Magnetic Phase Switching in a Tetraoxolene-Bridged Honeycomb

Article Cite This: Chem. Mater. 2017, 29, 10053−10059

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Magnetic Phase Switching in a Tetraoxolene-Bridged Honeycomb Ferrimagnet Using a Lithium Ion Battery System Kouji Taniguchi,*,†,‡,§ Jian Chen,†,‡ Yoshihiro Sekine,†,‡ and Hitoshi Miyasaka*,†,‡ †

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578, Japan § Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan ‡

S Supporting Information *

ABSTRACT: The design of molecular magnets using the “porosity” concept seen in metal−organic frameworks (MOFs) is a unique direction for potential applications such as magnetic sensors, switches, and nonvolatile electromagnets. In addition, the strategy of “magnet + porosity” could allow for the creation of postsynthesized magnets often exhibiting a higher magnetic phase transition temperature (Tc) than a selfassembled magnet. A class of paramagnetic MOFs with electron-acceptor ligands, which can accept electrons from an appropriate donor or electrode to form organic radicals, shows great promise, because their magnetic phase stability is tuned through an external electron-filling control. Here, we demonstrate the electrochemical switching of nonvolatile magnetic phases in the well-known honeycomb-layer ferrimagnet (NBu4)[MnIICrIII(Cl2An)3] (H2Cl2An = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone) using a Li ion battery (LIB) system, in which Li+ ions and electrons are simultaneously inserted into or extracted from the material. Ferrimagnetic phase stability is reversibly modulated using in situ LIB discharge/charge cycles.

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INTRODUCTION The design of molecule-based magnets with a high magnetic phase transition temperature (Tc) remains a challenge in molecular and material science.1−5 The usual approach for producing magnets is to combine the spin sources of transitionmetal ions6−9 using appropriate organic bridging ligands or organic radical ligands,10,11 which could form magnetic frameworks with superexchange spin coupling through an organic linker. However, in most cases using this self-assembly process, the magnetic order appears at a quite low Tc. This occurs because of the long distance between localized spins, which normally leads to a weak spin exchange. One daring idea for improving this situation is to turn a diamagnetic ligand (S = 0) linking paramagnetic metal centers into a paramagnetic radical ligand (S = 1/2). The introduction of organic radical ligands into frameworks could cause a significant overlap in ligand-based magnetic orbitals and metal magnetic orbitals via an effective charge-transfer, leading to a strong exchange interaction between the metal centers via the ligand spin.12 Eventually, this would produce a higher Tc than that in the original form using nonradical ligands.13,14 There are two common routes for introducing organic radicals into metal-spin-based magnetic networks: (I) preorganization of organic radicals in a self-assembly process and (II) postsynthetic modification of organic ligands into their radical forms in a framework, in which electrons are chemically or © 2017 American Chemical Society

electrochemically injected into (or extracted from) organic ligands. A class of redox-active networks involving intralattice electron transfer (ET) is one of the attractive candidates for materials synthesized via route I. The representative example is a series of charge-transfer (CT) materials composed of carboxylate-bridged paddlewheel-type diruthenium(II,II) complexes ([Ru2II,II]) as an electron donor (D) and 7,7,8,8tetracyano-p-quinodimethane derivatives (TCNQR) as an electron acceptor (A).15 In this example, multidimensional networks with a D2A formula (generally the D/A formulation is hereafter given as DmAn) provide various magnets with Tc ≈ 100 K, depending on the intralattice electron transfer that occurs during the self-assembly process. In this system, a key component of obtaining a high-Tc magnet is the formation of a radical form of TCNQR (i.e., TCNQR•−) created by intralattice one-electron transfer in D2A.16−21 Another group of CT magnetic materials can be found in a group of tetraoxolene-bridged honeycomb-layered metal assemblies, such as those using 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate (chloranilate, Cl2An2−) acting as A, which have recently been energetically studied.22 In particular, a D2A3 compound with a ferrous ion (Fe2+ as D), (Me2NH2)2[Fe2(Cl2An)3]· Received: August 31, 2017 Revised: October 25, 2017 Published: November 14, 2017 10053

