Dinitrile–Mononitrile-Based Electrolyte System for Lithium-Ion Battery

Article pubs.acs.org/JPCC

Dinitrile−Mononitrile-Based Electrolyte System for Lithium-Ion Battery Application with the Mechanism of Reductive Decomposition of Mononitriles Rupesh Rohan,† Tsung-Chieh Kuo,† Jing-Heng Lin,† Ya-Chu Hsu,† Chia-Chen Li,‡ and Jyh-Tsung Lee*,†,§ †

Department of Chemistry, National Sun Yat-sen University, Kaohsiung 80424, Taiwan Department of Material Science and Resource, National Taipei University of Technology, Taipei 10608, Taiwan § Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan ‡

S Supporting Information *

ABSTRACT: The development of electrolytes capable of performing at a high voltage (>5 V) is essential for the advancement of lithium-ion batteries. In the present work, we have investigated a dinitrile−mononitrile-based electrolyte system that can offer electrochemical stability up to 5.5 V at room temperature. The electrolytes consist of 1.0 M lithium bis(trifluoromethane)sulfonamide in various volume proportions of glutaronitrile, a dinitrile, and butyronitrile, a mononitrile (10/0; 8/2; 6/4; 4/6; 2/8; 10/0). The ionic conductivity of the electrolytes was found to be 3.1 × 10−3− 10.6 × 10−3 S cm−1 at 30 °C, comparable with commercially used carbonate-based electrolytes. However, butyronitrile reacts with Li metal to give 3-amino-2-ethylhex-2-ene-nitrile, 2,6-dipropyl-5-ethylpyrimidin-4-amine, and oligomers/polymers. These compounds have been characterized by nuclear magnetic resonance techniques, and based on these findings, a plausible mechanism of reactivity of mononitriles toward Li metal has been proposed. Finally, 5 wt % of vinylene carbonate is added to the glutaronitrile/butyronitrile (6/4 ratio) system to inhibit the reductive decomposition of butyronitrile. The resultant electrolyte system is used in the assembly of several coin cells consisting of a LiFePO4 composite cathode and a Li metal anode. The cells perform up to 3 C charge/discharge rate with reasonably good discharge capacity and also display a cycle life of more than 100 cycles at a 0.5 C rate with capacity retention above 95% at room temperature.

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INTRODUCTION The increasing adverse effects of greenhouse gases, which has been causing the climate change, and rapid exhaustion of fossil fuels have compelled a switch to renewable and clean energy resources. While a plethora of energy is available, courtesy of the sun, wind, and water, the issue of storage and delivery of energy has become an area requiring serious technological advancement.1−7 Lithium-ion battery technology, which has been powering portable gadgets and light transportation vehicles successfully for decades, could serve the purpose but necessitates significant improvements in energy and power density of the batteries.8−14 In this context, various high-voltage cathode-active materials, such as LiNiVO4, LiNi0.5Mn1.5O4, LiCrxMn2−xO4, and LiNiPO4, have been developed recently, which exhibit working voltages above 5.0 V vs Li/Li+.15−22 To utilize these cathode materials practically, a coherent electrolyte system is required, which can perform successfully above the 5.0 V benchmark. Unfortunately, the electrochemical stability of conventional carbonate electrolyte solvents is lower than 4.8 V vs Li/Li+.23−25 Although several high-voltage electrolyte © XXXX American Chemical Society

solvents have been reported, such as sulfones, ionic liquids, fluorinated carbonates, and dinitriles, all of these have other issues that limit their utility as a part of high-voltage electrolyte systems.24,26−37 By virtue of the inherent cathodic stability of cyano groups, the nitrile solvents, mononitriles and dinitriles, could serve as high-voltage electrolyte solvents. It has been well demonstrated that the electrolyte of lithium bis(trifluoromethane)sulfonamide (LiTFSI) in dinitrile systems possesses a wide working temperature range, excellent high electrochemical stability (∼7.0 V for a single nitrile; 6.0−6.5 V for binary and ternary solvents), and great dielectric permittivity.27,38,39 In addition, the dinitrile−LiTFSI electrolyte systems can form a protective layer on the Al current collector; this layer reduces corrosion owing to the LiTFSI salt at a high voltage.24,38−40 However, the high viscosity, which hampers the ionic Received: January 29, 2016 Revised: March 8, 2016

