New Desolvated Gel Electrolyte for Rechargeable Lithium Metal

Dec 1, 2014 - A specific capacity of 1276 mAh/g is retained under the DGE after 50 cycles at 60 mA/g current rate. It is indicated that the as-prepare...
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New Desolvated Gel Electrolyte for Rechargeable Lithium Metal Sulfurized Polyacrylonitrile (S-PAN) Battery Borong Wu,†,‡ Qi Liu,† Daobin Mu,*,†,‡ Yonghuan Ren,† Yu Li,† Lei Wang,† Hongliang Xu,† and Feng Wu†,‡ †

Beijing Key Laboratory of Environment Science and Engineering, School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China ‡ National Development Center of Hi-Tech Green Materials, Beijing, 100081, China S Supporting Information *

ABSTRACT: A new desolvated gel electrolyte (DGE) is investigated for its use in lithium metal sulfurized polyacrylonitrile (S-PAN) battery. Lithium dendrite growth is examined under the DGE by scanning electron microscope (SEM). The electrolyte desolvation is analyzed with IR and 1H NMR spectra as well as density functional theory (DFT) calculation using Gaussian09 package. The electrochemical performance of S-PAN cathode is compared under the DGE and a common electrolyte via galvanostatic charge/discharge. The growth mode of Li dendrite is schematically illustrated to elucidate the role of the DGE during the charge/discharge process. It is shown that the DGE can prevent the growth of dendrite from the Li anode surface. A specific capacity of 1276 mAh/g is retained under the DGE after 50 cycles at 60 mA/g current rate. It is indicated that the as-prepared gel electrolyte is desolvated, which is also confirmed with the theoretical calculation. The DGE weakens the solvation effect of the lithium ions and reduces the resistance of charge transfer at cathode/electrolyte interface; it increases lithium ion transference number as well, so enhancing the electrochemical performance of the cathode.

1. INTRODUCTION With the consumption of fossil fuels and the increasing demand for energy, human beings have no alternative but to develop renewable energy sources.1 Lithium-ion batteries have been used to store renewable energy for electrical appliances and vehicles.2,3 However, conventional lithium-ion battery has a limited energy density less than 300 mAh/g, which is far away from the requirements for high energy electrified transportations.4 Alternatively, the Li−S battery is receiving increasingly attention as a promising device for next-generation energy storage because its energy density is 5 times greater than that of conventional ones.5,6 The typical Li−S battery uses sulfur/carbon composite as cathode and metal Li as anode.7 The cathode has high theoretical specific capacity and energy density, which can deliver 1672 mAh/g and 2600 Wh/kg, respectively.8 Furthermore, lithium metal anode exhibits an extremely high theoretical specific capacity (3860 mAh/g), and it has low density (0.59 g cm−3), and the lowest negative potential (−3.040 V vs standard hydrogen electrode).9 The main challenges to the Li−S battery are the electrical insulation of sulfur and the dissolution of polysulfide as well as Li dendrites on the anode.10 The electrical insulation of sulfur has been fundamentally overcome by mixing with conductive additives such as kinds of carbons. However, the discharge intermediate products dissolve in organic electrolyte and © XXXX American Chemical Society

migrate from the cathode to the anode, leading to rapid capacity degradation. Also, with the growth of Li dendrite during the process of charge, it may pierce the membrane causing a short circuit. As a result, the battery is prone to suffer from safety issues.11−13 As for the cathode of the Li−S battery, much attention is paid to various kinds of sulfur/carbon composites,6,14−16 such as sulfur compounds with porous carbons or graphenes, carbon nanotubes, and so forth. Thereby, conductivity of the sulfur/ carbon composite is enhanced, and dissolution of the polysulfide in the electrolyte is restricted. In addition, the kinds of polymer matrices have also been studied as the host to sulfur.17−19 The polymer reacts with sulfur at high temperature, forming sulfurized polymer to inhibit the dissolution of intermediate products during the charge−discharge process. In order to solve the Li dendrite problem, polymer electrolytes and superionic conductors with high ionic conductivity, such as Li3PS4 and Li10GeP2S12, have been used in all-solid lithium batteries to improve the safety of the high-energy batteries.20−30 Besides, additives have been adopted to prevent the dendrite growth on the lithium metal anode.30−34 However, these Received: July 31, 2014 Revised: November 20, 2014

