Electrochemical Li-Ion Intercalation in Octacyanotungstate-Bridged

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Electrochemical Li-Ion Intercalation in Octacyanotungstate-Bridged Coordination Polymer with Evidence of Three Magnetic Regimes Jérôme Long,*,† Daisuke Asakura,*,‡ Masashi Okubo,§ Atsuo Yamada,§ Yannick Guari,† and Joulia Larionova† †

Institut Charles Gerhardt Montpellier, Ingénierie Moléculaire et Nano-Objets, UMR 5253 UM-CNRS-ENSM, Université de Montpellier, Place E. Bataillon, 34095 Montpellier, France ‡ National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Japan § Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-8-1, Bunkyo-ku, Tokyo, Japan S Supporting Information *

ABSTRACT: Discovery of novel compounds capable of electrochemical ion intercalation is a primary step toward development of advanced electrochemical devices such as batteries. Although cyano-bridged coordination polymers including Prussian blue analogues have been intensively investigated as ion intercalation materials, the solid-state electrochemistry of the octacyanotungstate-bridged coordination polymer has not been investigated. Here, we demonstrate that an octacyanotungstate-bridged coordination polymer Tb(H2O)5[W(CN)8] operates as a Li+-ion intercalation electrode material. The detailed magnetic measurements reveal that the tunable amount of intercalated Li+ ion in the solid-state redox reaction between paramagnetic [WV(CN)8]3− and diamagnetic [WIV(CN)8]4− in the framework enables the electrochemical control of different magnetic regimes. While the initial ferromagnetic long-range ordering is irreversibly lost upon lithium insertion, electrochemical switching between paramagnetic and short-range ordering regimes can be achieved.

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INTRODUCTION Porous coordination polymers (PCPs) or metal−organic frameworks (MOFs) belong to a wide family of moleculebased materials made of metal or lanthanide ions assembled through organic or inorganic ligands in two- or threedimensional architectures.1 They have attracted a great deal of attention during the last four decades not only from a fundamental point of view but also from their great potential for many applications including gas storage,2 magnetism,3 separation,4 catalysis,5 and energy conversion.6 One of the advantages of these molecule-based materials is related to a wide flexibility of the molecular structures, which permits to finely tune their physical and chemical properties and then to design multifunctional molecular materials.7 They may either present a simple coexistence of two or more functionalities, or they may ultimately exhibit a strong correlation between functions allowing the control of one function by another, realizing indeed novel switching devices. Thus, various bifunctional systems combining for instance magnetic properties with luminescence,8 ferroelectricity,9 conductivity,10 or porosity11 have been reported. However, molecule-based architectures exhibiting a real synergy/correlation between the properties are extremely scarce. We can cite, for instance, a chiral magnet showing a © XXXX American Chemical Society

change in their second-harmonic generation intensity above and below the magnetic ordering temperature12 or two different coordination polymers showing magneto-chiral effects.13 Additionally, a coordination network exhibiting an interference effect between ionics and magnetism, which could potentially be used for memory devices such as a magnetoresistive random access memory, has also been reported.8g Therefore, exploration of novel multifunctional materials presents a particular interest not only from the fundamental point of view but also due to their potential applications. Thus, the synthesis of these materials and investigation of the synergy between different functions is particularly challenging. The ability to store ions electrochemically is an important functionality, owing to the potential application to batteries.14,15 Many oxides, sulfides, and polyanionic compounds have extensively been studied as host materials for electrochemical ion intercalation due to a strong industrial demand for the wide deployment of high-performance batteries in electric vehicles and power grid.16,17 However, an electrochemical ion intercalation into PCPs/MOFs has been relatively poorly Received: May 3, 2016

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DOI: 10.1021/acs.inorgchem.6b01086 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

coordination network that occurs upon electrochemical process. In this work, we investigate solid-state electrochemistry of a bidimensional magnetic cyano-bridged coordination polymer Tb(H2O)5[W(CN)8]. We demonstrate that this octacyanotungstate-bridged coordination polymer exhibits an 80% reversible Li-ion (de)intercalation associated with a phase transformation between the crystalline−amorphous states. We show that three different magnetic regimes (a ferromagnetism, a short-range magnetic ordering with a slow relaxation, and a paramagnetism) can be obtained by the electrochemical Li-ion (de)intercalation amount. While the initial ferromagnetic state is irreversibly lost due to a small amount of trapped Li+ ion, an electrochemical switching between paramagnetic/short-range ordering regimes can be achieved.

