Polypyrrole-modified Prussian blue cathode material for potassium ion

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Polypyrrole-modified Prussian blue cathode material for potassium ion batteries via in situ polymerization coating Qing Xue, Li Li, Yongxin Huang, Ruling Huang, Feng Wu, and Renjie Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04579 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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ACS Applied Materials & Interfaces

Polypyrrole-modified Prussian blue cathode material for potassium ion batteries via in situ polymerization coating Qing Xue a, Li Li a, b, Yongxin Huang a, Ruling Huang a, Feng Wu a, b, and Renjie Chen a, b,* a Beijing

Key Laboratory of Environmental Science and Engineering, Beijing

Institute of Technology, Beijing 100081, China. b Collaborative

Innovation Center of Electric Vehicles in Beijing, Beijing

100081, China. KEYWORDS: Potassium ion batteries; Cathode; Prussian blue; Polypyrrolemodified; In situ polymerization coating

ABSTRACT: Potassium ion batteries (PIBs) have received significant attention due to the abundant potassium reserves and similar electrochemistry of potassium to lithium. Due to the open framework and structural controllability, Prussian Blue and its analogs (PB) are considered to be competitive cathodes of PIB. However, the intrinsic lattice defects and poor electronic conductivity of PBs induce poor cycling perfomance and rate capability. Herein, we propose a polypyrrole-modified Prussian blue material (KHCF@PPy) via in situ 1 ACS Paragon Plus Environment

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polymerization coating method for the first time. KHCF@PPy possess a low defect concentration and improved electronic conductivity, and the electrode was found to exhibit 88.9 mA h g–1 discharge capacities at 50 mA g–1 with 86.8% capacity retention after 500 cycles. At higher current density of 1000 mA g–1, the initial discharge capacity was 72.1 mA h g–1, which dropped slightly to 61.8 mA h g–1 after 500 cycles. The capacity decay rate was 0.03% per cycle. Detailed characterization showed a lack of phase transition during the charge and discharge process and determined that K ions were not completely extracted from the monoclinic structure, possibly contributing to the excellent cycling stability. This simple surface modification method represents a promising means of mitigating issues currently associated with PB-based cathodes for PIBs.

1.

Introduction Lithium ion batteries (LIBs) have been extensively used in consumer electronics and

are attracting increasingly attention in the field of electric vehicles. However, the high cost of lithium and its relative scarcity limit its applications in large-scale energy storage systems (EESs). 1-6

For this reason, sodium ion batteries (SIBs) and potassium

ion batteries (PIBs) have become promising candidates for EESs because of the rich abundance of Na and K and their similar electrochemical properties compared with lithium.7-12

Recently, PIBs have become the subject of much research because the

K+/K redox pair has a more negative redox potential than the Na+/Na pair.13-14 Graphitic 2 ACS Paragon Plus Environment

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anodes have also been successfully employed with potassium, thus further promoting the study of PIBs.15-16 It have been reported that PIBs using graphite negative electrodes can obtain reversible capacities over 200 mA h g–1, which is comparable with that obtained from LIBs.16-17 However, compared with the progress in anode design, appropriate PIBs cathodes have rarely been reported, possibly due to the larger ionic radius of K ions, which possibly lead to structural collapse or distortion.18-19 Consequently, the development of PIBs are still in their infant stage. Prussian blue and its analogues (PBs) have been extensively studied as components of SIBs over the past few years.20-21 This research interest stems from the structural stability, rich redox-active sites and cost advantage of these materials, and has resulted in remarkable achievements.6,

22

Even so, the electrochemical performance of PBs

remains unsatisfactory, due to vacancies in the open frameworks of these materials accompanying the introduction of coordinated water molecules.23 To eliminate this effect, researchers have used many approaches to reduce lattice defects and remove interstitial water in PBs-based cathodes, including morphological tailoring, surface modification, nanostructure design, and composition optimization.24-28 The large interstices and ion transport channels in PBs lattices facilitate the accommodation of large alkali metal ions, suggesting that these materials could be used as cathodes for PIBs.29-32 In addition, PBs preferentially store K+ over Na+ ions because of the smaller Stokes radius of K+ in electrolyte solutions and the lower Gibbs free energy associated with the combination of K+ with cavities in PBs.13, 33 Moreover, the water content of K-

