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Ionic liquid-based electrolyte with binary lithium salts for high performance lithium–sulfur batteries
Journal of Power Sources 296 (2015) 10e17
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Ionic liquid-based electrolyte with binary lithium salts for high performance lithiumesulfur batteries Feng Wu a, b, Qizhen Zhu a, Renjie Chen a, b, *, Nan Chen a, Yan Chen a, Yusheng Ye a, Ji Qian a, Li Li a, b a b
School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We prepare an ionic liquid-based electrolyte with binary lithium salts for LieS batteries. Various molar ratios of electrolyte systems were thoroughly investigated. The novel electrolytes show excellent discharge capacity and cycling performance. The SEI-forming ability of LiODFB protect Li anode from suffering lithium dendrites.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2015 Received in revised form 26 June 2015 Accepted 10 July 2015 Available online 18 July 2015
Rechargeable LieS batteries have suffered several technical obstacles, such as rapid capacity fading and low coulombic efficiency. To overcome these problems, we design new electrolytes containing Nmethoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr1,2O1TFSI) and tri(ethylene glycol)dimethyl ether (TEGDME) in mass ratio of 7:3. Moreover, Lithium difluoro(oxalate)borate (LiODFB) is introduced for the modification. Although the addition of LiODFB as additive lead to extremely high viscosity of electrolyte and inferior performance of the cells, the electrolyte containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 0.84 nm) and LiODFB (0.60 nm) mixture with a total molar concentration of 0.4 mol kg1 as binary lithium salt shows excellent electrochemical performance. The Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI/LiODFB binary lithium salts in mole ratio of 6:4 is obtained after optimizing ratio. The LieS cells containing this electrolyte system show excellent capacity and cycle performance, whose initial discharge capacity is 1264.4 mAh g1, and retains 911.4 mAh g1 after 50 cycles with the coulombic efficiency more than 95%. It can be attributed the solid-electrolyte interphase (SEI)-forming ability of LiODFB which protect Li anode from suffering lithium dendrites and prevent the shuttle phenomenon. The novel electrolytes provide good cycling stability and high coulombic efficiency for the LieS batteries, which is suggested as a promising electrolyte for LieS batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lithiumesulfur batteries Electrolyte Ionic liquid Binary lithium salts Solid-electrolyte interphase
1. Introduction * Corresponding author. School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail address: [email protected] (R. Chen). http://dx.doi.org/10.1016/j.jpowsour.2015.07.033 0378-7753/© 2015 Elsevier B.V. All rights reserved.
In recent years, the lithiumesulfur (LieS) battery has been focus of many recent scientific investigations as the new generation of
F. Wu et al. / Journal of Power Sources 296 (2015) 10e17
rechargeable electrochemical system due to the high theoretical capacity (1672 mAh g1) of its sulfur (S) cathode [1e4]. However, the LieS battery system may suffer from several technological problems, such as serious capacity fading and poor coulombic efficiency. The main reason is the dissolution of the cell reaction intermediates, soluble lithium polysulfides (Li2Sm) and the consequent “shuttle” effect. Modification of the electrolyte is one of the most important directions to overcome the problem. To prevent the Li2Sm from side-reacting and shuttle, the solvents with chemically stability and slow transfer of Li2Sm should be applied. Ionic liquid-based electrolyte is one of the most promising alternatives for LieS batteries for their chemical and thermal inertia [5e12]. Moreover, ether-based solvents have been widely used in LieS batteries as part of mixed electrolyte systems [13e17]. The combination of ionic liquid and ether solvents as LieS battery electrolyte can counterbalance the totally different physicochemical properties of the two components. It has been reported that electrolytes composed of ionic liquid and ether solvents such as dimethoxyethane and tetra(ethylene glycol) dimethyl ether show good cell performance [18e22]. Except for the “shuttling” problem, another challenge for the LieS battery is the lithium dendrite formation on the surface of the cycled lithium anode and the subsequent danger. The most widely used strategy to suppress the shuttle and lithium dendrite problems involve the formation of a passivation layer on the lithium metal anode [23e26]. For example, the addition of LiNO3 into the electrolyte was reported to be effective in preventing the redox shuttle reactions [27e32]. However, the strong oxidation of LiNO3 will increase the potential risks of the batteries and the formed surface film with inorganic species is devoid of tenacity which is hard to maintain stability during cycling. Recently, new additives such as lithium bis(oxalato) borate (LiBOB) was explored for the electrolyte in LieS batteries [33]. LiBOB is a salt facilitate the formation of an effective solid-electrolyte interphase (SEI) layer [34] which contributes to cycling stability of LieS batteries. However, its lower solubility in organic solvents [35] restricts the application in the electrolyte. Lithium tetrafluoroborate (LiBF4) has moderate ion conductivity in organic solvents, but its reactivity with lithium poses serious problem for LieS batteries [36]. Lithium difluoro(oxalate)borate (LiODFB) with half molecular moieties of LiBOB and LiBF4 possesses the combined advantages of the two lithium salts [37,38]. As a result, LiODFB with film-forming ability, good compatibility with the electrode and improved solubility in the organic solvents presents an interesting target for investigation [39]. In this work, we report a LieS battery electrolyte based on Nmethoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr1,2O1TFSI). The performance of the ionic liquid can be improved by cation functionalization [10]. The highly flexible alkoxy chains in etherfunctionalized ionic liquid provide the transport convenience for the adjacent molecules and hinder the crystallization [40], which can raise the conductivity and the Li2Sm solubility of the ionic liquid [41,42]. Tri(ethylene glycol)dimethyl ether (TEGDME) is used as the cosolvent in the electrolyte to further modulate the Li2Sm solubility and the conductivity of the electrolyte. The lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) and LiODFB were used as single lithium salt or part of binary lithium salts in the electrolyte. A series of Pyr1,2O1TFSI/TEGDME (7:3 in mass) electrolyte with binary lithium salts in different ratio were prepared. We characterized the electrochemical performance of the LieS cells containing the prepared electrolytes in order to determine the optimum ratio. The binary lithium salts synergetically create the optimum condition for passivation SEI formation which suppressed the “shuttle” phenomenon and protected the electrodes for LieS
