Fabrication of a Cyanide-Bridged Coordination Polymer Electrode for

Apr 9, 2012 - Daisuke Asakura,. † ... of Complexity Science and Engineering, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Ja...
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Fabrication of a Cyanide-Bridged Coordination Polymer Electrode for Enhanced Electrochemical Ion Storage Ability Daisuke Asakura,† Masashi Okubo,*,† Yoshifumi Mizuno,† Tetsuichi Kudo,† Haoshen Zhou,*,† Kazumichi Ikedo,‡ Takashi Mizokawa,‡ Atsushi Okazawa,§ and Norimichi Kojima§ †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ‡ Department of Complexity Science and Engineering, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan § Graduate School of Arts and Sciences, The University of Tokyo, 3-1-8 Komaba, Meguro, Tokyo 153-8902, Japan S Supporting Information *

ABSTRACT: Host frameworks with the ability to store guest ions are very important in a wide range of applications including electrode materials for Li-ion batteries. In this report, we demonstrate that the ion storage ability of the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) can be enhanced by suppressing vacancy formation. K-ions in the vacancy-suppressed PBA framework K1.72Mn[Mn(CN)6]0.93·□0.07·0.65H2O (□: a [Mn(CN)6]4− defect) were electrochemically extracted. The open circuit voltages (OCVs) during K-ion extraction exhibited two specific plateaus at 3.0 and 3.7 V vs Li/Li+. Ex situ X-ray diffraction and IR spectroscopy revealed drastic structural and electronic changes during K-ion extraction. Furthermore, after K-ion extraction, the vacancy-suppressed PBA framework was applied to the cathode material for a Li-ion battery. The charge/ discharge experiments revealed that the framework can accommodate a large amount of Li-ions.

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INTRODUCTION Electrochemical guest-ion storage in a host framework is an important fundamental issue in both science and technology.1 Ion insertion/extraction accompanied by a solid-state redox reaction of the host framework can be applied to electrochromic devices, ionic sensors, and electrode materials in rechargeable batteries.2−4 Ion storage requires the host framework to possess redox-active sites for electron storage as well as porous space for ion storage. Furthermore, electronic conduction/ionic diffusion paths for mixed conductivity of ions and electrons are essential for ion insertion/extraction. To satisfy these many requisites, porous coordination polymers (PCPs) have a great advantage due to their versatility and designability. In particular, cyanide-bridged PCPs known as Prussian blue analogues (PBAs) are coordination polymers that have the ability to store the ions.5 The electrochemical properties of PBAs have been intensively studied for decades.5−11 PBAs have the perovskite-type structure A2MII[M′II(CN)6] (A: alkali metal; M and M′: transition metal). Thus, the electrochemical reaction of PBAs is ideally given by: 2A+ + 2e− + MIII[M′III(CN)6] ⇔ A2MII[M′II(CN)6]. Realizing this ideal reaction would enable a high ion storage (2A+) to be attained. However, in practice, the conventional synthetic procedure p r o vi d e s P B A s w i t h t h e g e n e r a l f o r m u l a A x M[M′(CN)6]1−y·□y·nH2O (0 < x < 2, y < 1), where □ © 2012 American Chemical Society

represents a [M′(CN)6] vacancy occupied with coordinating and zeolitic water.12,13 The presence of vacancies is less efficient for ion storage. For example, the precipitation obtained in an aqueous solution by mixing Mn2+ and K3[Fe3+(CN)6] produces K0.1Mn2+[Fe3+(CN)6]0.7·□0.3·4.2H2O (MnFe-PBA),8 which undergoes the following ion storage reaction 0.8A+ + 0.8e− + Mn 2.1 +[Fe3 +(CN)6 ]0.7 ·□0.3· 4.2H 2O ⇔ A 0.8Mn 2 +[Fe 2 +(CN)6 ]0.7 ·□0.3·4.2H 2O

This ion storage is too low for practical applications. In the application to electrode materials for Li-ion batteries (A = Li), the MnFe-PBA exhibited a charge/discharge capacity of 60 mAh/g, which is much lower than those of conventional electrode materials.14,15 In contrast, the theoretical capacity for vacancy-free MIII[M′III(CN)6] is calculated to be 202 mAh/g (M, M′ = Mn). It is thus important to fabricate a PBA framework with the suppressed amount of vacancies. Here, we focus on K2MnII[MnII(CN)6]. This material has been studied from the viewpoint of magnetism16−18 and contains only less than 0.3 wt % of H, which implies that it has extremely few vacancies and very little zeolitic water. Thus, if Received: December 10, 2011 Revised: March 23, 2012 Published: April 9, 2012 8364

