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Highly concentrated carbonate electrolyte for Li-ion batteries with lithium metal and graphite anodes
Journal of Power Sources 450 (2020) 227657
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Highly concentrated carbonate electrolyte for Li-ion batteries with lithium metal and graphite anodes Jian-De Xie a, *, Jagabandhu Patra b, c, Purna Chandra Rath b, Weng-Jing Liu d, Ching-Yuan Su e, Sheng-Wei Lee d, Chung-Jen Tseng e, Yasser Ashraf Gandomi f, Jeng-Kuei Chang b, c, d, ** a
Fujian Provincial Key Laboratory of Functional Materials and Applications, Institute of Material Preparation and Applied Technology, School of Materials Science and Engineering, Xiamen University of Technology, China Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan c Hierarchical Green-Energy Materials (Hi-GEM) Research Center, National Cheng Kung University, Tainan, Taiwan d Institute of Materials Science and Engineering, National Central University, Taoyuan, Taiwan e Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan f Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States b
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
� An electrolyte containing LiFSI salt and carbonate was used for Li-ion batteries. � Influence of Li salt, concentration, sol vent, and composition was investigated. � An optimal composition, 5.5 M LiFSIDMC/EC and 5.5 M LiFSI-DEC/EC, was determined. � Raman spectroscopy was used to confirm the formation of ICP and AGG compounds. � Specific capacity and high-rate capa bility of Li and graphite anodes were improved. A R T I C L E I N F O
A B S T R A C T
Keywords: Li battery Dendritic Li structure Electrolyte composition Coulombic efficiency Solid electrolyte interface
Highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) salt dissolved in carbonate solvent is employed as a high-performance and robust organic electrolyte for Li-ion batteries. The influences of Li salt type, concentration, and solvent type (such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylene carbonate (EC)) on the electrochemical properties of Li metal and graphite anodes are systematically assessed. A superior electrolyte composition of 5.5 M LiFSI-DMC/EC is achieved, enhancing the anti-flammability, coulombic efficiency, and high rate capability. The optimal efficiency values of Li electrodeposition/stripping utilizing 5.5 M LiFSI-DMC/ EC are 97.0% and 94.5% at 0.4 and 6 mA cm 2, respectively. Such an enhanced performance is due to the formation of a three-dimensional ion network, composed of contact ion pairs (CIPs) and ion aggregates (AGGs) in the highly concentrated LiFSI electrolyte, which effectively decreases the number of free solvent molecules and inhibits the formation of undesired dendritic Li structures. Raman spectroscopy is employed to confirm the formation of CIP and AGG compounds within the electrolyte. The electrochemical data of the 5.5 M LiFSI-DMC/ EC electrolyte cell demonstrates a remarkable improvement in the specific capacity and rate capability of a graphite anode.
* Corresponding author. 1001 University Road, National Chiao Tung University, Hsinchu, 30010, Taiwan. ** Corresponding author. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan. E-mail addresses: [email protected] (J.-D. Xie), [email protected] (J.-K. Chang). https://doi.org/10.1016/j.jpowsour.2019.227657 Received 1 July 2019; Received in revised form 15 December 2019; Accepted 20 December 2019 Available online 24 December 2019 0378-7753/© 2019 Published by Elsevier B.V.
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constituent solvents. All the chemicals used in this work were of battery grades. To ensure uniformity, the electrolyte was continually stirred for 24 h inside an Ar-filled glove box. Prior to coin cell assembly, the electrode was cut in the form of circular disks (with a diameter of ~1.3 cm). The Cu disks were immersed in 1 M hydrochloric acid for 10 min and then rinsed with deionized water for 5 min. The purpose of the acid treatment on Cu was to remove the native copper oxide and any possible contamination [23]. Finally, all Cu disks were dried in a vacuum oven at 50 � C for 1 h. To prepare the graphite anodes, graphite powder (MG-16, China Steel Chemical Corporation, Taiwan), carbon black (Imerys Graphite & Carbon), and binder (polyvinylidene fluoride (PVDF)) were added into a homogenizer with a weight ratio of 80:10:10. Subsequently, N-methyl pyrrolidinone (NMP) was added into the solid mixture, and then a ho mogenizing step was performed at 300 rpm for 1 h. The electrode slurry was uniformly and continuously coated on Cu foil using a doctor blade with a gap of 300 μm. The resulting electrodes were placed in a vacuum oven at 110 � C. After baking for 3 h, the electrode sheets were cooled down to room temperature. The graphite anodes were cut into circular disks (with a diameter of ~1.3 cm). The final thickness of the graphite layer was about 150 μm, and the loading amount of graphite on Cu foil was approximately 5 mg cm 2. To separate the positive and negative electrodes, a glass fiber membrane was utilized. The flammability tests were conducted following a previously established procedure [24]. The glass fiber separator was soaked in the electrolyte (to make sure the separator is fully wet) and then placed at a distance of 123 mm from the top of an electric Bunsen burner. With such a testing condition, the electrolyte was burned out within 30 s, and the flame was no longer observed. A digital camera was employed to monitor the flammability of the separator. Raman spectroscopy (PTT UniRAM; with) was used to analyze the coordination structures of various electrolytes (a solid-state laser with a wavelength of 532 nm was adopted). The chemical composition of the solid electrolyte interface (SEI) layer on Li surface after 5 electrodeposition/stripping cycles was explored using X-ray photoelectron spectroscopy (XPS) equipped with an Al Kα radiation source. The microstructures of the electrodes were inspected using scanning electron microscopy (SEM, FEI Inspect F50). To conduct the chronopotentiometry experiments, a multi-channel electrochemical analyzer (Biologic, VSP-300) was utilized. The coin cells were charged and discharged at various current densities (i.e., 0.4, 1, 1.6, 2, and 6 mA cm 2). The graphite charge/discharge tests were performed at different C rates in a voltage range of 0.01–2 V. Electro chemical impedance spectroscopy (EIS) measurement of symmetric Li// Li cells was carried out at open-circuit potential with a frequency range of 100 kHz to 1 mHz and a potential amplitude of 10 mV. All electro chemical measurements were performed at ambient temperature.