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battery. Furthermore, the effect of LiODFB on the electrochemical behaviors of LieS cells was investigated to modify electrolyte/Li anode the interface. 2. Experimental 2.1. Materials To prepare the Pyr1,2O1TFSI/TEGDME composite electrolyte, Pyr1,2O1TFSI (99%, Shanghai Chengjie Chemical Co. LTD) and TEGDME (>99%, J&K Chemical) were mixed in mass ratio of 7:3 after dehydration for 12 h. The lithium salts, LiTFSI (99%, 3 M) and LiODFB (99%, Fosai), were added to the mixed electrolytes and stirred to dissolve. Confection plans for this study is listed in Table 1. All above operations were in the glove box under an argon atmosphere (H2O < 1 ppm, O2 <1 ppm). A simple melt-diffusion strategy [39] was used to prepare the multiple wall carbon nano tube (MWCNT, Shenzhen Nanoport, China)/S composite material. The S and MWCNT (1:1 in mass) were milled and mixed uniformly. The mixture powder was heated to 155 C for 24 h, and then cooled to room temperature to obtain the MWCNT/S material (S content of 50 wt%). 2.2. Preparation of electrodes and cells Cathodes used in the LieS electrochemical cells consisted of a mixture of 70 wt% MWCNT/S composite, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF). The mixture slurry with N-methyl-2-pyrrolidone (NMP) as the solvent stirred uniformly and then spread onto an aluminum foil current collector. After drying at 60 C under vacuum for 24 h, the MWCNT/S electrode was cut into pellets as the size of diameter 8 mm and the areal loading of about 4 mg cm2. The electrochemical cells were constructed with a MWCNT/S cathode, a lithium foil anode and the prepared electrolyte using a Celgard®2300 separator in the argon-filled glove box with oxygen and water contents less than 1 ppm. 2.3. Measurements and characterizations Cycling performance and coulombic efficiency of the LieS cells were carried out by the galvanostatic chargeedischarge test with the current density of 0.1C (160 mA g1) on a LAND CT2001A workstation (Wuhan, China). The cells charge and discharge cutoff voltages were set at 1.0 V and 3.0 V. The ionic conductivity of the electrolyte, AC impedance tests and cyclic voltammetry (CV) of the cells were performed on a CHI660D electrochemical workstation. Electrochemical impedance spectra (EIS) was used to measure the ionic conductivity and AC impedance with the AC amplitude of ±5 mV over the test frequency range of 105e10 Hz and 105e102 Hz, respectively. Electrochemical performances of the cells were recorded by CV curves at a scan rate of 0.1 mV s1 in the range of 1.0 V and 3.0 V to characterize the reversible behavior. The potentials throughout the paper are referenced to the Li/Liþ couple. The flammability test was taken by exposing the glass fiber containing the electrolyte to a butane flame. The self-extinguishing time of each sample was recorded for five times. The cycled electrodes were obtained by disassembling the cells after 5 cycles. The electrodes were washed with excess TEGDME and then dried under an argon atmosphere. Their surface morphology and chemical composition were characterized by scanning electron microscopy (SEM, HITACHI S-3500N, Japan) and energy dispersive spectrometry (EDS, HITACHI S-3500N, Japan). Xray photoelectron spectroscopy (XPS) measurements were employed to investigate the chemical element distribution on the surface of the cycled electrodes by means of an electron
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F. Wu et al. / Journal of Power Sources 296 (2015) 10e17 Table 1 Confection plans for preparing the series of ternary electrolytes. No.
Name
Composition
1 2 3 4 5 6 7 8 9
LiTFSI LiODFB 1 wt% 2 wt% 5 wt% 8:2 7:3 6:4 5:5
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
spectroscopy for chemical analysis (ESCA) spectrometer (PHI-1600, USA) with a monochromatic Al-Ka (1486.6 eV) source. The calculations were performed with the DMol3 module of the Materials Studio 5.5 program. The solvent molecule and radical structures were optimized and the possible reactions with energy change were calculated employing nonlocal density functional theory (DFT) with the BLYP functional based on a DNP group. 3. Results and discussion The chemical structures of components with the sizes in the electrolytes (along the longest expansion) are shown in Fig. 1. More detailed information on DFT calculation and the molecule size by other axis are exhibited in Table S1 and Fig. S1 in the Supporting information. The lithium salts, LiTFSI and LiODFB possessed sizes of 0.84 nm and 0.60 nm. The optimized Pyr1;2O1 þ cation and TFSI anion had structure sizes of 0.88 nm and 0.69 nm, respectively. The size of TEGDME along the longest expansion was 1.48 nm. The components with large sizes favor the “shuttle” suppression by slowing the Li2Sm diffusion [43]. Schematic illustration of the combination of the ionic liquid and the TEGDME as the composite electrolyte for LieS battery (Fig. 2(a)) reveals synergy which takes advantage of the two components leading to the acceptable electrochemical performance of the LieS cells. Fig. 2(b) shows the CV curves with a scan rate of 0.1 mV s1 for the LieS cells containing the Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI or LiODFB as lithium salts. The two reduction peaks in the CV curve of the cell with LiTFSI corresponded two steps for electrochemical reduction in LieS cells during discharge, which can be
mol mol mol mol mol mol mol mol mol
kg1 kg1 kg1 kg1 kg1 kg1 kg1 kg1 kg1
LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiODFB-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) þ 1 wt% LiODFB LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) þ 2 wt% LiODFB LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) þ 5 wt% LiODFB LiTFSI/LiODFB (8:2 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI/LiODFB (7:3 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI/LiODFB (6:4 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI/LiODFB (5:5 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w)
described as (1) from S8 to long chain Li2Sm at 2.2 Ve2.4 V and (2) from long chain Li2Sm to short chain Li2Sm below 2.0 V. The two reduction peaks for the Li2Sm formation shifted to the lower potentials than that of cells with organic electrolyte which evidenced the cell polarization behavior due to the high viscosity of the ionic liquid. However, the CV curve of the cell with LiODFB shows reduction/oxidation peaks with small area indicating the low utilization of the sulfur active material in the cells. The cycling performance and coulombic efficiency of the LieS cells containing the electrolyte with LiTFSI or LiODFB at 0.1C are shown in Fig. 2(c, d). The initial discharge capacity of the LieS cell containing LiTFSIPyr1,2O1TFSI/TEGDME was 1212.8 mAh g1 with the coulombic efficiency of 99.1% and retained a value of 896 mAh g1 after 50 cycles. The capacity fading and decrease of coulombic efficiency during cycling resulted from the cumulative loss of sulfur active material in the electrode. The cell with LiODFB-Pyr1,2O1TFSI/ TEGDME electrolyte exhibited undesired performance. One of the possible reasons is that LiODFB in the electrolyte continuously reductive decomposed during discharge process and the consequent products deposited on the surface of the electrodes forming the excessive passivation film, which hindered the electrochemical reduction of the sulfur active material. These results are consistent with the CV curves. The combination of the two lithium salts in the Pyr1,2O1TFSI/ TEGDME electrolyte were excepted to achieve good LieS cell performance. To suppress the shuttle phenomenon and stabilize the lithium surface, LiODFB with film-forming ability was introduced into the LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte system as additive with various amount. However, as shown in Fig. 3(a), the LieS cells
Fig. 1. Molecular structures and sizes of the electrolyte components: N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr1,2O1TFSI), tri(ethylene glycol)dimethyl ether (TEGDME), lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) and lithium difluoro(oxalate)borate (LiODFB).
