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tetraglyme electrolyte for rechargeable magnesium batteries
Journal of Power Sources 276 (2015) 255e261
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
High concentration magnesium borohydride/tetraglyme electrolyte for rechargeable magnesium batteries Feilure Tuerxun a, b, Yasen Abulizi a, b, Yanna NuLi a, b, *, Shuojian Su a, b, Jun Yang a, b, JiuLin Wang a, b a b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, PR China
h i g h l i g h t s A magnesium battery electrolyte based Mg(BH4)2 in TG with high safety is developed. Mg(BH4)2 in TG shows an improved performance than in DME and DGM reported before. The electrolyte based on Mg(BH4)2 in TG has higher concentration than in DME and DGM. The solution shows 2.4 V stability on SS, near 100% deposition/dissolution efficiency. Good compatibility with Mo6S8 cathode confirms the application in Mg batteries.
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Article history: Received 1 September 2014 Received in revised form 21 November 2014 Accepted 25 November 2014 Available online 27 November 2014
High concentration magnesium borohydride/tetraglyme electrolyte for rechargeable magnesium batteries is simply prepared by dissolving inorganic magnesium salt Mg(BH4) in tetraglyme (TG) ether solvent with good safety. 90 C heating treatment is performed in the preparation process and LiBH4 as a chelating agent is added to improve the electrochemical performance. Mg depositionedissolution performance and the electrochemical window of the electrolyte on non-inert stainless steel (SS), nickel (Ni), copper (Cu) electrodes and inert platinum (Pt) electrode are systematically studied by cyclic voltammetry and constant current dischargeecharge measurements. 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG ([LiBH4] ¼ 1.5 mol L1) solution shows good electrochemical performance with 2.4 V (vs. Mg RE) anodic stability on stainless steel, close to 100% Mg deposition/dissolution efficiency and high cycling reversibility. Furthermore, the reversible electrochemical process of Mg intercalation into Mo6S8 cathode with excellent cycling performance in the electrolyte indicates the feasible application in rechargeable magnesium batteries. © 2014 Elsevier B.V. All rights reserved.
Keywords: Deposition and dissolution Electrochemical reversibility Magnesium Electrolyte Rechargeable magnesium batteries
1. Introduction The demand for green rechargeable batteries for applications such as electric vehicles and large scale power storage systems is constantly increasing because of fossil energy shortage and environmental issues. Following the rapid development of electric automobiles, batteries are now needed to have not only high energy density, but also high power density and good safety. Rechargeable magnesium batteries with magnesium as the anode may be a
* Corresponding author. School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. E-mail address: [email protected] (Y. NuLi). http://dx.doi.org/10.1016/j.jpowsour.2014.11.113 0378-7753/© 2014 Elsevier B.V. All rights reserved.
potential candidate competable with post Li-ion batteries containing a lithium metal anode due to a relatively low price ($2700/ ton and $64,800/ton for Mg and Li, respectively), a high theoretical volumetric capacity (3832 mAh cm3 for Mg and 2062 mAh cm3 for Li) and a higher expected safety (less dendritic morphologies for magnesium deposits than lithium) for magnesium compared with lithium [1,2]. Different from Li batteries, Mg batteries suffer from the lack of suitable cathode materials and electrolyte systems. In many nonaqueous solutions, a reversible process of electrochemical Mg depositionedissolution is hard achieved because of the formation of compact passive film [3e6]. Therefore, it is necessary to discover a right electrolyte with reversible Mg deposition and a wide
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electrochemical window for the development of magnesium batteries. The significant progress is the 0.25 mol L1 Mg(AlCl2BuEt)2/ THF (Bu ¼ butyl, Et ¼ ethyl) electrolyte and 0.4 mol L1 (PhMgCl)2AlCl3/THF electrolyte proposed by Aurbach et al., which have high anodic stability (2.5 V and 3.3 V vs. Mg RE on inert Pt electrode, respectively) and Mg electrodes behave highly reversibly in them [7e11]. In 2011, Kim et al. reported firstly the non-nucleophilic electrolyte based on the crystal of [Mg2Cl3-6THF][HDMSAlCl3] for reversible magnesium depositionedissolution [1]. Recently, a family of novel boron based electrolytes with high ionic conductivity, excellent Mg deposition reversibility as well as high anodic stability were proposed [12,13]. On the other hand, our group reported a new phenolate-based electrolyte, which exhibits air insensitive character and excellent magnesium depositionedissolution performance [14]. However, these electrolytes are generally composed of organic magnesium salts and may corrode non-inert current collectors duo to the presence of halides in the cation and anion components of the electrolytes, although some of these electrolytes show impressive stability against electrochemical oxidation [12,15]. Hence, It is still necessary to find electrolytes with high stabilities on non-inert current collectors for the realizing a practical rechargeable Mg battery system. Recently, Mohtadi et al. [16] confirmed reversible magnesium depositionedissolution from a new class of electrolytes based on a relatively ionic and halide-free inorganic salt Mg(BH4)2 in both tetrahydrofuran (THF) and dimethoxyethane (DME) ether solvents. LiBH4 was employed as an additive to increase the electrochemical performance, and 94% coulombic efficiencies for Mg depositionedissolution were observed. Y.Y. Shao et al. [17] further developed Mg(BH4)2 based electrolyte with diglyme (DGM) as the solvent and LiBH4 also the additive, in which the coulombic efficiency of close to 100% was achieved in the electrolyte of 0.1 mol L1 Mg(BH4)2/LiBH4/DGM ([LiBH4] ¼ 1.5 mol L1). The research about the structure-property relationship showed that the coulombic efficiency in Mg(BH4)2-based electrolyte increases according to the series THF < DME < DGM because of the improved stability of the solvated Mg(BH4)2 complexes with the denticity of the solvent ligands (monodendate, bidentate, and tridentate, respectively). However, the solubility of Mg(BH4)2 in DGM and DME is 0.1 M/0.01 mol L1 with/without LiBH4 respectively. [17] The limited concentration of Mg(BH4)2 in solvents will lead to the limited rate performance of Mg batteries for practical application. Herein, quinquedentate ligand tetraglyme (TG) with higher safety (the boiling/flash points of TG, DGM, DME and THF are 275 C/ 141 C, 162 C/57 C, 85 C/2 C and 66 C/14 C, respectively) was further developed as the solvent of Mg(BH4)2. More importantly, a high concentration electrolyte based on Mg(BH4)2 in TG with a higher anodic stability on non-inert metal electrode was pursued for practical application. 2. Experimental 2.1. Chemicals and material synthesis Magnesium borohydride (Mg(BH4)2, 95%) and magnesium ribbon (1 mm diameter) were purchased from SigmaeAldrich. Lithium borohydride (LiBH4, 95%) was purchased from J&K Scientific. Tetrahydrofuran (THF), dimethoxyethane (DME), diglyme (DGM) and tetraglyme (TG) were purchased from Aladdin reagent and further dried using 3 Å molecular sieve. The synthetic work was conducted in an argon-filled glove box (Mbraun, Unilab, Germany) containing less than 2 ppm H2O and O2. The electrolytes were prepared by dissolving the predetermined amount of Mg(BH4)2 and LiBH4 in solvents under stirring at room temperature or 90 C for at least 2 h.