1. Introduction Lithium-ion batteries (LIBs) are usually the primary choice when high energy/power densities are needed [1,2]. Frequently used com ponents within the LIBs configuration are the electrodes (anode and cathode), separator, and the electrolyte. Commonly, graphite is employed as the anode material due to the low charge-discharge plateau, good mechanical stability, and high reversibility during Liþ insertion/extraction processes. However, graphite has a relatively low Li-storage capacity (theoretically 372 mAh g 1), and thus cannot meet the high capacity demand for next-generation LIBs [3,4]. Therefore, there is a significant need for developing safe, high-performance, du rable, and non-toxic alternative materials for the anode side [5,6]. Li metal is a promising anode candidate due to its high theoretical capacity (3860 mAh g 1), low density (0.59 g cm 3), and low redox potential ( 3.04 V vs. standard hydrogen electrode) [7]. However, Li anodes commonly demonstrate short cycle life, low coulombic efficiency, and poor structural stability due to high reactivity with the electrolyte. Accordingly, it is necessary to further improve the electrolyte compo sition for next-generation high-capacity rechargeable Li batteries (e.g., Li-oxide, Li–S, and Li-air batteries) [7,8]. High reactivity between the electrolyte and Li metal usually results in the formation of dendritic Li structures during cycling. Uncontrolled growth of the dendritic Li metal on the anode results in a needle-like structure that can puncture the separator, short-circuit the battery, and subsequently result in hazardous thermal runaway [9–12]. Considering the critical roles of electrolyte on the battery performance, engineering the electrolyte composition is of great importance for pre venting the formation of the dendritic Li structures. The electrolyte is usually composed of a Li salt, solvent, and supporting additives. An ideal electrolyte should provide high ionic conductivity (which is usually inversely proportional to its viscosity) and high electrochemical and thermal stability [11,13,14]. Several previous efforts have been dedi cated to improving the electrolyte composition. Recently, the develop ment of organic electrolytes has suggested an effective approach for avoiding undesired Li dendrite formation. Yamada et al. have reported the use of highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in organic elec trolyte for improving the electrochemical performance of graphite an odes [15,16]. The concentrated electrolyte has also been utilized for stabilizing the Li metal deposition and suppressing the unwanted growth of Li dendrites [17–19]. Some other studies have reported on the application of LiFSI as an alternative to LiPF6 salt to improve the chemical/thermal stability, suppressing HF generation and thus unde sired side reactions [20–22]. Previous studies in the field have confirmed the practicality of LiFSI as a promising electrolyte salt for LIBs. However, further development is required for improving the stability and enhancing the coulombic effi ciency of the electrolyte during cycling. Therefore, this work has been dedicated to developing a series of LiFSI-based electrolytes for Li metal and graphite anodes. The influences of Li salt type and concentration, solvent, and electrolyte coordination status on the Li metal and graphite anode properties are systematically investigated. Finally, an optimized electrolyte composition including highly concentrated LiFSI salt along with DMC/EC solvent has been proposed. It has been shown that the unique electrolyte coordination status of the highly concentrated LiFSI electrolyte significantly improves the coulombic efficiency as well as the rate capability of both Li metal and graphite anodes.
3. Results and discussion Organic carbonate-based electrolytes have been widely used for LIBs due to the wide electrochemical stability window as well as superior compatibility with the intercalated electrodes. However, the use of carbonate-based electrolytes with Li metal anodes substantially lowers the coulombic efficiency. The major reason for this is the formation of dendrites during Li deposition/stripping cycling [25–28]. Therefore, it is of great importance to explore the effects of electrolyte composition on the formation/growth of the dendritic structures during cycling. To this end, we first characterized the influences of Li salt type and concentration on the coulombic efficiency of Li anodes. The deposition/ stripping data have been compared with the Li//Cu half cells. The total charge density for Li electrodeposition was 2 mAh cm 2 and the coulombic efficiency was assessed via calculating the ratio of discharge (stripping) to charge (deposition) capacity. Fig. 1(a)–(b) show the chronopotentiometric characteristics of the cells with 1 M LiPF6-DEC/ EC and 1 M LiFSI-DEC/EC electrolytes. The coulombic efficiency values for the former cell were 90.3 and 80.0% at 0.4 and 6 mA cm 2,
2. Experimental To prepare the electrolytes, LiPF6 (KISHIDA Chemical Co., purity: 99.9%) or LiFSI (KISHIDA Chemical Co., purity: 99.9%) was dissolved in various kinds of organic solvent (i.e., diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylene carbonate (EC)). For the binary DEC/EC and DMC/EC systems, a volume ratio of 1:1 was used for the two 2
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respectively. As for the 1 M LiFSI-DEC/EC cell, the values increased to 93.1 and 85.2% under the same operating conditions (i.e., 0.4 and 6 mA cm 2). The enhanced efficiency can be ascribed to the better passivation ability of the LiFSI-derived SEI [29], which prevents excessive decom position of the electrolyte [30]. To explore the effects of Li salt concentration on the cell perfor mance, the 3 and 5.5 M LiFSI-DEC/EC cells were also assessed (see Fig. 1 (c)–(d)). Herein 5.5 M LiFSI in DEC/EC is around the maximum solu bility. Therefore, we cannot further increase the concentration. As shown in Table 1, the concentration of LiFSI plays a crucial role in determining the coulombic efficiency. The use of 5.5 M LiFSI-DEC/EC electrolyte enhanced the coulombic efficiency to 95.5% at 0.4 mA cm 2 and 93.3% at 6 mA cm 2. Generally, increased coulombic effi ciency indicates reduced Li consumption during the Li deposition/ stripping process. At lower LiFSI concentrations, the free uncoordinated solvent molecules existing in the electrolyte have a high tendency to react with Li metal, generating Li2CO3, Li2O, and other Li compounds [31–33]. This SEI film is a discontinuous layer that allows the electrolyte penetration, inducing further SEI formation. As the repetitive charge/ discharge cycles progress, the deposited Li is gradually encased by a thick SEI layer and is disconnected from the bulk Li metal, eventually forming electrically inert, “dead” Li particles. Such an undesired for mation of dendrites further reduces the coulombic efficiency. In contrast, increasing the concentration of LiFSI reduces the number of free solvent molecules and thus increases the coulombic efficiency. Meanwhile, the anions are easily coordinated with the Liþ ions, creating a cross-linked network capable of inhibiting the growth of unwanted Li dendritic structures [25]. Fig. S1 (see Electronic Supporting Information) compares the cycling data of symmetric Li//Li cells with 1 M and 5.5 M LiFSI-DEC/EC electrolytes. The polarization of the former cell increased more rapidly, indicating faster interfacial resistance growth. Fig. S2 shows the cycling data of Cu//LiFePO4 cells with the two electrolytes. Again, the cell with 1 M LiFSI-DEC/EC electrolyte exhibited more sig nificant capacity decay upon cycling. Both data indicate the benefits of using the high-concentration electrolyte for improving the coulombic efficiency and cycleability. As shown in Fig. 2, a significant percentage of free solvent molecules exists within the 1 M electrolyte. However, the intensities of contact ion
Table 1 Coulombic efficiencies of Li deposition/stripping measured at various current densities in various electrolytes. Electrolyte\Current density
0.4 mA/ cm2
1 mA/ cm2
1.6 mA/ cm2
2 mA/ cm2
6 mA/ cm2
1 M LiPF6-DEC/EC 1 M LiFSI- DEC/EC 3 M LiFSI-DEC/EC 5.5 M LiFSI-DEC/ EC 5.5 M LiFSI-DMC/ EC 5.5 M LiFSI-DMC
90.3% 93.1% 94.9% 95.5%
90.1% 93.0% 94.4% 95.2%
89.0% 92.6% 94.0% 94.9%
87.0% 90.9% 93.8% 94.7%
80.0% 85.2% 92.0% 93.3%
97.0%
95.6%
95.4%
95.0%
94.5%
65.0%
59.0%
55.2%
50.0%
45.0%