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Fig. 2. (a) Schematic illustration of the Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI as lithium salt in LieS cell; (b) CVs of the LieS cells containing the Pyr1,2O1TFSI/TEGDME electrolytes with LiTFSI and LiODFB. Each cathode contains the MWCNT/S active material of about 1.41 mg; cycling performance (c) and coulombic efficiency (d) of the LieS cells with the LiTFSI-Pyr1,2O1TFSI/TEGDME and LiODFB-Pyr1,2O1TFSI/TEGDME electrolytes. The cycling constant current density is 0.1C.
Fig. 3. (a) Discharge capacities and coulombic efficiencies of the LieS cells containing LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte with various contents of LiODFB as additive at 0.1C; EIS graphs of the LieS cells as-prepared (b) and after 50 cycles (c) with the equivalent circuit diagram in the inset.
containing LiTFSI-Pyr1,2o1TFSI/TEGDME electrolyte with LiODFB as additive exhibited poor discharge capacities although their cycling stabilities were improved. LiODFB plays a part in improving the coulombic efficiency of the cells. The highest coulombic efficiency was achieved when the electrolyte contained 2 wt% LiODFB; the coulombic efficiency of the cell reached 98% after 10 cycles. However, the discharge capacities of the LieS cells with 1 wt%, 2 wt%
and 5 wt% LiODFB addition after 50 cycles were 744.9 mAh g1, 803.5 mAh g1 and 488.1 mAh g1, respectively. The reduction of the LieS cell capacity after LiODFB addition resulted from the overcrowding lithium salts which decouple the ion motion and limit the sulfur utilization of the cells. EIS graphs of the LieS cells containing electrolytes with and without LiODFB before and after 50 cycles are displayed in Fig. 3(b, c). As shown in the inset in
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Fig. 3(c), the LieS cell resistance contains bulk resistance (Rb) corresponding to the intercept of the semicircle at the highest frequency with the real axis, Liþ migration resistance (RSEI), charge transfer resistance (Rct) and Warburg resistance (W). The electrolyte containing LiODFB had higher viscosity, which caused larger Rb of the as-prepared cells. LiODFB facilitated SEI layer growth on the surface of electrodes leading to the larger RSEI of electrolyte with LiODFB The increasing Rct of the cells with LiODFB can be attributed to the suppression of ion motion due to the overcrowding lithium salts, and the high Rct of the cells with LiODFB is responsible for their inferior cycle performance. As a result, the ionic liquid-based electrolyte differing from the organic one with LiODFB as additive cannot afford satisfactory cycling performance for LieS battery, and the else approach to synergistically incorporate the two lithium salts should be investigated. The conductivity of the binary electrolyte containing different ionic liquids is far exceeding those of the single component materials. It was demonstrated that the improved conductivity and difficulty for the mixtures to crystallize result from the difference in size of the ions [44]. Because of difference in sizes of LiTFSI and LiODFB anions, we used the LiTFSI/LiODFB with a total molar concentration of 0.4 M as binary lithium salts in the Pyr1,2o1TFSI/ TEGDME mixture to prepare the new electrolyte systems. The CV curves of the cells with the electrolyte systems are illustrated in Fig. 4. LiODFB had an appreciable effect on the electrochemical behavior of the cells. With the increase of the LiODFB ratio in the binary lithium salts, the reduction peak corresponding to the long chain Li2Sm formation became larger and the peak area indicating the short chain Li2Sm formation decreased. The negligible reduction peak can be observed for the cell with LiTFSI/LiODFB binary lithium salts in the mole ratio of 5:5, which evidenced the thick passivation layer resulting from the excessive LiODFB in the electrolyte/electrode interface impeding the cell electrochemical reactions. Furthermore, the differences in the ionic conductivity among the electrolytes with binary lithium salts in various mole ratios are rather small as shown in the inset. All the electrolyte samples with binary lithium salts had the ionic conductivities above 3.5 mS cm1 at room temperature, which satisfies the demand of lithium battery application. Fig. 5 shows the cycling performances of the LieS cells containing the binary lithium salts-Pyr1,2O1TFSI/TEGDME electrolytes
Fig. 4. CV curves of the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolytes with LiTFSI/LiODFB binary lithium salts in different ratios. The CV scan rate is 0.1 mV s1. The inset shows the ionic conductivities of the electrolytes.
with various molar ratios of LiTFSI/LiODFB. In the electrolyte containing LiTFSI/LiODFB binary lithium salts with the optimum mole ratio of 6:4, the LieS cells show good cycling stability and coulombic efficiency. The LieS cells with 0.24 M LiTFSI/0.16 M LiODFB-Pyr1,2O1TFSI/TEGDME electrolyte exhibit initial discharge capacity of 1264.4 mAh g1 with the coulombic efficiency of 102.3%, and retain the discharge capacity of 824.3 mAh g1 with a coulombic efficiency of 94.7% after 100 cycles. The electrolyte with the binary lithium salts in the optimum ratio can facilitate the SEI film forming on the surface of the lithium anode to successfully operate the LieS cells. Besides, the high coulombic efficiency can be attributed to the decomposition of the electrolyte and the subsequent SEI formation. The decrease in the molar ratio of LiODFB added below 40% cannot be adequate to form the stable SEI; while the increase may thick the layer causing large cell resistance. The discharge/charge profiles of the LieS cells containing electrolytes with LiTFSI and binary lithium salts are shown in Fig. 5(d). Compared to the cells with LiTFSI, the profiles of the cells with LiTFSI/LiODFB in mole ratio of 6:4 exhibited higher reversible capacity and lower overpotentials between discharge and charge plateaus after 50 cycles, which is in good agreement with the cell cycling performance. Moreover, according to the flammability test, the self-extinguishing time of the electrolytes based on Pyr1,2O1TFSI/TEGDME is below 5 s g1, revealing good flame resistance. The ratios of the binary lithium salts take little effect on the flammability of the electrolyte systems (Table