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parameters (B3) exchange functional along with the Lee− Yang−Parr (LYP) nonlocal correlation functional (B3LYP). All complexes were treated at the B3LYP/6-31G(d,p) level for full geometry optimization. Natural bond orbital (NBO) analysis were performed at the B3LYP/6-31G(d,p) level using the NBO program to obtain a quantitative result. All of the DFT and NBO calculations were performed using the Gaussian 09 program package.35 The desolvation of lithium ions was investigated by IR and 1H NMR spectra.

problems have not been substantially overcome yet, such as in Li dendrite growth and cycle failure, etc., which limits the application of the battery, and there are still many opportunity to modify the Li−S battery system from various aspects. This work presents a new desolvated gel electrolyte (DGE) by controlling Li deposit morphology to suppress its dendrite growth. It is also expected to retard the dissolution of intermediates so as to maintain the capacity of the cathode. In addition, the desolvation may lower charge transfer resistance enhancing electrochemical performance. Herein, the DGE was examined in detail, the ability of DGE to control Li dendrites was characterized, the electrochemical performance of S-PAN cathode under the DGE was investigated, and the role of the electrolyte in the modification was analyzed as well.

3. RESULTS AND DISCUSSION 3.1. Ionic Conductivity and Electrochemical Window of the DGE. Figure 1 shows the ionic conductivity of the DGE

2. EXPERIMENTAL SECTION 2.1. Preparation of DGE and Common Electrolyte. Equal quantities of EC and DMC were mixed in an Isotope flask, and then LiPF6 was slowly added to the flask under stirring until the solution changed to a gel, which was named DGE. The ratio of LiPF6 to (EC+DMC) equaled 0.72 in mass. The operation was performed in a glovebox filled with argon. SPAN material was provided by Tsinghua University, the content of sulfur is 41 wt %, and the preparation details can be seen in refs 17,18. In order to compare the experiment data, the electrolyte of 1 M LiPF6 and equal quantities of EC and DMC is named the common electrolyte. The method of preparation is similar to that of DGE. 2.2. Li Deposition and Preparation of S-PAN Composite Electrode. Li was deposited on Cu foil substrate making use of a coin cell of Cu/electrolyte/Li under constant current density of 0.1 mA cm−2. DGE and common electrolyte were used in the cell, respectively. The deposition electricity is 10 C cm−2 and diameter = 1 cm. After deposition, the cell was disassembled, and the Cu foil was washed with DMC to remove the residual electrolyte and dried under vacuum. Li was also deposited on the Li foil substrate at a diameter of 1 cm following a similar process as above. The current density was 0.1 mA cm−2, and the deposition electricity was 200 C cm−2. S-PAN electrode was prepared by mixing the powders of SPAN composite, super-P, and polyacrylonitrile (6:3:1 by weight) in N,N-dimethylformamide to form a viscous slurry. The slurry was pasted onto an Al foil current collector, and then the electrode was dried at 80 °C for 24 h in an oven. The loading of active mass sulfur is 1.52 mg cm−2. 2.3. Characterization. Ionic conductivities of the DGE and the common electrolyte were measured by electrochemical impedance spectroscopy. The impedance of the Pt/DGE/Pt cell was tested at a range of −20 to 60 °C in an Ar atmosphere. The deposits were analyzed by scanning electron microscope (SEM) with an accelerating voltage of 20.00 kV. An argon-filled valve bag was used to transfer the deposited sample into an SEM chamber to avoid air contamination. The electrochemical stability of the DGE was evaluated by cyclic voltammetry performed on a Li/DGE/stainless steel cell at a scan rate of 1 mV s−1; the scan range is −0.5 to 5 V. Charge−discharge measurements of the lithium metal S-PAN cell were conducted on battery testing system (BT2000). The test was carried out within the 1−3 V range at room temperature. The impedance plot of the S-PAN/electrolyte/Li cell was recorded from 100 kHz to 10 mHz. Quantum chemical calculations were completed through the DFT method with Becke’s three

Figure 1. (a) Arrhenius ionic conductivity plots of DGE and (b, c, d) impedance plots data from high to low temperatures; (c) is the enlargement of (d).

and the impedance plots (see Figure 1b). Ionic conductivity is measured according to the sum of the interface impedance and the ohmic resistance. The viscosity of DGE increases when the temperature decreases, which depresses the flowability of the gel electrolyte. Meanwhile, the interface impedance and ohmic resistance increase. When the temperature drops to −20 °C, the DGE is almost transformed into a solid state, leading to a sharp increase of the impedance (see Figure 1d). Figure 1c is the enlargement of Figure1d. The plot of Figure 1a basically conforms to the Arrhenius equation23 ⎛ E ⎞ σ = σ0 exp⎜ − a ⎟ ⎝ RT ⎠