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based PBs is very low as a result of the weak hydration ability of K+ during the hydrothermal preparation of these materials.34-36Generally speaking, it should be possible to develop K+ insertion cathodes having good structural integrity based on PBs. Lei and co-workers29 investigated K0.22Fe[Fe(CN)6]0.805·0.195·4.01H2O as a potential cathode material for non-aqueous PIBs. This material showed a reversible capacities of ~73 mA h g–1 at 50 mA g–1 with a high discharge voltage between 3.1 and 3.4 V. However, the high potential polarization resulted from the low electronic conductivity of such cathodes and the poor cycling performance caused by side reactions with electrolyte should be ameliorated to allow the further development of PBs in PIBs.37-40 Based on research regarding the use of PBs in SIBs, approaches, such as structural optimization and surface modification, could be examined to enhace the cycling performance and rate capability of PBs in PIBs. However, as far as we know, there are no reports regarding the surface modification of PBs for use in PIBs. Herein, we synthesized a polypyrrole (PPy)-coated K-rich iron hexacyanoferrate composite (KHCF@PPy) with a long cycle life and excellent rate capacity, for use as a PIB cathode material. In this synthesis, pyrrole was polymerized in situ on the surfaces of KHCF particles utilizing the intrinsic oxidizing ability of KHCF and with no additional oxidant. The introduction of this conductive polymer enhanced the electronic conductivity of the KHCF, thus improving the rate capability. Simultaneously, the KHCF itself was reduced during the oxidative polymerization to obtain a potassium-rich iron hexacyanoferrate. Moreover, excess [Fe(CN)6]4– ions in


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the solution were doped into the PPy chains during the polymerization process, resulting in improved electronic conductivity as well as increased capacity. It has been reported that the redox potential of [Fe(CN)6]4– ions is close to the Fermi level of PPy,41 such that these ions can not only act as dopants to enhance the electronic conductivity of PPy, but also work as redox-active centers to increase the capacity of the doped PPy. These effects mitigate the decrease in specific capacity caused by applying a conductive coating to the KHCF. In summary, a modified PB was synthesized as a good candidate for PIB cathode material, because of its excellent cycling stability and rate capability. The one-pot coating method demonstrated herein is both simple and energy-effective, which is applicable to other electrode materials modifaction. 2.

Experimental section

Material Preparation All the chemical reagents employed in this study were directly used without any purification. In the synthetic process, 3.315 g K4Fe(CN)6·H2O was dissolved in 135 mL deionized water (DI) in a 250 mL three-necked flask to obtain solution A, while 1.48 g hydrochloric acid (37%) was added to 15 mL DI to form solution B. Then Solution B was slowly added to solution A drop by drop with continuous stirring. The resulting light yellow mixture was kept at 60 °C with vigorous stirring, during which it gradually took on a blue coloration. After 4 h, 75 μL pyrrole was added to the KHCF suspension with magnetic stirring and while bubbling nitrogen gas through the solution. The polymerization was carried out at 0 to 5 °C for 3 h, after which the PPy-coated KHCF 5 ACS Paragon Plus Environment


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precipitates were separated by centrifugation. Finally, KHCF@PPy was gained after drying in an oven at 80 °C overnight. As a contrast, pure KHCF material was prepared by similar method but with no addition of the pyrrole. Structural Characterization Rigaku Ultima IV-185 type X-ray diffraction (XRD) instrument was used to characterize the crystal structures of the as-prepared powders. The radiation source is Cu Kα. Raman spectra at the range of 200 to 3200 cm-1was obtained on Renishaw inVia spectrometer with laser wavelength of 633 nm. The chemical compositions of synthesized powders were determined by assessing the mass ratios of K, Fe, C, N, and H using inductively coupled plasma (ICP) combined with elemental analysis. Additionally, thermogravimetric analysis (TGA) was conducted on Netzsch STA 449F3 to determine the water content in each sample. X-ray photoelectron spectroscopy (XPS) were performed on Uivac-PHI with Al Kα as radiation source to assess the element valence values. Iron-57 Mössbauer spectra was determined by Wissel accelerated drive Mössbauer spectrometer with