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understand the phenomenon, two Li foils were separately placed in the glutaronitrile and butyronitrile solvents. In the Li metal−butyronitrile system, a color change occurred from transparent to yellow, inferring the instability of the Li metal in mononitriles. To reconfirm the process occurring, the experiment was repeated with propionitrile, another mononitrile. The byproducts of both mononitrile systems were purified and characterized, and based on the findings, a likely reaction mechanism is suggested. Finally, the 1.0 M LiTFSI− glutaronitrile−butyronitrile electrolyte, which offers comparable ionic conductivity with commercially used carbonate electrolyte systems, was added with different quantities of VC to prevent the Li anode corrosion, and the resultant systems were analyzed for their electrochemical behavior. The battery performance of the systems in the coin cells consisting of the LiFePO4 composite cathode and Li metal anode was also investigated. The detailed methodology is discussed under the subsequent sections. Preparation of Electrolytes. First, a set of 1.0 M LiTFSI− glutaronitrile−butyronitrile electrolytes was prepared by varying the volume proportion of glutaronitrile and butyronitrile (10/0; 8/2; 6/4; 4/6; 2/8; 0/10). The operation was performed inside a glovebox, and the solutions were stirred at room temperature for 6 h. Later, the solution of 6/4 ratio of glutaronitrile/butyronitrile was taken in four different bottles. Each bottle was added a different volume (0, 2, 5, and 10 w/v %) of VC and then stirred for 10 h at room temperature, inside a glovebox. Electrode Preparation and Cell Fabrication. Either a MCMB or LiFePO4 composite electrode was prepared to execute CV in the electrolytes. The MCMB composite electrode consists of MCMB (93 wt %) and PVdF (7 wt %).46 A copper foil was used as a current collector. The LiFePO4 composite electrode comprises LiFePO4 (85 wt %), PVdF (5 wt %), and Super-P carbon (10 wt %). All the ingredients were mixed NMP. The prepared solution was cast onto an aluminum foil. Both MCMB and LiFePO4 composite electrodes were dried in a vacuum oven at 80 °C for 12 h. The dried cathodes were then punched in a circular shape to be used in coin cells. Coin cells (CR2032) were assembled using a polypropylene separator (Celgard 2500) between the Li metal anode and either the MCMB or LiFePO4 composite cathode inside a glovebox. The CV test was performed on an electrochemical analyzer (CH Instruments, 6081C) at a scan rate of 0.1 mV s−1 in the operating voltage ranges of the respective cathode materials. To measure the battery performance, a multichannel battery-testing system (BAT-750B, Acu Tech System, Taiwan) was used. The charge and discharge capacities of the coin cells were measured at different C-rates at 30 °C. Electrochemical Characterizations. The electrochemical stability of the electrolytes was measured using the linear sweep voltammetry (LSV) technique. For the analysis, 4 mL of the electrolyte solution was taken in a sealed transparent bottle fitted with a platinum electrode as a working electrode and a Li metal electrode as a reference and counter electrode. The measurements were performed on the electrochemical analyzer (CH Instruments, 6081C) with a scan rate of 1 mV s−1 in the voltage range from open-circuit voltage (OCV) to 7.0 V. The ionic conductivity of the electrolytes was measured by electrochemical impedance spectroscopy using an impedance analyzer (CH Instruments, 6081C) over a frequency range of 1 × 105 to 0.1 Hz with an oscillating voltage of 10 mV. The