(1)

where Ea is the activation energy, σ0 is the pre-exponential factor, and R is the universal gas constant. The result of Figure 1a deviates from the Arrhenius equation from 0 °C to −20 °C, which might be caused by the fast phase transition at low temperature. From Figure 1a, the ionic conductivity of the DGE is 8.84 × 10−4 S cm−1 at 25 °C, lower than the value of the common electrolyte which can be 10−3 S cm−1 at room temperature.36 However, the lithium ion transference number of DGE is found to be 0.69, which is higher than the value (0.34) of the common electrolyte. The details of the measurement can be found in section 1 of the Supporting Information. It is generally acknowledged that the lithium ion transference number increases while electrolyte salinity increases according to Zugmann.37 It is of importance to the B

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electrochemical performance of lithium ion battery.38 Its increase may enhance the rate performance of the cathode and offset the influence of a small decrease in the ionic conductivity of the DGE. Figure 2 exhibits the current−potential curve of Li/DGE/ stainless steel cell with a lithium reference electrode. During the

electrolyte within the same deposition time (see Figure 3b,e). The SEM morphology of the Li deposition cross section also exhibits that DGE gives the advantage of controlling the growth of the Li dendrites (see Supporting Information Figure S1). It may be caused by space charges emerging on the surface of the electrode.41,42 Anions and cations uniformly distribute in the electrolyte if uncharged. When the battery is charged, the cations rapidly gather to the anode, forming space charges. The electric field intensity across the interface between electrolyte and electrode can reach to 104 V cm−1 in the common electrolyte, which is 103 times greater than that in the bulk solution (see Figure 4a). The distinct increase of electric field intensity will unavoidably lead to uneven deposition. The protuberant tips carry more charges than the smooth Li film, which spurs Li+ preferentially to obtain negative charge on the tips, forming Li dendrites (see Figure 4b). Meanwhile, the drastic increase of the current density on the tips results in lithium irreversible deposition and exfoliation, producing dead lithium. However, as a high salinity gel electrolyte, the DGE provides enough PF6− ions near the metal lithium surface, which tends to reduce the space charges, and thus lower the interface current density and raise the electrode reaction reversibility. As a result, Li dendrite growth is restricted. In addition, the high viscosity property of the gel electrolyte decreases the current density in the interface, increasing thermodynamic reversibility of the electrode reaction, which will make a contribution to the uniform deposition on the surface of the anode. This can be explained by the following eqs 2 and 3:

Figure 2. Current−potential curve of Li/DGE/stainless steel cell: stainless steel as the working electrode and Li as the counter and the reference electrode, at a scan rate of 1 mV s−1.

first negative scan, the small reduction peak around 1.5 V may be ascribed to the reduction of water or oxygen impurities.39,40 Subsequently, a strong current peak is seen along with a further negative scan, corresponding to lithium deposition (Li+ + e− → Li). In addition, the peak at 0.25 V is ascribed to the dissolution (Li → Li+ + e−), and no other oxidation peaks are observed during the following positive scan. It is clearly shown that the oxidation potential of the DGE exceeds 5 V, suggesting its strong oxidation resistance. So, the DGE completely meets the requirements for lithium metal S-PAN battery whose charge voltage is usually ended at 3 V. 3.2. Li Dendrite Growth and Control with DGE. Li dendrite formation and growth were also investigated under the DGE and the common electrolyte. Figure 3 is the SEM images of the deposited surface. Figure3a and d shows the surfaces of the fresh Cu and fresh Li, respectively. It is clear that in the case of the DGE, a smooth Li film is attained as seen in Figure 3c,f, unlike the dendritic and mossy one obtained in the common

j = 2nDi

Di =

ci0 − ciS δ

kT 6πriη

(2)

(3)

where j is the diffusion current density, n is the number of moles of electrons transferred, Di is the diffusion coefficient, c0i is the ion concentration of bulk solution, csi is the ion concentration of electrode surface, δ is the thickness of diffusion layer, k is the Avogadro’s constant, T is the absolute

Figure 3. SEM images of the morphologies of Li deposited on Cu (a, b, c) and Li deposited on Li (d, e, f) under different electrolytes at a current density of 0.1 mA cm−2. (a) Fresh Cu, (d) fresh Li (b, e) common electrolyte, (c, f) DGE. C

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Figure 4. (a) Space charge mechanism of Li deposition process; (b) mode of Li dendrite growth on the surface of Li metal in charge process.