57Co/Pd

radiation source at room

temperature. The spectra were fitted and resolved using the least squares method. Morphology characterization was performed using Hitachi S-4800 scanning electron microscopy (SEM) combined with JEM-2100F transmission electron microscopy (TEM). Computational method


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Density functional theory (DFT) calculations were perfomed employing the CASTEP software package (Materials Studio 2018). The exchange-correlation function was calculated according to the generalized gradient approximation with parameter setting of Perdew–Burke–Ernzerhof. Hubbard U term values of 7.0 and 3.0 eV were applied for the high-spin and low-spin Fe 3d orbitals, and cutoff value of 450 eV for kinetic energy of plane wave expansion was adopted to fit the valence electrons. Ultrasoft pseudopotentials were used to describe the core–valence interactions, and the Brillouin region is integrated with the k-point mesh density of 4 × 2 × 2. Additionally, the maximum stress was 0.02 GPa and the maximum force was 0.01 eV Å−1, respectively. Electrochemical characterization The KHCF@PPy and KHCF electrodes were prepared by casting a slurry of active powder, conductive Super P and polyvinylidene fluoride (PVDF) (with weight ratio of 8:1:1) onto Al foil. The thickness of the resulting layer was controlled at 150 μm. After drying at 80 °C overnight, the as-prepared electrode piece were cut into small discs, with a material loading on each disc of approximately 2.1 mg cm-2. The electrochemical performances of the as-prepared electrodes were tested in CR2032-type coin cells which were assembled in a glove box filled with argon. In each cell, the K metal anode and the as-prepared cathode was separated by Whatman glass fiber separator. The electrolyte used was 0.8 M KPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) with volume ratio of 1:1. The cells were cycled in the range of 2–4.2 V at different current densities using a Land Test System. Cyclic voltammetry (CV) 7 ACS Paragon Plus Environment


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was conducted with 0.1 mV s-1 scan rate on a SP-150-type Biologic workstation. The specific capacity were calculated based on the combined KHCF and PPy mass. In preparation for ex situ XRD analyses, the cathode were separated from the disassembled cells of different charge/discharge states, then washed with 1, 3-Dioxolane (DOL) and dried sufficiently under vacuum. 3.

Results and discussion The reaction mechanism associated with the KHCF@PPy synthesis is schematically

illustrated in Figure 1 (a). In an acidic environment, Fe2+ ions slowly dissociated from [Fe(CN)6]4– and were oxidized to Fe3+ by reaction with ambient air. In addition, the undecomposed [Fe(CN)6]4– reacted with Fe2+/Fe3+ to form cubic KHCF nuclei that gradually grew as the reaction proceeded. After four hours, the pyrrole was added to the suspension containing Fe3+ ions, which oxidized the pyrrole to form the polymer. Simultaneously, the residual [Fe(CN)6]4– in the solution was doped into the PPy. In this manner, high quality KHCF coated with [Fe(CN)6]4–-doped PPy was obtained. The morphological features were characterized by SEM and TEM and the resulting images are shown in Figs. 1(b-d). Numerous standard nanocubic structures with smooth surfaces can be observed in the pure KHCF, the edge lengths of which varies between 500 and 700 nm (Figure 1(b) and Figure S1 (a)). In comparison, the coated KHCF exhibits coarse surfaces with some small aggregations on the surfaces, and these features are related to the PPy coating (Figure 1(c) and Figure S1 (b)). The highresolution TEM images further indicating the existence of a PPy coating layer on the 8 ACS Paragon Plus Environment