explored to date, in part due to their intrinsically low electronic conductivity. In contrast, cyano-bridged coordination polymers, called also Prussian blue analogous (PBA), possess a moderate electronic conductivity via strong orbital hybridization between transition metal ions and cyanide ligands, which enables an electrochemical ion intercalation.18,19 These compounds constitute the most investigated materials of the PCPs family due to a wide range of applications, such as magnetism,20 photomagnetism,21 materials for electrode,22 ion capture,23 and gas adsorption.24 Their general chemical formula can be described as AaM[M′(CN)6]b□c·xH2O, where A is an alkali ion, M and M′ are transition metal ions and □ represents the cyanometallate vacancies that may be present to ensure the electroneutrality of the coordination network. Some of those exhibit a highly reversible (de)intercalation of various ions (Li+, Na+, K+, Mg2+, Al3+, etc.) in aqueous/nonaqueous electrolytes.25 For example, Na2Fe[Fe(CN)6]·0.1H2O delivers a large capacity of 155 mAh/g in a nonaqueous sodium-ion electrolyte,26 while KaCu[Fe(CN)6]b□c·xH2O operates as a positive electrode material in high-power aqueous batteries.27 Furthermore, cyano-bridged coordination polymers exhibit wide structural and electronic versatilities to modulate functionalities through the variation of transition metal ions nature, its coordination number, the size and the amount of alkali cations making them promising candidates for the rational design of novel multifunctional materials associating for instance magnetic properties with an ion storage ability. The previous studies on intercalation properties of PBA have been focused on PBA AaM[Fe(CN)6]b□c·xH2O frameworks consisting of hexacyanometallates such as [Fe(CN)6]3−/4−. It has been demonstrated that a gradual shift from a long-range ferroor ferrimagnetic ordering to a paramagnetic phase is induced by electrochemical reduction of the hexacyanoferrate moiety.28 Related octacyanometallate coordination polymer networks exhibit a vast structural diversity, strikingly different from the usual face-centered cubic (fcc) structure of PBA. The various geometries of the octacyanometallate [M′(CN)8]3−/4− (M′ = Mo, W) building blocks as well as its coordination flexibility due to the higher coordination number of the transition metal ion lead to coordination networks with various dimensionalities (1D, 2D, and 3D) and topologies.29 Additionally, the octacyanometallate moieties can exhibit two stable redox states, a paramagnetic [MV(CN)8]3− and a diamagnetic [MIV(CN)8]4− species, making them building blocks of choice for studying an ion insertion. Surprisingly, there is only one example of such octacyanomolybdate-manganese [Mn(H 2 O)][Mn(HCOO) 2 / 3 (H2O)2/3]3/4[Mo(CN)8]·H2O compound that shows a reversible ion intercalation upon sodium and lithium ions.30 In the similar way to that for PBAs, a gradual switch from a ferrimagnetic to a paramagnetic behavior is observed upon the ion insertion. Furthermore, whatever the nature of the cyanometallate building block, the dynamic magnetic behavior of final and intermediate compositions have never been thoroughly investigated, most likely because of the weak magnetic anisotropy of these systems. In this sense, some of us have described the first octacyanometallate−terbium Tb(H2O)5[W(CN)8] network combining a ferromagnetic ordering with a lanthanide photoluminescence.8b The strong magnetic anisotropy of the lanthanide ion, arising from the spin−orbit coupling, constitutes an interesting local probe to directly monitor the dynamic change in magnetization of the