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the K+ ions are completely removed, a vacancy-free PBA framework MnIII[MnIII(CN)6] can be fabricated. In this report, electrochemical K-ion extraction from K2Mn[Mn(CN)6] was investigated with the aim of fabricating MIII[M′III(CN)6]. We also report the Li-ion insertion/extraction properties of the host framework for an electrode material in Li-ion batteries.

0.28), which suggests the presence of a small amount of [Mn(CN)6]4− vacancy (7%) in contrast with earlier reports.16−18 However, the amount of vacancies and zeolitic water was remarkably suppressed compared to typical PBAs (e.g., MnFe-PBA contains 30% vacancies).8 Figure 1a shows the XRD pattern of K1.72Mn[Mn(CN)6]0.93·□0.07·0.65H2O and the Rietveld refinement result.

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EXPERIMENTAL METHODS K2Mn[Mn(CN)6] was synthesized by a precipitation method. An aqueous solution of MnCl2·4H2O (0.25 M) was added to an aqueous solution of KCN (1.28 M) under a N2 atmosphere. The precipitate was centrifuged, washed twice with 40 mL of distilled H2O, and then dried in vacuum for 24 h. The composition was determined by the standard microanalytical method for C, H, and N elements and by inductively coupled plasma mass spectroscopy (ICP-MS) for K, Mn, and Fe elements. Scanning electron microscopy (SEM) (Carl Zeiss Gemini Supra) and transmission electron microscopy (TEM) (Hitachi H-7100FA) were used to estimate the particle size. Each sample (50 mg, 75 wt %) was ground with acetylene black (13.3 mg, 20 wt %) and polytetrafluoroethylene (PTFE) (3.3 mg, 5 wt %) into a paste for electrochemical experiments. Lithium metal was used for the reference and counter electrodes. For the electrolyte, a 1 M LiClO4 ethylene carbonate/diethyl carbonate solution (1:1 v/v %) was used. The cutoff voltages were 4.3 V for charging (K- and Li-ion extraction) and 2.0 V for discharging (Li-ion insertion). The open circuit voltage (OCV) was recorded by a galvanostatic intermittent titration technique (GITT)19 where we alternately repeated the slow charge/discharge at 18 mA/g for 10 min and then interruption for 60 min. All measurements were performed at 298 K. The powder X-ray diffraction (XRD) experiments were carried out at BL19B2, SPring-8.20−23 The wavelength of the Xray was set to λ = 0.5010 Å. Ex situ XRD measurements were performed using a Bruker D8 Advance with Cu Kα radiation in steps of 0.01° over a 2θ range of 10−80°. The RIETAN-FP was used for the Rietveld refinement.24 The unit cell parameters were calculated by least-squares fitting. Each sample was washed with ethanol before ex situ XRD measurements. For X-ray photoemission spectroscopy (XPS) experiments, a JEOL JPS9200 analyzer with an Al Kα X-ray source (hν = 1486.6 eV) was employed. The total energy resolution was 0.6 eV. After complete K-ion extraction, the sample surface was washed with ethanol to remove the electrolyte and dried in a vacuum for 24 h before the measurement. A JASCO FT/IR-6200 spectrometer was used for ex situ Fourier transform infrared (FT-IR) spectroscopy. To dilute the samples on the transparent-mode FT-IR, the sample of 0.05 mg was mixed into KBr powder of 150 mg, and the mixed powder was pelletized for 10 min.

Figure 1. (a) Rietveld refinement result for the XRD pattern of K1.72Mn[Mn(CN)6]0.93·□0.07·0.65H2O. The wavelength of the X-ray was set to λ = 0.5010 Å. Red dots, diffraction data; green lines, simulated result; blue lines, residues. (b) The crystal structure. (c) An SEM image of K1.72Mn[Mn(CN)6]0.93·□0.07·0.65H2O.