S2). Compared to the ether-based electrolyte, the lithium salts-Pyr1,2O1TFSI/TEGDME electrolyte is more promising for safe LieS batteries. To clarify the influence of the LiODFB on the electrochemical stability, the interfacial resistance of the Li/S cells with electrolytes with LiTFSI and the binary lithium salts at different voltages in the first discharge/charge process are shown in Fig. 6. The plateaus in the discharge/charge voltage vs capacity plots are influenced by the intermediate species. Upon discharging, the redox couple of a LieS cell is described by the reaction S8 þ 16Liþ þ 16e / 8Li2S (cathode) (Fig. 6(a, b)). It consists of two portions, the reactions from S to Li2Sm (4 m 8) at 2.15 Ve2.4 V, and the electrochemical reductions of Li2Sm (4 m 8) to Li2Sm (2 m 4) below 2.1 V. At the end of discharge in the LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte, the insulating Li2S generates and piles up on the surface of electrode leading to a dramatic increase in the cell resistance. Li2S is difficult to participate in further electrochemical reactions limiting sulfur utilization of the cells which results in low discharge capacity. The cells containing LiTFSI/LiODFB-Pyr1,2O1TFSI/TEGDME electrolyte exhibit the different process for cell resistance variation. The resistance increase at about 1.6 V is assumed corresponding to the reduction of LiODFB which facilitate SEI layer formation [45]. The conductive SEI layer prevents excessive accumulation of Li2S and make the cell resistances below 1.4 V retain small values. The reverse reaction occurs through decomposition of Li2Sm back to Li and S (Fig. 6(b, c)). The resistances of cells with LiTFSI/LiODFBPyr1,2O1TFSI/TEGDME electrolyte remain stable for the voltage below 2.2 V, indicating the smooth running of electrochemical reaction on the electrode-electrolyte interface. The morphology and chemical composition of the surface of the cycled MWCNT/S material cathodes were characterized by SEM and EDS analysis (Fig. 7). The film covering the cathode surface after cycling in electrolyte containing LiODFB and its lower sulfur content than in LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte validate the SEI formation. Combined investigations of DFT modeling, SEM and XPS analysis were used to reveal the surface morphology of SEI formation on the Li anodes. As seen in Fig. 8(a), the surface of the Li anode cycled in the electrolyte without LiODFB cracked and grown Li dendrites which may lead to capacity fading and safety problem; while the
F. Wu et al. / Journal of Power Sources 296 (2015) 10e17
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Fig. 5. Cycling performance (a, c) and coulombic efficiency (b) of the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI/LiODFB in various mole ratios at 0.1C; (d) discharge/charge profiles of the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI and LiTFSI/LiODFB in mole ratio of 6:4 for the first and 50th cycles.
Fig. 6. Resistance variation of the LieS cells at different voltages containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI or binary lithium salts (0.24 M LiTFSI/0.16 M LiODFB) during first discharging (a) and charging (c); (b) the intermediate species formed in the discharge/charge process.
binary lithium salts can overcome them. The smooth surface morphology (Fig. 8(b)) covered a compact and uniform film was observed on the surface of Li anode cycled in the LiTFSI/LiODFBPyr1,2O1TFSI/TEGDME electrolyte. The F, S elements distribution of the film forming in the modified electrolyte system to protect the Li anodes is listed in Fig. 8(c, d). The role of the fluorinated electrolyte systems for film forming were demonstrated in previous studies for both lithium-ion batteries [46] and LieS batteries [47e49]. In the F 1s pattern, peak at 688.2 eV attributes to LiyC2Fx, and the peak at 684.3 eV evidences LiF in SEI [50]. The S 2p peaks in 160e163 eV are corresponding to LieS indicating the presence of Li2S or Li2S2. The peaks in 166e170 eV corresponding to SeO are assigned with LixSOy [51], which may result from the SEI layer. Fig. 8(e) shows schematic illustration of the SEI layer formation in the electrolyte with LiODFB. To provide an additional support to rationalize the SEI forming, the reaction mechanism of the mixed electrolytes with binary lithium salts was studied by calculation. LiODFB facilitate the SEI formation by reduction of the F2B[ox] anions in discharging process at 1.6 V [45]. The equilibrium is F2 B ½ox þ e$ /½oxB$ þ 2F
with elimination of F which contributes to the formation of the inorganic inner layer of the SEI. The calculated reactions between the [ox]B radical and the solvent molecule with their energy changes are demonstrated in Fig. 8(f). The main terminal product according to the principle of lowest energy is C4H8O2BOCOCOOC2H4OCH3 with the molecular size of 1.17 nm adsorbing on the Li anode surface and taking part in the outer layer of SEI formation. However, the SEI formation is a highly complex process in LieS batteries which is still not fully understood because of too many factors. Here the influence of Li2Sm and ionic liquid that do exist were not taken into consideration and the corresponding search is in process. Overall, LiODFB can promote the SEI layer formation through reductive reactions, and the SEI layer prevents the shuttle effect and protects the Li anode in LieS batteries. 4. Conclusions The ionic liquid-based electrolyte systems with TEGDME as cosolvents were designed, and the electrochemical performance of the LieS cells with the electrolytes were investigated. The
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Fig. 7. SEM and EDS of the S electrodes in 0.4 M LiTFSI-Pyr1,2O1TFSI/TEGDME(a, c) and 0.24 M LiTFSI/0.16 M LiODFB-Pyr1,2O1TFSI/TEGDME(b, d) electrolytes after 50 cycles.
Fig. 8. SEM images of the Li anodes after cycling in the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI (a) or 0.24 M LiTFSI/0.16 M LiODFB (b); (c, d) XPS spectra (F 1s; S 2p) of the Li anodes surfaces in the binary lithium salts-Pyr1,2O1TFSI/TEGDME electrolyte after 50 cycles; (e) schematic illustration of the SEI layer formation in the electrolyte with LiODFB; (f) possible reactions of oxalatoboryl radical on the interface between the electrolyte containing LiODFB and Li anode in discharge process.