pair (CIP) and ion aggregate (AGG) become more evident with increasing the LiFSI concentration. The higher ratios of CIP and AGG reflect the formation of a three-dimensional ion network and confirm the reduced number of free solvent molecules [34]. That is, the solvent molecules in the electrolyte are mainly coordinated with the Li salt, and the concentration of free solvent molecules is considerably decreased. When the concentration reaches a critical level, the ions within the electrolyte coordinate to form a liquid-phase network improving the anti-flammability (see the following section for further details). The less amount of free solvent molecules also reduces the undesired side re actions between the electrolyte and Li metal. To delineate the formation of Li dendrites, SEM observation was performed to characterize the Li morphologies on the Cu substrates upon Li-deposition/stripping cycling. Fig. 3(a)‒3(d) present top-view SEM images of the Li deposited in various electrolytes, including 1 M LiPF6DEC/EC, 1 M LiFSI-DEC/EC, 3 M LiFSI-DEC/EC, 5.5 M LiFSI-DEC/EC, after five consecutive charge-discharge cycles. It can be seen from Fig. 3(a) that using traditional 1 M LiPF6-DEC/EC electrolyte tends to generate a slender needle-like structure, so-called a “dendritic structure” on Cu foil. As discussed earlier, the formation of the dendritic structure is expected when the Li metal anode is used in conjunction with LiPF6 carbonate electrolyte. When the LiPF6 was replaced with LiFSI salt (see Fig. 3(b)), the resulting structure still exhibited a needle-like topog raphy. However, at high electrolyte concentration, the microstructures of the Li deposits changed to knotted round structures, as depicted in
Fig. 1. Chronopotentiometry curves of various Li//Cu cells containing (a) 1 M LiPF6-DEC/EC, (b) 1 M LiFSI-DEC/EC, (c) 3 M LiFSI-DEC/EC, and (d) 5.5 M LiFSIDEC/EC electrolytes measured at different current densities. 3
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Fig. 3(c)–(d). For comparison, an SEM image of the Li deposited in 5.5 M LiFSI-DEC/EC after 20 cycles is given in Fig. S3. The Li morphology still maintained a knot-type structure without any dendrites after cycling. This finding demonstrates that the high-concentration electrolyte is effective in suppressing the formation/growth of needle-like structures. It is well known in the field that the chemical composition of SEI is strongly related to the electrolyte composition; it is also reported that the SEI layer directly affects the behavior of Li deposition [35]. Gener ally, LiF is formed by degradation of Li salt within the electrolyte. The presence of LiF in the SEI layer is helpful for improving the coulombic efficiency due to its good ionic conductivity as well as enhanced me chanical stability [35]. To further explore the SEI properties, XPS was employed to characterize the chemical composition of the SEI layers on the deposited Li. The narrow-scan C1s, F 1s, and Li 1s spectra of the SEI films formed in 1 M LiPF6-DEC/EC and 5.5 M LiFSI-DEC/EC electrolytes are illustrated in Fig. 4. First, we found that the high concentrations of – C–O, and CO3 of the1 M LiPF6-DEC/EC sample are associated C–O, O– with solvent decomposition, which is considered unfavorable for Liþ transport. Second, the LiF content within the SEI layer was significantly increased using 5.5 M LiFSI-DEC/EC electrolyte. The high LiF content, induced by the high concentration of LiFSI salt, improved the coulombic efficiency of the Li metal anodes. In addition, high concentration of LiFSI promoted the formation of F–S bonding within the SEI layer. As compared to PF6 anion, the FSI anion, in either carbonate or ether sol vents, demonstrates superior compatibility toward Li metal [36,37]. As a consequence, better reversibility of Li deposition/stripping can be obtained. Next, it is crucial to explore the influence of solvent composition on the reversibility of Li deposition/stripping. Fig. 5(a) shows the chro nopotentiometric characteristics of the Li//Cu cells using 5.5 M LiFSIDMC/EC electrolyte. As compared to the 5.5 M LiFSI-DEC/EC cell (see Fig. 1(d)), the coulombic efficiency was enhanced via using DMC in the
Fig. 2. Raman spectra of various LiFSI-DEC/EC electrolytes.
Fig. 3. Top-view SEM images of the Li deposited in (a) 1 M LiPF6-DEC/EC, (b) 1 M LiFSI-DEC/EC, (c) 3 M LiFSI-DEC/EC, and (d) 5.5 M LiFSI-DEC/EC electrolytes. 4
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Fig. 4. XPS (a) C 1s, (a) F 1s, and (b) Li 1s spectra of the Li samples deposited in 1 M LiPF6-DEC/EC and 5.5 M LiFSI-DEC/EC electrolytes.
electrolyte (see Table 1). The efficiency values the 5.5 M LiFSI-DMC/EC cells were increased to 97.0% and 94.5% at 0.4 and 6 mA cm 2, respectively. This reveals that the application of DMC/EC is preferable over DEC/EC for a Li metal anode, probably related to the high con ductivity and lower viscosity of the former electrolyte (see Table 2). The influence of the EC addition in the electrolyte on the coulombic effi ciency was also examined; the data are shown in Fig. 5(b). Without the presence of EC, the cell using 5.5 M LiFSI-DMC electrolyte showed a poor coulombic efficiency, especially at high rates. Increasing the cur rent density to 6 mA cm 2 resulted in a significant reduction in the coulombic efficiency (to ~ 45%). It is known that the viscosity of EC is
higher than DMC. Therefore, addition of EC increases the electrolyte viscosity. However, EC has a higher dielectric constant (89.8 vs. 3.1 for DMC), which promotes Li salt dissociation, increasing the ionic con ductivity (see Table 2). Therefore, it is inferred that addition of EC in the electrolyte can enhance formation of a SEI and thus reversibility of Li deposition/stripping. The highly concentrated LiFSI-DMC/EC electro lyte is thought to be promising for Li metal anodes. Top-view SEM images of the Li deposited in 5.5 M LiFSI-DMC/EC and 5.5 M LiFSI-DMC are given in Fig. 5(c) and (d), respectively. There are relatively flat Li structures formed in the former electrolyte. Such a favorable microstructure is beneficial for maintaining the coulombic
Fig. 5. Chronopotentiometry curves of Li//Cu cells containing (a) 5.5 M LiFSI-DMC/EC and (b) 5.5 M LiFSI-DMC electrolytes measured at different current densities. SEM images of the Li deposited in (c) 5.5 M LiFSI-DMC/EC and (d) 5.5 M LiFSI-DMC electrolytes. 5
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electrochemical properties of graphite anodes, the charge-discharge experiments were conducted with 1 M LiPF6-DEC/EC, 5.5 M LiFSIDEC/EC, 5.5 M LiFSI-DMC, and 5.5 M LiFSI-DMC/EC electrolytes at various rates, as shown in Fig. 7(a)-7(d). The specific capacities of the graphite anode with a conventional electrolyte (i.e., 1 M LiPF6-DEC/EC) were 320 and 48 mAh g 1 at 0.1 and 2 C, respectively. Using 5.5 M LiFSIDMC and 5.5 M LiFSI-DMC/EC electrolytes, the specific capacitances were enhanced to 349 and 355 mAh g 1 at 0.1 C, approaching the theoretical capacity of graphite (i.e., 372 mAh g 1 according to the formation of LiC6 intercalation compound) [4]. The specific capacities of the graphite anodes measured at various C rates in different electrolytes are listed in Table S1 and illustrated in Fig. 7(e). It is worth noting that with the implementation of high concentration LiFSI (even without the use of EC), the graphite anode can deliver a higher specific capacity than that found in the conventional 1 M LiPF6-DEC/EC electrolyte. The rea sons for the improved performance may include: (i) the high-concentration Li salt tends to generate the CIP and AGG coordi nation states, which increase the ion dissociation kinetics at the elec trode interface, (ii) the SEI layer, formed in the highly concentrated electrolyte, facilitates Liþ ion transport, and (iii) the high Liþ concen tration results in high lithiation flux into the graphitic anode [40]. We also demonstrate that with the presence of EC in the electrolyte, the rate capability of the graphite anode can be improved. In addition, DMC/EC solvent seems to be superior to DEC/EC solvent, especially at high C rate. At 2 C, the optimal capacity of the graphite anode is 120 mAh g 1, which is found for 5.5 M LiFSI-DMC/EC electrolyte. One of the major concerns utilizing the conventional organic elec trolyte is the potential fire hazard. When the Li metal electrode is used as the anode within the cell, the electrolyte design is particularly crucial to address the safety issues. Fig. 8 shows the results of flammability tests conducted for 1 M LiPF6-DEC/EC and 5.5 M LiFSI-DMC/EC electrolytes. As shown in Fig. 8(a), the flammability is very intense for the conven tional 1 M LiPF6-DEC/EC electrolyte. However, the flammability of 5.5 M LiFSI-DMC/EC is much less, as revealed in Fig. 8(b). The improved anti-flammability of the latter electrolyte is related to the coordination between the Liþ ions and FSI anions and solvent molecules, forming a three-dimensional polymeric network that reduces the electrolyte volatility [17–19]. Since most of the solvent molecules are coordinated with Liþ ions, the number of free solvent molecules decreased, sup pressing the reactivity of the electrolyte. The low flammability reflects high safety of the proposed 5.5 M LiFSI-DMC/EC electrolyte.