Synthesis route of Mo6S8 followed the literature [18]. In briefly, the mixture of 1 g MoS2 (99%, Aldrich), 0.398 g CuS (99.5%, Cerac), 0.602 g Mo (99%, Aldrich) and 4 g KCl (99%, Aldrich) was pestled for over 10 min and then heated at 850 C for 60 h. The product was washed by deionized water and sonicated to obtain Cu2Mo6S8. Then, Mo6S8 was prepared by chemical removal of the copper from Cu2Mo6S8 using an aqueous HCl solution through the following reaction:
Cu2 Mo6 S8 ðsÞ þ 8HClðaqÞ þ O2 /Mo6 S8 ðsÞ þ 2H2 O þ 2½CuCl4 2 ðaqÞ þ 4Hþ ðaqÞ
(1)
2.2. Measurement procedures and apparatus Cyclic voltammograms (CVs) were conducted in three-electrode cells inside an argon-filled glove box using an electrochemical instrument of CHI604A Electrochemical Workstation (Shanghai, China). The working electrode was a stainless steel, nickel, copper and platinum disk (geometric area ¼ 3.14 102 cm2), which was polished with a corundum suspension and rinsed with dry acetone before use, and magnesium ribbon as counter and reference electrodes. Electrochemical magnesium depositionedissolution cycles were examined with CR2016 experimental coin cells on a land battery measurement system (Wuhan, China). Stainless steel foil (Ф12 mm) was served as the working electrode (substrate). Mg disc as the counter electrode. An Entek PE and a fiber membrane as the separator. The cells were assembled in the glove box. Magnesium was deposited onto the stainless steel substrate for fixed periods of 30 min followed by stripping to a fixed potential limit of 0.8 V vs. Mg at a constant current density of 0.088 mA cm2. There was a 30 s rest between deposition and dissolution. The magnesium deposition and dissolution on the substrate were referred to as the discharge and charge process, respectively. The time of charge divided by the time of discharge was defined as the depositionedissolution efficiency. The Mo6S8 electrode was prepared by casting and pressing a 8:1:1 weight-ratio mixture of Mo6S8, super-P carbon powder and polyvinylidene fluoride (PVDF) binder dissolved in N-methyl-2pyrrolidinone (NMP) onto stainless steel current collector followed by drying in vacuum at 120 C. CR2016 coin cells were fabricated using Mo6S8 cathode, Mg disc anode, Entek PE and fiber membrane separator, and 0.5 mol L1 heated Mg(BH4)2/TG or Mg(BH4)2/LiBH4/ TG ([LiBH4] ¼ 1.5 mol L1) electrolyte. The chargeedischarge tests of the coin cells were carried out on the land battery measurement system with the cutoff voltage of 1.6/0.5 V vs. Mg. IR analysis of the solutions was run using a Spectrum 100 FT-IR spectrometer (Perkin Elmer, Inc., USA). Nuclear magnetic resonance (NMR) measurements were performed using a Bruker Avance 400 MHz spectrometer equipped with a CP/MAS probe. The spectra were reported in part per million (ppm) with referenced to tetramethylsilane (TMS) and BF3$O(CH2CH3)2 as the external standards of 1H and 11B, respectively. X-ray diffraction (XRD) analysis of the magnesium deposits was conducted on a Rigaku diffractometer D/MAX-2200/PC equipped with Cu Ka radiation. The morphology of the deposits was observed using scanning electron microscopy (SEM) on a FEI SIRION200 fieldemission microscope. Before the analysis of the deposits, the sample deposited for 10 h at 0.088 mA cm2 was washed in the glove box with drying THF solvent to remove soluble residue and then transferred out of the box and kept without exposure to the atmosphere.
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3. Results and discussion
Fig. 1. CVs of Mg electrochemical depositionedissolution on SS disk electrode from the solutions consisting of 0.1 mol L1 Mg(BH4)2 in the solvents of (a) DGM, (b) TG and DME (upper clear solution). The inset of (a) shows the solubility of 0.1 mol L1 Mg(BH4)2 in THF, TG, DGM and DME.
Fig. 1a shows CVs of the electrochemical Mg deposition and dissolution on non-inert stainless steel (SS) disk electrode from the electrolytes of 0.1 mol L1 Mg(BH4)2 in DGM, TG and DME at 50 mV s1. Inert Pt electrode can ensure that electrochemical experiments characterizing redox events related solely to the magnesium electrolyte and eliminate reactions between the electrode and electrolyte. However, the electrochemical performance on SS was mainly evaluated experimentally considering the limit of charging in a coin cell battery configuration due to the utilization of stainless steel in the casing and current collector material. Compared with that (Fig. 1a) from the solution with DGM as the solvent, the curve (Fig. 1b) obtained from TG shows a more obvious oxidation peak at approximately 0.25 V after a reduction process with onset at approximately 0.6 V, suggesting a more reversible Mg depositionedissolution process. It has been reported that the O donor denticity, i.e. ligand strength of the ethereal solvents in corresponding solvated Mg(BH4)2 solutions plays a significant role in enhancing coulombic efficiency of Mg depositionedissolution [17]. As a more effective donating ligand for magnesium than DGM, TG significantly improves the dissolution process through synergetic effects. Herein, it should be further noted that the clarity of the solution, which probably influences the morphology of Mg deposition, also has an effect on the coulombic efficiency. The inset of Fig. 1a shows the solubility of 0.1 mol L1 Mg(BH4)2 in THF, DME, DGM and TG, respectively. The solution is transparent for Mg(BH4)2 in THF and TG, turbid in DGM. There is precipitation for the solution of DME. A more reversible Mg depositionedissolution process can be obtained from Mg(BH4)2/DME upper clear solution than Mg(BH4)2/DGM turbid solution. Higher denticity of TG than DME probably decreases the mobility of solvated Mg2þ, thus a lower deposition current and a higher potential for Mg deposition and dissolution, as shown in Fig. 1b. Although the overpotential for Mg deposition/dissolution from DME is smaller, Mg depositionedissolution efficiency from TG is higher, which is partially related to the different morphology of Mg deposition due to different species adsorbed on the electrode in each solution. Fig. 2 compares the SEM images of electrodeposits (deposition was
Fig. 2. SEM images of the electrodeposits on SS substrate at 3.168C cm2 charge from upper clear solution of Mg(BH4)2/DME and transparent solution of Mg(BH4)2/TG.
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performed at 0.088 mA cm2 for 10 h) on SS substrate from transparent solution of Mg(BH4)2/TG and upper clear solution of Mg(BH4)2/DME, respectively. The results indicate that the deposits from these two solvents show different morphology in term of both particle size and shape. The particle shape is irregular and the deposited layer is loose and non-uniform from DME. However, the particle size is smaller and the deposited layer is compact and smooth from TG, which is more favorable for subsequent dissolution process. Fig. 3 shows CVs of Mg depositionedissolution on SS disk electrode from Mg(BH4)2/TG with the different concentrations (0.1, 0.175, 0.2, 0.25 and 0.5 mol L1). The increased concentration speeds up the depositionedissolution kinetics at electrode surface. 0.5 mol L1 Mg(BH4)2/TG solution shows the highest Mg depositionedissolution currents, which is related to a high conductivity (2.78 and 12.32 ms cm1 for 0.1 and 0.5 mol L1, respectively). However, transparent solution could not be obtained at the concentration of 0.5 mol L1. In order to improve the solubility of Mg(BH4)2 in TG, 0.5 mol L1 Mg(BH4)2/TG solution was heated at 90 C under stirring. Fig. 4a compares the CVs of Mg electrochemical depositionedissolution on SS disk electrode from 0.5 mol L1 Mg(BH4)2/TG solution with or without heating. Although totally transparent solution could not obtained after the heat-treatment, the depositionedissolution currents and the anodic stability improve dramatically. The former is resulted from the enhanced conductivity (38.57 ms cm1 for 0.5 mol L1 heated Mg(BH4)2/TG solution). The latter is probably related to the change of electronic structure of the Mg complexes in the solution after the heat-treatment, which is confirmed by NMR measurement shown in Fig. 5. The inset of Fig. 4a shows the XRD pattern of the resulting deposit on SS substrate from 0.5 mol L1 heated Mg(BH4)2/TG solution. The main diffraction peaks at 32.2, 34.4, 36.6 and 63.1 can be attributed to metallic Mg (JCPDS file 35-0821) and the peaks at 43.7,
Fig. 3. CVs of Mg electrochemical depositionedissolution on SS disk electrode from Mg(BH4)2/TG electrolyte with different concentrations.