DOI: 10.1021/acs.chemmater.7b03691 Chem. Mater. 2017, 29, 10053−10059

Article

Chemistry of Materials 2H2O·6DMF, was reported to be a ferrimagnet with Tc = 80 K14 via the formation of Cl2An•3− by an intralattice ET from Fe2+ to Cl2An2− (S = 0) in the self-assembly process, which produces a radical spin state, Cl2An•3− (S = 1/2) (Figure 1a).14

migration. For these reasons, metal−organic framework (MOF) type materials could be useful candidates (vide infra)24−28 In the case of an LIB system, electrons and Li+ ions are simultaneously inserted into/extracted from a target material via discharge/charge processes, realizing a reversible transformation between [(D0)m(A0)n] and Lix[(D0)m(A)n−x(A•−)x] and leading to switchable control of the radical spin. For the modification from A0 to A•−, the negative charge of [(D0)m(A)n−x(A•−)x]x− is neutralized by Li+ ions introduced to a material. The electrons are introduced to acceptor ligands, while Li+ ions are accommodated in framework channels or pores. Thus, paramagnetic MOFs composed of D and A components (i.e., D/A-MOFs) are suitable for magnetism control using ion transport. In fact, reversible magnetic phase switching using an LIB system has been realized in layered D2A-type MOFs composed of [Ru2II,II] as D and TCNQR as A.24,25 In these materials, the nonvolatile phase stability of paramagnetic and ferrimagnetic states was switched via a transformation between TCNQR0 (S = 0) and TCNQR•− (S = 1/2) in LIB discharge/charge procedures. This method is applicable for various materials that meet two conditions: (i) the material is composed of redox-active ligands that possibly create an electronic conjugation mediating metal spin when the ligand is reduced or oxidized, and (ii) the material is porous, with through-space columns or pores where counterions can migrate. A group of tetraoxolene-bridged honeycomb-layered metal assemblies is one of the most promising materials for electrochemical switchability. This type of bis-bidentate bridging ligand has been widely investigated in the multidimensional assemblies29 of discrete molecules, 12,30 one-dimensional (1-D) chains, 31,32 2-D layers,32,33 and 3-D infinite networks.33 Except for the abovementioned materials with Fe2+, these compounds exhibit spin correlations mediated by a simple magnetic superexchange interaction between paramagnetic metal centers through a nonradical tetraoxolene ligand (e.g., Cl2An2−). Hence, the magnetic phase stability in these materials could be tuned by the electron-filling control of the Cl2An2− ligand, because of the formation of a radical spin (Cl2An•3−) between metal spins is possible (Figure 1b).14,34 Atzori et al. synthesized a family of tetraoxolene-bridged honeycomb-layered bimetallic ferrimagnets with the formulation of A[MIIMIII(X2An)3] {A = [(H3O)(phz)3]+ (phz = phenazine) or NBu4+; MII = Mn, Fe, Co, etc.; MIII = Cr, Fe; X = Cl, Br, I, H}.35 These compounds exhibited long-range ferrimagnetic ordering at temperatures below 10 K, although the Tc value was dependent on X. The low Tc suggested that a weak superexchange interaction between metal spins through X2An2− is the dominant factor in determining long-range ordering. Here, we selected (NBu4)[MnIICrIII(Cl2An)3], the magnetic sources of which consist of Mn2+ (S = 5/2) and Cr3+ (S = 3/2) ions, as a target material for magnetism control using an LIB system. The structure of this compound is composed of alternating hexagonal anionic [MnIICrIII(Cl2An)3]− layers and cationic (NBu4)+ layers in the [001] direction (Figure 2).35 In this study, we demonstrate the reversible modification of the magnetic phase stability of (NBu4)[MnIICrIII(Cl2An)3] with a Tc value between 10 K for the pristine and 36 K for the reduced form (discharged). This is achieved through the electron-filling control of the Cl2An2− ligands via Li+ ion insertion/extraction using an LIB system.