Pyr1,2O1TFSI and TEGDME synergistically afford acceptable capacity and coulombic efficiency for LieS batteries. Further improvement of LieS battery performance was achieved by using the LiTFSI/ LiODFB binary lithium salts. The LieS cells containing the Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI/LiODFB binary lithium salts in mole ratio of 6:4 possess excellent cycling performance whose initial discharge capacity is 1264.4 mAh g1 with the coulombic efficiency of 102.3%, and retains 911.4 mAh g1 with the coulombic efficiency of 96.8% after 50 cycles. Because LiODFB in the electrolyte can facilitate the SEI layer formation on the surface of the electrodes, it prevents the shuttle phenomenon and protects
the Li anode in the LieS batteries. The good cycling stability and high coulombic efficiency of the LieS cells suggest that the ionic liquid-based electrolyte with the binary lithium salts promising electrolyte for the LieS batteries. Acknowledgments This work was supported by the National Natural Science Foundation of China (21373028), National Key Program for Basic Research of China (2015CB251100), Major Achievements Transformation Project for Central University in Beijing, Beijing Science
F. Wu et al. / Journal of Power Sources 296 (2015) 10e17
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Ionic liquid-based electrolyte with binary lithium salts for high performance lithiumesulfur batteries Feng Wu a, b, Qizhen Zhu a, Renjie Chen a, b, *, Nan Chen a, Yan Chen a, Yusheng Ye a, Ji Qian a, Li Li a, b a b
School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We prepare an ionic liquid-based electrolyte with binary lithium salts for LieS batteries. Various molar ratios of electrolyte systems were thoroughly investigated. The novel electrolytes show excellent discharge capacity and cycling performance. The SEI-forming ability of LiODFB protect Li anode from suffering lithium dendrites.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2015 Received in revised form 26 June 2015 Accepted 10 July 2015 Available online 18 July 2015
Rechargeable LieS batteries have suffered several technical obstacles, such as rapid capacity fading and low coulombic efficiency. To overcome these problems, we design new electrolytes containing Nmethoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr1,2O1TFSI) and tri(ethylene glycol)dimethyl ether (TEGDME) in mass ratio of 7:3. Moreover, Lithium difluoro(oxalate)borate (LiODFB) is introduced for the modification. Although the addition of LiODFB as additive lead to extremely high viscosity of electrolyte and inferior performance of the cells, the electrolyte containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 0.84 nm) and LiODFB (0.60 nm) mixture with a total molar concentration of 0.4 mol kg1 as binary lithium salt shows excellent electrochemical performance. The Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI/LiODFB binary lithium salts in mole ratio of 6:4 is obtained after optimizing ratio. The LieS cells containing this electrolyte system show excellent capacity and cycle performance, whose initial discharge capacity is 1264.4 mAh g1, and retains 911.4 mAh g1 after 50 cycles with the coulombic efficiency more than 95%. It can be attributed the solid-electrolyte interphase (SEI)-forming ability of LiODFB which protect Li anode from suffering lithium dendrites and prevent the shuttle phenomenon. The novel electrolytes provide good cycling stability and high coulombic efficiency for the LieS batteries, which is suggested as a promising electrolyte for LieS batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Lithiumesulfur batteries Electrolyte Ionic liquid Binary lithium salts Solid-electrolyte interphase
1. Introduction * Corresponding author. School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail address: [email protected] (R. Chen). http://dx.doi.org/10.1016/j.jpowsour.2015.07.033 0378-7753/© 2015 Elsevier B.V. All rights reserved.
In recent years, the lithiumesulfur (LieS) battery has been focus of many recent scientific investigations as the new generation of
F. Wu et al. / Journal of Power Sources 296 (2015) 10e17
rechargeable electrochemical system due to the high theoretical capacity (1672 mAh g1) of its sulfur (S) cathode [1e4]. However, the LieS battery system may suffer from several technological problems, such as serious capacity fading and poor coulombic efficiency. The main reason is the dissolution of the cell reaction intermediates, soluble lithium polysulfides (Li2Sm) and the consequent “shuttle” effect. Modification of the electrolyte is one of the most important directions to overcome the problem. To prevent the Li2Sm from side-reacting and shuttle, the solvents with chemically stability and slow transfer of Li2Sm should be applied. Ionic liquid-based electrolyte is one of the most promising alternatives for LieS batteries for their chemical and thermal inertia [5e12]. Moreover, ether-based solvents have been widely used in LieS batteries as part of mixed electrolyte systems [13e17]. The combination of ionic liquid and ether solvents as LieS battery electrolyte can counterbalance the totally different physicochemical properties of the two components. It has been reported that electrolytes composed of ionic liquid and ether solvents such as dimethoxyethane and tetra(ethylene glycol) dimethyl ether show good cell performance [18e22]. Except for the “shuttling” problem, another challenge for the LieS battery is the lithium dendrite formation on the surface of the cycled lithium anode and the subsequent danger. The most widely used strategy to suppress the shuttle and lithium dendrite problems involve the formation of a passivation layer on the lithium metal anode [23e26]. For example, the addition of LiNO3 into the electrolyte was reported to be effective in preventing the redox shuttle reactions [27e32]. However, the strong oxidation of LiNO3 will increase the potential risks of the batteries and the formed surface film with inorganic species is devoid of tenacity which is hard to maintain stability during cycling. Recently, new additives such as lithium bis(oxalato) borate (LiBOB) was explored for the electrolyte in LieS batteries [33]. LiBOB is a salt facilitate the formation of an effective solid-electrolyte interphase (SEI) layer [34] which contributes to cycling stability of LieS batteries. However, its lower solubility in organic solvents [35] restricts the application in the electrolyte. Lithium tetrafluoroborate (LiBF4) has moderate ion conductivity in organic solvents, but its reactivity with lithium poses serious problem for LieS batteries [36]. Lithium difluoro(oxalate)borate (LiODFB) with half molecular moieties of LiBOB and LiBF4 possesses the combined advantages of the two lithium salts [37,38]. As a result, LiODFB with film-forming ability, good compatibility with the electrode and improved solubility in the organic solvents presents an interesting target for investigation [39]. In this work, we report a LieS battery electrolyte based on Nmethoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr1,2O1TFSI). The performance of the ionic liquid can be improved by cation functionalization [10]. The highly flexible alkoxy chains in etherfunctionalized ionic liquid provide the transport convenience for the adjacent molecules and hinder the crystallization [40], which can raise the conductivity and the Li2Sm solubility of the ionic liquid [41,42]. Tri(ethylene glycol)dimethyl ether (TEGDME) is used as the cosolvent in the electrolyte to further modulate the Li2Sm solubility and the conductivity of the electrolyte. The lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) and LiODFB were used as single lithium salt or part of binary lithium salts in the electrolyte. A series of Pyr1,2O1TFSI/TEGDME (7:3 in mass) electrolyte with binary lithium salts in different ratio were prepared. We characterized the electrochemical performance of the LieS cells containing the prepared electrolytes in order to determine the optimum ratio. The binary lithium salts synergetically create the optimum condition for passivation SEI formation which suppressed the “shuttle” phenomenon and protected the electrodes for LieS