50.6, and 74.6 to the SS substrate, indicating that deposition of pure metal Mg was obtained from the solution. Fig. 4b compares the CVs of Mg electrochemical depositionedissolution on Pt and SS disk electrode from 0.5 mol L1 heated Mg(BH4)2/TG solution. Although the solution shows higher reversibility on Pt electrode duo to an apparent limitation of SS electrode to intermediate efficiencies of Mg deposition/dissolution [12], a higher anodic stability is obtained on SS electrode (2.4 V and 1.5 V for SS and Pt, respectively). The phenomenon is different from those in most organic magnesium salts system, which generally corrodes the non-insert metal due to the chlorine ion contained in the organic magnesium salts [19,20]. In order to understand coordination structures of Mg(BH4)2 in TG, TG solvated Mg(BH4)2 complex was characterized by 1H NMR and 11B NMR spectroscopies in non-coordinating CD2Cl2, which does not interrupt the structures of the solvated Mg(BH4)2 complexes with TG. In the 1H NMR spectra (Fig. 5a), TG solvated Mg(BH4)2 species shows proton resonances at 3.628 ppm (CH2), 3.511 ppm (CH2) and 3.330 ppm (CH3) for the coordinating TG, downfield shifting in comparison with those of free TG in CDCl2
Fig. 4. CVs of Mg electrochemical depositionedissolution on (a) SS disk electrode from 0.5 mol L1 unheated and heated Mg(BH4)2/TG solution, (b) Pt and SS disk electrode from 0.5 mol L1 heated Mg(BH4)2/TG solution. The inset of (a) shows the XRD of deposits on SS substrate from 0.5 mol L1 heated Mg(BH4)2/TG solution.
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Fig. 5. (a) 1H NMR spectra of TG and Mg(BH4)2/TG solution recorded in CD2Cl2. (b) 11B and 1H (inset) NMR spectra of 0.5 mol L1 unheated and heated Mg(BH4)2/TG solutions recorded in CD2Cl2. (c) IR spectra for 0.5 mol L1 unheated and heated Mg(BH4)2/TG solutions. (d) Coordination structure of Mg(BH4)2 in TG.
Fig. 6. The cycling curves (a), a typical curve (b), and cycling efficiency (c) of Mg depositionedissolution on SS substrate from 0.5 mol L1 heated Mg(BH4)2/TG solution.
Fig. 7. The cycling curves (a), a typical curve (b), and cycling efficiency (c) of Mg depositionedissolution on SS substrate from 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG solution ([LiBH4] ¼ 1.5 mol L1).
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(3.544 ppm (CH2), 3.454 ppm (CH2) and 3.292 ppm (CH3), respectively). This is resulted from the de-shielding effect of TG metalation for the TG solvated Mg(BH4)2 species. 11B and 1H NMR spectra of unheated and heated Mg(BH4)2/TG solutions are represented in Fig. 5b to confirm the improvement of heating-treatment process to anodic stability of the solutions. In 11B NMR spectrum, the unheated solution displays the characteristic quintet at 41.37 ppm. After heating-treatment, the characteristic peak shifts to 41.28 ppm, indicating a decreased boron shielding. In 1H NMR spectrum shown in the inset of Fig. 5b, a major quartet at the chemical shift 0.36 ppm corresponds to BH 4 . There is also a reduced proton shielding for the heated solution. These results indicate a lower electron cloud density for the radical group after heatingtreatment, resulting in an improved anodic stability, which is consistent with the result of Fig. 4a. IR spectroscopic analyses were further conducted to characterize the electroactive species in unheated and heated Mg(BH4)2/TG solutions. In Fig. 5c, the bands between 1000e1400 cm1 and 2000e2500 cm1 represent BeH bending and BeH stretching vibrations, respectively [21,22], the bands between 2700 and 4000 cm1 are related with TG solvent. Those peaks change after heating, especially for the BeH stretching modes. The intensity of BeH stretching bands is obviously weaker,
Fig. 8. CVs of Mg electrochemical depositionedissolution on (a) SS and (b) Pt, Ni, Cu disk electrode from 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG solution ([LiBH4] ¼ 1.5 mol L1).
suggesting an enhanced ionic character duo to more free BH 4 [16], in accord with the increased electronic conductivity (from 12.32 to 38.57 ms cm1). It has been reported that the coordination structure of Mg(BH4)2 in DGM, DME and THF is Mg(BH4)2DGM, Mg2(BH4)4(DME)3, Mg(BH4)2(THF)3, respectively [17]. Considering that TG is a quinquedentate pincer ligand and each BH 4 anion coordinates with Mg via two hydrides, the composition of Mg(BH4)2 complex in TG can be identified as seven coordinated Mg5(BH4)10(TG)3 with a ratio of Mg(BH4)2:TG ¼ 5:3, as shown in Fig. 5d. For electrolyte, the assembled battery has an efficient cycling characteristic is very important. Fig. 6a demonstrates the galvanostatic cycling curves of Mg deposition and dissolution on SS substrate in 0.5 mol L1 heated Mg(BH4)2/TG solution via CR2016 experimental coin cells. The Mg deposition and dissolution potentials in a typical curve (shown in Fig. 6b) are about 0.45 V and 0.35 V, respectively, which are obviously too high for electrolyte. Fig. 6c shows the Mg depositionedissolution cycling efficiencies on SS substrate in heated 0.5 mol L1 Mg(BH4)2/TG solution, which are calculated according to the ratios of the charge amounts of magnesium dissolution to those of magnesium deposition. The
Fig. 9. The initial dischargeecharge curve at 0.05C of Mg/Mo6S8 coin-cell at room temperature using the electrolyte of 0.5 mol L1 heated (a) Mg(BH4)2/TG, inset is the cyclic voltammogram at 0.02 mV s1 and (b) Mg(BH4)2/LiBH4/TG ([LiBH4] ¼ 1.5 mol L1), inset is the cycling stability.