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KHCF surface. Compared with the pure crystalline KHCF (insert to Figure 1(d)), Figure 1(d) demonstrates that the edges of the KHCF@PPy have a uniform, thin amorphous layer, indicative of a homogeneous PPy coating. The data used for the structural analysis of the KHCF@PPy powders are presented in Figure 2. The crystal properties of KHCF@PPy were assessed by XRD, and the associated Rietveld refinement patterns are shown in Figure 2(a). According to the Rietveld refinement (Rwp = 10.88%, Rp = 7.51%) based on the Reflex procedure, the lattice parameters were determined to be a=10.091 Å, b=7.341 Å, c=7.048 Å, α= 90.00°, β=89.58° and γ=90°, which can be readily indexed to a monoclinic phase with space group P21/n. This is a typical highly stable, K-rich configuration, in which FeN6 and FeC6 octahedra are linked by (C ≡ N)¯ ligands. The rigid open framework of this structure, which contains large interstitial sites, can accommodate the relatively large K+ ions, while simultaneously providing suitable channels for the diffusion of these ions. In contrast, the pure KHCF exhibits a typical face-centered cubic structure (Figure S2), which is consistent with that of Fe-HCF synthesized via a traditional single iron source co-precipitation method.42 The KHCF@PPy Raman spectrum in Figure 2 (b) contains four characteristic PPy peaks at 925.2, 1050.3, 1328.8 and 1601.9 cm-1, confirming the introduction of PPy onto the KHCF surface through the in situ polymerization coating method. Peaks related to the ν (CN¯) band appear at 2115 and 2079 cm-1, suggesting that the Fe in this compound was primarily bivalent. This was confirmed by the XPS data in Figure 2(c). In the Fe 2p spectrum obtained from the



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KHCF@PPy, the primary peaks centered at 708.63 and 721.43 eV are assigned to FeII 2p3/2 and FeII 2p1/2, respectively, indicating that Fe2+ was the primary ion. Considering the high concentration of divalent iron in the compound, it is likely that the PB had a high potassium content, which is also demonstrated by the chemical composition analysis. Based on the ICP and element analysis results, the chemical composition of the KHCF@PPy was characterized as K1.87Fe[Fe(CN)6]0.97·0.03 (where  represents a [Fe(CN)6]4- vacancy), which obviously corresponds to a K-rich PB compound. The TGA curves obtained from this material (Figure S3) show a mass loss of 4.2% below 250 °C, indicating 0.84 water molecules per molecule. Consequently, the chemical formula of the KHCF@PPy is K1.87Fe[Fe(CN)6]0.97·0.03·0.84H2O. Using the same method, the formula of the KHCF was found to be K0.68Fe[Fe(CN)6]0.86·0.14·1.68H2O, suggesting that there was a lower K content and a greater concentration of defects in the pure KHCF. It is assumed that the higher K concentration in the KHCF@PPy can be attributed to the reduced number of vacancies in the framework as well as the increased level of Fe2+ resulting from the oxidation effect during the pyrrole polymerization. Iron-57 Mössbauer spectra were acquired at room temperature (Figure 2(d)) to identify the Fe valence as well as the K content in the KHCF@PPy. The black stars and red solid line in this figure represent the experimental data and fitted results, respectively, and the detail of the fitted results are listed in Table S1. The isomer shift (IS) values of 0.07 and 0.95 mm s–1 are ascribed to Fe3+ and Fe2+, with corresponding weight percentages of 0.44 and 0.56, respectively. These results further demonstrate that the K content in the KHCF was high relative to previously reported values.42-44 We 10 ACS Paragon Plus Environment