11
battery. Furthermore, the effect of LiODFB on the electrochemical behaviors of LieS cells was investigated to modify electrolyte/Li anode the interface. 2. Experimental 2.1. Materials To prepare the Pyr1,2O1TFSI/TEGDME composite electrolyte, Pyr1,2O1TFSI (99%, Shanghai Chengjie Chemical Co. LTD) and TEGDME (>99%, J&K Chemical) were mixed in mass ratio of 7:3 after dehydration for 12 h. The lithium salts, LiTFSI (99%, 3 M) and LiODFB (99%, Fosai), were added to the mixed electrolytes and stirred to dissolve. Confection plans for this study is listed in Table 1. All above operations were in the glove box under an argon atmosphere (H2O < 1 ppm, O2 <1 ppm). A simple melt-diffusion strategy [39] was used to prepare the multiple wall carbon nano tube (MWCNT, Shenzhen Nanoport, China)/S composite material. The S and MWCNT (1:1 in mass) were milled and mixed uniformly. The mixture powder was heated to 155 C for 24 h, and then cooled to room temperature to obtain the MWCNT/S material (S content of 50 wt%). 2.2. Preparation of electrodes and cells Cathodes used in the LieS electrochemical cells consisted of a mixture of 70 wt% MWCNT/S composite, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF). The mixture slurry with N-methyl-2-pyrrolidone (NMP) as the solvent stirred uniformly and then spread onto an aluminum foil current collector. After drying at 60 C under vacuum for 24 h, the MWCNT/S electrode was cut into pellets as the size of diameter 8 mm and the areal loading of about 4 mg cm2. The electrochemical cells were constructed with a MWCNT/S cathode, a lithium foil anode and the prepared electrolyte using a Celgard®2300 separator in the argon-filled glove box with oxygen and water contents less than 1 ppm. 2.3. Measurements and characterizations Cycling performance and coulombic efficiency of the LieS cells were carried out by the galvanostatic chargeedischarge test with the current density of 0.1C (160 mA g1) on a LAND CT2001A workstation (Wuhan, China). The cells charge and discharge cutoff voltages were set at 1.0 V and 3.0 V. The ionic conductivity of the electrolyte, AC impedance tests and cyclic voltammetry (CV) of the cells were performed on a CHI660D electrochemical workstation. Electrochemical impedance spectra (EIS) was used to measure the ionic conductivity and AC impedance with the AC amplitude of ±5 mV over the test frequency range of 105e10 Hz and 105e102 Hz, respectively. Electrochemical performances of the cells were recorded by CV curves at a scan rate of 0.1 mV s1 in the range of 1.0 V and 3.0 V to characterize the reversible behavior. The potentials throughout the paper are referenced to the Li/Liþ couple. The flammability test was taken by exposing the glass fiber containing the electrolyte to a butane flame. The self-extinguishing time of each sample was recorded for five times. The cycled electrodes were obtained by disassembling the cells after 5 cycles. The electrodes were washed with excess TEGDME and then dried under an argon atmosphere. Their surface morphology and chemical composition were characterized by scanning electron microscopy (SEM, HITACHI S-3500N, Japan) and energy dispersive spectrometry (EDS, HITACHI S-3500N, Japan). Xray photoelectron spectroscopy (XPS) measurements were employed to investigate the chemical element distribution on the surface of the cycled electrodes by means of an electron
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F. Wu et al. / Journal of Power Sources 296 (2015) 10e17 Table 1 Confection plans for preparing the series of ternary electrolytes. No.
Name
Composition
1 2 3 4 5 6 7 8 9
LiTFSI LiODFB 1 wt% 2 wt% 5 wt% 8:2 7:3 6:4 5:5
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
spectroscopy for chemical analysis (ESCA) spectrometer (PHI-1600, USA) with a monochromatic Al-Ka (1486.6 eV) source. The calculations were performed with the DMol3 module of the Materials Studio 5.5 program. The solvent molecule and radical structures were optimized and the possible reactions with energy change were calculated employing nonlocal density functional theory (DFT) with the BLYP functional based on a DNP group. 3. Results and discussion The chemical structures of components with the sizes in the electrolytes (along the longest expansion) are shown in Fig. 1. More detailed information on DFT calculation and the molecule size by other axis are exhibited in Table S1 and Fig. S1 in the Supporting information. The lithium salts, LiTFSI and LiODFB possessed sizes of 0.84 nm and 0.60 nm. The optimized Pyr1;2O1 þ cation and TFSI anion had structure sizes of 0.88 nm and 0.69 nm, respectively. The size of TEGDME along the longest expansion was 1.48 nm. The components with large sizes favor the “shuttle” suppression by slowing the Li2Sm diffusion [43]. Schematic illustration of the combination of the ionic liquid and the TEGDME as the composite electrolyte for LieS battery (Fig. 2(a)) reveals synergy which takes advantage of the two components leading to the acceptable electrochemical performance of the LieS cells. Fig. 2(b) shows the CV curves with a scan rate of 0.1 mV s1 for the LieS cells containing the Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI or LiODFB as lithium salts. The two reduction peaks in the CV curve of the cell with LiTFSI corresponded two steps for electrochemical reduction in LieS cells during discharge, which can be
mol mol mol mol mol mol mol mol mol
kg1 kg1 kg1 kg1 kg1 kg1 kg1 kg1 kg1
LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiODFB-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) þ 1 wt% LiODFB LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) þ 2 wt% LiODFB LiTFSI-Pyr1,2O1TFSI/TEGDME (7/3, w/w) þ 5 wt% LiODFB LiTFSI/LiODFB (8:2 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI/LiODFB (7:3 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI/LiODFB (6:4 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w) LiTFSI/LiODFB (5:5 in mole)-Pyr1,2O1TFSI/TEGDME (7/3, w/w)
described as (1) from S8 to long chain Li2Sm at 2.2 Ve2.4 V and (2) from long chain Li2Sm to short chain Li2Sm below 2.0 V. The two reduction peaks for the Li2Sm formation shifted to the lower potentials than that of cells with organic electrolyte which evidenced the cell polarization behavior due to the high viscosity of the ionic liquid. However, the CV curve of the cell with LiODFB shows reduction/oxidation peaks with small area indicating the low utilization of the sulfur active material in the cells. The cycling performance and coulombic efficiency of the LieS cells containing the electrolyte with LiTFSI or LiODFB at 0.1C are shown in Fig. 2(c, d). The initial discharge capacity of the LieS cell containing LiTFSIPyr1,2O1TFSI/TEGDME was 1212.8 mAh g1 with the coulombic efficiency of 99.1% and retained a value of 896 mAh g1 after 50 cycles. The capacity fading and decrease of coulombic efficiency during cycling resulted from the cumulative loss of sulfur active material in the electrode. The cell with LiODFB-Pyr1,2O1TFSI/ TEGDME electrolyte exhibited undesired performance. One of the possible reasons is that LiODFB in the electrolyte continuously reductive decomposed during discharge process and the consequent products deposited on the surface of the electrodes forming the excessive passivation film, which hindered the electrochemical reduction of the sulfur active material. These results are consistent with the CV curves. The combination of the two lithium salts in the Pyr1,2O1TFSI/ TEGDME electrolyte were excepted to achieve good LieS cell performance. To suppress the shuttle phenomenon and stabilize the lithium surface, LiODFB with film-forming ability was introduced into the LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte system as additive with various amount. However, as shown in Fig. 3(a), the LieS cells
Fig. 1. Molecular structures and sizes of the electrolyte components: N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr1,2O1TFSI), tri(ethylene glycol)dimethyl ether (TEGDME), lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) and lithium difluoro(oxalate)borate (LiODFB).