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coulombic efficiencies increase gradually from initial 83% with the progressive cycle number and reach to 90% after 4 cycles and are stable at above 95% after 15 cycles. However, higher efficiencies are necessary for the purpose of practical applications. In order to improve the electrochemical performance of 0.5 mol L1 heated Mg(BH4)2/TG solution, 1.5 mol L1 LiBH4 as a coordination ligand was further added in the solution since it has been shown to have a remarkable effect to Mg(BH4)2/THF and Mg(BH4)2/DGM solutions [16,17]. The Mg(BH4)2/TG solution became more homogeneous after adding LiBH4. Fig. 7a demonstrates the galvanostatic cycling curves of Mg deposition and dissolution on SS substrate in 0.5 mol L1 heated Mg(BH4)2/LiBH4/ TG solution ([LiBH4] ¼ 1.5 mol L1) via CR2016 experimental coin cells. The Mg deposition and dissolution potentials in the typical curve (shown in Fig. 7b) are about 0.2 V and 0.15 V, respectively, decreasing obviously compared with the solution without LiBH4. Furthermore, the solution demonstrates close to 100% coulombic efficiencies after 15 cycles and stable cycling performance, as shown in Fig. 7c. The improved Mg depositionedissolution performance is mainly due to the improved ionic conductivity (9.66 mS cm1 for 0.5 mol L1 Mg(BH4)2/LiBH4/TG solution ([LiBH4] ¼ 1.5 mol L1)) as a result of increasing Mg(BH4)2 dissociation [16]. Moreover, the anodic stability of the solution on SS electrode can still maintain at approximately 2.4 V with higher current densities compared with that without adding LiBH4 (Fig. 8a), although the stability on Cu or Ni non-inert electrode still needs to be improved, as shown in Fig. 8b. The research on the electrolytes with a wide electrochemical window on non-inert electrodes and prepared by scalable chemical processes is under development. The coin cell was constructed using 0.5 mol L1 heated Mg(BH4)2/TG and Mg(BH4)2/LiBH4/TG ([LiBH4] ¼ 1.5 mol L1) solutions as the electrolyte, model Mo6S8 (122.8 mAh g1 theoretical capacity) as a positive electrode, and a Mg disc as a negative electrode to verify the compatibility of the electrolyte with a magnesium ion intercalation cathode. As shown in Fig. 9a , the cell with Mo6S8 cathode and 0.5 mol L1 heated Mg(BH4)2/TG electrolyte delivers 65.9 mAh g1 initial discharge capacity and 63.4 mAh g1 initial charge capacity at 0.05C rate. Cyclic voltammogram (inset of Fig. 9a) shows reversible Mg insertion (peak at 0.9 V) and deinsertion (peak at 1.39 V) in Mo6S8 cathode. After adding LiBH4 in the electrolyte, the cell shown in Fig. 9b delivers higher discharge and charge capacity (76.8 mAh g1 and 75.7 mAh g1, respectively) with lower voltage polarization compared with that without LiBH4. The discharge capacity drops slightly at the second cycle and is stabilized at approximately 71 mAh g1 with 92.4% capacity retention for the remaining 107 cycles (inset of Fig. 9b), indicating good compatibility of the electrolyte with Mg intercalation cathode material. 4. Conclusions A electrolyte for rechargeable magnesium batteries based on an inorganic magnesium salt Mg(BH4)2 dissolved in a potentially safer ether solvent TG with 0.5 mol L1 high concentration is developed.
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The chelation property of TG with five oxygen sites per molecule 2þ results in an enhanced dissociation between BH 4 and Mg , thus an improved electrochemical performance for Mg deposition and dissolution compared with DME and DGM as the solvents. 90 C heat-treatment and LiBH4 additive are employed to further improve the anodic stability and electronic conductivity of the solution. 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG 1 ([LiBH4] ¼ 1.5 mol L ) solution demonstrates 2.4 V anodic satiability on non-inert stainless steel electrode and close to 100% coulombic efficiency for reversible Mg deposition and dissolution. Furthermore, the compatibility with the Mo6S8 cathode indicates that the Mg(BH4)2/TG solution could be used as a potential electrolyte for rechargeable Mg battery system. Acknowledgments This work was supported by the National Nature Science Foundation of China (project no. 21273147) and the Shanghai Municipal Science and Technology Commission (project no. 11JC1405700). References [1] H.S. Kim, T.S. Arthur, G.D. Allred, J. Zajicek, J.G. Newman, A.E. Rodnyansky, A.G. Oliver, W.C. Boggess, J. Muldoon, Nat. Commun. 2 (2011) 427. [2] M. Matsui, J. Power Sources 196 (2011) 7048. [3] T.D. Gregory, R.J. Hoffman, R.C. Winterton, J. Electrochem. Soc. 137 (1990) 775. [4] D. Aurbach, A. Schechter, M. Moshkovich, Y. Cohen, J. Electrochem. Soc. 148 (2001) A1004. [5] Y. Viestfried, M.D. Levi, Y. Gofer, D. Aurbach, J. Electroanal. Chem. 576 (2005) 183. [6] Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach, J. Electroanal. Chem. 466 (1999) 203. [7] D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi, Nature 274 (2000) 724. [8] D. Aurbach, H. Gizbar, A. Schechter, O. Chusid, H.E. Gottlieb, Y. Gofer, I. Goldberg, J. Electrochem. Soc. 149 (2002) A115. [9] Y. Gofer, O. Chusid, H. Gizbar, Y. Viestfrid, H.E. Gottlieb, V. Marks, D. Aurbach, Electrochem. Solid. State Lett. 9 (2006) A257. [10] O. Mizrahi, N. Amir, E. Pollak, O. Chusid, V. Marks, H. Gottlieb, L. Larush, E. Zinigrad, D. Aurbach, J. Electrochem. Soc. 155 (2008) A103. [11] D. Aurbach, G.S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid, M. Brunelli, Adv. Mater. 19 (2007) 4260. [12] J. Muldoon, C.B. Bucur, A.G. Oliver, T. Sugimoto, M. Matsui, H.S. Kim, G.D. Allred, J. Zajicek, Y. Kotani, Energy Environ. Sci. 5 (2012) 5941. [13] Y.S. Guo, F. Zhang, J. Yang, F.F. Wang, Y.N. NuLi, S.I. Hirano, Energy Environ. Sci. 5 (2012) 9100. [14] F.F. Wang, Y.S. Guo, J. Yang, Y.N. NuLi, S.I. Hirano, Chem. Commun. 48 (2012) 10763. [15] Q. Chen, Y.N. NuLi, J. Yang, K. Kailibinuer, J.L. Wang, Acta Phys. Chim. Sin. 28 (2012) 2625. [16] R. Mohtadi, M. Matsui, T.S. Arthur, S.J. Hwang, Angew. Chem. Int. Ed. 51 (2012) 9780. [17] Y.Y. Shao, T.B. Liu, G.S. Li, M. Gu, Z.M. Nie, M. Engelhard, J. Xiao, D.P. Lv, C.M. Wang, J.G. Zhang, J. Liu, Sci. Rep. 3 (2013) 3130. [18] E. Lancry, E. Levi, A. Mitelman, S. Malovany, D. Aurbach, J. Solid State Chem. 179 (2006) 1879. [19] J. Muldoon, C.B. Bucur, A.G. Oliver, J. Zajicek, G.D. Allred, W.C. Boggess, Energy Environ. Sci. 6 (2013) 482. [20] D.P. Lv, T. Xu, P. Saha, M.K. Datta, M.L. Gordin, A. Manivannan, P.N. Kumta, D.H. Wang, J. Electrochem. Soc. 160 (2013) A351. [21] Y. Filinchuk, R. Cerný, H. Hagemann, Chem. Mater. 21 (2009) 925. [22] Y. Filinchuk, B. Richter, T.R. Jensen, V. Dmitriev, D. Chernyshov, H. Hagemann, Angew. Chem. Int. Ed. 50 (2011) 11162.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
High concentration magnesium borohydride/tetraglyme electrolyte for rechargeable magnesium batteries Feilure Tuerxun a, b, Yasen Abulizi a, b, Yanna NuLi a, b, *, Shuojian Su a, b, Jun Yang a, b, JiuLin Wang a, b a b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, PR China
h i g h l i g h t s A magnesium battery electrolyte based Mg(BH4)2 in TG with high safety is developed. Mg(BH4)2 in TG shows an improved performance than in DME and DGM reported before. The electrolyte based on Mg(BH4)2 in TG has higher concentration than in DME and DGM. The solution shows 2.4 V stability on SS, near 100% deposition/dissolution efficiency. Good compatibility with Mo6S8 cathode confirms the application in Mg batteries.