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EXPERIMENTAL SECTION

Synthesis. The synthesis was performed under aerobic conditions at room temperature. Reagents and solvents were used as received. Tb(NO3)3·6H2O (99.9%) was purchased from ABCR. The precursor [N(C4H9)4]3[W(CN)8] was prepared as already described in the literature.31 Tb(H2O)5[W(CN)8] was synthesized according to the procedure already published by some of us.8b Briefly, Tb(NO3)3·6H2O (0.05 mmol) and [N(C4H9)4]3[W(CN)8] (0.06 mmol) were mixed in 9 mL of acetonitrile. Slow diffusion of diethyl ether allows the appearance of orange crystals after several days, which were washed with acetonitrile and dried in the air. Electrochemical Measurements. The electrochemical experiments were conducted by using a three-electrode glass cell with lithium metal as counter and reference electrodes. Each sample (75 wt %) was ground with acetylene black (20 wt %) and polytetrafluoroethylene (5 wt %) into a paste and used as the working electrode. One molar LiClO4 ethylene carbonate (EC)−diethyl carbonate (DEC) solution (1/1 v/v %) was used as the electrolyte. The cutoff voltages were 2.5 and 4.3 V versus Li/Li+ for lithiation and delithiation, respectively. The galvanostatic intermittent titration technique (GITT) was conducted by repeated application of a low-density current for 10 min and followed by application of 30 min of equilibration. The ex situ experiments (X-ray absorption spectroscopy (XAS) and powder X-ray diffraction) were performed with the (de)lithiated samples washed with ethanol. Infrared Spectroscopy. A JASCO FT/IR-6200 spectrometer was used for ex situ Fourier transform infrared (FT-IR) spectroscopy. Each sample (0.05 mg) was mixed with KBr powder of 150 mg and then pelletized for the measurement. X-ray Absorption Spectroscopy. XAS was performed using synchrotron radiation on beamline BL-7C of the Photon Factory. The (de)lithiated samples were prepared by the GITT. The transmission spectra were recorded at room temperature. The X-ray energy was calibrated by the corresponding metal-foil edge. The experimental data were analyzed using REX2000 software (Rigaku). Magnetic Measurements. Magnetic susceptibility data were collected with a Quantum Design MPMS-XL SQUID magnetometer working in the range of 1.8−350 K with the magnetic field up to 7 T. The sample weights were estimated from the ratio of coordination network used for the electrochemistry experiments. The data were corrected for the sample holder, and the diamagnetic contributions were calculated from the Pascal’s constants.32 A temperatureindependent paramagnetic contribution was also subtracted in the case of the lithiated and delithiated compounds.

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RESULTS AND DISCUSSION The coordination framework Tb(H2O)5[WV(CN)8] (TbW) was synthesized by mixing Tb3+ and [WV(CN)8]3− precursors in acetonitrile. The X-ray diffraction pattern of crushed crystals, Figure 1a, is consistent with the simulated pattern based on the previously reported crystal structure, which belongs to the B


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Inorganic Chemistry

The electrochemical properties of TbW with a nonaqueous Li+ electrolyte was studied by the cyclic voltammetry (CV). The CV curves for TbW in 1 M LiClO4/EC-DEC (Figure 2)

Figure 2. CV curves for TbW in a nonaqueous Li+ electrolyte. The scan rate is 0.5 mV/s.

show a pair of cathodic/anodic peaks centered around 3.3 V versus Li/Li+ at each cycle, indicating the reversible redox of the [WV(CN)8]3−/[WIV(CN)8]4− couple, which should be associated with the reversible Li+ (de)intercalation into the Tb(H2O)5[W(CN)8] framework. The redox potential of the [WV(CN)8]3−/[WIV(CN)8]4− couple centered at 3.3 V versus Li/Li+ is higher than the standard redox potential of the WV/ WIV couple in the oxides (−0.03 V vs NHE, 3.01 V vs Li/Li+), because the strong crystal field splitting of the cyanide ligands lowers the energy level of the frontier 5d orbital. In comparison with octacyanomolybdates , the redox potential of the [MoV(CN)8]3−/[MoIV(CN)8]4− couple is ca. 3.45 V versus Li/Li + in [Mn(H 2 O)][Mn(HCOO) 2/3 (H 2 O) 2/3 ] 3/4 [Mo(CN)8]·H2O, which is slightly higher than that of the [WV(CN)8]3−/[WIV(CN)8]4− couple, because the energy level of the 4d orbital of Mo is lower than that of the 5d orbital of W, leading to the higher redox potential of Mo. However, it should also be mentioned that the octacyanomolybdate in [Mn(H 2O)][Mn(HCOO) 2/3 (H2 O) 2/3 ]3/4[Mo(CN)8]·H2O has a bicapped trigonal prism geometry (C2v)30 in contrast to the square antiprism geometry of the octacyanotungstate in TbW, which may also cause the difference in the redox potential. After the irreversible current flow at the first cathodic scan presumably due to the irreversible surface/contaminant reduction, the CV curves are highly reversible in the subsequent cycles. Thus, TbW is robust against Li+ (de)intercalation. Although TbW has one redoxactive couple, that is, [WV(CN)8]3−/[WIV(CN)8]4−, in the structure, the CV curves in Figure 2 show multiple current flows in both the cathodic/anodic scans. Most likely, the enthalpy change associated with the structural change, Li−Li interaction, and/or electron−electron interaction, may cause the distribution of the reaction Gibbs function. The galvanostatic charge−discharge measurements were conducted to analyze the Li+ (de)intercalation into TbW more quantitatively. Figure 3a shows the charge−discharge curves for TbW at constant specific current of 10 mA/g. Here, charge denotes anodic process (Li+ deintercalation), while discharge denotes cathodic process (Li+ intercalation). At the first discharge, TbW delivers a discharge capacity of 29 mAh/g at the average potential of ca. 3.3 V, which corresponds to ∼0.7