conductivity of the electrolytes, and the inability to form a solid electrolyte interface (SEI) layer on carbonaceous anodes are the two major issues that still need to be addressed. The latter problem has been partially solved by the use of SEI additives such as vinylene carbonates (VCs), fluoroethylene carbonates, and lithium bis(oxalato)borate.38,41−45 To achieve a higher ionic conductivity of dinitrile electrolyte systems, the addition of low viscous solvents could be a rational approach as these electrolytes should remain stable at high voltage. Compared with dinitriles, short-chain alkyl mononitriles have low viscosities and also offer high-voltage stability.27 Therefore, alkyl mononitriles could serve as the cosolvent(s) for dinitrile electrolyte systems to enhance the ionic conductivity, analogous to linear carbonates in the carbonate-based electrolytes.31,32 With this approach, we report here a detailed study of 1.0 M LiTFSI in dinitrile/mononitrile electrolyte systems, by varying dinitrile/mononitrile proportion. The properties of the electrolytes are studied using ionic conductivity, cyclic voltammetry (CV), and scanning electron microscopy (SEM). The battery performance of the cells with the nitrile-based electrolytes is also measured. Since the decomposition of mononitriles is observed when the Li metal is immersed in the mononitrile solvents the reductive byproducts of the mononitrile-based electrolytes are purified and characterized, and a plausible mechanism for the decomposition of mononitriles is also proposed.

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EXPERIMENTAL SECTION Materials. Glutaronitrile (99%, TCI), butyronitrile (99%, Alfa−Aesar), vinylene carbonate (VC, 98%, Acros Organics), lithium bis(trifluoromethane)sulfonamide (LiTFSI, 99%, Acros Organics), LiFePO4 (Advanced Lithium Electrochemistry Co.), Li metal (UBIQ Technology Co. LTD), N-methyl-2pyrrolidone (NMP, 99%, Janssen), Super-P carbon (Timcal), mesophase microbeads (MCMB-2528 Osaka Gas, 25 μm diameter), and polyvinylidene fluoride (PVdF, 1100, Kureha) were used as received. The chemical structures of the electrolyte salt, nitrile solvents, and the additive used are depicted in Figure 1.

Figure 1. Structures of glutaronitrile, butyronitrile, propionitrile, vinylene carbonate (VC), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

Methods. In the measurement of ionic conductivity, a set of 1.0 M LiTFSI−glutaronitrile−butyronitrile electrolyte system, with a varying proportion of the two-solvent electrolyte, was performed. Based on the result, the electrolyte with an ionic conductivity close to carbonate-based electrolytes was chosen for the further study. However, during the electrochemical stability test, the color of the electrolyte solution was changed from transparent to yellow−brown, presumably owing to corrosion of the Li metal used in the measurement. To B

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Figure 2. (a) Arrhenius plots and (b) activation energies of 1.0 M LiTFSI in the varying proportions of glutaronitrile/butyronitrile.

Gel permeation chromatography (GPC) was performed on a Waters GPC column (Wat 054460) in association with a refractive index detector (Jasco RI-2031 Plus) and an HPLC pump (Jasco PU-2080 Plus). Tetrahydrofuran was used as an eluent, and polystyrene standards were used as calibration standards to determine molecular weights.