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a b s t r a c t
Article history: Received 1 September 2014 Received in revised form 21 November 2014 Accepted 25 November 2014 Available online 27 November 2014
High concentration magnesium borohydride/tetraglyme electrolyte for rechargeable magnesium batteries is simply prepared by dissolving inorganic magnesium salt Mg(BH4) in tetraglyme (TG) ether solvent with good safety. 90 C heating treatment is performed in the preparation process and LiBH4 as a chelating agent is added to improve the electrochemical performance. Mg depositionedissolution performance and the electrochemical window of the electrolyte on non-inert stainless steel (SS), nickel (Ni), copper (Cu) electrodes and inert platinum (Pt) electrode are systematically studied by cyclic voltammetry and constant current dischargeecharge measurements. 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG ([LiBH4] ¼ 1.5 mol L1) solution shows good electrochemical performance with 2.4 V (vs. Mg RE) anodic stability on stainless steel, close to 100% Mg deposition/dissolution efficiency and high cycling reversibility. Furthermore, the reversible electrochemical process of Mg intercalation into Mo6S8 cathode with excellent cycling performance in the electrolyte indicates the feasible application in rechargeable magnesium batteries. © 2014 Elsevier B.V. All rights reserved.
Keywords: Deposition and dissolution Electrochemical reversibility Magnesium Electrolyte Rechargeable magnesium batteries
1. Introduction The demand for green rechargeable batteries for applications such as electric vehicles and large scale power storage systems is constantly increasing because of fossil energy shortage and environmental issues. Following the rapid development of electric automobiles, batteries are now needed to have not only high energy density, but also high power density and good safety. Rechargeable magnesium batteries with magnesium as the anode may be a
* Corresponding author. School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. E-mail address: [email protected] (Y. NuLi). http://dx.doi.org/10.1016/j.jpowsour.2014.11.113 0378-7753/© 2014 Elsevier B.V. All rights reserved.
potential candidate competable with post Li-ion batteries containing a lithium metal anode due to a relatively low price ($2700/ ton and $64,800/ton for Mg and Li, respectively), a high theoretical volumetric capacity (3832 mAh cm3 for Mg and 2062 mAh cm3 for Li) and a higher expected safety (less dendritic morphologies for magnesium deposits than lithium) for magnesium compared with lithium [1,2]. Different from Li batteries, Mg batteries suffer from the lack of suitable cathode materials and electrolyte systems. In many nonaqueous solutions, a reversible process of electrochemical Mg depositionedissolution is hard achieved because of the formation of compact passive film [3e6]. Therefore, it is necessary to discover a right electrolyte with reversible Mg deposition and a wide
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electrochemical window for the development of magnesium batteries. The significant progress is the 0.25 mol L1 Mg(AlCl2BuEt)2/ THF (Bu ¼ butyl, Et ¼ ethyl) electrolyte and 0.4 mol L1 (PhMgCl)2AlCl3/THF electrolyte proposed by Aurbach et al., which have high anodic stability (2.5 V and 3.3 V vs. Mg RE on inert Pt electrode, respectively) and Mg electrodes behave highly reversibly in them [7e11]. In 2011, Kim et al. reported firstly the non-nucleophilic electrolyte based on the crystal of [Mg2Cl3-6THF][HDMSAlCl3] for reversible magnesium depositionedissolution [1]. Recently, a family of novel boron based electrolytes with high ionic conductivity, excellent Mg deposition reversibility as well as high anodic stability were proposed [12,13]. On the other hand, our group reported a new phenolate-based electrolyte, which exhibits air insensitive character and excellent magnesium depositionedissolution performance [14]. However, these electrolytes are generally composed of organic magnesium salts and may corrode non-inert current collectors duo to the presence of halides in the cation and anion components of the electrolytes, although some of these electrolytes show impressive stability against electrochemical oxidation [12,15]. Hence, It is still necessary to find electrolytes with high stabilities on non-inert current collectors for the realizing a practical rechargeable Mg battery system. Recently, Mohtadi et al. [16] confirmed reversible magnesium depositionedissolution from a new class of electrolytes based on a relatively ionic and halide-free inorganic salt Mg(BH4)2 in both tetrahydrofuran (THF) and dimethoxyethane (DME) ether solvents. LiBH4 was employed as an additive to increase the electrochemical performance, and 94% coulombic efficiencies for Mg depositionedissolution were observed. Y.Y. Shao et al. [17] further developed Mg(BH4)2 based electrolyte with diglyme (DGM) as the solvent and LiBH4 also the additive, in which the coulombic efficiency of close to 100% was achieved in the electrolyte of 0.1 mol L1 Mg(BH4)2/LiBH4/DGM ([LiBH4] ¼ 1.5 mol L1). The research about the structure-property relationship showed that the coulombic efficiency in Mg(BH4)2-based electrolyte increases according to the series THF < DME < DGM because of the improved stability of the solvated Mg(BH4)2 complexes with the denticity of the solvent ligands (monodendate, bidentate, and tridentate, respectively). However, the solubility of Mg(BH4)2 in DGM and DME is 0.1 M/0.01 mol L1 with/without LiBH4 respectively. [17] The limited concentration of Mg(BH4)2 in solvents will lead to the limited rate performance of Mg batteries for practical application. Herein, quinquedentate ligand tetraglyme (TG) with higher safety (the boiling/flash points of TG, DGM, DME and THF are 275 C/ 141 C, 162 C/57 C, 85 C/2 C and 66 C/14 C, respectively) was further developed as the solvent of Mg(BH4)2. More importantly, a high concentration electrolyte based on Mg(BH4)2 in TG with a higher anodic stability on non-inert metal electrode was pursued for practical application. 2. Experimental 2.1. Chemicals and material synthesis Magnesium borohydride (Mg(BH4)2, 95%) and magnesium ribbon (1 mm diameter) were purchased from SigmaeAldrich. Lithium borohydride (LiBH4, 95%) was purchased from J&K Scientific. Tetrahydrofuran (THF), dimethoxyethane (DME), diglyme (DGM) and tetraglyme (TG) were purchased from Aladdin reagent and further dried using 3 Å molecular sieve. The synthetic work was conducted in an argon-filled glove box (Mbraun, Unilab, Germany) containing less than 2 ppm H2O and O2. The electrolytes were prepared by dissolving the predetermined amount of Mg(BH4)2 and LiBH4 in solvents under stirring at room temperature or 90 C for at least 2 h.