Figure 1. (a) Powder X-ray diffraction pattern for Tb(H2O)5[WV(CN)8]. The simulated pattern is also shown as reference. Perspectives views of the two-dimensional Tb(H2O)5[WV(CN)8] structure along (b) the c and (c) a axes. Color code: W, orange; Tb, purple; C, gray; N blue; O, red. H atoms are omitted for clarity.

tetragonal crystal system with a P4/nmm space group. The least-squares fitting of the diffraction peaks gives the unit cell parameters of a = 10.876(7) Å, c = 7.363(7) Å, and V = 871(1) Å3, which which are in agreement with the previously reported values.8b The structure can be described as a 2D coordination polymer formed by corrugated layers of Tb(H2O)5[WV(CN)8] in the ab plane with an intermetallic Tb3+-W5+ distance of 5.762 Å (Figure 1b). A strong structural disorder is observed in the coordination sphere of both, lanthanide and transition metal ions, giving rise to a distribution of coordination spheres. In addition to four nitrogen atoms belonging to cyano groups, the Tb3+ ion completes its coordination sphere with five water molecules. Interlayers interactions are mediated through directional hydrogen bonds between these water molecules and nitrogen atoms from a terminal cyano group belonging to the adjacent layer (Figure 1c). The shortest interlayer distance involving metal ions (Tb3+-Tb3+ and W5+-W5+) is found to be 7.163 Å. C


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Inorganic Chemistry

Figure 4. OCV as a function of Li content x in Lix{Tb(H2O)5[W(CN)8]}. The dotted and solid lines are the potential profile during (de)lithiation in the GITT mode.

Figure 3. (a) Charge−discharge curves and (b) cycle stability for TbW at 10 mA/g.

Li+ intercalation per the formula unit of TbW. The following first charge gives a specific capacity of 31 mA/g almost reversibly. Thus, 0.7 Li+ is reversibly intercalated/deintercalated in TbW. Compared with PBAs, where almost all cyanoferrates are reduced/oxidized reversibly,21 TbW exhibits the redox reaction of 70% octacyanotungstates in the framework. It is most likely that the important structural changes associated with Li+ intercalation inhibit the complete redox reaction of the octacyanotungstates. Though the available capacity slightly decreases during a few initial cycles, TbW retains 86% of the initial capacity after 10 cycles (Figure 3b), which is consistent with the stable CV curves. The open-circuit voltage (OCV) measurements further clarify the Li+ intercalation properties of TbW. The OCV as a function of Li content x in Lix{Tb(H2O)5[W(CN)8]} (LixTbW; Figure 4, open triangles) shows that, after 0.9 Li+ intercalation at the first lithiation, 0.7 Li+ is deintercalated/ intercalated almost reversibly at the average potential of 3.3 V versus Li/Li+. The polarization between the lithiation and delithiation is small, supporting the high reversibility of the (de)lithiation processes. Since the potential profile is S-shaped, it is presumed that the lithiation proceeds entirely via the solid solution process rather than the biphasic process. The initial slight irreversible lithiation may suggest that 0.2 Li+ is trapped presumably at the surface and/or defects of TbW. The residual Li+ explains the irreversible initial cathodic peak in the CV, as well as the capacity fade in the initial charge−discharge cycles. Infrared spectroscopy (IR) appears as particularly relevant to monitor changes in the oxidation state of the octacyanometallate moiety. Ex situ IR spectroscopy was performed in the cyanide vibration window (2000−2200 cm−1) for the three samples (initial, delithiated, and lithiated) to get further insights into the mechanism of Li+ ion insertion/deinsertion (Figure 5).

