Communication pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Reversible Electrochemical Lithium-Ion Insertion into the Rhenium Cluster Chalcogenide−Halide Re6Se8Cl2 Andrea M. Bruck,† Jiefu Yin,† Xiao Tong,‡ Esther S. Takeuchi,†,§,⊥ Kenneth J. Takeuchi,†,§ Lisa F. Szczepura,*,∥ and Amy C. Marschilok*,†,§ †
Department of Chemistry and §Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States ‡ Center for Functional Nanomaterials and ⊥Energy Sciences Directorate, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, United States S Supporting Information *
which can undergo dimensional reduction in the presence of Cs+ or Tl+ halides. These 2D cluster frameworks should be considered as a new class of potential electroactive materials based on this rigid cluster−cluster bonding. Herein, we use Re6Se8Cl2 as a model material to illustrate the potential of these frameworks to exhibit multiple electron transfer by electrochemical Li+ insertion and extraction. The structure of the two-dimensional Re6Se8Cl2 contains layers of [Re6Se8]2+ cores linked by Re2Se2 rhombic interactions (along the a and b axes).25,26 Each cluster core also contains two terminal chloride ligands arranged in a trans fashion, creating 2D layers that are only connected by van der Waals interactions. The formulation [Re6Sei4Sei−a4/2]Sea−i4/2Cla2, (i indicates the inner and a the outer portion of a single core within the structure, (Figure S1), best describes the repeat pattern in the solid.25,26 The as-synthesized Re6Se8Cl2 has a triclinic crystal structure with the space group P1̅ with unit cell parameters that correspond well to the previously reported structure.26 In Figure 1, the observed X-ray diffraction (XPD) pattern and corresponding refinement are shown with the crystal structure as the inset, with the full refinement parameters provided in Table S1. The
ABSTRACT: The cluster-based material Re6Se8Cl2 is a two-dimensional ternary material with cluster−cluster bonding across the a and b axes capable of multiple electron transfer accompanied by ion insertion across the c axis. The Li/Re6Se8Cl2 system showed reversible electron transfer from 1 to 3 electron equivalents (ee) at high current densities (88 mA/g). Upon cycling to 4 ee, there was evidence of capacity degradation over 50 cycles associated with the formation of an organic solid− electrolyte interface (between 1.45 and 1 V vs Li/Li+). This investigation highlights the ability of cluster-based materials with two-dimensional cluster bonding to be used in applications such as energy storage, showing structural stability and high rate capability.
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etal chalcogenides have been identified as high-interest materials for energy storage, electronics, optoelectronics, and catalysis.1,2 Recently, there has been a heightened interest in exploring how changing the dimensionality of metal chalcogenides exploits unique electrical and optical properties that evolve from a changing electronic structure and the accompanying surface effects.3 To expand on the current understanding of the electronic structure of metal chalcogenides, Re6Se8Cl2, from the family of hexarhenium cores [Re6Q4+nX4−n](6−n)+ (where Q = chalcogen, X = halogen, and n = 0−4),4 was investigated with respect to its ability to function for reversible electrochemical energy storage in a lithium-based system. Previous studies focused on the luminescent and electrochemical properties of the molecular [Re6Se8]2+ unit,5−13 as well as using the cluster core as a building block to prepare materials for a variety of applications.14−18 This investigation focuses on Re6Se8Cl2 with the [Re6Se8]2+ cluster unit isostructural to that found in the superconducting Mo6Q8 Chevrel-phase core.19 Theoretical and experimental studies have explored the ability of Chevrel-phase materials (Mo6S8) to reversibly transfer four electron equivalents (ee) per formula unit.20−22 Unlike the molybdenum analogue, the hexarhenium core has been studied as a “building unit” in extended solid frameworks, where motifs of three, two, and one dimensionality are possible.23,24 These frameworks consist of rigid cluster−cluster linkages, some of © XXXX American Chemical Society
Figure 1. Observed XPD pattern with Rietveld refinement of pristine Re6Se8Cl2. The inset is the crystal structure with the (001) plane identified. Received: February 23, 2018
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DOI: 10.1021/acs.inorgchem.8b00499 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