Synthesis route of Mo6S8 followed the literature [18]. In briefly, the mixture of 1 g MoS2 (99%, Aldrich), 0.398 g CuS (99.5%, Cerac), 0.602 g Mo (99%, Aldrich) and 4 g KCl (99%, Aldrich) was pestled for over 10 min and then heated at 850 C for 60 h. The product was washed by deionized water and sonicated to obtain Cu2Mo6S8. Then, Mo6S8 was prepared by chemical removal of the copper from Cu2Mo6S8 using an aqueous HCl solution through the following reaction:
Cu2 Mo6 S8 ðsÞ þ 8HClðaqÞ þ O2 /Mo6 S8 ðsÞ þ 2H2 O þ 2½CuCl4 2 ðaqÞ þ 4Hþ ðaqÞ
(1)
2.2. Measurement procedures and apparatus Cyclic voltammograms (CVs) were conducted in three-electrode cells inside an argon-filled glove box using an electrochemical instrument of CHI604A Electrochemical Workstation (Shanghai, China). The working electrode was a stainless steel, nickel, copper and platinum disk (geometric area ¼ 3.14 102 cm2), which was polished with a corundum suspension and rinsed with dry acetone before use, and magnesium ribbon as counter and reference electrodes. Electrochemical magnesium depositionedissolution cycles were examined with CR2016 experimental coin cells on a land battery measurement system (Wuhan, China). Stainless steel foil (Ф12 mm) was served as the working electrode (substrate). Mg disc as the counter electrode. An Entek PE and a fiber membrane as the separator. The cells were assembled in the glove box. Magnesium was deposited onto the stainless steel substrate for fixed periods of 30 min followed by stripping to a fixed potential limit of 0.8 V vs. Mg at a constant current density of 0.088 mA cm2. There was a 30 s rest between deposition and dissolution. The magnesium deposition and dissolution on the substrate were referred to as the discharge and charge process, respectively. The time of charge divided by the time of discharge was defined as the depositionedissolution efficiency. The Mo6S8 electrode was prepared by casting and pressing a 8:1:1 weight-ratio mixture of Mo6S8, super-P carbon powder and polyvinylidene fluoride (PVDF) binder dissolved in N-methyl-2pyrrolidinone (NMP) onto stainless steel current collector followed by drying in vacuum at 120 C. CR2016 coin cells were fabricated using Mo6S8 cathode, Mg disc anode, Entek PE and fiber membrane separator, and 0.5 mol L1 heated Mg(BH4)2/TG or Mg(BH4)2/LiBH4/ TG ([LiBH4] ¼ 1.5 mol L1) electrolyte. The chargeedischarge tests of the coin cells were carried out on the land battery measurement system with the cutoff voltage of 1.6/0.5 V vs. Mg. IR analysis of the solutions was run using a Spectrum 100 FT-IR spectrometer (Perkin Elmer, Inc., USA). Nuclear magnetic resonance (NMR) measurements were performed using a Bruker Avance 400 MHz spectrometer equipped with a CP/MAS probe. The spectra were reported in part per million (ppm) with referenced to tetramethylsilane (TMS) and BF3$O(CH2CH3)2 as the external standards of 1H and 11B, respectively. X-ray diffraction (XRD) analysis of the magnesium deposits was conducted on a Rigaku diffractometer D/MAX-2200/PC equipped with Cu Ka radiation. The morphology of the deposits was observed using scanning electron microscopy (SEM) on a FEI SIRION200 fieldemission microscope. Before the analysis of the deposits, the sample deposited for 10 h at 0.088 mA cm2 was washed in the glove box with drying THF solvent to remove soluble residue and then transferred out of the box and kept without exposure to the atmosphere.
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3. Results and discussion
Fig. 1. CVs of Mg electrochemical depositionedissolution on SS disk electrode from the solutions consisting of 0.1 mol L1 Mg(BH4)2 in the solvents of (a) DGM, (b) TG and DME (upper clear solution). The inset of (a) shows the solubility of 0.1 mol L1 Mg(BH4)2 in THF, TG, DGM and DME.
Fig. 1a shows CVs of the electrochemical Mg deposition and dissolution on non-inert stainless steel (SS) disk electrode from the electrolytes of 0.1 mol L1 Mg(BH4)2 in DGM, TG and DME at 50 mV s1. Inert Pt electrode can ensure that electrochemical experiments characterizing redox events related solely to the magnesium electrolyte and eliminate reactions between the electrode and electrolyte. However, the electrochemical performance on SS was mainly evaluated experimentally considering the limit of charging in a coin cell battery configuration due to the utilization of stainless steel in the casing and current collector material. Compared with that (Fig. 1a) from the solution with DGM as the solvent, the curve (Fig. 1b) obtained from TG shows a more obvious oxidation peak at approximately 0.25 V after a reduction process with onset at approximately 0.6 V, suggesting a more reversible Mg depositionedissolution process. It has been reported that the O donor denticity, i.e. ligand strength of the ethereal solvents in corresponding solvated Mg(BH4)2 solutions plays a significant role in enhancing coulombic efficiency of Mg depositionedissolution [17]. As a more effective donating ligand for magnesium than DGM, TG significantly improves the dissolution process through synergetic effects. Herein, it should be further noted that the clarity of the solution, which probably influences the morphology of Mg deposition, also has an effect on the coulombic efficiency. The inset of Fig. 1a shows the solubility of 0.1 mol L1 Mg(BH4)2 in THF, DME, DGM and TG, respectively. The solution is transparent for Mg(BH4)2 in THF and TG, turbid in DGM. There is precipitation for the solution of DME. A more reversible Mg depositionedissolution process can be obtained from Mg(BH4)2/DME upper clear solution than Mg(BH4)2/DGM turbid solution. Higher denticity of TG than DME probably decreases the mobility of solvated Mg2þ, thus a lower deposition current and a higher potential for Mg deposition and dissolution, as shown in Fig. 1b. Although the overpotential for Mg deposition/dissolution from DME is smaller, Mg depositionedissolution efficiency from TG is higher, which is partially related to the different morphology of Mg deposition due to different species adsorbed on the electrode in each solution. Fig. 2 compares the SEM images of electrodeposits (deposition was
Fig. 2. SEM images of the electrodeposits on SS substrate at 3.168C cm2 charge from upper clear solution of Mg(BH4)2/DME and transparent solution of Mg(BH4)2/TG.
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performed at 0.088 mA cm2 for 10 h) on SS substrate from transparent solution of Mg(BH4)2/TG and upper clear solution of Mg(BH4)2/DME, respectively. The results indicate that the deposits from these two solvents show different morphology in term of both particle size and shape. The particle shape is irregular and the deposited layer is loose and non-uniform from DME. However, the particle size is smaller and the deposited layer is compact and smooth from TG, which is more favorable for subsequent dissolution process. Fig. 3 shows CVs of Mg depositionedissolution on SS disk electrode from Mg(BH4)2/TG with the different concentrations (0.1, 0.175, 0.2, 0.25 and 0.5 mol L1). The increased concentration speeds up the depositionedissolution kinetics at electrode surface. 0.5 mol L1 Mg(BH4)2/TG solution shows the highest Mg depositionedissolution currents, which is related to a high conductivity (2.78 and 12.32 ms cm1 for 0.1 and 0.5 mol L1, respectively). However, transparent solution could not be obtained at the concentration of 0.5 mol L1. In order to improve the solubility of Mg(BH4)2 in TG, 0.5 mol L1 Mg(BH4)2/TG solution was heated at 90 C under stirring. Fig. 4a compares the CVs of Mg electrochemical depositionedissolution on SS disk electrode from 0.5 mol L1 Mg(BH4)2/TG solution with or without heating. Although totally transparent solution could not obtained after the heat-treatment, the depositionedissolution currents and the anodic stability improve dramatically. The former is resulted from the enhanced conductivity (38.57 ms cm1 for 0.5 mol L1 heated Mg(BH4)2/TG solution). The latter is probably related to the change of electronic structure of the Mg complexes in the solution after the heat-treatment, which is confirmed by NMR measurement shown in Fig. 5. The inset of Fig. 4a shows the XRD pattern of the resulting deposit on SS substrate from 0.5 mol L1 heated Mg(BH4)2/TG solution. The main diffraction peaks at 32.2, 34.4, 36.6 and 63.1 can be attributed to metallic Mg (JCPDS file 35-0821) and the peaks at 43.7,
Fig. 3. CVs of Mg electrochemical depositionedissolution on SS disk electrode from Mg(BH4)2/TG electrolyte with different concentrations.