Figure 5. Ex situ IR spectra of Tb(H2O)5[W(CN)8] before cycle (a), after the first lithiation (b), and after the first delithiation (c). The black solid line accounts for the experimental data, while the red solid line corresponds to the cumulative curve from the deconvolution.

The main stretching vibrations of the pristine TbW material are located at 2124(s), 2148(sh), 2162(s), and 2179(s) cm−1. While the presence of ν(CN) in the range of 2120−2130 cm−1 arises from terminal cyano groups, the bands located at D


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Inorganic Chemistry

reversible redox reaction of octacyanotungstate in a solid-state matrix. To further clarify the reaction mechanism, ex situ X-ray diffraction patterns were recorded (Figure 7). After lithiation of

higher wavenumbers confirm the presence of bridging cyanides ν(WV−CN−TbIII). As it can be observed, the insertion of Li+ induces the disappearance of the higher wavenumber bands in Li0.9TbW (at 2179 and 2162 cm−1) and the shift of most of others bands toward lower wavenumbers. Such behavior directly reflects the reduction from [W V (CN) 8 ] 3− to [WIV(CN)8]4− due to population’s increase in the antibonding highest occupied molecular orbital of cyano ligands.33,34 The IR spectra of the delithiated Li0.2TbW compound show a recovery of the bands that were initially located at higher wavenumbers. However, the spectra correspond to the envelope of the pristine and lithiated compounds indicating some irreversibility. Nevertheless, the deconvolution of the spectra for the delithiated sample reveals that all the bands of the pristine material can be recovered upon lithium deinsertion. The XAS further confirms the role of the octacyanotungstate center during the (de)lithiation of TbW. The W L1 edge spectra (Figure 6) show that the lithiation induces a decrease of

Figure 7. Ex situ X-ray diffraction patterns for TbW before cycle, after the first lithiation, and after the first delithiation.

TbW to 2.5 V, the X-ray diffraction pattern shows no diffraction peak, suggesting that the initial crystalline TbW is transformed to the amorphous state (or at least to the poor crystalline state). However, when the compound is delithiated, the XRD pattern shows the sharp diffraction peaks similar to those observed for the pristine state, which indicates the recovery to the initial crystalline phase. Therefore, the reversible phase transformation between the crystalline and the amorphous states occurs during the (de)lithiation process, while maintaining the fundamental cyano-bridged framework. Presumably, the Li-ion intercalation induces disorder in the weak stacking of the neutral TbW layers leading to the amorphous state, whereas the fundamental structure of the TbW layers is preserved to recover the initial crystalline state by delithiation. As for the sample after the delithiation, the calculated unit cell parameters are a = 10.79(1) Å, c = 7.35(2) Å, and V = 856(3) Å3, which are slightly smaller than those found for the sample before the (de)lithiation. It is most likely that the trapped Li+ after the first lithiation prevents the complete recovery of the initial state. In the case of PBAs, the intercalated alkaline cation is located in the tetrahedral sites of the fcc structure. However, in our case and although the structural changes associated with a Li+ ion insertion remain unclear due to the amorphization of the framework, the hypothesis on the lithium ion localization may be done on the basis of the crystal structure of the previously reported K(H2O)[Tb(H2O)4][WIV(CN)8] compound:37 K+ ion is situated between the cyano-bridged Tb3+/W5+ bimetallic layers and forms weak interactions with terminal cyano groups and terbium coordinated water molecules to give a 3D framework. Having demonstrated the reversible solid-state redox reaction between paramagnetic [WV(CN)8]3− (5d1) and diamagnetic [WIV(CN)8]4− (5d2), the magnetic measurements were conducted to clarify the change in the magnetic properties during the (de)lithiation process. For the lithiated Li0.9TbW compound, the room-temperature value of χT is equal to 10.93 cm3 K mol−1, which is in a good agreement with the value expected for a single Tb3+ ion (11.75 cm3 K mol−1) considering than the WIV is diamagnetic (Figure 8). When cooled, a decrease of χT is observed to reach the

Figure 6. Ex situ X-ray absorption spectra of Tb(H2O)5[W(CN)8] before cycle, after the first lithiation, and after the first delithiation.