Figure 1. (a) Redox reaction of deprotonated benzoquinoid linkers based on chloranilic acid: left to right, Cl2An2− (S = 0) and Cl2An•3− (S = 1/2). (b) Conceptual figure of magnetic phase stability tuning by electron-filling control of benzoquinoid linkers using an LIB system. This schematic figure describes the magnetic phase switching scenario expected in the temperature range of Tc (charged) < T ≤ Tc(discharged). One-electron filling of the Cl2An2− in (NBu4)[MnCr(Cl2An)3] produces an antiferromagnetic kinetic exchange interaction (J < 0) between the radical spin of Cl2An•3− and the spin of metal cation [Mn2+ (S = 5/2)/Cr3+ (S = 3/2)]. Electron extraction from the Cl2An•3− radical (S = 1/2) during the charging process of an LIB system removes kinetic exchange interactions. The schematic figure of the paramagnetic state, in which magnetic moments are thermally fluctuated, is displayed for a certain moment. The schematic figure of the ferrimagnetic state is speculated from the antiferromagnetic interaction.

Reflecting radical spin generation, the Tc reaching 80 K, which is high compared with those in most of other layered moleculebased magnets, was observed even in low-dimensional layered systems.10,11 An intriguing technique following the research on route II was recently reported by DeGayner et al.,23 in which the abovementioned compound (Me2NH2)2[Fe2(Cl2An)3]·2H2O·6DMF was soaked in a DMF solution of cobaltocene (Cp2Co) to synthesize a complete radical form with (Cl2An)39− in D2A3 (i.e., Cp2Co + Cl2An2− → Cp2Co+ + Cl2An•3−; Figure 1a). Consequently, the Tc of the compound increased to 105 K. Another notable aspect of route II is a potential for tunability of magnetism. In the recent years, an electrochemical electrondoping technique based on a Li ion battery (LIB) system was developed for magnetism modulation.24−26 This method involves counterion insertion/migration in a material, and hence, it is necessary to secure paths in materials for ion 10054


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Cycle Test of a Li Ion Battery. Cycle testing of an LIB system was performed using 2032-type coin cells, which were assembled in an Arfilled glovebox. A LiPF6 electrolyte dissolved at 1 mol/dm3 in ethylene carbonate−dimethyl carbonate with a 3:7 volume ratio was used. The galvanostatic charge/discharge test was conducted using a potentiogalvanostat (Solartron, 1470E) at 298 K. The cell voltage in the galvanostatic charge/discharge measurements was cycled at 5 mA/g between 3.5 and 2.0 V vs Li/Li+. Li+ Ion Insertion/Extraction Process for Magnetism Control. In the ex situ measurements, a galvanostatic intermittent titration technique (GITT),37 for which a constant low-density current (5.18 mA/g) was applied for 1 h followed by an interval of 1 h to achieve an electrochemical equilibrium state, was employed to insert lithium ions into 1. The open circuit voltage (OCV) was recorded with a potentiogalvanostat (Solartron, 1470E) at 298 K. In the in situ measurements, the LIB cell was discharged/charged to the target voltage under a constant current, 5 mA/g; each voltage was maintained for 2 h. Discharge/charge control was carried out using a potentio-galvanostat (Solartron, 1287) at 300 K. Ex Situ Magnetic Measurements. Magnetic data were collected by a Quantum Design MPMS magnetometer (MPMS-XL). Magnetization measurements were performed by applying a magnetic field of 100 Oe in a temperature range from 1.8 to 300 K. After the discharge process finished, the coin cells were disassembled, and the 1 cathodes were extracted in the Ar-filled glovebox. In Situ Magnetic Measurements. The in situ LIB cell was inserted into a Quantum Design MPMS-7S. Discharging/charging operations for the in situ LIB cell were conducted at 300 K. Electronfilling control was carried out on the basis of the battery voltage to avoid the effects of irreversible capacity. After the discharging/charging process, magnetization was measured in the OCV state. The temperature dependence of the magnetization was measured by applying a magnetic field of 100 Oe between 8 and 120 K. Magnetic field dependence was also measured at 15 K by applying a magnetic field between −7 and 7 T.