50.6, and 74.6 to the SS substrate, indicating that deposition of pure metal Mg was obtained from the solution. Fig. 4b compares the CVs of Mg electrochemical depositionedissolution on Pt and SS disk electrode from 0.5 mol L1 heated Mg(BH4)2/TG solution. Although the solution shows higher reversibility on Pt electrode duo to an apparent limitation of SS electrode to intermediate efficiencies of Mg deposition/dissolution [12], a higher anodic stability is obtained on SS electrode (2.4 V and 1.5 V for SS and Pt, respectively). The phenomenon is different from those in most organic magnesium salts system, which generally corrodes the non-insert metal due to the chlorine ion contained in the organic magnesium salts [19,20]. In order to understand coordination structures of Mg(BH4)2 in TG, TG solvated Mg(BH4)2 complex was characterized by 1H NMR and 11B NMR spectroscopies in non-coordinating CD2Cl2, which does not interrupt the structures of the solvated Mg(BH4)2 complexes with TG. In the 1H NMR spectra (Fig. 5a), TG solvated Mg(BH4)2 species shows proton resonances at 3.628 ppm (CH2), 3.511 ppm (CH2) and 3.330 ppm (CH3) for the coordinating TG, downfield shifting in comparison with those of free TG in CDCl2
Fig. 4. CVs of Mg electrochemical depositionedissolution on (a) SS disk electrode from 0.5 mol L1 unheated and heated Mg(BH4)2/TG solution, (b) Pt and SS disk electrode from 0.5 mol L1 heated Mg(BH4)2/TG solution. The inset of (a) shows the XRD of deposits on SS substrate from 0.5 mol L1 heated Mg(BH4)2/TG solution.
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Fig. 5. (a) 1H NMR spectra of TG and Mg(BH4)2/TG solution recorded in CD2Cl2. (b) 11B and 1H (inset) NMR spectra of 0.5 mol L1 unheated and heated Mg(BH4)2/TG solutions recorded in CD2Cl2. (c) IR spectra for 0.5 mol L1 unheated and heated Mg(BH4)2/TG solutions. (d) Coordination structure of Mg(BH4)2 in TG.
Fig. 6. The cycling curves (a), a typical curve (b), and cycling efficiency (c) of Mg depositionedissolution on SS substrate from 0.5 mol L1 heated Mg(BH4)2/TG solution.
Fig. 7. The cycling curves (a), a typical curve (b), and cycling efficiency (c) of Mg depositionedissolution on SS substrate from 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG solution ([LiBH4] ¼ 1.5 mol L1).
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(3.544 ppm (CH2), 3.454 ppm (CH2) and 3.292 ppm (CH3), respectively). This is resulted from the de-shielding effect of TG metalation for the TG solvated Mg(BH4)2 species. 11B and 1H NMR spectra of unheated and heated Mg(BH4)2/TG solutions are represented in Fig. 5b to confirm the improvement of heating-treatment process to anodic stability of the solutions. In 11B NMR spectrum, the unheated solution displays the characteristic quintet at 41.37 ppm. After heating-treatment, the characteristic peak shifts to 41.28 ppm, indicating a decreased boron shielding. In 1H NMR spectrum shown in the inset of Fig. 5b, a major quartet at the chemical shift 0.36 ppm corresponds to BH 4 . There is also a reduced proton shielding for the heated solution. These results indicate a lower electron cloud density for the radical group after heatingtreatment, resulting in an improved anodic stability, which is consistent with the result of Fig. 4a. IR spectroscopic analyses were further conducted to characterize the electroactive species in unheated and heated Mg(BH4)2/TG solutions. In Fig. 5c, the bands between 1000e1400 cm1 and 2000e2500 cm1 represent BeH bending and BeH stretching vibrations, respectively [21,22], the bands between 2700 and 4000 cm1 are related with TG solvent. Those peaks change after heating, especially for the BeH stretching modes. The intensity of BeH stretching bands is obviously weaker,
Fig. 8. CVs of Mg electrochemical depositionedissolution on (a) SS and (b) Pt, Ni, Cu disk electrode from 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG solution ([LiBH4] ¼ 1.5 mol L1).
suggesting an enhanced ionic character duo to more free BH 4 [16], in accord with the increased electronic conductivity (from 12.32 to 38.57 ms cm1). It has been reported that the coordination structure of Mg(BH4)2 in DGM, DME and THF is Mg(BH4)2DGM, Mg2(BH4)4(DME)3, Mg(BH4)2(THF)3, respectively [17]. Considering that TG is a quinquedentate pincer ligand and each BH 4 anion coordinates with Mg via two hydrides, the composition of Mg(BH4)2 complex in TG can be identified as seven coordinated Mg5(BH4)10(TG)3 with a ratio of Mg(BH4)2:TG ¼ 5:3, as shown in Fig. 5d. For electrolyte, the assembled battery has an efficient cycling characteristic is very important. Fig. 6a demonstrates the galvanostatic cycling curves of Mg deposition and dissolution on SS substrate in 0.5 mol L1 heated Mg(BH4)2/TG solution via CR2016 experimental coin cells. The Mg deposition and dissolution potentials in a typical curve (shown in Fig. 6b) are about 0.45 V and 0.35 V, respectively, which are obviously too high for electrolyte. Fig. 6c shows the Mg depositionedissolution cycling efficiencies on SS substrate in heated 0.5 mol L1 Mg(BH4)2/TG solution, which are calculated according to the ratios of the charge amounts of magnesium dissolution to those of magnesium deposition. The
Fig. 9. The initial dischargeecharge curve at 0.05C of Mg/Mo6S8 coin-cell at room temperature using the electrolyte of 0.5 mol L1 heated (a) Mg(BH4)2/TG, inset is the cyclic voltammogram at 0.02 mV s1 and (b) Mg(BH4)2/LiBH4/TG ([LiBH4] ¼ 1.5 mol L1), inset is the cycling stability.