electrolyte solution (2 mL) was placed in a Teflon cuboid cell fitted with two polished stainless steel electrodes and sealed in a glovebox under an argon atmosphere. The measurements were performed at different temperatures ranging from 30 to 80 °C. SEI Observation. To study the SEI formation ability of the electrolyte systems, the coin cells were run for four CV cycles. Subsequently, the post-mortem of the batteries was performed, and the SEI film formation on the respective cathodes was observed by field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) with an accelerating voltage of 10 kV and a current of 10 μA. The samples were sputtered with platinum, prior to the SEM analysis. Purification and Characterization of the Byproducts of Li−Mononitrile Systems. To study the reductive reaction mechanism of mononitriles with Li, both of the reacted solutions of lithium−propionitrile and lithium−butyronitrile were purified and characterized. After 1 week, the solutions had turned yellow. The propionitrile solution showed some crystal formation. The crystals were analyzed by single-crystal X-ray diffraction (XRD), 1H NMR, and 13C NMR on a single-crystal diffractometer (Bruker APEX DUO), a Bruker Avance-300 spectrometer (300 MHz), and a Varian 500 NMR spectrometer (125 MHz), respectively. Later, both the solutions were purified via a silica gel (Si 60, Merck) column chromatography (eluent: ethyl acetate/ether: 1/1, v/v) to isolate the byproducts and were characterized using 1H NMR and 13C NMR spectroscopies. For 2,6-diethyl-5-methylpyrimidin-4-amine, a trimer formed from the decomposition of propoinitrile: 1H NMR (300 MHz, CDCl3) δ 1.20 (t, 3H, J = 7.7 Hz), 1.27 (t, 3H, J = 7.7 Hz), 2.03 (s, 3H), 2.68 (q, 2H, J = 7.6 Hz), 2.70 (q, 2H, J = 7.6 Hz), 4.7 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 11.0, 13.0, 13.1, 28.3, 32.3, 106.4, 161.9, 167.5, 168.6. For 3amino-2-methylpent-2-enenitrile, a dimer formed from the decomposition of propionitrile: 1H NMR (300 MHz, CDCl3) δ 1.18 (t, 3H, J = 7.7 Hz), 1.67 (s, 3H), 2.42 (q, 2H, J = 7.6 Hz), 4.08 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 12.6 (2C), 27.5, 71.6, 123.2, 159.6. For 2,6-dipropyl-5-ethylpyrimidin-4-amine, a trimer formed from the decomposition of butyronitrile: 1H NMR (300 MHz, CDCl3) δ 0.95 (q, J = 7.0, 3H), 0.97 (q, J = 6.9, 3H), 1.13 (t, J = 7.7, 3H), 1.60−1.80 (m, 4H), 2.46 (q, 2H, J = 7.60 Hz), 2.55−2.64 (m, 4H), 4.86 (s, 2H). 13C NMR (125 MHz, CDCl3) δ 12.5, 14.0, 14.2, 19.0, 22.3, 22.9, 36.3, 41.1, 112.8, 161.4, 166.0, 167.4. For 3-amino-2-ethylhex-2-ene-nitrile, a dimer formed from the decomposition of butyronitrile: 1H NMR (300 MHz, CDCl3) δ 0.97 (t, 3H, J = 7.2 Hz), 1.12 (t, 3H, J = 7.5 Hz), 1.60 (sextet, 2H, J = 6.84 Hz), 2.00 (q, 2H, J = 7.50 Hz), 2.37 (t, 2H, J = 7.5 Hz), 4.13 (s, 2H). 13C NMR (300 MHz, CDCl3) δ 12.3, 13.4, 19.9, 21.5, 36.4, 80.8, 122.1, 156.8.

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RESULTS AND DISCUSSION Ionic Conductivity. The ionic conductivity of 1.0 M LiTFSI in glutaronitrile, butyronitrile, and the blends of both solvents (glutaronitrile/butyronitrile) in various proportions (10/0; 8/2; 6/4; 4/6; 2/8; 0/10, v/v) were measured at various temperature ranging from 30 to 80 °C. A graph plotted of the logarithmic values of the ionic conductivity of the electrolytes against the inverse of absolute temperatures is depicted in Figure 2a. At 30 °C, the ionic conductivity of the pure glutaronitrile and butyronitrile systems was found to be 3.14 × 10 −3 and 16.2 × 10 −3 S cm−1, respectively. For the glutaronitrile/butyronitrile blends, it was found to be 4.8 × 10−3, 6.0 × 10−3, 8.7 × 10−3, and 10.6 × 10−3 S cm−1 at 30 °C for 8/2, 6/4, 4/6, and 2/8 ratio electrolytes, respectively. The observed increment in the ionic conductivity with increasing content of butyronitrile may be attributed to the reduction in the viscosity of the electrolyte systems caused by its addition. The ionic conductivity of the electrolytes increases with increase in temperature; the corresponding curves exhibit a similar trend to the Arrhenius type curve. Therefore, their activation energies (Ea) were calculated using the Arrehenius equation and plotted in Figure 2b. The Ea of the electrolytes with 0%, 20%, 40%, 60%, 80%, and 100% of butyronitrile content was found to be 18.2, 16.8, 13.8, 11.5, 9.1, and 5.6 kJ mol−1, respectively. The decrease in Ea values with an increase in the percentage of butyronitrile predominantly corresponds to the low melting point and low viscosity of butyronitrile, which eventually enhances the ionic conductivity of the nitrilebased electrolytes. Since the blend containing 40% butyronitrile offers ionic conductivity that is quantitatively similar to the carbonate-based electrolytes,47 further study was conducted with the same blend, i.e., the 6/4 ratio of glutaronitrile/ butyronitrile electrolyte system with 1.0 M LiTFSI. Decomposition Phenomenon of Li−Mononitrile Systems. As stated in the Experimental Section, the color change in the glutaronitrile/butyronitrile blend electrolyte system was caused by butyronitrile. To understand the phenomenon, two Li metal strips were dipped in butyronitrile and propionitrile solvents separately. In propionitrile, when the Li foil reacted with the solvent, the solution became yellow. After a week, some colorless crystals were formed in the solution (Figure 3a). C