Figure 2. Crystal structure of 1 showing the packing mode projected in the [001] direction, where Cl, O, Mn/Cr, and C of the framework are displayed in green, red, pink, and black, respectively. Disordered NBu4+ groups located between the honeycomb layers are portrayed in pale gray. The structure of 1 was reported in ref 35.

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EXPERIMENTAL SECTION

Materials Synthesis. All reagents were purchased from commercial sources and used without further purification. The precursor (NBu4)3[Cr(Cl2An)3] was synthesized in accordance with literature methods.32 Polycrystalline compound 1 was synthesized using methods modified from the literature.35 MnCl2·4H2O (21.8 mg, 0.11 mmol) and precursor (NBu4)3[Cr(Cl2An)3] (154.0 mg, 0.11 mmol) were mixed with a solvent consisting of 12 mL of methanol and 3 mL of dichloromethane. Then, the mixture was transferred to a 20 mL Teflon-lined autoclave and hydrothermally treated at 100 °C for 24 h. After cooling to room temperature, the violet powder sample was collected at a 77% yield by suction filtration, washed with dichloromethane, and finally dried in a vacuum for 48 h. Anal. Calcd for C34H36Cl6MnCrNO12: C 42.09, H 3.74, N 1.44. Found: C 42.27, H 4.04, N 1.42. IR (νmax/cm−1, KBr pellets): 2962 (m), 2934 (w), 2875 (m), 1608 (m), 1516 (vs), 1357 (s), 1310 (w), 1007 (s), 878 (w), 857 (s), 737 (m), 629 (m), 578 (m), 509 (m), 458 (m). Measurements of Physical Characteristics. Infrared (IR) spectra were recorded using KBr pellets at room temperature with a JASCO FT/IR-4200 spectrometer. Powder X-ray diffraction (PXRD) patterns were collected for a polycrystalline powder of compound 1, filled into a glass capillary (ϕ 0.5 mm) and installed on a RIGAKU Ultima IV diffractometer, with Cu Κα radiation (λ = 1.5418 Å). The measurements were conducted at 0.02° steps. Electron spin resonance (ESR) spectra were recorded on a JEOL JEA-FA100 using near 9 GHz microwaves (X-band) for cathode samples at 290 K. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a PHI5000 VersaProbe II (ULVAC-PHI, Inc.) with a monochromatic Al Kα X-ray source. Cathode and Li Ion Battery Cell Preparation. Li+ ion insertion into/extraction from the target material was carried out with an LIB system, in which Li metal was used as an anode (a half-cell). A cathode was fabricated using a sample of 1, acetylene black, and polytetrafluoroethylene at a mass ratio of 5:4:1. The mixture was pressed onto an aluminum net current collector and dried overnight at 60 °C in a vacuum. A coin-type cell (2032-type) for ex situ magnetization modulation or miniature LIB quartz cell (15 × 7 × 5 mm)25 for in situ magnetization tuning was assembled in an Ar-filled glovebox. The cells consisted of 1 as cathode, a lithium metal anode, and an electrolyte. A lithium hexafluorophosphate (LiPF6) electrolyte dissolved at 1 mol/dm3 in ethylene carbonate−dimethyl carbonate at a 3:7 volume ratio and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) electrolyte dissolved at 0.30 mol/kg in 1-ethyl-3methylimidazolium bis(fluorosulfonyl)amide (EMI-FSA)36 were used as the LIB electrolytes for ex situ and in situ measurements, respectively. In the in situ cell, a Pt plate was used as a current collector for the electrode.