the intensity of the absorption peak centered at 12104 eV corresponding to the excitation from 2s to 6p-5d (dz2, admitting the square antiprism geometry of the octacyanotungstate) hybridized orbital, indicating the reduction of [WV(CN)8]3− (5d1) to [WIV(CN)8]4− (5d2). The subsequent delithiation provokes an increase of the absorption peak intensity at 12 104 eV induced by the [WIV(CN)8]4− to [WV(CN)8]3−oxidation. The small changes observed in the absorption spectrum upon lithium insertion could be ascribed to the high degree of covalency in cyanometallate systems as previously evidenced in hexacyano35 or octacyanometallate coordination compounds.30 Therefore, the electrochemical (de)lithiation of TbW is associated with the reversible redox reaction of [WV(CN)8]3−/[WIV(CN)8]4−. However, note that the spectrum profile does not turn back exactly to the initial shape even after delithiation up to 4.3 V versus Li/Li+. As observed in IR spectroscopy, the irreversibility of the spectrum change is explained by the residual (trapped) Li+ after the first lithiation, suggested by the CV and the charge−discharge curve. In parallel, the highly energetic X-ray beam most likely caused radiation damages as previously observed in octacyanomolybdate systems.36 Although the photomagnetic properties of octacyanomolybdate systems have been toughly studied, this is, to the best of our knowledge, the first demonstration of the E


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Inorganic Chemistry

Figure 8. (left) χT vs T plots for the pristine, lithiated, and delithiated compounds. (right) Field dependence of the magnetization at 1.8 K (inset) normalized magnetization curves with a logarithmic scale.

Figure 9. Temperature dependence of the in-phase, χ′, and out-of-phase, χ″, components of the AC susceptibility at various frequencies for: (a) the pristine TbW, (b) the lithiated Li0.9TbW, and (c) deliathed Li0.2TbW compounds.

value of 7.18 cm3 K mol−1 at 1.8 K. This decrease originates from the thermal depopulation of the Stark sublevels from the Tb3+ ion. This behavior clearly contrasts with the magnetic behavior of the pristine TbW coordination polymer in which a dramatic increase of χT from the room-temperature value of 12.125 cm3 K mol−1 (the sum of 11.75 cm3 K mol−1 for a Tb3+ ion and 0.375 cm3 K mol−1 for a WV ion) as the temperature decreases, indicates the presence of a ferromagnetic interaction between the W5+ and Tb3+ ions. The field dependence of the

magnetization at low temperature brings also a supplementary proof for the reduction of the octacyanotungstate moiety in the lithiated compound. The magnetization value of 4.55 μB observed under 50 kOe is close to the value of 5.00 μB expected for the Tb3+ ion considering an Ising spin of S = 1/ 2 and a strong uniaxial anisotropy of the g tensor (g⊥ = 10; g∥ = 0), while the TbW compound exhibits a higher value of 6.25 μB due to the presence of the W5+ paramagnetic ion (S = 1/2). F


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Inorganic Chemistry The room-temperature value of χT for the delithiated sample Li0.2TbW, equal to 11.89 cm3 K mol−1, is slightly lower than the one observed for the pristine non-lithiated compound (χT = 12.125 cm3 K mol−1). When cooled, a decrease of χT is observed to reach the minimum value of 10.18 cm3·K·mol−1 at 13 K. Below this temperature, an increase of χT is observed, which originates from ferromagnetic interactions between the WV and the TbIII ions. This behavior is in well accordance with the pristine TbW compound, although the maximum value of χT is slightly lower than the one observed for the pristine compound. This confirms the results from the electrochemical experiments indicating that 0.2 Li+ per neutral coordination polymer formula cannot be removed from the structure. The field dependence of the magnetization leads to a value of 4.67 μB observed under 50 kOe, which is lower in comparison with the value of 5.96 μB observed for the pristine TbW compound, confirming that the delithiation process is not fully reversible, as indicated in the CV and charge−discharge curves. This value is also lower than the expected value of 5.80 μB based on the Li0.2TbW formula. One possibility could be related to a modification of the Tb3+ ion symmetry caused by an interaction of the terbium-coordinated water molecules with the Li+ ion during the lithiation/delithiation process in accordance with the structural changes evidenced by PXRD. Such mechanism has been supported by the reported cyano-bridged 3D framework NaxEu[Fe(CN)6]·4H2O in which the insertion of sodium ions induces a long-range cooperative rotation of the hexacyanoferrate moieties, which induces an adaptation of the lattice including significant changes in the crystallographic distances of the Eu3+ coordination sphere.38 Additionally, the field dependence of the magnetization at low fields (0−10 kOe) for Li0.2TbW (inset, Figure 8) shows that the slope is intermediate between the pristine and delithiated samples. This suggests the occurrence of ferromagnetic interactions between WV and TbIII ions but without the presence of a ferromagnetic ordering. Alternative current (AC) measurements were performed to probe any changes in the dynamic behavior upon lithium insertion. In contrast with the pristine TbW coordination polymer, which exhibits a long-range magnetic ordering below Tc = 2.8 K (Figure 9a), the absence of an out-of-phase component (χ″) of the AC susceptibility for the lithiated sample Li0.9TbW (Figure 9b) indicates the disappearance of a long-range ordering that can be explained by the breaking of the Tb3+-W5+ magnetic interaction between the spin carriers through the cyano bridge upon reduction of the octacyanotungstate moiety. In contrast, the dynamic behavior of the delithiated compound Li0.2TbW shows a clear frequency dependence indicating a slow relaxation of the magnetization (Figure 9c). The temperature dependence of the relaxation time was fitted with an Arrhenius law, τ = τ0 exp(Ea/kT), where Ea is the energy barrier and τ0 is the attempt time (Figure 10). This gives the value of the activation barrier of 26 K and the τ0 value equal to 5.0 × 10−10 s (Table 1). Note that the latter is in accordance with the ones usually observed for classical superparamagnetic systems (10−8 to 10−12 s). This is further confirmed by the value of the Mydosh parameter defined as φ = (Tmax − Tmin)/ (Tmax × log νmax − log νmin), which indicates the amplitude of the out-of-phase maxima’s shift with frequency.39 It is equal to 0.155, which is in the range of values classically observed for superparamagnets (>0.1). However, the frequency dependence of the AC susceptibilities was also measured at 1.8 K (Figure S1). The resulting Cole−Cole plots (Figure S2) show a strong