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molecular weight (Mw) of 1956, and a polydispersity index (PDI, = Mw/Mn) of 1.04. Therefore, the yellow color may be due to the long conjugated length containing oligomers or polymers; the molecular weight confirms the formation of oligomers or polymers, along with the dimer and trimer, as the product of the reduction of mononitrile by the Li metal. After a week, the butyronitrile solution containing a Li strip had also turned to yellow, but no crystals formed in the solution. However, a dimer and a cyclic trimer were found after purification by a silica gel column, which were then characterized by 1H NMR and 13C NMR spectroscopies (Figures S6, S7, S8, and S9). The crystal of cyclic trimer could not be found by crystallization, which may be attributed to the presence of longer side chains to suppress the crystallization. As with propionitrile, the yellow color compound can be purified further, and its GPC analysis determines Mn of 1259, Mw of 1297, and PDI of 1.03. The above results confirm that mononitriles are unstable with lithium to form dimers, trimers, and oligomers/polymers. Plausible Reaction Mechanism. Based on these findings, a plausible reaction mechanism to depict the instability of mononitriles toward Li, which may lead to the formation of dimer, trimer, and oligomers/polymer byproducts, is presented in Scheme 1. The α-hydrogen of nitriles (pKa ∼ 30) can be easily extracted by bases.48,49 So, Li may react with nitrile to give lithium nitrile and lithium hydride that can further react with another nitrile to form lithium nitrile and hydrogen gas. Bubbles were evident in the solutions, thus confirming the formation of H2 gas. Subsequently, the nitrile anion, acting as a nucleophile, may instigate a nucleophilic addition reaction by attacking the nitrile carbon of another nitrile molecule to form the lithium alkyl amide, which eventually, after rearrangements, forms a dimer. In a similar fashion, the lithium dimer amide may also attack another nitrile molecule to form a lithium trimeric amide to form a cyclic trimer. Finally, lithium trimeric

Figure 3. (a) Trimer crystals formed by reductive decomposition of propionitrile and (b) cystal structure of the trimer crystal characterized by single-crystal XRD.

These crystals were characterized as 2,6-diethyl-5-methylpyrimidin-4-amine, a trimer, by single-crystal XRD, as shown in Figure 3b. The chemical structure of the trimer was also confirmed by 1H NMR and 13C NMR spectroscopies (Figures S1 and S2 in Supporting Information). Subsequently, the reaction mixture was also purified by column chromatography. The other compound was isolated and characterized as 3amino-2-methylpent-2-enenitrile, a dimer, by 1H NMR and 13C NMR spectroscopy (Figures S3 and S4). Despite the solution being a yellow color, the dimer solution and the trimer crystal (Figure S5) were both colorless; this may be due to the conjugated lengths being too short to absorb visible light. However, the yellow compounds cannot be purified further by column chromatography because the yellow compounds became adsorbed at the top of a column. The thin-layer chromatography result also shows that the spot of the yellow compounds remained in the original spot, which indicates that yellow compound may be a mixture of oligomers or polymers. The GPC analysis of the yellow compounds shows a numberaverage molecular weight (Mn) of 1868, a weight-average

Scheme 1. Proposed Mechanisms for the Reaction of Li Metal and the Mononitriles

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Figure 4. First four CV cycles for the MCMB∥Li coin cell with 1.0 M LiTFSI in glutaronitrile/butyronitrile (6/4) with (a) 0%, (b) 2%, (c) 5%, and (d) 10% additions of VC, at a scanning rate of 0.1 mV s−1 at 30 °C.