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RESULTS AND DISCUSSION The PXRD pattern of the freshly prepared polycrystalline sample of 1 was consistent with that in previous literature [Figure S1, Supporting Information (SI)].35 The temperature dependence of the field-cooled magnetization (FCM) of 1 was measured under a 1 kOe magnetic field and showed a rapid increase in magnetization at temperatures below approximately 15 K (Figure S2a, SI). A cusp structure appeared at 10 K, followed by further increases in magnetization with decreasing temperatures. The rapid increase in magnetization indicated the onset of ferrimagnetic long-range ordering, which is in good agreement with the data reported in ref 35. However, the Tc for the present compound (compound 1) may be somewhat higher than the Tc of the original compound (Tc = 5.5 K). The cusp feature in the FCM might indicate the presence of antiferromagnetic interactions between layers. This was not described in the original paper, but the effects of interlayer interaction, as well as the X-substitution effect of the X2An2− ligand series, could be crucial for deciding Tc.35 The ferrimagnetically ordered state of 1 was also confirmed from the data for the field dependence of the magnetization. A fieldinduced magnetization hysteresis curve was observed at 2 K. Its small coercive field (i.e., soft magnet) was consistent with that of the original compound (Figure S2b, SI).35 Sample 1 was used as a cathode material for subsequent LIB measurements. The electron-filling control of 1 was conducted using Li+ ion insertion by an LIB system. The reduction reaction with supplemental Li+ ions was carried out using a galvanostatic intermittent titration technique (GITT); the details of this are described in the Experimental Section. Figure 3a displays an 10055


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decrease of OCV in the discharge process shown in Figure 3a is ascribed to an increase in the chemical potential for electrons in the cathode material resulting from electron doping via Li+ ion insertion; the chemical potential for electrons in Li metal does not change during the discharge process. Figure 3b shows the absolute value of the derivative of capacity (Q) with respect to voltage (V), |dQ/dV|. A distinct peak was observed at approximately 2.6 V vs Li/Li+, indicating the first reduction potential of 1. Notably, no other distinct peak (reduction) was observed in the voltage range from 3.4 to 2.2 V (vs Li/Li+). Considering that the respective reduction potentials of Mn2+ and Cr3+ are reported to be around ∼0.5 V vs Li/Li+40 and ∼0.2 V vs Li/Li+,41 the observed reduction potential, ∼2.6 V vs Li/Li+, should correspond to the ligand-based reduction of Cl2An2− + e− → Cl2An•3− in the material (vide infra).12 In fact, in the measurement of the X-ray photoelectron spectroscopy (XPS) of 1 (x = 0) and Li+-inserted 1 (x = 3), no sign of a reduction reaction for Mn2+ and Cr3+ was observed (Figure S4, SI). Taking into account this result, the tail structure at around 2.2 V seems to be ascribed to the second reduction potential on the ligand molecule, which is the remaining redox active species, in the material; Cl2An•3− + e− → Cl2An4−.23 Figure 3c shows the temperature dependence of FCM (H = 100 Oe; closed circles), as well as the remnant magnetization (RM after the FCM measurement at 100 Oe; open circles) of the extracted LIB cathode materials, which is measured at several OCVs: 3.38 V (initial state), 2.55 V, and 2.17 V vs Li/ Li+. The OCV points for the material are also displayed as the corresponding colored spots in Figure 3a,b. The FCM curve of a pristine cathode material having OCV = 3.38 V vs Li/Li+ (without Li+ ion insertion; red curve) exhibits a ferrimagnetic transition at around 10 K, as observed in the bulk sample of 1. When Li+ ions and electrons are introduced into the cathode by following the discharge operation to an OCV value of 2.55 V vs Li/Li+ (orange curve), the Tc shifts from 10 K to a higher temperature of 36 K (Tc was estimated from the vanishing temperature of RM). Considering that the electron is doped to 1 without generating other magnetic impurity phases in the discharge process (Figures S3 and S4), the increase in Tc should be attributed to the enhancement of exchange interactions with additional spins. The enhancement of exchange interaction was confirmed by the increase of Weiss temperature (θ); θ = −12.8 K for 3.38 V vs Li/Li+ (pristine) and θ = −39.8 K for 2.55 V vs Li/Li+ (discharged). This could result from the generation of Cl2An•3− radical spins that produces new kinetic exchange interactions with the neighboring spins of Mn2+ (S = 5/2)/Cr3+ (S = 3/2). The formation of Cl2An•3− radical spins was reflected in the electron spin resonance (ESR) spectra, in which a sharpened signal with g ≈ 2 was only observed in the discharged process, while a broadened signal with g ≈ 2 arising from Mn2+/Cr3+ ions was observed in the pristine sample (Figure S5, SI). The generation of Cl2An•3− radical spins was also confirmed by the increase of Curie constant after the discharging process; C = 5.54 emu K mol−1 for 3.38 V vs Li/Li+ (pristine) and C = 8.34 emu K mol−1 for 2.55 V vs Li/Li+ (discharged) (Figure S6, SI). On the other hand, the further decrease in OCV from 2.55 to 2.17 V vs Li/Li+ does not allow any further shifts in Tc, but the magnitude of the magnetization increases (see Figure 3c). This fact indicates that the single domain fraction of a ferrimagnetic phase with Tc ∼ 36 K increases in the discharge process in an OCV range from 2.55 to 2.17 V vs Li/Li+.