F. Wu et al. / Journal of Power Sources 296 (2015) 10e17
13
Fig. 2. (a) Schematic illustration of the Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI as lithium salt in LieS cell; (b) CVs of the LieS cells containing the Pyr1,2O1TFSI/TEGDME electrolytes with LiTFSI and LiODFB. Each cathode contains the MWCNT/S active material of about 1.41 mg; cycling performance (c) and coulombic efficiency (d) of the LieS cells with the LiTFSI-Pyr1,2O1TFSI/TEGDME and LiODFB-Pyr1,2O1TFSI/TEGDME electrolytes. The cycling constant current density is 0.1C.
Fig. 3. (a) Discharge capacities and coulombic efficiencies of the LieS cells containing LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte with various contents of LiODFB as additive at 0.1C; EIS graphs of the LieS cells as-prepared (b) and after 50 cycles (c) with the equivalent circuit diagram in the inset.
containing LiTFSI-Pyr1,2o1TFSI/TEGDME electrolyte with LiODFB as additive exhibited poor discharge capacities although their cycling stabilities were improved. LiODFB plays a part in improving the coulombic efficiency of the cells. The highest coulombic efficiency was achieved when the electrolyte contained 2 wt% LiODFB; the coulombic efficiency of the cell reached 98% after 10 cycles. However, the discharge capacities of the LieS cells with 1 wt%, 2 wt%
and 5 wt% LiODFB addition after 50 cycles were 744.9 mAh g1, 803.5 mAh g1 and 488.1 mAh g1, respectively. The reduction of the LieS cell capacity after LiODFB addition resulted from the overcrowding lithium salts which decouple the ion motion and limit the sulfur utilization of the cells. EIS graphs of the LieS cells containing electrolytes with and without LiODFB before and after 50 cycles are displayed in Fig. 3(b, c). As shown in the inset in
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F. Wu et al. / Journal of Power Sources 296 (2015) 10e17
Fig. 3(c), the LieS cell resistance contains bulk resistance (Rb) corresponding to the intercept of the semicircle at the highest frequency with the real axis, Liþ migration resistance (RSEI), charge transfer resistance (Rct) and Warburg resistance (W). The electrolyte containing LiODFB had higher viscosity, which caused larger Rb of the as-prepared cells. LiODFB facilitated SEI layer growth on the surface of electrodes leading to the larger RSEI of electrolyte with LiODFB The increasing Rct of the cells with LiODFB can be attributed to the suppression of ion motion due to the overcrowding lithium salts, and the high Rct of the cells with LiODFB is responsible for their inferior cycle performance. As a result, the ionic liquid-based electrolyte differing from the organic one with LiODFB as additive cannot afford satisfactory cycling performance for LieS battery, and the else approach to synergistically incorporate the two lithium salts should be investigated. The conductivity of the binary electrolyte containing different ionic liquids is far exceeding those of the single component materials. It was demonstrated that the improved conductivity and difficulty for the mixtures to crystallize result from the difference in size of the ions [44]. Because of difference in sizes of LiTFSI and LiODFB anions, we used the LiTFSI/LiODFB with a total molar concentration of 0.4 M as binary lithium salts in the Pyr1,2o1TFSI/ TEGDME mixture to prepare the new electrolyte systems. The CV curves of the cells with the electrolyte systems are illustrated in Fig. 4. LiODFB had an appreciable effect on the electrochemical behavior of the cells. With the increase of the LiODFB ratio in the binary lithium salts, the reduction peak corresponding to the long chain Li2Sm formation became larger and the peak area indicating the short chain Li2Sm formation decreased. The negligible reduction peak can be observed for the cell with LiTFSI/LiODFB binary lithium salts in the mole ratio of 5:5, which evidenced the thick passivation layer resulting from the excessive LiODFB in the electrolyte/electrode interface impeding the cell electrochemical reactions. Furthermore, the differences in the ionic conductivity among the electrolytes with binary lithium salts in various mole ratios are rather small as shown in the inset. All the electrolyte samples with binary lithium salts had the ionic conductivities above 3.5 mS cm1 at room temperature, which satisfies the demand of lithium battery application. Fig. 5 shows the cycling performances of the LieS cells containing the binary lithium salts-Pyr1,2O1TFSI/TEGDME electrolytes
Fig. 4. CV curves of the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolytes with LiTFSI/LiODFB binary lithium salts in different ratios. The CV scan rate is 0.1 mV s1. The inset shows the ionic conductivities of the electrolytes.
with various molar ratios of LiTFSI/LiODFB. In the electrolyte containing LiTFSI/LiODFB binary lithium salts with the optimum mole ratio of 6:4, the LieS cells show good cycling stability and coulombic efficiency. The LieS cells with 0.24 M LiTFSI/0.16 M LiODFB-Pyr1,2O1TFSI/TEGDME electrolyte exhibit initial discharge capacity of 1264.4 mAh g1 with the coulombic efficiency of 102.3%, and retain the discharge capacity of 824.3 mAh g1 with a coulombic efficiency of 94.7% after 100 cycles. The electrolyte with the binary lithium salts in the optimum ratio can facilitate the SEI film forming on the surface of the lithium anode to successfully operate the LieS cells. Besides, the high coulombic efficiency can be attributed to the decomposition of the electrolyte and the subsequent SEI formation. The decrease in the molar ratio of LiODFB added below 40% cannot be adequate to form the stable SEI; while the increase may thick the layer causing large cell resistance. The discharge/charge profiles of the LieS cells containing electrolytes with LiTFSI and binary lithium salts are shown in Fig. 5(d). Compared to the cells with LiTFSI, the profiles of the cells with LiTFSI/LiODFB in mole ratio of 6:4 exhibited higher reversible capacity and lower overpotentials between discharge and charge plateaus after 50 cycles, which is in good agreement with the cell cycling performance. Moreover, according to the flammability test, the self-extinguishing time of the electrolytes based on Pyr1,2O1TFSI/TEGDME is below 5 s g1, revealing good flame resistance. The ratios of the binary lithium salts take little effect on the flammability of the electrolyte systems (Table S2). Compared to the ether-based electrolyte, the lithium salts-Pyr1,2O1TFSI/TEGDME electrolyte is more promising for safe LieS batteries. To clarify the influence of the LiODFB on the electrochemical stability, the interfacial resistance of the Li/S cells with electrolytes with LiTFSI and the binary lithium salts at different voltages in the first discharge/charge process are shown in Fig. 6. The plateaus in the discharge/charge voltage vs capacity plots are influenced by the intermediate species. Upon discharging, the redox couple of a LieS cell is described by the