Table 2 Viscosity and ionic conductivity of various electrolytes. Electrolyte
Viscosity (cP)
Conductivity (mS/cm)
1 M LiPF6-DEC/EC 1 M LiFSI-DEC/EC 3 M LiFSI-DEC/EC 5.5 M LiFSI-DEC/EC 5.5 M LiFSI-DMC/EC 5.5 M LiFSI-DMC
4.8 3.6 7.8 36.3 28.9 26.2
8.22 7.46 1.82 0.46 0.69 0.51
efficiency even upon prolonged charge/discharge cycling. In contrast, in the absence of the EC (i.e., 5.5 M LiFSI-DMC), the resulting Li morphology is relatively rough and irregular. An extended charge/discharge cycling along with EIS analyses were conducted with a symmetric Li//Li cell with 5.5 M LiFSI-DMC/EC electrolyte; the obtained data are shown in Fig. 6. The voltage profile (see Fig. 6(a)) confirms a stable performance for the symmetric cell during the long-term cycling (more than 20 days of continuous cycling). There is no significant polarization increase in the profile, which can be attributed to the formation of a robust SEI and the suppression of Li dendrite growth [38,39]. Fig. 6(b) confirms that the electro de/electrolyte interfacial resistance is rather steady upon 500 cycles. The cycling stability data of a symmetric Li//Li cell with 1 M LiPF6-DMC/EC electrolyte are shown in Fig. S4. The initial overpotential amplitude was similar to that of the 5.5 M LiFSI-DMC/EC cell, inidcating that although the ionic conductivity of 1 M LiPF6-DMC/EC was considerably lower (see Table 2) the corresponding SEI resistance and interfacial charge transfer resistance are higher. Moreover, as shown in Fig. S4, the polarization rapidly increases with increasing the cycle number, reflecting a poor interface stability. This 1 M LiPF6-DMC/EC electrolyte is thus not adequate to be used for Li metal anodes. To probe the effects of the electrolyte composition on
4. Conclusions This study demonstrates that highly concentrated LiFSI-DEC/EC and LiFSI-DMC/EC can be utilized as high-performance and stable electro lytes for Li batteries. A series of experiments with various Li salt types, concentrations, and solvent types of the electrolytes were conducted to assess their influences on the performance of Li metal and graphite an odes. It was shown that the application of 5.5 M LiFSI-DMC/EC or 5.5 M LiFSI-DEC/EC electrolyte (instead of 1 M LiPF6-DEC/EC electrolyte) effectively improved the coulombic efficiency and rate capability of Li metal anodes. In the former electrolyte, the coulombic efficiency values were 97.0% at 0.4 mA cm 2 and 94.5% at 6 mA cm 2. Such an improvement is related to the reduce solvent reactivity (e.g., the free solvent amount), better SEI stability, and the knot-type Li morphology. This study also demonstrated that the highly concentrated LiFSI elec trolyte, especially 5.5 M LiFSI-DMC/EC, can lead to a high specific ca pacity and great rate capability of a graphite anode. Moreover, the low flammability of this electrolyte was also confirmed. The proposed electrolyte has shown great potential for high-performance and highsafety battery applications. Declaration of competing interest
Fig. 6. (a) Voltage profile of Li/Li symmetric cell with 5.5 M LiFSI-DMC/EC electrolyte cycled at 1 mA cm 2 for 500 times. (b) variation of EIS spectra of above cell upon cycling.
The authors declare that they have no known competing financial 6
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Fig. 7. Charge-discharge curves of graphite anodes measured in (a) 1 M LiPF6-DEC/EC, (b) 5.5 M LiFSI-DEC/EC, (c) 5.5 M LiFSI-DMC, and (d) 5.5 M LiFSI-DMC/EC electroelytes. (e) Rate capability comparison of various cells.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The financial support provided for this work by the Ministry of Sci ence and Technology (MOST) of Taiwan is gratefully appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227657. References [1] Z.S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.M. Cheng, ACS Nano 4 (2010) 3187–3194. [2] P. Guo, H. Song, X. Chen, Electrochem. Commun. 11 (2009) 1320–1324. [3] P. Lian, X. Zhu, H. Xiang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 56 (2010) 834–840. [4] G. Wang, X. Shen, J. Yao, J. Park, Carbon 47 (2009) 2049–2053. [5] M.S. Whittingham, Proc. IEEE 100 (2012) 1518–1534. [6] W.A. Henderson, J. Phys. Chem. B 110 (2006) 13177–13183. [7] J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J. G. Zhang, Nat. Commun. 6 (2015) 6362 1–9. [8] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19–29. [9] E. Mengeritsky, P. Dan, I. Weissman, A. Zaban, D. Aurbach, J. Electrochem. Soc. 143 (1996) 2110–2116.
Fig. 8. Flammability tests of glass fiber membranes containing (a) 1 M LiPF6DEC/EC and (b) 5.5 M LiFSI-DMC/EC electrolytes.