Figure 5. Model scheme of (a) the solvation effect of lithium ion in common electrolyte and (b) desolvation of lithium ion in the DGE.

temperature, ri is the ionic radius, and η is the viscosity of solution. The two equations suggest that diffusion current density will decrease with reduced diffusion coefficient. There is a reverse relationship between diffusion coefficient and viscosity. Therefore, the diffusion current density decreases when the viscosity increases. The DGE enjoys high viscosity; in this way, the reversibility of electrodeposition is improved. Accordingly, the dead lithium and Li dendrite can be mitigated by the DGE. On the other hand, the high lithium ion transference number of DGE raises the lithium ionic mass transfer rate between the electrolyte and metallic lithium electrode, enhancing the uniformity of lithium deposition as well.43 Therefore, the results prove that the DGE contributes to controlling Li dendrite growth morphology during the process of Li deposition. It means that the safety of the battery can be enhanced by using the DGE. 3.3. DGE Desolvation. Furthermore, the desolvation of the DGE was also investigated. The solvation effect represents the interaction forming complexes between solvent molecules and ions, lowering the activity of ions.44 EC and DMC both have CO and CH groups. Moreover, the CO group exhibits stronger polarity than the CH group. The common electrolyte contains fewer lithium salts at about 1 mol/L; the number of solvent molecules is about 14 times more than that of lithium ions. The polar CO group will interact with Li+ forming a larger complex when lithium salt is dissolved into the solvent. One lithium ion can interact with four solvent molecules to form a complex,45−47 resulting in the increase of solvated lithium ionic radius (see Figure 5a), which restricts the movement of lithium ion in the electrolyte. Therefore, it can be explained that lithium ion transference number is no more than

0.5 in the common electrolyte, while in the DGE, the content of lithium salt is about 7 times more than that in the common electrolyte, which means that the number of solvent molecules is just around 2 times more than the number of lithium ions. Thus, at most, two solvent molecules can interact with one lithium ion to form a complex. Reasonably, the CO double bond is weakened. Figure 5b shows the model of desolvation in the DGE. Obviously, two EC molecules, two DMC molecules, or one EC and DMC molecule interact with one lithium ion, forming complexes. The size of the complex ion may be reduced under this circumstance, on account of decreasing the number of solvent molecules, and the free mobility of these complex ions in the DGE is increased. Accordingly, the lithium ion transference number rises with increased salt concentration. The model reveals that the DGE may decrease the solvation effect of lithium ion. An IR spectrum was measured to demonstrate the desolvation of lithium ions in the DGE and the solvation effect of lithium ions in the common electrolyte. In IR spectra (see Figure 6), the following functional groups are identified in the DGE and the common electrolyte, CO stretching vibration (1725−1812 cm−1) and CH stretching vibration (2850−2960 cm−1). The absorption peak intensity of CO group is higher than that of the CH group, which demonstrates that the polarity of the CO group is stronger than that of the CH group. The IR spectrum from DGE exhibits that the absorption frequency of the CO group shifts to lower wavenumber, indicating that the double bond of the CO group is relatively stretched in the DGE. It is in accord with the theoretical analysis of desolvation (see Figure 5b). Therefore, the solvation effect of the lithium ion exists in the D

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Table 1. Distances of All CO Double Bonds after Optimizationa Li+−EC Li+−2EC Li+−3EC Li+−4EC EC Li+−DMC Li+−EC+DMC DMC a

1.22966 1.22133 1.21545 1.20992 1.19435 1.24899 1.22247 1.21770

1.22134 1.21567 1.20863 1.24024 -

1.21562 1.21051 -

1.20908 -

Units are Å.