Figure 5. (a) SEM photograph of the virgin MCMB electrode. SEM photographs of MCMB electrodes of Li∥MCMB coin cell with 1.0 M LiTFSI in glutaronitrile/butyronitrile (6/4) with (b) 0, (c) 2, (d) 5, and (e) 10% additions of VC after 4 cycles of CV, performed with a scanning rate of 0.1 mV s−1 at 30 °C. E

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Figure 6. First four CV cycles for LiFePO4∥Li coin cell with 1.0 M LiTFSI in glutaronitrile/butyronitrile (6/4) with (a) 0%, (b) 2%, (c) 5%, and (d) 10% of VC, at a scanning rate of 0.1 mV s−1 at 30 °C.

amide may attack nitrile again to form conjugated oligomers and polymers. Unfortunately, as discussed previously, the conjugated oligomers and polymers were not purified by column chromatography. It is important to consider that the great solubility of the byproducts of the mononitriles in organic solvents may suppress the formation of a protecting layer on Li metal which causes the further decomposition of mononitrile, in the Li ambience. On the contrary, the dinitriles, with two cyano groups in one molecule, may form cross-linked structures leading to insolubility in the solvents and forming a protective layer on the Li electrode, which inhibits any obvious further reaction on the Li metal. Electrochemical Performances of Nitrile-Based Electrolytes with VC Additive. It is well documented that VC can be a good additive for formation of SEI films.45,50 To prevent the decomposition of butyronitrile, the 1.0 M LiTFSI− glutaronitrile/butyronitrile (6/4 ratio) electrolyte solution was added with 2, 5, and 10 wt % of VC separately. Figure 4 depicts the CV curves of the MCMB∥Li cells, comprising these electrolytes. There are neither anodic nor cathodic peaks found for the 0% VC addition (Figure 4a), which infers the absence of any SEI films on the MCMB electrode, as well as their absence on the Li metal. Consequently, the Li electrode reacts with butyronitrile readily, and the lithiated MCMB electrode may also react with butyronitrile. Therefore, no lithium intercalation/deintercalation occurs in the cell. For the 2% VC addition (Figure 4b), the appearance of anodic and cathodic peaks confirms that the lithium intercalation/deintercalation processes do occur, but only for the first cycle, and disappear in subsequent cycles. This finding suggests that the amount of VC may be insufficient to form proper SEI films to cover the surfaces of Li and lithiated MCMB. This is confirmed in the case of 5% and 10% VC additions (Figure 4c and 4d), where stable redox peaks for lithium intercalation/deintercalation are found during each of the first four cycles. The CV results

suggest that 5% or more addition of VC in the nitrile-based electrolyte can significantly suppress the reductive decomposition of the butyronitrile. Subsequently, the post-mortem of the CV-cycled cells was performed, and the respective MCMB electrodes were observed by SEM (Figure 5). However, the MCMB electrodes belonging to the cells with 0% and 2% VC addition electrolytes were covered with some material(s); in the case of 0% VC, complete coverage was seen, whereas in the 2% VC sample only partial coverage occurred (Figure 5b and 5c). The material(s) might be formed owing to the reductive decomposition of the mononitrile component of the electrolyte. In the case of the MCMB electrodes, belonging to the cells with 5% and 10% VC addition electrolytes, no obvious deposit material could be observed (Figure 5d and 5e), and the surface morphology of the electrodes remained similar to the asprepared virgin MCMB electrode, mainly attributed to the formation of the SEI film by VC. The SEM results show good agreement with the results of the CV test and reaffirm the statement that the addition of 5% or more VC could form a formidable SEI film, which eventually suppresses the mononitrile decomposition. To extend and validate the findings further, the same CV testing was performed for LiFePO4∥Li cells consisting of the same set of the electrolytes (Figure 6). No obvious redox couple was found for the 0% VC addition (Figure 6a). When the VC addition is increased to 2%, the redox peaks do appear, but almost vanish after 2 cycles (Figure 6b), which is due to the insufficiency of VC to form SEI films on the Li electrode. For the 5% VC addition, the redox peaks are reversible, repeatable, and stable for all the cycles (Figure 6c). Similar behavior has been also observed for the 10% VC electrolyte (Figure 6d). However, the difference between the anodic and cathodic peak potentials increases as the cycling number increases, which may be attributed to an increase in the SEI resistance during the cycling process. Overall, the CV results show coherence for F