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Journal of Power Sources 450 (2020) 227657 [25] X.B. Cheng, R. Zhang, C.Z. Zhao, F. Wei, J.G. Zhang, Q. Zhang, Adv. Sci. 3 (2016) 1–20, 1500213. [26] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.G. Zhang, Energy Environ. Sci. 7 (2014) 513–537. [27] X.B. Cheng, Q. Zhang, J. Mater. Chem. 3 (2015) 7207–7209. [28] N. Togasaki, T. Momma, T. Osaka, J. Power Sources 294 (2015) 588–592. [29] L.E.C. Forero, T.W. Smith, P.B. Balbuena, J. Phys. Chem. C 121 (2017) 182–194. [30] H.B. Han, S.S. Zhou, D.J. Zhang, S.W. Feng, L.F. Li, K. Liu, W.F. Feng, J. Nie, H. Li, X.J. Huang, M. Armand, J. Power Sources 196 (2011) 3623–3632. [31] K. Kanamura, H. Tamura, S. Shiraishi, Z.I. Takehara, J. Electrochem. Soc. 142 (1995) 340–347. [32] Y. Lu, Z. Tu, J. Shu, L.A. Archer, J. Power Sources 279 (2015) 413–418. [33] C.R. Yang, Y.Y. Wang, C.C. Wan, J. Power Sources 72 (1998) 66–70. [34] Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc. 136 (2014) 5039–5046. [35] C. Wan, S. Xu, M.Y. Hu, R. Cao, J. Qian, Z. Qin, J. Liu, K.T. Mueller, J.G. Zhang, J. Z. Hu, ACS Appl. Mater. Interfaces 9 (2017) 14741–14748. [36] B. Duan, W. Wang, H. Zhao, A. Wang, M. Wang, K. Yuan, Z. Yu, Y. Yang, ECS Electrochem. Lett. 2 (2013) A47–A51. [37] Q. Ma, Z. Fang, P. Liu, J. Ma, X. Qi, W. Feng, J. Nie, Y.S. Hu, H. Li, X. Huang, L. Chen, ChemElectroChem 3 (2016) 531–536. [38] X. Fan, L. Chen, X. Ji, T. Deng, S. Hou, J. Chen, J. Zheng, F. Wang, J. Jiang, K. Xu, C. Wang, Chem 4 (2018) 174–185. [39] G.M.A. Girard, M. Hilder, D. Nucciarone, K. Whitbread, S. Zavorine, M. Moser, M. Forsyth, D.R. MacFarlane, P.C. Howlett, J. Phys. Chem. C 121 (2017) 21087–21095. [40] Y. Yamada, M. Yaegashi, T. Abe, A. Yamada, Chem. Commun. 49 (2013) 11194–11196.
[10] M. Gauthier, T.J. Carney, A. Grimaud, L. Giordano, N. Pour, H.H. Chang, D. P. Fenning, S.F. Lux, O. Paschos, C. Bauer, F. Maglia, J. Phys. Chem. Lett. 6 (2015) 4653–4672. [11] K. Xu, Chem. Rev. 104 (2004) 4303–4417. [12] S.K. Jeong, H.Y. Seo, D.H. Kim, H.K. Han, J.G. Kim, Y.B. Lee, Y. Iriyama, T. Abe, Z. Ogumi, Electrochem. Commun. 10 (2008) 635–638. [13] T. Abe, F. Sagane, M. Ohtsuka, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 152 (2005) A2151–A2154. [14] T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 151 (2004) A1120–A1123. [15] Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc. 136 (2014) 5039–5046. [16] Y. Yamada, K. Usui, C.H. Chiang, K. Kikuchi, K. Furukawa, A. Yamada, ACS Appl. Mater. Interfaces 6 (2014) 10892–10899. [17] D. Aurbach, J. Electrochem. Soc. 134 (1987) 1611–1620. [18] D. Aurbach, J. Electrochem. Soc. 142 (1995) 2873–2882. [19] G.V. Zhuang, K. Xu, H. Yang, T.R. Jow, P.N. Ross, J. Phys. Chem. B 109 (2005) 17567–17573. [20] J. Wang, Y. Yamada, K. Sodeyama, C.H. Chiang, Y. Tateyama, A. Yamada, Nat. Commun. 7 (2016) 12032 1–9. [21] S.T. Myung, Y. Sasaki, S. Sakurada, Y.K. Sun, H. Yashiro, Electrochim. Acta 55 (2009) 288–297. [22] X. Zhang, T.M. Devine, J. Electrochem. Soc. 153 (2006) B344–B351. [23] A.A. Assegie, C.C. Chung, M.C. Tsai, W.N. Su, C.W. Chen, B.J. Hwang, Nanoscale 11 (2019) 2710–2720. [24] C. Arbizzani, G. Gabrielli, M. Mastragostino, J. Power Sources 196 (2011) 4801–4805.
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Highly concentrated carbonate electrolyte for Li-ion batteries with lithium metal and graphite anodes Jian-De Xie a, *, Jagabandhu Patra b, c, Purna Chandra Rath b, Weng-Jing Liu d, Ching-Yuan Su e, Sheng-Wei Lee d, Chung-Jen Tseng e, Yasser Ashraf Gandomi f, Jeng-Kuei Chang b, c, d, ** a
Fujian Provincial Key Laboratory of Functional Materials and Applications, Institute of Material Preparation and Applied Technology, School of Materials Science and Engineering, Xiamen University of Technology, China Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan c Hierarchical Green-Energy Materials (Hi-GEM) Research Center, National Cheng Kung University, Tainan, Taiwan d Institute of Materials Science and Engineering, National Central University, Taoyuan, Taiwan e Department of Mechanical Engineering, National Central University, Taoyuan, Taiwan f Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States b
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
� An electrolyte containing LiFSI salt and carbonate was used for Li-ion batteries. � Influence of Li salt, concentration, sol vent, and composition was investigated. � An optimal composition, 5.5 M LiFSIDMC/EC and 5.5 M LiFSI-DEC/EC, was determined. � Raman spectroscopy was used to confirm the formation of ICP and AGG compounds. � Specific capacity and high-rate capa bility of Li and graphite anodes were improved. A R T I C L E I N F O
A B S T R A C T
Keywords: Li battery Dendritic Li structure Electrolyte composition Coulombic efficiency Solid electrolyte interface
Highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) salt dissolved in carbonate solvent is employed as a high-performance and robust organic electrolyte for Li-ion batteries. The influences of Li salt type, concentration, and solvent type (such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylene carbonate (EC)) on the electrochemical properties of Li metal and graphite anodes are systematically assessed. A superior electrolyte composition of 5.5 M LiFSI-DMC/EC is achieved, enhancing the anti-flammability, coulombic efficiency, and high rate capability. The optimal efficiency values of Li electrodeposition/stripping utilizing 5.5 M LiFSI-DMC/ EC are 97.0% and 94.5% at 0.4 and 6 mA cm 2, respectively. Such an enhanced performance is due to the formation of a three-dimensional ion network, composed of contact ion pairs (CIPs) and ion aggregates (AGGs) in the highly concentrated LiFSI electrolyte, which effectively decreases the number of free solvent molecules and inhibits the formation of undesired dendritic Li structures. Raman spectroscopy is employed to confirm the formation of CIP and AGG compounds within the electrolyte. The electrochemical data of the 5.5 M LiFSI-DMC/ EC electrolyte cell demonstrates a remarkable improvement in the specific capacity and rate capability of a graphite anode.
* Corresponding author. 1001 University Road, National Chiao Tung University, Hsinchu, 30010, Taiwan. ** Corresponding author. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan. E-mail addresses: [email protected] (J.-D. Xie), [email protected] (J.-K. Chang). https://doi.org/10.1016/j.jpowsour.2019.227657 Received 1 July 2019; Received in revised form 15 December 2019; Accepted 20 December 2019 Available online 24 December 2019 0378-7753/© 2019 Published by Elsevier B.V.