Figure 10. Temperature dependence of the relaxation time for the deliathed Li0.20TbW compound using the AC susceptibilities data under a zero DC-field.

deviation from perfect semicircles. This reflects a wide distribution of relaxation processes. Indeed, it was not possible to obtain a reasonable fit using the Debye model, indicating a wide distribution of relaxation times. AC magnetic measurements were also performed in the presence of various direct-current (DC) magnetic fields (Figure 11). It appears that the maximum of χ″ progressively decreases upon increasing the magnetic field to 200 Oe. However, for 500 and 1000 Oe fields, a clear maximum at a higher temperature (2.6 K) is evidenced before almost disappearing at 2000 Oe. This behavior does not correspond to what is found in traditional spin glasses, where the transition temperature shifts to lower temperature upon increasing magnetic field.40 The temperature dependence of the AC susceptibilities with various frequencies measured under a 900 Oe DC field shows a clear frequency dependence (Figure 12). Fitting the relaxation time with an Ahrrenius law gives Ea = 42 K and τ0 = 1.1 × 10−11 s, which is in the range to what is found in a superparamagnetic regime. Similarly to the data obtained at zero DC field, the Cole−Cole plots under this optimum field show flattened semicircles (Figure S3). Fitting with a generalized Debye gives very high value of the α parameter ranging from 0.71 to 0.90 (Table S1), reflecting a very wide distribution of relaxation process. Such slow relaxation of the magnetization may originate from the presence of isolated magnetic WV−TbIII fragments formed by the rupture of the long-range correlation between the spin carriers as frequently observed in similar systems.41 This could explain the wide distribution of relaxation times suggested from the Cole−Cole plots. In this sense, we have previously reported a slow relaxation of the magnetization originating from a spinglass behavior in the coordination polymer Eu0.5Tb0.5(H2O)5[W(CN)8].8d At low temperature, the diamagnetic Eu3+ ion breaks the propagation of the long-range ordering and induces a spin frustration. In the present system, the magnetic behavior of the coordination polymers may be switched from an initial long-range magnetic ordering to a short-range ordering regime upon lithium insertion. To further check this phenomenon, two additional samples of respective compositions Li0.37Tb(H2O)5{[WV(CN)8]0.63[WIV(CN)8]0.37} ( L i 0 . 3 7 T b W ) a n d Li 0 . 6 0 T b ( H 2 O) 5 { [ W V ( C N ) 8 ] 0 . 4 0 [WIV(CN)8]0.60} (Li0.60TbW) with intermediate amounts of Li+ were characterized by AC susceptibility measurements. While the latter does not show any frequency dependence, the former exhibits weak frequency dependence. Fitting the G