molecule complexes increase, the CO double bonds are shortened. Until no Li+ interaction with solvent molecules exists, the CO double bond distance reaches the minimum. For instance, the average distance of CO double bonds is 1.22966 Å, 1.221335 Å, 1.21558 Å, 1.209535 Å, and 1.19435 Å in Li+(EC), Li+(EC)2, Li+(EC)3, Li+(EC)4, and EC, respectively. For the Li+(DMC+EC) complex, the EC(CO) double bond distance is 1.22247 Å, which is longer than the value of 1.19435 Å in pure EC, and the DMC(CO) double bond distance is 1.24024 Å longer than that of pure DMC of 1.121770 Å. The result is in agreement with the analysis in IR spectrum that the CO double bonds are stretched in the DGE. Therefore, it is confirmed that the solvation size of the Li+-solvent complex decreases in the DGE, demonstrating the desolvation of lithium ion. The impact of the electrolyte desolvation on the battery performance will be discussed in detail. 3.4. Electrochemical Performance of S-PAN Cathode under the DGE. Electrochemical impedance analyses were conducted on lithium metal S-PAN cells with the DGE and the common electrolyte. The measurements were carried out after the third cycle. As shown in Figure 8, the intercept obtained at

Figure 6. Infrared spectra of common electrolyte and DGE.

common electrolyte. Nevertheless, DGE effectively achieves the desolvation of lithium ion. Also, the IR spectra from both DGE and common electrolyte show that the absorption peaks of C H groups are fundamentally in accordance with each other and no evident shift of wavenumber occurs. The result manifests that there is no obvious change on the intensity of CH bonds in both DGE and common electrolyte. The solvation of the lithium ion was also calculated by DFT method with 6-31G basis set. EC and DMC molecules and their complexes including Li+ were optimized. The optimized structures are shown in Figure 7. Figure 7a−e represents EC,

Figure 8. Impedance plots of the S-PAN/electrolyte/Li cell with the DGE and common electrolyte after the third cycle. Figure 7. Optimized ball and stick structures of all complexes. Dark gray, white, red, and violet spheres denote C, H, O, and Li atoms/ions, respectively.

high frequency corresponds to the total ohmic resistance of the cell, reflecting the migration of lithium ions in the electrolyte. The value under the DGE is higher than the case in the common electrolyte due to the high viscosity of the DGE. In addition, the semicircle indicates the charge transfer resistance in the middle frequency range, relating to the charge transfer between the electrode and electrolyte interface. Also, the inclined line in the low frequency represents Warburg impedance, which is associated with the solid-state diffusion of lithium ions in the electrode material. The plots clearly show a much smaller semicircle under the DGE than the common electrolyte, and the charge transfer resistance is reduced from 80 Ω for the common electrolyte to 27 Ω for the DGE. It is

Li+(EC), Li+(EC)2, Li+(EC)3, and Li+(EC)4 complexes, respectively. Figure 7f is an optimized DMC molecule. Li+(DMC) and Li+(DMC+EC) complexes are shown in Figure 7g and h, respectively. The calculated result suggests that Li+ could interact with the solvent molecules to form complexes, and the size of the complex becomes larger with increasing number of solvent molecules. It also exhibits that the size of the complex in the common electrolyte is larger than that in the DGE. Table 1 lists the distances of all CO double bonds after optimization. It is clearly seen that as the size of Li+-solvent E

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first discharge curve presents a low voltage plateau, indicating that the discharge process is different from the latter ones. It is similar to that reported by Michael R. Buchmeiser, where the elemental sulfur in the sample is neatly removed, and only chemically bonded sulfur and the PAN-derived backbone perform lithium storage,49 showing the low platform in the first discharge. Figure 9d shows the galvanostatic charge/discharge profile of the S-PAN composite cathode under the common electrolyte. Compared to the DGE, the charge/discharge capacity fades dramatically, and the platform declines from the third discharge markedly, because the discharge intermediate products are dissolved into the electrolyte during the cycling process. To sum up, the desolvation of the DGE increases the lithium ion transference number and reduces the resistance of charge transfer at the cathode/electrolyte interface, enhancing the electrochemical performance of the S-PAN cathode. 3.5. Weak Solvation of the Anion. Finally, the weak interaction between the C−H group and PF6− was investigated by 1H NMR spectra. Figure 10 presents the 1H NMR spectra of

ascribed that the desolvation of the DGE is advantageous to charge transfer at the cathode/electrolyte interface, which brings about the decrease of charge transfer resistance in the DGE. Therefore, the desolvation effect of the DGE may improve the electrochemical performance of the battery. To demonstrate the DGE benefits to S-PAN electrode in comparison with the common electrolyte, the charge/discharge performance was also tested with the DGE and the common electrolyte, respectively. The capacities in Figure 9a,b were