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constituent solvents. All the chemicals used in this work were of battery grades. To ensure uniformity, the electrolyte was continually stirred for 24 h inside an Ar-filled glove box. Prior to coin cell assembly, the electrode was cut in the form of circular disks (with a diameter of ~1.3 cm). The Cu disks were immersed in 1 M hydrochloric acid for 10 min and then rinsed with deionized water for 5 min. The purpose of the acid treatment on Cu was to remove the native copper oxide and any possible contamination [23]. Finally, all Cu disks were dried in a vacuum oven at 50 � C for 1 h. To prepare the graphite anodes, graphite powder (MG-16, China Steel Chemical Corporation, Taiwan), carbon black (Imerys Graphite & Carbon), and binder (polyvinylidene fluoride (PVDF)) were added into a homogenizer with a weight ratio of 80:10:10. Subsequently, N-methyl pyrrolidinone (NMP) was added into the solid mixture, and then a ho mogenizing step was performed at 300 rpm for 1 h. The electrode slurry was uniformly and continuously coated on Cu foil using a doctor blade with a gap of 300 μm. The resulting electrodes were placed in a vacuum oven at 110 � C. After baking for 3 h, the electrode sheets were cooled down to room temperature. The graphite anodes were cut into circular disks (with a diameter of ~1.3 cm). The final thickness of the graphite layer was about 150 μm, and the loading amount of graphite on Cu foil was approximately 5 mg cm 2. To separate the positive and negative electrodes, a glass fiber membrane was utilized. The flammability tests were conducted following a previously established procedure [24]. The glass fiber separator was soaked in the electrolyte (to make sure the separator is fully wet) and then placed at a distance of 123 mm from the top of an electric Bunsen burner. With such a testing condition, the electrolyte was burned out within 30 s, and the flame was no longer observed. A digital camera was employed to monitor the flammability of the separator. Raman spectroscopy (PTT UniRAM; with) was used to analyze the coordination structures of various electrolytes (a solid-state laser with a wavelength of 532 nm was adopted). The chemical composition of the solid electrolyte interface (SEI) layer on Li surface after 5 electrodeposition/stripping cycles was explored using X-ray photoelectron spectroscopy (XPS) equipped with an Al Kα radiation source. The microstructures of the electrodes were inspected using scanning electron microscopy (SEM, FEI Inspect F50). To conduct the chronopotentiometry experiments, a multi-channel electrochemical analyzer (Biologic, VSP-300) was utilized. The coin cells were charged and discharged at various current densities (i.e., 0.4, 1, 1.6, 2, and 6 mA cm 2). The graphite charge/discharge tests were performed at different C rates in a voltage range of 0.01–2 V. Electro chemical impedance spectroscopy (EIS) measurement of symmetric Li// Li cells was carried out at open-circuit potential with a frequency range of 100 kHz to 1 mHz and a potential amplitude of 10 mV. All electro chemical measurements were performed at ambient temperature.
1. Introduction Lithium-ion batteries (LIBs) are usually the primary choice when high energy/power densities are needed [1,2]. Frequently used com ponents within the LIBs configuration are the electrodes (anode and cathode), separator, and the electrolyte. Commonly, graphite is employed as the anode material due to the low charge-discharge plateau, good mechanical stability, and high reversibility during Liþ insertion/extraction processes. However, graphite has a relatively low Li-storage capacity (theoretically 372 mAh g 1), and thus cannot meet the high capacity demand for next-generation LIBs [3,4]. Therefore, there is a significant need for developing safe, high-performance, du rable, and non-toxic alternative materials for the anode side [5,6]. Li metal is a promising anode candidate due to its high theoretical capacity (3860 mAh g 1), low density (0.59 g cm 3), and low redox potential ( 3.04 V vs. standard hydrogen electrode) [7]. However, Li anodes commonly demonstrate short cycle life, low coulombic efficiency, and poor structural stability due to high reactivity with the electrolyte. Accordingly, it is necessary to further improve the electrolyte compo sition for next-generation high-capacity rechargeable Li batteries (e.g., Li-oxide, Li–S, and Li-air batteries) [7,8]. High reactivity between the electrolyte and Li metal usually results in the formation of dendritic Li structures during cycling. Uncontrolled growth of the dendritic Li metal on the anode results in a needle-like structure that can puncture the separator, short-circuit the battery, and subsequently result in hazardous thermal runaway [9–12]. Considering the critical roles of electrolyte on the battery performance, engineering the electrolyte composition is of great importance for pre venting the formation of the dendritic Li structures. The electrolyte is usually composed of a Li salt, solvent, and supporting additives. An ideal electrolyte should provide high ionic conductivity (which is usually inversely proportional to its viscosity) and high electrochemical and thermal stability [11,13,14]. Several previous efforts have been dedi cated to improving the electrolyte composition. Recently, the develop ment of organic electrolytes has suggested an effective approach for avoiding undesired Li dendrite formation. Yamada et al. have reported the use of highly concentrated lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in organic elec trolyte for improving the electrochemical performance of graphite an odes [15,16]. The concentrated electrolyte has also been utilized for stabilizing the Li metal deposition and suppressing the unwanted growth of Li dendrites [17–19]. Some other studies have reported on the application of LiFSI as an alternative to LiPF6 salt to improve the chemical/thermal stability, suppressing HF generation and thus unde sired side reactions [20–22]. Previous studies in the field have confirmed the practicality of LiFSI as a promising electrolyte salt for LIBs. However, further development is required for improving the stability and enhancing the coulombic effi ciency of the electrolyte during cycling. Therefore, this work has been dedicated to developing a series of LiFSI-based electrolytes for Li metal and graphite anodes. The influences of Li salt type and concentration, solvent, and electrolyte coordination status on the Li metal and graphite anode properties are systematically investigated. Finally, an optimized electrolyte composition including highly concentrated LiFSI salt along with DMC/EC solvent has been proposed. It has been shown that the unique electrolyte coordination status of the highly concentrated LiFSI electrolyte significantly improves the coulombic efficiency as well as the rate capability of both Li metal and graphite anodes.