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Inorganic Chemistry Table 1. Arrhenius Fit and Mydosh Parameters Values Using the AC Susceptibilities Data compound

frequency dependence

Li0.20TbW Li0.37TbW Li0.60TbW Li0.90TbW a

yes yes no no

τ0 (s) −10

(5.0 ± 0.8) × 10 (1.08 ± 0.07) × 10−7

Ea (K)

φa

τ0 (s) at 900 Oe

Ea (K) at 900 Oe

26 ± 2 16.8 ± 0.1

0.155 0.265

(1.1 ± 0.9) × 10−11 (7 ± 3) × 10−13

42 ± 1 50.2 ± 0.8

Mydosh parameter.

amount of trapped Li+ ion, the switch between short-range ordering/paramagnetic states may be electrochemically achieved based on the electrochemical results. Besides, we would like to point out that a slow relaxation of the magnetization has never been evidenced in PBA upon Li+ or Na+ insertion. In such a system, a switch from ferromagnetism to paramagnetism is usually observed with a gradual decrease of the ordering temperature.28 Thus, electrochemistry constitutes a way for designing intriguing magnetic molecular materials.

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CONCLUSION

For the first time, the electrochemical storage behavior of the octacyanotungstate-based coordination polymer has been evaluated. Upon lithium insertion, reduction of the [WV(CN)8]3− to [WIV(CN)8]4− moiety is observed and has been evidenced through IR, XAS, and magnetic measurements. Such process is mostly reversible, and remarkably, the crystallinity of the network is recovered upon lithium deinsertion. Such changes may be monitored though magnetic measurements, which shows an occurrence of three different magnetic regimes depending on the lithium content. While the pristine coordination polymer is a ferromagnet at low temperature, a low Li+ content (x < 0.4) is sufficient to irreversibly break the long-range magnetic ordering due to the partial octacyanotungstate moiety reduction and induces an appearance of a short-range magnetic ordering with a slow relaxation of the magnetization arising from the presence of isolated WV−TbIII fragments. Increasing the Li+ content yields to a pure paramagnetic regime. Such studies indicate that octacyanometallate coordination polymers may be used to design cathode materials for batteries and that the amount of the inserted Li+ may control the magnetic behavior of these multifunctional materials.

Figure 11. Temperature dependence of the out-of-phase, χ″, component of the AC susceptibility (1000 Hz) measured for various DC fields for the deliathed Li0.2TbW compound.

relaxation time with the Arrhenius law yields Ea = 16.8 K and τ0 = 1.08 × 10−7 s, which is slightly lower that what is found in superparamagnetic systems. Similarly, applying a DC field of 900 Oe leads to pronounced frequency dependence of both the in-phase and out-of-phase susceptibilities (Figure S4). The parameters obtained from the Arrhenius law (Figure S5) are summarized in Table 1. Consequently, it appears that both, the static and the dynamic magnetic responses of the coordination polymer Tb(H2O)5[W(CN)8] can be easily modulated based on the amount of the inserted Li+ ion. Namely, a low Li+ content is sufficient to irreversibly switch from a ferromagnetic long-range ordering to a short-range ordering with a slow relaxation of the magnetization that may arise from the presence of isolated WV−TbIII fragments. Further introduction of Li+ with the concomitant reduction of the octacyanotungstate moiety leads to a pure paramagnetic behavior. To our knowledge, this is the first time that different magnetic regimes can be electrochemically observed in a single coordination network. While the initial ferromagnetic state is not recovered due to a residual

Figure 12. Temperature dependence of the in-phase, χ′, and out-of-phase, χ″, components of the AC susceptibility measured at various frequencies and under a 900 Oe DC field for the deliathed Li0.2TbW compound. H


Article

Inorganic Chemistry

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01086. Magnetic data plotted to show frequency dependence of in-phase and out-of-phase components of AC susceptibility under a zero DC field for Li0.2TbW at 1.8 K, Cole− Cole plots, temperature dependence plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (J.L.) *E-mail: [email protected]. (D.A.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Univ. of Montpellier, CNRS, Plateforme d’Analyse et de Caractérisation Balard ICGM. M.O. was financially supported by Japan Science and Technology Agency, SICORP, Molecular Technology: Molecular Materials for Mg Batteries (MoMa). The X-ray absorption spectroscopy was conducted under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2014G044).

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REFERENCES

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Electrochemical Li-Ion Intercalation in Octacyanotungstate-Bridged

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