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attempted to use elemental analysis and TGA results to determine the PPy content in the composite but, unfortunately, the exact value was difficult to obtain because C and N were present in both the PPy and KHCF, and the temperature ranges over which pyrrole decomposed and coordinated water evaporated were overlapped. Therefore, a pyrrole content of about 5 wt% was roughly estimated based on the mass difference between the KHCF and KHCF@PPy. The electrochemical behaviors of the KHCF and KHCF@PPy were initially measured by CV in K-half cells and the results are displayed in Figure 3. The CV curves obtained from the KHCF@PPy (Figure 3(a)) show a 3.68/3.3 V couple peak during the first scan, ascribed to the redox potential of low spin FeIII/FeII connected to C atoms. The following two CV profiles exhibit the similar two peaks, implying identical redox mechanism and the high reversibility of depotassiation/potassiation. It is noteworthy that the sharp peak at approximately 4.15 V that appeared during the first cycle is attributed to solid electrolyte interphase (SEI) formation, based on prior reports. Subsequently, the 4.15 V peak weakened and the polarization voltage gradually decreased to a steady value of 350 mV, suggesting complete and stable SEI film formation. The CV behavior of the KHCF as depicted in Figure 3(b) was basically the same as that of the KHCF@PPy, except that its polarization voltage was slightly larger (approximately 500 mV).The charge-discharge profiles of KHCF@PPy at 50 mA g–1 are provided in Figure 3(c). Two obvious plateaus can be observed at approximately 3.6 and 3.3 V, which is consistent with the CV measurements. The first charge and



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discharge capacities of the KHCF@PPy were 122.2 and 88.8 mA h g–1, respectively, with an coulombic efficiency of 72.67%. This value was higher than that obtained from the KHCF (64%) in Figure 3(d). This enhanced coulombic efficiency may have originated from the lower coordinated water content and reduced defect concentration in the KHCF@PPy, as noted in the structural characterization discussion. During the subsequent long cycles, there was no significant voltage decay or polarization voltage increase in the case of the KHCF@PPy, while the KHCF exhibited poor kinetics by comparison (Figure 3(d)). The detailed electrochemical performance of the KHCF@PPy is shown in Figure 4. After 500 cycles at a current density of 50 mA g–1, the KHCF@PPy electrode maintained 86.8% of its initial discharge capacity, with a value of 77.1 mA h g–1 (Figure 4(a)), while the KHCF only delivered a capacity of 54.5 mA h g–1, equal to 60% of its initial value (Figure S4). The coulombic efficiency (CE) gradually increased to a stable level of above 98% after several cycles, which may have been related to the presence of interstitial water in the material.45 During the first several cycles, the decomposition of interstitial water during the charge process would be expected to lead to a low CE. Throughout the subsequent process, the decreased amount of residual water would give an enhanced CE value. The rate performance of the KHCF@PPy at different current densities ranged from 50 to 1000 mA g–1 are displaed in Figure 4(b). The reversible capacity at 100 mA g–1 was 80 mA h g–1, equal to approximately 95% of its capacity at current density of 50 mA g–1. For larger current densities, such as 200, 500 and 1000


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mA g–1, the specimen was still able to discharge 77, 69 and 60 mA h g–1, respectively, all of which are higher than the values obtained from the KHCF (Figure S5). After cycling at high rates, the capacity of the KHCF@PPy returned to 83 mA h g–1 when the current density was lowered back to 50 mA g–1, demonstrating minimal damage to the structure and the framework interface. The initial low coulombic efficiency gradually increased and stabilized at 98% along with an increase in the current density. Prolonged cycling at a high rate of 1000 mA g–1 was further investigated and the results are presented in Figure 4(c) and Figure S6. The KHCF@PPy material owned an initial discharge capacity of 72.1 mA h g–1 and retained 61.8 mA h g–1 after 500 cycles with a capacity retention of 85% and a capacity decay rate of 0.03% per cycle. The coulombic efficiency was greater than 98% after several cycles, indicating remarkable reversibility of K+ extraction and insertion. The enhanced rate capability can possibly be ascribed to the improved electronic conductivity conferred by the PPy coating, as demonstrated by the electrochemical impedance spectra in Figure 4(d). In these data, the KHCF@PPy produced a smaller semicircle than the KHCF, indicating enhanced electronic response and decreased charge transfer resistance. Ex situ XRD analyses were conducted to assess the phase evolution of the KHCF@PPy during the charge-discharge process, and Figure 5(b) shows the detailed XRD profiles acquired at different charge-discharge states (as marked in Figure 5(a)). It can be seen that there are no obvious changes in the diffraction peaks during the entire charge-discharge process, suggesting no phase transition during the electrochemical