reaction S8 þ 16Liþ þ 16e / 8Li2S (cathode) (Fig. 6(a, b)). It consists of two portions, the reactions from S to Li2Sm (4 m 8) at 2.15 Ve2.4 V, and the electrochemical reductions of Li2Sm (4 m 8) to Li2Sm (2 m 4) below 2.1 V. At the end of discharge in the LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte, the insulating Li2S generates and piles up on the surface of electrode leading to a dramatic increase in the cell resistance. Li2S is difficult to participate in further electrochemical reactions limiting sulfur utilization of the cells which results in low discharge capacity. The cells containing LiTFSI/LiODFB-Pyr1,2O1TFSI/TEGDME electrolyte exhibit the different process for cell resistance variation. The resistance increase at about 1.6 V is assumed corresponding to the reduction of LiODFB which facilitate SEI layer formation [45]. The conductive SEI layer prevents excessive accumulation of Li2S and make the cell resistances below 1.4 V retain small values. The reverse reaction occurs through decomposition of Li2Sm back to Li and S (Fig. 6(b, c)). The resistances of cells with LiTFSI/LiODFBPyr1,2O1TFSI/TEGDME electrolyte remain stable for the voltage below 2.2 V, indicating the smooth running of electrochemical reaction on the electrode-electrolyte interface. The morphology and chemical composition of the surface of the cycled MWCNT/S material cathodes were characterized by SEM and EDS analysis (Fig. 7). The film covering the cathode surface after cycling in electrolyte containing LiODFB and its lower sulfur content than in LiTFSI-Pyr1,2O1TFSI/TEGDME electrolyte validate the SEI formation. Combined investigations of DFT modeling, SEM and XPS analysis were used to reveal the surface morphology of SEI formation on the Li anodes. As seen in Fig. 8(a), the surface of the Li anode cycled in the electrolyte without LiODFB cracked and grown Li dendrites which may lead to capacity fading and safety problem; while the
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Fig. 5. Cycling performance (a, c) and coulombic efficiency (b) of the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI/LiODFB in various mole ratios at 0.1C; (d) discharge/charge profiles of the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI and LiTFSI/LiODFB in mole ratio of 6:4 for the first and 50th cycles.
Fig. 6. Resistance variation of the LieS cells at different voltages containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI or binary lithium salts (0.24 M LiTFSI/0.16 M LiODFB) during first discharging (a) and charging (c); (b) the intermediate species formed in the discharge/charge process.
binary lithium salts can overcome them. The smooth surface morphology (Fig. 8(b)) covered a compact and uniform film was observed on the surface of Li anode cycled in the LiTFSI/LiODFBPyr1,2O1TFSI/TEGDME electrolyte. The F, S elements distribution of the film forming in the modified electrolyte system to protect the Li anodes is listed in Fig. 8(c, d). The role of the fluorinated electrolyte systems for film forming were demonstrated in previous studies for both lithium-ion batteries [46] and LieS batteries [47e49]. In the F 1s pattern, peak at 688.2 eV attributes to LiyC2Fx, and the peak at 684.3 eV evidences LiF in SEI [50]. The S 2p peaks in 160e163 eV are corresponding to LieS indicating the presence of Li2S or Li2S2. The peaks in 166e170 eV corresponding to SeO are assigned with LixSOy [51], which may result from the SEI layer. Fig. 8(e) shows schematic illustration of the SEI layer formation in the electrolyte with LiODFB. To provide an additional support to rationalize the SEI forming, the reaction mechanism of the mixed electrolytes with binary lithium salts was studied by calculation. LiODFB facilitate the SEI formation by reduction of the F2B[ox] anions in discharging process at 1.6 V [45]. The equilibrium is F2 B ½ox þ e$ /½oxB$ þ 2F
with elimination of F which contributes to the formation of the inorganic inner layer of the SEI. The calculated reactions between the [ox]B radical and the solvent molecule with their energy changes are demonstrated in Fig. 8(f). The main terminal product according to the principle of lowest energy is C4H8O2BOCOCOOC2H4OCH3 with the molecular size of 1.17 nm adsorbing on the Li anode surface and taking part in the outer layer of SEI formation. However, the SEI formation is a highly complex process in LieS batteries which is still not fully understood because of too many factors. Here the influence of Li2Sm and ionic liquid that do exist were not taken into consideration and the corresponding search is in process. Overall, LiODFB can promote the SEI layer formation through reductive reactions, and the SEI layer prevents the shuttle effect and protects the Li anode in LieS batteries. 4. Conclusions The ionic liquid-based electrolyte systems with TEGDME as cosolvents were designed, and the electrochemical performance of the LieS cells with the electrolytes were investigated. The
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Fig. 7. SEM and EDS of the S electrodes in 0.4 M LiTFSI-Pyr1,2O1TFSI/TEGDME(a, c) and 0.24 M LiTFSI/0.16 M LiODFB-Pyr1,2O1TFSI/TEGDME(b, d) electrolytes after 50 cycles.
Fig. 8. SEM images of the Li anodes after cycling in the LieS cells containing Pyr1,2O1TFSI/TEGDME electrolyte with 0.4 M LiTFSI (a) or 0.24 M LiTFSI/0.16 M LiODFB (b); (c, d) XPS spectra (F 1s; S 2p) of the Li anodes surfaces in the binary lithium salts-Pyr1,2O1TFSI/TEGDME electrolyte after 50 cycles; (e) schematic illustration of the SEI layer formation in the electrolyte with LiODFB; (f) possible reactions of oxalatoboryl radical on the interface between the electrolyte containing LiODFB and Li anode in discharge process.
Pyr1,2O1TFSI and TEGDME synergistically afford acceptable capacity and coulombic efficiency for LieS batteries. Further improvement of LieS battery performance was achieved by using the LiTFSI/ LiODFB binary lithium salts. The LieS cells containing the Pyr1,2O1TFSI/TEGDME electrolyte with LiTFSI/LiODFB binary lithium salts in mole ratio of 6:4 possess excellent cycling performance whose initial discharge capacity is 1264.4 mAh g1 with the coulombic efficiency of 102.3%, and retains 911.4 mAh g1 with the coulombic efficiency of 96.8% after 50 cycles. Because LiODFB in the electrolyte can facilitate the SEI layer formation on the surface of the electrodes, it prevents the shuttle phenomenon and protects
the Li anode in the LieS batteries. The good cycling stability and high coulombic efficiency of the LieS cells suggest that the ionic liquid-based electrolyte with the binary lithium salts promising electrolyte for the LieS batteries. Acknowledgments This work was supported by the National Natural Science Foundation of China (21373028), National Key Program for Basic Research of China (2015CB251100), Major Achievements Transformation Project for Central University in Beijing, Beijing Science
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