Figure 3. Magnetic phase stability tuning by an LIB system under ex situ conditions. (a) Open circuit voltage (OCV) as a function of capacity measured in an LIB coin cell combined with the 1 cathode (open circles), where the solid line shows the variations in voltage of an LIB cell during a discharge process at a constant current. The red, orange, and blue closed circles display the states of the samples taken for ex situ measurements, such as the magnetization and PXRD shown in part c and Figure S3 (SI). x is the nominal Li composition per formula unit, estimated from the discharge capacity. (b) Absolute values of the derivative of the capacity (Q) in part a with respect to the voltage (V), |dQ/dV|. (c) Temperature dependence of the cathode magnetization of Lix(NBu4)[MnCr(Cl2An)3] under ex situ conditions at each equilibrium electrochemical potential vs Li/Li+, where the closed and open circles represent field-cooled magnetization at 100 Oe and remnant magnetization, respectively. The nominal Li composition (x) per formula unit estimated from the discharged capacity, which might include contributions from the irreversible capacity of the LIB cell, is displayed in addition to the OCV.

OCV curve as a function of capacity, which is the electric charge quantity per unit weight, measured in the discharge process of an LIB incorporating 1 as the cathode. In the discharge process, the structural stability was checked using ex situ PXRD measurements (Figure S3, SI). The main crystal structure of 1 was likely maintained in the discharge process, and no additional peak of impurity phases, which might be produced by some conversion-type reactions,38 was observed. This result indicates that the reduction reaction (i.e., electronfilling) in the discharge process involves Li+ ion insertion into the pores of a crystal, while the essential framework skeleton is maintained. Considering the fact that the OCV of LIB is given by the difference in the electrochemical potentials for electrons between the cathode and anode in an electrolyte,39 the 10056