The crystal structure was successfully refined with the space group of P21/n (see Table S1 in the Supporting Information), which is almost consistent with the crystal structure of K2Mn[Mn(CN)6] reported in the literature.18 Slight differences in the refinement, e.g., the presence of the small amount of [Mn(CN)6]4− vacancy, may be attributed to the differences in the synthetic procedure. Most likely, the addition rate of the Mn solution to the KCN solution was slightly faster than that in the previous procedure, resulting in the rapid precipitation of the PBA with a small amount of the vacancy. Figure 1b illustrates the crystal structure of K 1 . 7 2 Mn[Mn(CN)6]0.93·□0.07·0.65H2O, in which both the three-dimensional cyanide-bridged framework and the three-dimensional porous channel exist. Both SEM (Figure 1c) and TEM (Figure S1,

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RESULTS AND DISCUSSION K2Mn[Mn(CN)6] was synthesized by the precipitation method described in refs 16−18. Although the hydrogen content was measured to be 0.28 wt %, which is similar to that (0.2 wt %) reported by Her et al.,18 the precise composition was determined to be K1.72Mn[Mn(CN)6]0.93·□0.07·0.65H2O (□: a [Mn(CN)6]4− vacancy) by the elemental analyses (standard microanalysis and ICP-MS; anal. calcd for K1.72Mn[Mn(CN)6]0.93·□0.07·0.65H2O: K 20.38, Mn 32.20, C 20.30, N 23.68, H 0.39; found: K 20.4, Mn 32.1, C 19.82, N 22.41, H 8365

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structure, while the electrons may move by thermally activated hopping as polarons. The total amount of extracted K-ions calculated by the Faraday law suggests that all the K-ions were successfully extracted. To confirm complete K-ion extraction, K 2s core-level XPS spectra were recorded (Figure 2c). Prior to K-ion extraction, the peak of the K 2s core level was clearly observed at 337 eV. However, after electrochemical K-ion extraction, the K 2s peak completely disappeared. This result shows complete K-ion extraction from the host within the error bars due to XPS measurements (probably a few %). The XPS spectra also indicate that insoluble KClO4 did not form at the electrode/ electrolyte interface. Since KClO4 at the interface could act as an insulating layer to deteriorate the electrode performance, it is important to confirm the nonexistence of interfacial KClO4. Thus, Mn[Mn(CN)6]0.93·□0.07·0.65H2O was successfully fabricated by K-ion extraction. As shown in Figure 2b, the OCV change during K-ion extraction exhibits specific potential plateaus at 3.0 and 3.7 V. To clarify the structural changes that occur during K-ion extraction, ex situ XRD experiments were performed as shown in Figure 3a. When K-ion extraction starts, new peaks indexed to a cubic phase appear in addition to the initial monoclinic phase. On extracting the K-ion (1.72 > x > 1.0), the intensities of the cubic phase peaks increase, while those of the initial monoclinic phase decrease. This result clearly suggests that Kion extraction for 1.72 > x > 1.0 proceeds via a two-phase process. For 1.0 > x, the peaks for the initial monoclinic phase

Supporting Information) images indicated that the average particle size is about 1 μm. To fabricate a vacancy-suppressed PBA framework, K-ions were extracted electrochemically from the compound. Since it is important to completely extract all K-ions, we performed electrochemical K-ion extraction by repeatedly applying a lowdensity current (18 mA/g) for 10 min, with a 60 min interruption to allow the K-ion concentration in the PBA particle to equilibrate (GITT).19 Figure 2a shows typical time

Figure 2. (a) Time dependences of the potential and current during K-ion extraction by GITT mode. OCVs were recorded after each interruption of 60 min. The current of 22.9 μA corresponds to 18 mA/ g. (b) Voltage change and OCVs during electrochemical K-ion extraction. (c) K 2s core-level XPS spectra before (x = 1.72) and after (x = 0) K-ion extraction.

dependence of the electric current and cell voltage during the GITT. During the interruption, the electrochemical cell voltage showed a relaxation behavior to the equilibrium state. Figure 2b shows OCVs as a function of x in K x Mn[Mn(CN)6]0.93·□0.07·0.65H2O. The solid line in Figure 2b indicates the voltage change when an electric current is applied. The amount of extracted K-ions was calculated based on the Faraday's law. The OCV profile (open squares in Figure 2b) during K-ion extraction exhibited two potential plateaus. While K-ion extraction occurs at 3.0 V for 1.72 > x > 1.0, the voltage is raised to 3.7 V for 1.0 > x > 0. During K-ion extraction, K-ions should diffuse the porous network in the perovskite-type PBA