3. Results and discussion Organic carbonate-based electrolytes have been widely used for LIBs due to the wide electrochemical stability window as well as superior compatibility with the intercalated electrodes. However, the use of carbonate-based electrolytes with Li metal anodes substantially lowers the coulombic efficiency. The major reason for this is the formation of dendrites during Li deposition/stripping cycling [25–28]. Therefore, it is of great importance to explore the effects of electrolyte composition on the formation/growth of the dendritic structures during cycling. To this end, we first characterized the influences of Li salt type and concentration on the coulombic efficiency of Li anodes. The deposition/ stripping data have been compared with the Li//Cu half cells. The total charge density for Li electrodeposition was 2 mAh cm 2 and the coulombic efficiency was assessed via calculating the ratio of discharge (stripping) to charge (deposition) capacity. Fig. 1(a)–(b) show the chronopotentiometric characteristics of the cells with 1 M LiPF6-DEC/ EC and 1 M LiFSI-DEC/EC electrolytes. The coulombic efficiency values for the former cell were 90.3 and 80.0% at 0.4 and 6 mA cm 2,
2. Experimental To prepare the electrolytes, LiPF6 (KISHIDA Chemical Co., purity: 99.9%) or LiFSI (KISHIDA Chemical Co., purity: 99.9%) was dissolved in various kinds of organic solvent (i.e., diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylene carbonate (EC)). For the binary DEC/EC and DMC/EC systems, a volume ratio of 1:1 was used for the two 2
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respectively. As for the 1 M LiFSI-DEC/EC cell, the values increased to 93.1 and 85.2% under the same operating conditions (i.e., 0.4 and 6 mA cm 2). The enhanced efficiency can be ascribed to the better passivation ability of the LiFSI-derived SEI [29], which prevents excessive decom position of the electrolyte [30]. To explore the effects of Li salt concentration on the cell perfor mance, the 3 and 5.5 M LiFSI-DEC/EC cells were also assessed (see Fig. 1 (c)–(d)). Herein 5.5 M LiFSI in DEC/EC is around the maximum solu bility. Therefore, we cannot further increase the concentration. As shown in Table 1, the concentration of LiFSI plays a crucial role in determining the coulombic efficiency. The use of 5.5 M LiFSI-DEC/EC electrolyte enhanced the coulombic efficiency to 95.5% at 0.4 mA cm 2 and 93.3% at 6 mA cm 2. Generally, increased coulombic effi ciency indicates reduced Li consumption during the Li deposition/ stripping process. At lower LiFSI concentrations, the free uncoordinated solvent molecules existing in the electrolyte have a high tendency to react with Li metal, generating Li2CO3, Li2O, and other Li compounds [31–33]. This SEI film is a discontinuous layer that allows the electrolyte penetration, inducing further SEI formation. As the repetitive charge/ discharge cycles progress, the deposited Li is gradually encased by a thick SEI layer and is disconnected from the bulk Li metal, eventually forming electrically inert, “dead” Li particles. Such an undesired for mation of dendrites further reduces the coulombic efficiency. In contrast, increasing the concentration of LiFSI reduces the number of free solvent molecules and thus increases the coulombic efficiency. Meanwhile, the anions are easily coordinated with the Liþ ions, creating a cross-linked network capable of inhibiting the growth of unwanted Li dendritic structures [25]. Fig. S1 (see Electronic Supporting Information) compares the cycling data of symmetric Li//Li cells with 1 M and 5.5 M LiFSI-DEC/EC electrolytes. The polarization of the former cell increased more rapidly, indicating faster interfacial resistance growth. Fig. S2 shows the cycling data of Cu//LiFePO4 cells with the two electrolytes. Again, the cell with 1 M LiFSI-DEC/EC electrolyte exhibited more sig nificant capacity decay upon cycling. Both data indicate the benefits of using the high-concentration electrolyte for improving the coulombic efficiency and cycleability. As shown in Fig. 2, a significant percentage of free solvent molecules exists within the 1 M electrolyte. However, the intensities of contact ion
Table 1 Coulombic efficiencies of Li deposition/stripping measured at various current densities in various electrolytes. Electrolyte\Current density
0.4 mA/ cm2
1 mA/ cm2
1.6 mA/ cm2
2 mA/ cm2
6 mA/ cm2
1 M LiPF6-DEC/EC 1 M LiFSI- DEC/EC 3 M LiFSI-DEC/EC 5.5 M LiFSI-DEC/ EC 5.5 M LiFSI-DMC/ EC 5.5 M LiFSI-DMC
90.3% 93.1% 94.9% 95.5%
90.1% 93.0% 94.4% 95.2%
89.0% 92.6% 94.0% 94.9%
87.0% 90.9% 93.8% 94.7%
80.0% 85.2% 92.0% 93.3%
97.0%
95.6%
95.4%
95.0%
94.5%
65.0%
59.0%
55.2%
50.0%
45.0%
pair (CIP) and ion aggregate (AGG) become more evident with increasing the LiFSI concentration. The higher ratios of CIP and AGG reflect the formation of a three-dimensional ion network and confirm the reduced number of free solvent molecules [34]. That is, the solvent molecules in the electrolyte are mainly coordinated with the Li salt, and the concentration of free solvent molecules is considerably decreased. When the concentration reaches a critical level, the ions within the electrolyte coordinate to form a liquid-phase network improving the anti-flammability (see the following section for further details). The less amount of free solvent molecules also reduces the undesired side re actions between the electrolyte and Li metal. To delineate the formation of Li dendrites, SEM observation was performed to characterize the Li morphologies on the Cu substrates upon Li-deposition/stripping cycling. Fig. 3(a)‒3(d) present top-view SEM images of the Li deposited in various electrolytes, including 1 M LiPF6DEC/EC, 1 M LiFSI-DEC/EC, 3 M LiFSI-DEC/EC, 5.5 M LiFSI-DEC/EC, after five consecutive charge-discharge cycles. It can be seen from Fig. 3(a) that using traditional 1 M LiPF6-DEC/EC electrolyte tends to generate a slender needle-like structure, so-called a “dendritic structure” on Cu foil. As discussed earlier, the formation of the dendritic structure is expected when the Li metal anode is used in conjunction with LiPF6 carbonate electrolyte. When the LiPF6 was replaced with LiFSI salt (see Fig. 3(b)), the resulting structure still exhibited a needle-like topog raphy. However, at high electrolyte concentration, the microstructures of the Li deposits changed to knotted round structures, as depicted in
Fig. 1. Chronopotentiometry curves of various Li//Cu cells containing (a) 1 M LiPF6-DEC/EC, (b) 1 M LiFSI-DEC/EC, (c) 3 M LiFSI-DEC/EC, and (d) 5.5 M LiFSIDEC/EC electrolytes measured at different current densities. 3
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Fig. 3(c)–(d). For comparison, an SEM image of the Li deposited in 5.5 M LiFSI-DEC/EC after 20 cycles is given in Fig. S3. The Li morphology still maintained a knot-type structure without any dendrites after cycling. This finding demonstrates that the high-concentration electrolyte is effective in suppressing the formation/growth of needle-like structures. It is well known in the field that the chemical composition of SEI is strongly related to the electrolyte composition; it is also reported that the SEI layer directly affects the behavior of Li deposition [35]. Gener ally, LiF is formed by degradation of Li salt within the electrolyte. The presence of LiF in the SEI layer is helpful for improving the coulombic efficiency due to its good ionic conductivity as well as enhanced me chanical stability [35]. To further explore the SEI properties, XPS was employed to characterize the chemical composition of the SEI layers on the deposited Li. The narrow-scan C1s, F 1s, and Li 1s spectra of the SEI films formed in 1 M LiPF6-DEC/EC and 5.5 M LiFSI-DEC/EC electrolytes are illustrated in Fig. 4. First, we found that the high concentrations of – C–O, and CO3 of the1 M LiPF6-DEC/EC sample are associated C–O, O– with solvent decomposition, which is considered unfavorable for Liþ transport. Second, the LiF content within the SEI layer was significantly increased using 5.5 M LiFSI-DEC/EC electrolyte. The high LiF content, induced by the high concentration of LiFSI salt, improved the coulombic efficiency of the Li metal anodes. In addition, high concentration of LiFSI promoted the formation of F–S bonding within the SEI layer. As compared to PF6 anion, the FSI anion, in either carbonate or ether sol vents, demonstrates superior compatibility toward Li metal [36,37]. As a consequence, better reversibility of Li deposition/stripping can be obtained. Next, it is crucial to explore the influence of solvent composition on the reversibility of Li deposition/stripping. Fig. 5(a) shows the chro nopotentiometric characteristics of the Li//Cu cells using 5.5 M LiFSIDMC/EC electrolyte. As compared to the 5.5 M LiFSI-DEC/EC cell (see Fig. 1(d)), the coulombic efficiency was enhanced via using DMC in the
Fig. 2. Raman spectra of various LiFSI-DEC/EC electrolytes.