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Chemistry of Materials The reversibility of Li+ ion insertion/extraction for the cathode of 1 was confirmed by conducting LIB discharge/ charge cycles applying a constant current of 5 mA/g at 298 K (Figure 4a). The redox plateau is observed in both the

cycled cathode with that of the pristine cathode (Figure S7, SI). Though the intensity of PXRD pattern decreases may be due to the crystallinity lowering induced by mechanical stress to the lattice in the Li+ ion insertion process, the main peaks in the PXRD pattern are maintained even after the discharge/charge cycles, indicating the structural stability of 1 in discharge/ charge cycles involving Li+ ion insertion/extraction. These results imply that the LIB system is available for the reversible electron-filling control of 1. Taking advantage of the rechargeability of the LIB, the reversible magnetic modification performance of the 1 cathode was demonstrated in situ via discharge/charge cycles at room temperature (Figure 5). In this attempt, we used a miniature in situ LIB cell,25 which enables discharging/charging in a commercial superconducting quantum interference device (SQUID). The battery cycle was conducted between 3.1 and 2.35 V vs Li/Li+ (Figure 5c). Magnetization measurements were started from a charged state of 3.1 V vs Li/Li+, which was sufficiently higher than the reduction potential of Cl2An2− at ca. 2.6 V vs Li/Li+. Figure 5a displays the temperature dependence of FCM (closed circles) and RM (open circles) for discharged or charged states. For the charged states at 3.1 V vs Li/Li+ (red curves), the original magnetization behavior of 1 with a ferrimagnetic order at ∼10 K can be repeatedly confirmed. Meanwhile, all discharged states at 2.35 V vs Li/Li+ (blue curves) display the onset of ferrimagnetic ordering at Tc ∼ 33 K. Thus, the stability of the ferrimagnetic phase is successfully modulated with the switching of Tc between ∼10 and ∼33 K by electron-filling control via Li+ ion insertion into/extraction from 1 (Figure 5d). This implies that magnetic phase switching between paramagnetic and ferrimagnetic states is also possible in the temperature range of 10 K < T ≤ 33 K (i.e., between Tc

Figure 4. Cyclability of a Li ion battery with a cathode of (NBu4)[MnCr(Cl2An)3]. (a) Charge/discharge curve of the LIB cell with a 1 cathode measured at 298 K. The charge and discharge curves are represented by the red and blue lines, respectively. (b) Cycle performance of the LIB cell with a 1 cathode in relation to capacity. The closed red and blue circles represent the capacities of the charging process and discharging process, respectively.

discharging process at around 2.6 V vs Li/Li+ and the charging process at around 2.7 V vs Li/Li+, proving that a single redox dominates the cycle. Figure 4b shows the cyclability of the 1 cathode over 10 cycles. The discharge and charge capacities have almost the same values in each cycle. The discharge capacity for the 10th cycle is 62 mAh/g, which is 78% of that for the first cycle. The structural state was also checked after the discharge/charge cycle by comparing the PXRD pattern of the

Figure 5. In situ reversible control of magnetic phase stability. (a) Reversible change of magnetism in the discharge/charge cycles of an LIB system, where ΔM(T) ≡ M(T) − M(100K). The closed circles and open circles represent the field-cooled magnetization (FCM) measured by applying 100 Oe and the remnant magnetization (RM) after cooling with 100 Oe, respectively. The blue and red curves represent the magnetization of the discharged (Li+ ion inserted) and charged (Li+ ion extracted) states, respectively. (b) Magnetic field dependence of magnetization at 15 K for discharged (2.35 V vs Li/Li+) and charged states (3.1 V vs Li/Li+). The magnetization variation from the initial charged paramagnetic state at 3.1 V vs Li/Li+ (Minitial), ΔM(H) ≡ M(H) − Minitial(H), is displayed. Reversible magnetic phase switching between paramagnetic and ferrimagnetic states is demonstrated. (c) Repetitive cycling of discharged and charged LIB voltages. (d) Reversible switching of the ferrimagnetic transition temperature (Tc). (e) Reversible magnetization variation at 7 T due to phase switching between paramagnetic and ferrimagnetic states. In parts c−e, the red and blue closed circles represent the charged and the discharged states, respectively. 10057