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CONCLUSIONS In the quest to develop electrolytes compatible with a highvoltage cathode material, we have presented a dinitrile-/ mononitrile-based electrolyte. This electrolyte has a wide electrochemical window (at least up to 5.5 V vs Li/Li+) and an ionic conductivity comparable with commercially available carbonate-based electrolytes. The electrolyte consists of 1.0 M LiTFSI salt in glutaronitrile/butyronitrile (6/4, v/v) solution with 5 wt % VC additive. The cell assembled with the electrolyte using a LiFePO4 composite cathode and a Li metal anode displayed high discharge capacity with a long cycle life. In the absence of the VC additive, the mononitrile component reacts with Li metal and decomposes. The decomposition products were purified and characterized as dimer, trimer, and oligomer/polymer by NMR techniques. Based on these findings, a probable mechanism of the reductive reaction has been proposed. The understanding of the mechanism could assist some constructive approaches to utilize mononitriles as electrolytes for lithium-ion batteries. Admittedly, the use of a high-voltage electrode material could have been a more rational approach to investigate the electrolyte performance for high-voltage application, but owing to an unavailability of any standard high-voltage cathode material, this study has chosen to use LiFePO4 cathodes. Nevertheless, the study may shed light on the potential of nitrile-based electrolytes for lithium-based batteries, and the suggested electrolyte could be investigated explicitly with some high-voltage cathode materials.

both MCMB and LiFePO4 cells and infer that 5% VC addition for the dinitrile-/mononitrile-based electrolyte could be sufficient to achieve the optimal redox behavior. The ionic conductivity of the 1.0 M LiTFSI in glutaronitrile/ butyronitrile (6/4) with 5% VC was further measured at different temperatures, and a plot of the log values of the ionic conductivity vs absolute temperature is portrayed in Figure S10. The ionic conductivity was found to be 7.3 × 10−3 S cm−1 at 30 °C, which is slightly higher than the same solution without any VC additive. The electrochemical stability of the electrolyte, measured by LSV, was found to be 5.7 V vs Li/Li+, which is comfortably high to use with high-voltage electrode materials (Figure 7).

Figure 7. Linear sweep voltammetry curve of 1.0 M LiTFSI in glutaronitrile/butyronitrile (6/4) with 5% VC electrolyte, at a scan rate of 1 mV s−1, in the voltage range from OCV to 7.0 V vs Li/Li+ at room temperature.

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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.jpcc.6b00980. 1 H NMR and 13C NMR spectra of the dimers and the trimers, the photograph of 2,6-diethyl-5-methylpyrimidin-4-amine, a trimer, and the Arrhenius plot of 1.0 M LiTFSI in glutaronitrile/butyronitrile (= 6/4) with 5% VC electrolyte (PDF) Crystallographic information for C9 H15 N3 (CIF)

Finally, the battery performance of the electrolyte was measured in LiFePO4∥Li cells at 30 °C. Figure 8a depicts the C-rate performance of the LiFePO4∥Li cell with the 1.0 M LiTFSI in glutaronitrile/butyronitrile (6/4) with 5% VC electrolyte. The discharge capacity at 0.2, 0.5, 1, and 3 C was 149, 144, 138, and 112 mAh g−1, respectively, which confirms the suitability of the electrolyte. The cyclic-life test (Figure 8b) displays the discharge capacity of 135 mAh g−1 at 0.5 C rate with a capacity retention of more than 90% after 100 cycles. The performance confirms that the electrolyte could perform very well with sufficient high discharge capacity and is able to retain stability during the cycle life test of the battery cell.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +886-7-525-3951. Notes

The authors declare no competing financial interest.

Figure 8. (a) Charge and discharge profiles of the LiFePO4∥Li cell with the 1.0 M LiTFSI in glutaronitrile/butyronitrile (6/4) with 5% VC electrolyte at various C-rates. (b) Cycle-life performance of the coin cell at a charge/discharge rate of 0.5 C. G

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ACKNOWLEDGMENTS The authors gratefully acknowledge support of the Ministry of Science and Technology in Taiwan through Grant NSC 1022628-M-110-001-MY3.

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