Figure 3. (a) Ex situ XRD patterns during K-ion extraction. The red and blue indexes, respectively, correspond to those for the monoclinic and cubic phase. In (a), OCVs the same as Figure 2b are displayed for correspondence with the XRD patterns. (b) Change in the unit cell parameters during K-ion extraction. The precise values for a and (b2 + c2)1/2 of the monoclinic phase for 1.3 > x > 1.0 cannot be calculated due to the weak peak intensities. 8366

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disappear, and only the cubic phase peaks were observed, indicating that K-ion extraction at 3.7 V vs Li/Li+ proceeds via a solid-solution process. Figure 3b shows the x dependence of the unit cell parameters. Note that a and (b2 + c2)1/2 for the monoclinic phase are equivalent to a for the cubic phase. In the two-phase region (1.72 > x > 1.0), the initial monoclinic phase coexists with the cubic phase. The lattice expands by 17%, suggesting there is a high mechanical strain at the domain boundary. During the two-phase process, the unit cell parameters of both phases do not change greatly. This is consistent with the fact that K-ion extraction proceeds by a fractional change between the two phases. As mentioned above, the two potential plateaus have different K-ion extraction mechanisms. To investigate the mechanism, especially the redox-active site, we performed the ex situ IR spectroscopy (Figure S2, Supporting Information) because the CN-stretching frequency νCN can determine the valence state of the C-coordinating metal site.16 The IR spectrum for x = 1.72 has a single νCN peak at 2057 cm−1, which corresponds to the valence state of Mn2+−CN−Mn.18 This peak is not affected by K-ion extraction at 3.0 V; thus, the N-coordinating Mn (MnN) rather than the C-coordinating Mn (MnC) is redox active at 3.0 V. However, a new peak becomes visible at 2144 cm−1 when K ions are extracted at 3.7 V. With progression of K-ion extraction, the intensity of the peak at 2057 cm−1 decreases, while that of the new peak increases. Furthermore, the initial peak of Mn2+−CN−Mn is not visible after K-ion extraction has completed. As reported in previous studies, the new peaks should be ascribed to the valence state of Mn3+−CN−Mn.17,25 Therefore, MnC is redox active during Kion extraction at 3.7 V, and it is almost fully oxidized to Mn3+ after the K-ion extraction. Finally, we address the application of the host framework to the electrode materials for Li-ion batteries. If the host framework has Li-ion storage ability, it can be directly used as the electrode material of Li-ion batteries. Therefore, we performed electrochemical Li-ion insertion/extraction experiments for Mn[Mn(CN)6]0.93·□0.07·0.65H2O. Figure 4a shows the OCVs during Li-ion insertion/extraction obtained by the GITT and the discharge−charge curves under a constant current density of 30 mA/g. The OCVs contain two potential plateaus at 3.7 and 3.0 V for the Li-ion insertion/ extraction reactions, which are similar to those during K-ion extraction. However, the potential plateaus for Li-ion insertion/ extraction are not as flat as those for K-ion extraction. This may be due to the large polarization in the PBA particle induced by slow Li-ion diffusion. The first discharge capacity of 197 mAh/g corresponds to 1.91 Li+ insertion. This value is slightly higher than the theoretical value for Mn[Mn(CN)6]0.93·□0.07·0.65H2O (1.72 Li+; 176 mAh/g) presumably due to the electrochemical side reactions such as the conversion (decomposition) reaction.26,27 Nevertheless, the discharge capacity is almost equal to the charge capacity, which indicates reversible Li-ion insertion/extraction. In fact, the Coulombic efficiency shown in Figure 4b reaches almost 100%. On the other hand, as shown in Figure 4b, the discharge/charge capacity gradually decreases after several cycles. To confirm reversible Li-ion insertion/extraction from a structural viewpoint, we used ex situ XRD to investigate the structural change that occurs during the first discharge/charge cycle (see Supporting Information). Figure S3a shows the XRD patterns, and Figure S3b (Supporting Information) shows the