suppress the oxidation/reduction hysteresis by keeping large amounts of side reactions from occurring, where at cycle 20 (at a rate of 88 mA/g; Figure 2B) the redox capacities are 2.9 and 2.8 ee with less than 0.1 ee hysteresis. At 22, 44, and 88 mA/g, the Coulombic efficiencies are all >90% and the reversible ee are ∼3.7, 3.2, and 2.8, respectively. With the lower voltage limit of 1 V, the reduction profile shown in Figure 2D at various rates shows a new plateau at 2.0− 1.8 V. This new plateau was not observed in the higher voltage cutoff reactions and is therefore associated with the side reactions occurring below 1.45 V. At a rate of 4.4 mA/g, this plateau contributes up to 1 ee of capacity to the reduction process, which explains the elevated hysteresis between the reduction and oxidation capacity at the slow rates. Interestingly, as the rate increases up to 88 mA/g, the voltage plateau at 2.0 V becomes suppressed and does not significantly contribute to the reduction capacity. The effect of the rate on the electrochemical properties can be clearly identified by increasing the Columbic efficiency from 87.3% at 4.4 mA/g to 96.9% at 88 mA/g (Figure 2B). Variations in the Coulombic efficiency and voltage plateaus imply the redox mechanism is rate-dependent. X-ray photoelectron spectroscopy (XPS) was used to evaluate the reactions causing additional capacity delivered during the first cycle via evidence of electrolyte decomposition and [Re6Se8]2+ redox activity. In Figure S5, the binding energy from 0 to 700 eV is shown for a pristine Re6Se8Cl2 electrode and an electrode that was electrochemically reduced to 1.0 V. The most striking feature is the dramatic increase in the O 1s intensity after the electrode is reduced compared to the pristine electrode. Upon further analysis of this region (Figure 3A), this increase in the intensity is
most prominent reflection at 2θ = 1.29° (d = 8.13 Å) corresponds to the interlayer distances between clusters. The van der Waals interactions between the terminal chloride ligands (see Figure S2 for further detail) lead to the large interlayer spacing. Because of the weak interactions of Cl− in its local environment and the large distances between atoms in the interlayer spacing, it can be envisioned that this material provides adequate channels for Li+ insertion. The electrochemistry in a lithium-ion-based system was explored using cyclic voltammetry (CV) at a slow scan rate of 0.01 mV/s, as shown in Figure 2A. The first reduction peak is at
Figure 2. Li/Re6Se8Cl2 coin cells. (A) CV at a rate of 0.01 mV/s for 1 cycle for 1−3 and 1.45−3 V vs Li/Li+. (B) Galvanostatic cycling rate capability test with the corresponding Coulombic efficiencies and voltage profiles at various rates with (C) 1.45 and (D) 1 V cutoff voltages. The voltage profiles correspond to cycles 5, 10, 15, and 20 in part B. Cycle 1 voltage profiles for C and D shown in Figures S3 and S4.
1.72 V with the second at 1.47 V. Quasi reversible electrochemistry is observed with oxidation peaks at 1.98 and 1.79 V. However, at voltages below ∼1.3 V during reduction there is a substantial increase in current indicative of side reactions occurring until the 1 V limit, consistent with prior reports for lithium based electrochemical cells.27 Since this large oxidation occurred, CV was also tested with a 1.45 V cutoff voltage with little variation to the corresponding oxidation peaks. Lithium-ion half-cells were also cycled galvanostatically at the 1 V limit and at a higher voltage limit of 1.45 V. When cycled with a 1.45 V cutoff voltage the first reduction cycle at a rate of 4.4 mA/g yielded 3.1 ee (Figure S3), which decreased to 2.4 ee by cycle 5, as shown by Figure 2B (where 1 ee = 11.9 mAh/g). At a high rate of 88 mA/g, a reversibility of approximately 1.6 ee with a 1.45 V cutoff was observed. After cycling to the 1.45 V cutoff, the redox profiles at each rate show little to no variation after 25 cycles (Figure 2C). The plateau observed during reduction is at approximately 1.7 V, with the corresponding oxidation plateau at approximately 2.0 V, which correlates well with the CV and shows good reversibility. When cycled to a lower voltage cutoff of 1.0 V, the first reduction (Figure S4) is well above 15 ee, indicating the occurrence of side reactions. This large excess of electron reduction is not reversible upon oxidation, as indicated by the Coulombic efficiency of ∼26%. The hysteresis between reduction and oxidation reversibility decreased from a 10 electron difference on the first cycle to a
