB2 particles increases the contact area between the materials and the electrolyte, resulting in lower diffusion impedance. However, when the current density increases to 1600 mA g−1, the OPB2 electrode only has a capacity of 8.5 mA h g−1, corresponding to rapidly increased diffusion impedance. Therefore, there is much room for improvement to obtain an ideal cathode material. Further investigation of the electrochemical reaction on the electrode was carried out using electrochemical impedance spectroscopy (EIS). The Nyquist plots of OPB2 electrodes before and after cycling under an open-circuit voltage (Fig. 6(d)) are composed of a depressed semicircle in the high-frequency region related to the charge-transfer process (Rct) and a sloped line in the low-frequency region corresponding to the Warburg diffusion process (Zw). The combination of Rct 11



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and Zw is called the Faradaic impedance. The initial Rct value of the OPB2 electrode is calculated to be 215 Ω. After cycling, the stable Rct value remains at 616 Ω in a controllable range. This is much less than the values of other general PB materials36, indicating faster kinetics for Na+ ion insertion/extraction in OPB2. The porous structure with high BET surface area contributes to increase the speed of Na+ diffusion. In summary, the specific capacity, cycling performance, and rate performance of OPB1 and OPB2 electrodes have been significantly enhanced (as shown in Table. 1). This proves that Fe4[Fe(CN)6]3 is a promising material for large-scale energy storage.

3.5 Performance improvement analysis The porous structure of Fe4[Fe(CN)6]3 submicron cubes provides accessible porosity, which is confirmed by nitrogen adsorption–desorption isotherms. Therefore, the BET surface areas of the samples before and after etching treatment are important to investigate their porosity. As shown in Fig. 7(a), the BET surface area of porous OPB2 particles (9.0217 m2 g−1) is about three times higher than that of solid OPB1 particles (3.4042 m2 g−1). Significantly, PB particles with a complex porous structure exhibit a standard type-III isotherm with the adsorption amount increasing with increasing relative pressure, which can be interpreted as pore filling characteristics37. Owing to the enlarged surface area, the contact area between the electrolyte and active substance also increased, indicating that a large amount of Na+ ions can be rapidly inserted/extracted. The superior electrochemical performance of the OPB2 electrode can be attributed to the specific properties of the porous submicron cubic structure. When Na+ ions are inserted into solid PB, there will be considerable volume expansion of the material, resulting in large mechanical strain and collapse of the crystal framework (as shown in Fig. 7(b)). However, this situation will not occur in porous PB. Because of the spacious interior of OPB2, it can both expand outward and contract inward during the process of Na-ion insertion/extraction. The structural stability and adaptability are improved because of optimization of the microscopic structure. OPB2 particles with a staggered internal channel can provide a short Na+ ion diffusion distance (as shown in Fig. 7(c)), which plays a crucial role in the ability for rapid Na+ ion insertion/extraction. For this reason, the rate performance is markedly enhanced. 12


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4. CONCLUSIONS In summary, we have successfully produced a PB material by two-step optimization, which consists of a slow precipitation process and a chemical etching process. On one hand, the slow settling rate effectively reduces the amounts of vacancies and coordinating water in the material, leading to high capacity and long cycle life. On the other hand, the porous structure obtained by chemical etching provides a high BET surface area for a fast Na+ ion intercalation/extraction process, which is beneficial for the rate performance and Coulombic efficiency of Na-ion batteries. PVP not only acts as a preservative, but also contributes to the well-defined micromorphology of the particles. As a result, OPB1 with a small amount of vacancies and coordinating water exhibits a high specific capacity of 113.4 mA h g−1 and maintains 93.0 mA h g−1 after 150 cycles. In addition, the OPB2 electrode exhibits a higher discharge capacity (115.2 mA h g−1) than OPB1 (113.4 mA h g−1), good cycling stability without an apparent decrease in capacity after 150 charge–discharge cycles, and a high Coulombic efficiency of ∼100%. The rate performance of OPB2 is also enhanced owing to its porous structure with high BET surface area. The present work may provide new synthetic routes for PB and its analogues. We believe that this green and convenient method for synthesizing porous particles with high electrochemical performance can be used to produce cathodes for Na-ion batteries.

ASSOCIATED CONTENT Supporting Information ICP data, EDS spectrum, Raman spectroscopy, XPS spectrum, ATR-FTIR spectra and TG and DSC curves for OPB1 and OPB2 samples.

AUTHOR INFORMATION Corresponding Author *Tel./Fax: +86-10-68451429. E-mail: [email protected]. Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21373028), Major achievements Transformation Project for Central University in Beijing, National Key Program for Basic Research of China (2015CB251100) and Beijing Science and Technology Project (D151100003015001).

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Fig. 1 (a) Measured XRD profile of the OPB1 material alongside a profile fitting; (red dots) measured; (black curve) calculated; (blue curve) difference plot; (olive green bars) Bragg reflections; (inset) the schematic crystal structure of Fe4[Fe(CN)6]3 phase; (b and c) Local magnification of characteristic peaks.

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Fig. 2 (a and b) SEM images of the OPB1 submicron cubes; SEM image of a single OPB1 particle (c) and its EDX-elemental mapping image (d); (e) TEM images of OPB1 naocubes; (f) High-resolution TEM images and corresponding fast Fourier transform (FFT) images of OPB1 submicron cubes.

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Fig. 3 (a-d) SEM images of the OPB2 microboxs; SEM image of a single OPB2 particle (e) and its EDX-elemental mapping image (f); (g and h) TEM images of OPB1 microboxs.

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Fig. 4 Schematic illustration of synthesis process of OPB2 submicron cubes; inset describes the role of PVP.

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Fig. 5

Electrochemical characterizations of the OPB1 electrodes: all the cells were cycled

between 2.0 V and 4.2 V; (a) Cycling performance and coulombic efficiency at a constant current density of 50 mA g-1; inset is capacity fading histogram; (b) Voltage profiles at different cycles at a current density of 50 mA g-1; (c) Rate performance at various current densities from 50 mA g-1 to 1600 mA g-1; (d) Schematic illustration of synthetic processes of TPB [15] and OPB1 submicron cubes, respectively.

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Fig. 6

Electrochemical characterizations of the OPB2 electrodes: all the cells were cycled

between 2.0 and 4.2 V; (a) Cycling performance and coulombic efficiency at a constant current density of 50 mA g-1; inset is capacity fading histogram; (b) Voltage profiles at different cycles at a current density of 50 mA g-1, (insert) CV curves measured at a scan rate of 0.1 mV s-1; (c) Rate performance at various current densities between 50 mA g-1 and 1600 mA g-1; (d) Impedance spectra of electrode before and after cycle from 0.01 Hz to 1×105 Hz.

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Fig. 7

(a) N2-150 °C isotherms of the OPB1 and OPB2 materials curves; (b) Schematic

illustration of the volume change during the Na+ insertion of solid PB and porous PB; (c) Schematic illustration of the Na+ diffusion process of solid PB and porous PB.

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Table 1 Comparison of electrochemical performance for the TPB, OPB1 and OPB2 electrodes. Sample

Initial capacity

50th capacity

Coulombic efficiency

Capacity at 200 mA g-1

name

(mA h g-1)

(mA h g-1)

(%, 1st→50th)

(mA h g-1)

TPB[15]

95

71.3

100→100

81.5

OPB1

113.4

104.1

100→100

83.2

OPB2

115.2

113.1

100→100

99.6

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Table of Contents:

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Preparation of Prussian Blue Submicron Particles ... - ACS Publications

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