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redox process. In the enlarged patterns between 2θ values of 34 and 37°, the (400) peaks shift to larger angles, suggesting that the lattice parameter was reduced during the charge process (A-C) due to K+ extraction. Conversely, when the K+ ions were inserted into the crystal (discharge process, D-E), the lattice parameter increased, as reflected in the shift of the (400) peaks to smaller angles. Therefore, K+ storage in the KHCF@PPy proceeded via a reversible interaction mechanism without two-phase reactions. This was confirmed by theoretical calculations. Based on the DFT calculation results, if the K+ ions are completely extracted from the monoclinic structure, it will transition to a cubic phase (Figure 5(c)), which is in conflict with the ex situ XRD results. Moreover, the energy barrier for a single K+ insertion into a K-free unit cell is calculated as 6.99 eV, which is higher than that for a K-rich structure (6.5 eV), as shown in the table in Figure 5. Consequently, it can be inferred that K+ are instead partially extracted from/inserted into the monoclinic crystal during the charge and discharge process. The residual K+ in the crystal structure can preserve the structural stability of the PB framework, which may be benefit for the excellent cycling stability of the KHCF@PPy. 4.

Conclusions

In this work, a K-rich Prussian blue cathode material (KHCF@PPy) was synthesized by an in situ polymerization coating method. As a result of the PPy coating, the composite had a low defect concentration with enhanced electronic conductivity, and exhibited excellent cycling stability and rate capability. The as-prepared electrode made from this compound showed a high capacity retention of 86.8% after 500 cycles at 50 14 ACS Paragon Plus Environment

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mA g–1, and delivered an initial discharge capacity of 72.1 mA h g–1 while retaining 61.8 mA h g–1 after 500 cycles at 1000 mA g–1, with a capacity decay rate of 0.03% per cycle. Ex situ XRD combined with DFT calculations revealed that K+ storage in the KHCF@PPy is associated with a reversible interaction mechanism without a phase transition, which is beneficial with regard to structural stability. This novel modification method provides a promising route to the application of PBs-based cathodes in PIBs.



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FIGURES

Figure 1. (a) Illustration scheme of the synthesis process; SEM of pure KHCF (b) and KHCF@PPy (c); TEM of KHCF@PPy (d) and KHCF (insert of d).


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Figure 2. (a) XRD of KHCF@PPy; (b) Raman spectra of KHCF and KHCF@PPy; (c) Fe 2p XPS spectra of KHCF@PPy; (d) Iron-57 Mössbauer spectra of KHCF@PPy



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Figure 3. CV curves of KHCF@PPy (a) and KHCF (b) at a scan rate of 0.1 mV s–1; charge/discharge profiles of KHCF@PPy (c) and KHCF (d) at different cycles under 50 mA g–1.


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Figure 4. Electrochemical performances of KHCF@PPy: (a) cycling performance and Coulombic efficiency at 50 mA g–1; (b) rate capability and Coulombic efficiency at various current densities; (c) long cycling performance at 1000 mA g–1; (d) electrochemical impedance spectra measured from 10 mHz to 100 kHz.



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Figure 5. (a) First galvanostatic charge-discharge curves of KHCF@PPy electrodes; (b) Ex situ XRD patterns and the enlarged patterns (right column) at different charge and discharge states as marked in (a); (c) The crystal structural evolution of KHCF@PPy based on the calculation results and the illustration of calculation of energy barriers (table).


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ASSOCIATED CONTENT Supporting Information. XRD pattern of KHCF, TGA curves of KHCF@PPy and KHCF, cycling performance of pure KHCF, rate performance of pure KHCF, iron-57 Mössbauer spectra parameters of KHCF@PPy. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China “New Energy Project for Electric Vehicle” (2016YFB0100204), the National Natural Science Foundation of China (51772030), the Joint Funds of the National Natural Science Foundation of China (U1564206), Major achievements Transformation Project for Central University in Beijing, and Beijing Key Research and Development Plan (Z181100004518001).



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Graphical abstract


Polypyrrole-modified Prussian blue cathode material for potassium ion

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