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values). Indeed, the magnetic field dependence of magnetization (M−H) for the charged and discharged states at 15 K demonstrated a distinct change between paramagnetism and ferrimagnetism (Figure 5b). In the discharged states at 2.35 V vs Li/Li+ (blue curves), soft magnet behavior is observed, indicating the appearance of a ferrimagnetic phase at 15 K. In contrast, the nonlinear M−H curve observed at 2.35 V vs Li/Li+ completely disappears in the all-charged states at 3.1 V vs Li/ Li+ (red curves), showing a typical paramagnetic M−H feature. These results describe the fact that the reversible phase transformation between paramagnetic and ferrimagnetic states is realized in conjunction with the voltage switching of an LIB system incorporating 1 as a cathode (Figure 5c,e). Similar magnetization variations were confirmed with ex situ measurements for the extracted cathode material (Figure S8, SI), proving that the observed magnetization response originates from the 1 cathode in the inserted LIB cell.

ORCID

Hitoshi Miyasaka: 0000-0001-9897-0782 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank R. Ohta (Tohoku University) for his help in preparing the target sample and Dr. N. Hoshino, N. Shito, and Prof. T. Akutagawa (Tohoku University) for their support in the measurement of ESR spectra. This work was supported by a Grant-in-Aid for Scientific Research (Grant No. 16H02269, 26810029 and 16K05738) and a MEXT program ‘Elements Strategy Initiative to Form Core Research Center’ (since 2012) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan; a Grand-in-Aid for Scientific Research on Innovative Areas (“π-System Figuration” Area 2601, no. JP17H05137; “Coordination Asymmetry” Area 2802, no JP17H05350) from JSPS, a Support Program for Interdisciplinary Research (FRIS project); and the E-IMR project. J.C. gratefully acknowledges financial support from the Chinese Scholarship Council (CSC) and Institute for Materials Research, Tohoku University.

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CONCLUSION In this study, we have succeeded in switching the magnetic phase stability of (NBu4)[MnCr(Cl2An)3] using LIB discharge/ charge cycles. Phase stability is modulated by controlling the radical spin generation/annihilation on the bridging Cl2Ann− ligand through electron-filling tuning via Li+ ion insertion/ extraction. The redox reaction of tetraoxolene Cl2An2− ligands is crucial to magnetic control, demonstrating the occurrence of a reversible redox reaction of Cl2An2− + e− ⇌ Cl2An•3−, even in the metal-assembled network, where the generated Cl2An•3− radical ligand newly induces an antiferromagnetic exchange interaction with Mn2+ and Cr3+ spins, which is produced via πtype overlap between t2g-orbitals of Mn2+/Cr3+ in the Ohsymmetry approximation and LUMO of Cl2An2− (Figure S9, SI). Tc is reversibly modified by LIB charge/discharge cycles under in situ conditions. This is the first demonstration of reversible magnetism control using a radical spin species other than TCNQ derivatives. Taking advantage of the Tc change in the discharge/charge states, reversible phase switching between paramagnetic and ferrimagnetic states was also demonstrated in the temperature range of Tc(charged) < T ≤ Tc(discharged). Magnetism control based on electrochemical radical spin generation paves the way toward the postsynthetic design of artificial molecule-based magnets. Notably, this might be an appropriate method for producing high-Tc magnets.

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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.chemmater.7b03691. The PXRD pattern of the polycrystalline sample of 1, magnetization of 1, PXRD patterns of the cathode of 1 and lithiated 1, XPS spectra, ESR spectra of the cathode of 1 and lithiated 1, Curie−Weiss fitting results, PXRD patterns of the pristine and the discharge/charge cycled cathode, magnetic field dependence of pristine and discharged cathode of 1 (ex situ data), and schematic figures of exchange interaction between the spins of Mn2+/Cr3+ and Cl2An•3− (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*K.T. e-mail: [email protected]. 10058


Article

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