Figure 4. (a) OCVs obtained by the GITT mode and discharge/ charge curves for Mn[Mn(CN)6]0.93·□0.07·0.65H2O during Li-ion insertion/extraction. (b) The cyclability and Coulombic efficiency during 10 cycles.

change in the unit cell parameters. The results indicate that the phase diagram during Li-ion insertion/extraction is quite similar to that during K-ion extraction: the solid solution process of the cubic phase exists in the low ion concentration region, and the two-phase process between the cubic and monoclinic phases exists in the high ion concentration region (Scheme 1). Scheme 1. Reversible A-Ion Extraction/Insertion in Mn[Mn(CN)6]0.93·□0.07·0.65H2Oa

The upper figures show crystal structures for x = 1.72, 1.0, and 0. In the middle, schematics of structural changes for a particle are displayed. In the lower part, the redox reactions for the Mn−CN−Mn framework are shown. a

Compared to the cubic phase with a low ion concentration, the unit cell volume for the monoclinic phase shrinks by 17%. This is a much larger volume change than that for conventional electrode materials such as LiFePO4 (6%).15 This large volume change induces strong mechanical strain at the two-phase boundary, producing cracks or causing the electrodes to 8367

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The Journal of Physical Chemistry C decompose. As shown in Figure S3a (Supporting Information), several small impurity peaks (e.g., the peaks around 2θ = 20°) were observed after the charge/discharge cycle. This possible generation of impurities may be one of the causes for the poor cyclability (Figure 4b). Furthermore, we also performed ex situ IR measurements (Figure S4, Supporting Information) to investigate the electronic structure. The IR results could indicate that the electronic structure change during Li-ion insertion was almost equivalent to that during K-ion extraction: the 3.7 and 3.0 V plateaus could correspond to the redox reactions on MnC and MnN, respectively. On the other hand, for x > 0.6, a new peak was observed at 2106 cm−1, which is near νCN = 2112 cm−1 for K3[Mn3+(CN)6].28 This result indicated that the bridging Mn− CN−Mn bond would be broken during the charge/discharge processes to form nonbridging CN (Mn3+−CN). Breaking the bridging CN bond, apparently due to the large mechanical strain during the phase transformation, may be another possible cause for the poor cyclability. Therefore, to improve the cyclability of the vacancy-suppressed PBA, the large lattice expansion/shrinkage during the charge/discharge processes should be suppressed, for example, by the heteromeal substitution.

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CONCLUSION

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by Industrial Technology Research Grant Program in 2010 from New Energy and Industrial Development Organization (NEDO). The synchrotron XRD measurement was conducted under a SPring-8 Industrial Application Proposal (Proposal No. 2010B1934). The authors are grateful to Dr. K. Osaka and Mr. T. Matsumoto at Japan Synchrotron Radiation Research Institute (JASRI) for the synchrotron XRD measurements. The authors thank Ms. M. Ohno for experimental support.

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The enhanced ion storage ability of a cyanide-bridged coordination polymer electrode has been demonstrated. The vacancy-suppressed PBA framework showed the complete Kion extraction accompanied by the oxidation of both MnC and MnN. This two-electron redox behavior is distinctly different from that of conventional PBAs. We also studied the cathode performance of the vacancy-suppressed PBA for Li-ion batteries and found that the charge/discharge capacity reached 197 mAh/g. Since the average charge/discharge potential is estimated to be 3.15 V, the energy density per cathode weight (vs Li/Li+) is calculated as 620 Wh/kg. These values are considerably higher than those of typical PBAs (e.g., 60 mAh/g and 200 Wh/kg for LixMnFe-PBA),8 and they are comparable to those of conventional transition-metal-oxide cathodes such as LiMn2O4 (120 mAh/g, 480 Wh/kg)29 and LiCoO2 (137 mAh/g, 550 Wh/kg)14 and those of LiFePO4 (170 mAh/g, 600 Wh/kg).15 Although the appearance of nonbridging CN during the charge/discharge process and resulting poor cyclability of the vacancy-suppressed PBA were serious problems for practical applications, the enhanced ion storage ability of the cyanide-bridged coordination polymer should open new perspectives in the possible material designs and applications of PCPs.

S Supporting Information *

Additional experimental details and Table S1 and Figures S1− S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest. 8368

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