Figure 9. Electrochemical performance of S-PAN composite using DGE and common electrolyte: (a) cycling performance and (b) rate performance. Discharge/charge voltage profiles of S-PAN composite using (c) DGE and (d) common electrolyte. Figure 10. 1H NMR spectra of DGE and common electrolyte.

calculated on the basis of sulfur mass. The discharge/charge voltage profiles in Figure 9c,d were calculated by the mass of composite. As can be seen in Figure 9a, the S-PAN composite electrode using DGE exhibits superior cycling performance at a current rate of 60 mA/g, delivering a reversible capacity of 1276 mAh/g after 50 cycles. In contrast, a lower reversible capacity (851 mAh/g) is delivered at the end of the 50th cycle for the common electrolyte. The enhanced performance is largely ascribed to the limit of high salinity DGE to the dissolution of the discharge intermediate products. It may be different from the traditional polysulfidesthe dissolved products may be a big group containing S ions in the reaction process, because PAN can also dissolve into EC and PC.48 The cycle performance of the S-PAN composite electrode at the rate up to 600 mA/g was studied in the potential range of 1 to 3.0 V, as shown in Figure 9b. A stable rate capability at various charge/ discharge rates is observed in the case of DGE. In the rate cycle, the average reversible capacities are 1575 mAh/g, 1455 mAh/g, 1261 mAh/g, and 610 mAh/g for the current rates of 60 mA/g, 120 mA/g, 360 mA/g, and 600 mA/g. A reversible discharge capacity of 1426 mAh/g is recovered when the current rate is back to 60 mA/g. However, the reversible capacity under the common electrolyte is apparently lower than the value in the DGE. The electrolyte could thereby make both the capacity and the rate performance of the composite electrode increase, and the desolvation of the DGE may be the main reason for modifying the rate performance by reducing the charge transfer resistance at the cathode/electrolyte interface. Figure 9c exhibits the galvanostatic charge/discharge profile of the S-PAN composite cathode under the DGE in the 1st, 2nd, 5th, 25th, and 50th cycle at a current rate of 60 mA/g. The

the DGE and the common electrolyte. The peaks, observed at 4.478 and 4.471 ppm for the common electrolyte and the DGE, respectively, can be indexed to the chemical shift of the CH−O group. In addition, it is shown that the common electrolyte is at 3.690 ppm and the DGE at 3.685 ppm, which reflects the chemical shift of the CH3−O group. The result exhibits that the peak of the DGE moves to the high magnetic field (right of the spectra) compared to the common electrolyte. It may be ascribed to more ions of PF6− in the DGE than in the common electrolyte, leading to the weak interaction between C−H group and PF6− in the DGE. The interaction contributes to the increase of the extranuclear electronic cloud density of hydrogen in the DGE. Therefore, the peak of the DGE moves to a lower chemical shift. The results indicate that the weak desolvation of PF6− may exist in the DGE. The mechanism of weak solvation about PF6− cannot definitively be explained at this stage, and more work is still needed for understanding the relationship between the solvation of anions and the electrochemical performance of the cell.

4. CONCLUSION A new gel electrolyte whose oxidation potential exceeds 5 V is presented for rechargeable lithium metal S-PAN battery. Diffusion current density and electric field intensity between electrolyte and electrode are important factors for the growth of the lithium dendrite. The use of the DGE could reduce the growth of Li dendrites by lowering the electric field intensity of space charge. Its high viscosity is also prone to decrease the current density in the interface, mitigating the Li dendrites. It is F

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shown that a specific capacity of 1276 mAh/g is retained after 50 cycles at 60 mA/g current rates if the mass of sulfur is taken into account and 523 mAh/g based on the total mass of sulfurPAN, and the rate capability from 60 to 600 mA/g current rates exhibits good performance. It is primarily attributed to the desolvation of DGE reducing the charge transfer resistance at cathode/electrolyte interface, and the DGE could restrict the dissolution of intermediate discharge products. The desolvation of the electrolyte also increases the lithium ion transference number, enhancing the electrochemical performance of the battery. It is suggested that the DGE would be a promising electrolyte for rechargeable lithium metal batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Measurement of transference number of the DGE and common electrolyte and the SEM of Li deposition cross section. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-10-6891-8770. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the National 863 Program (2013AA050903) of China, Beijing Municipal Science and Technology Project (Z131100003413002), Beijing Key Laboratory of Environmental Science and Engineering (Grant No. 20131039031), Beijing Higher Institution Engineering Research Center for Power Battery and Chemical Energy Materials (Grant No. 2012039032).

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