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coulombic efficiencies increase gradually from initial 83% with the progressive cycle number and reach to 90% after 4 cycles and are stable at above 95% after 15 cycles. However, higher efficiencies are necessary for the purpose of practical applications. In order to improve the electrochemical performance of 0.5 mol L1 heated Mg(BH4)2/TG solution, 1.5 mol L1 LiBH4 as a coordination ligand was further added in the solution since it has been shown to have a remarkable effect to Mg(BH4)2/THF and Mg(BH4)2/DGM solutions [16,17]. The Mg(BH4)2/TG solution became more homogeneous after adding LiBH4. Fig. 7a demonstrates the galvanostatic cycling curves of Mg deposition and dissolution on SS substrate in 0.5 mol L1 heated Mg(BH4)2/LiBH4/ TG solution ([LiBH4] ¼ 1.5 mol L1) via CR2016 experimental coin cells. The Mg deposition and dissolution potentials in the typical curve (shown in Fig. 7b) are about 0.2 V and 0.15 V, respectively, decreasing obviously compared with the solution without LiBH4. Furthermore, the solution demonstrates close to 100% coulombic efficiencies after 15 cycles and stable cycling performance, as shown in Fig. 7c. The improved Mg depositionedissolution performance is mainly due to the improved ionic conductivity (9.66 mS cm1 for 0.5 mol L1 Mg(BH4)2/LiBH4/TG solution ([LiBH4] ¼ 1.5 mol L1)) as a result of increasing Mg(BH4)2 dissociation [16]. Moreover, the anodic stability of the solution on SS electrode can still maintain at approximately 2.4 V with higher current densities compared with that without adding LiBH4 (Fig. 8a), although the stability on Cu or Ni non-inert electrode still needs to be improved, as shown in Fig. 8b. The research on the electrolytes with a wide electrochemical window on non-inert electrodes and prepared by scalable chemical processes is under development. The coin cell was constructed using 0.5 mol L1 heated Mg(BH4)2/TG and Mg(BH4)2/LiBH4/TG ([LiBH4] ¼ 1.5 mol L1) solutions as the electrolyte, model Mo6S8 (122.8 mAh g1 theoretical capacity) as a positive electrode, and a Mg disc as a negative electrode to verify the compatibility of the electrolyte with a magnesium ion intercalation cathode. As shown in Fig. 9a , the cell with Mo6S8 cathode and 0.5 mol L1 heated Mg(BH4)2/TG electrolyte delivers 65.9 mAh g1 initial discharge capacity and 63.4 mAh g1 initial charge capacity at 0.05C rate. Cyclic voltammogram (inset of Fig. 9a) shows reversible Mg insertion (peak at 0.9 V) and deinsertion (peak at 1.39 V) in Mo6S8 cathode. After adding LiBH4 in the electrolyte, the cell shown in Fig. 9b delivers higher discharge and charge capacity (76.8 mAh g1 and 75.7 mAh g1, respectively) with lower voltage polarization compared with that without LiBH4. The discharge capacity drops slightly at the second cycle and is stabilized at approximately 71 mAh g1 with 92.4% capacity retention for the remaining 107 cycles (inset of Fig. 9b), indicating good compatibility of the electrolyte with Mg intercalation cathode material. 4. Conclusions A electrolyte for rechargeable magnesium batteries based on an inorganic magnesium salt Mg(BH4)2 dissolved in a potentially safer ether solvent TG with 0.5 mol L1 high concentration is developed.
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The chelation property of TG with five oxygen sites per molecule 2þ results in an enhanced dissociation between BH 4 and Mg , thus an improved electrochemical performance for Mg deposition and dissolution compared with DME and DGM as the solvents. 90 C heat-treatment and LiBH4 additive are employed to further improve the anodic stability and electronic conductivity of the solution. 0.5 mol L1 heated Mg(BH4)2/LiBH4/TG 1 ([LiBH4] ¼ 1.5 mol L ) solution demonstrates 2.4 V anodic satiability on non-inert stainless steel electrode and close to 100% coulombic efficiency for reversible Mg deposition and dissolution. Furthermore, the compatibility with the Mo6S8 cathode indicates that the Mg(BH4)2/TG solution could be used as a potential electrolyte for rechargeable Mg battery system. Acknowledgments This work was supported by the National Nature Science Foundation of China (project no. 21273147) and the Shanghai Municipal Science and Technology Commission (project no. 11JC1405700). References [1] H.S. Kim, T.S. Arthur, G.D. Allred, J. Zajicek, J.G. Newman, A.E. Rodnyansky, A.G. Oliver, W.C. Boggess, J. Muldoon, Nat. Commun. 2 (2011) 427. [2] M. Matsui, J. Power Sources 196 (2011) 7048. [3] T.D. Gregory, R.J. Hoffman, R.C. Winterton, J. Electrochem. Soc. 137 (1990) 775. [4] D. Aurbach, A. Schechter, M. Moshkovich, Y. Cohen, J. Electrochem. Soc. 148 (2001) A1004. [5] Y. Viestfried, M.D. Levi, Y. Gofer, D. Aurbach, J. Electroanal. Chem. 576 (2005) 183. [6] Z. Lu, A. Schechter, M. Moshkovich, D. Aurbach, J. Electroanal. Chem. 466 (1999) 203. [7] D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi, Nature 274 (2000) 724. [8] D. Aurbach, H. Gizbar, A. Schechter, O. Chusid, H.E. Gottlieb, Y. Gofer, I. Goldberg, J. Electrochem. Soc. 149 (2002) A115. [9] Y. Gofer, O. Chusid, H. Gizbar, Y. Viestfrid, H.E. Gottlieb, V. Marks, D. Aurbach, Electrochem. Solid. State Lett. 9 (2006) A257. [10] O. Mizrahi, N. Amir, E. Pollak, O. Chusid, V. Marks, H. Gottlieb, L. Larush, E. Zinigrad, D. Aurbach, J. Electrochem. Soc. 155 (2008) A103. [11] D. Aurbach, G.S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid, M. Brunelli, Adv. Mater. 19 (2007) 4260. [12] J. Muldoon, C.B. Bucur, A.G. Oliver, T. Sugimoto, M. Matsui, H.S. Kim, G.D. Allred, J. Zajicek, Y. Kotani, Energy Environ. Sci. 5 (2012) 5941. [13] Y.S. Guo, F. Zhang, J. Yang, F.F. Wang, Y.N. NuLi, S.I. Hirano, Energy Environ. Sci. 5 (2012) 9100. [14] F.F. Wang, Y.S. Guo, J. Yang, Y.N. NuLi, S.I. Hirano, Chem. Commun. 48 (2012) 10763. [15] Q. Chen, Y.N. NuLi, J. Yang, K. Kailibinuer, J.L. Wang, Acta Phys. Chim. Sin. 28 (2012) 2625. [16] R. Mohtadi, M. Matsui, T.S. Arthur, S.J. Hwang, Angew. Chem. Int. Ed. 51 (2012) 9780. [17] Y.Y. Shao, T.B. Liu, G.S. Li, M. Gu, Z.M. Nie, M. Engelhard, J. Xiao, D.P. Lv, C.M. Wang, J.G. Zhang, J. Liu, Sci. Rep. 3 (2013) 3130. [18] E. Lancry, E. Levi, A. Mitelman, S. Malovany, D. Aurbach, J. Solid State Chem. 179 (2006) 1879. [19] J. Muldoon, C.B. Bucur, A.G. Oliver, J. Zajicek, G.D. Allred, W.C. Boggess, Energy Environ. Sci. 6 (2013) 482. [20] D.P. Lv, T. Xu, P. Saha, M.K. Datta, M.L. Gordin, A. Manivannan, P.N. Kumta, D.H. Wang, J. Electrochem. Soc. 160 (2013) A351. [21] Y. Filinchuk, R. Cerný, H. Hagemann, Chem. Mater. 21 (2009) 925. [22] Y. Filinchuk, B. Richter, T.R. Jensen, V. Dmitriev, D. Chernyshov, H. Hagemann, Angew. Chem. Int. Ed. 50 (2011) 11162.