Fig. 3. Top-view SEM images of the Li deposited in (a) 1 M LiPF6-DEC/EC, (b) 1 M LiFSI-DEC/EC, (c) 3 M LiFSI-DEC/EC, and (d) 5.5 M LiFSI-DEC/EC electrolytes. 4
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Fig. 4. XPS (a) C 1s, (a) F 1s, and (b) Li 1s spectra of the Li samples deposited in 1 M LiPF6-DEC/EC and 5.5 M LiFSI-DEC/EC electrolytes.
electrolyte (see Table 1). The efficiency values the 5.5 M LiFSI-DMC/EC cells were increased to 97.0% and 94.5% at 0.4 and 6 mA cm 2, respectively. This reveals that the application of DMC/EC is preferable over DEC/EC for a Li metal anode, probably related to the high con ductivity and lower viscosity of the former electrolyte (see Table 2). The influence of the EC addition in the electrolyte on the coulombic effi ciency was also examined; the data are shown in Fig. 5(b). Without the presence of EC, the cell using 5.5 M LiFSI-DMC electrolyte showed a poor coulombic efficiency, especially at high rates. Increasing the cur rent density to 6 mA cm 2 resulted in a significant reduction in the coulombic efficiency (to ~ 45%). It is known that the viscosity of EC is
higher than DMC. Therefore, addition of EC increases the electrolyte viscosity. However, EC has a higher dielectric constant (89.8 vs. 3.1 for DMC), which promotes Li salt dissociation, increasing the ionic con ductivity (see Table 2). Therefore, it is inferred that addition of EC in the electrolyte can enhance formation of a SEI and thus reversibility of Li deposition/stripping. The highly concentrated LiFSI-DMC/EC electro lyte is thought to be promising for Li metal anodes. Top-view SEM images of the Li deposited in 5.5 M LiFSI-DMC/EC and 5.5 M LiFSI-DMC are given in Fig. 5(c) and (d), respectively. There are relatively flat Li structures formed in the former electrolyte. Such a favorable microstructure is beneficial for maintaining the coulombic
Fig. 5. Chronopotentiometry curves of Li//Cu cells containing (a) 5.5 M LiFSI-DMC/EC and (b) 5.5 M LiFSI-DMC electrolytes measured at different current densities. SEM images of the Li deposited in (c) 5.5 M LiFSI-DMC/EC and (d) 5.5 M LiFSI-DMC electrolytes. 5
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electrochemical properties of graphite anodes, the charge-discharge experiments were conducted with 1 M LiPF6-DEC/EC, 5.5 M LiFSIDEC/EC, 5.5 M LiFSI-DMC, and 5.5 M LiFSI-DMC/EC electrolytes at various rates, as shown in Fig. 7(a)-7(d). The specific capacities of the graphite anode with a conventional electrolyte (i.e., 1 M LiPF6-DEC/EC) were 320 and 48 mAh g 1 at 0.1 and 2 C, respectively. Using 5.5 M LiFSIDMC and 5.5 M LiFSI-DMC/EC electrolytes, the specific capacitances were enhanced to 349 and 355 mAh g 1 at 0.1 C, approaching the theoretical capacity of graphite (i.e., 372 mAh g 1 according to the formation of LiC6 intercalation compound) [4]. The specific capacities of the graphite anodes measured at various C rates in different electrolytes are listed in Table S1 and illustrated in Fig. 7(e). It is worth noting that with the implementation of high concentration LiFSI (even without the use of EC), the graphite anode can deliver a higher specific capacity than that found in the conventional 1 M LiPF6-DEC/EC electrolyte. The rea sons for the improved performance may include: (i) the high-concentration Li salt tends to generate the CIP and AGG coordi nation states, which increase the ion dissociation kinetics at the elec trode interface, (ii) the SEI layer, formed in the highly concentrated electrolyte, facilitates Liþ ion transport, and (iii) the high Liþ concen tration results in high lithiation flux into the graphitic anode [40]. We also demonstrate that with the presence of EC in the electrolyte, the rate capability of the graphite anode can be improved. In addition, DMC/EC solvent seems to be superior to DEC/EC solvent, especially at high C rate. At 2 C, the optimal capacity of the graphite anode is 120 mAh g 1, which is found for 5.5 M LiFSI-DMC/EC electrolyte. One of the major concerns utilizing the conventional organic elec trolyte is the potential fire hazard. When the Li metal electrode is used as the anode within the cell, the electrolyte design is particularly crucial to address the safety issues. Fig. 8 shows the results of flammability tests conducted for 1 M LiPF6-DEC/EC and 5.5 M LiFSI-DMC/EC electrolytes. As shown in Fig. 8(a), the flammability is very intense for the conven tional 1 M LiPF6-DEC/EC electrolyte. However, the flammability of 5.5 M LiFSI-DMC/EC is much less, as revealed in Fig. 8(b). The improved anti-flammability of the latter electrolyte is related to the coordination between the Liþ ions and FSI anions and solvent molecules, forming a three-dimensional polymeric network that reduces the electrolyte volatility [17–19]. Since most of the solvent molecules are coordinated with Liþ ions, the number of free solvent molecules decreased, sup pressing the reactivity of the electrolyte. The low flammability reflects high safety of the proposed 5.5 M LiFSI-DMC/EC electrolyte.
Table 2 Viscosity and ionic conductivity of various electrolytes. Electrolyte
Viscosity (cP)
Conductivity (mS/cm)
1 M LiPF6-DEC/EC 1 M LiFSI-DEC/EC 3 M LiFSI-DEC/EC 5.5 M LiFSI-DEC/EC 5.5 M LiFSI-DMC/EC 5.5 M LiFSI-DMC
4.8 3.6 7.8 36.3 28.9 26.2
8.22 7.46 1.82 0.46 0.69 0.51
efficiency even upon prolonged charge/discharge cycling. In contrast, in the absence of the EC (i.e., 5.5 M LiFSI-DMC), the resulting Li morphology is relatively rough and irregular. An extended charge/discharge cycling along with EIS analyses were conducted with a symmetric Li//Li cell with 5.5 M LiFSI-DMC/EC electrolyte; the obtained data are shown in Fig. 6. The voltage profile (see Fig. 6(a)) confirms a stable performance for the symmetric cell during the long-term cycling (more than 20 days of continuous cycling). There is no significant polarization increase in the profile, which can be attributed to the formation of a robust SEI and the suppression of Li dendrite growth [38,39]. Fig. 6(b) confirms that the electro de/electrolyte interfacial resistance is rather steady upon 500 cycles. The cycling stability data of a symmetric Li//Li cell with 1 M LiPF6-DMC/EC electrolyte are shown in Fig. S4. The initial overpotential amplitude was similar to that of the 5.5 M LiFSI-DMC/EC cell, inidcating that although the ionic conductivity of 1 M LiPF6-DMC/EC was considerably lower (see Table 2) the corresponding SEI resistance and interfacial charge transfer resistance are higher. Moreover, as shown in Fig. S4, the polarization rapidly increases with increasing the cycle number, reflecting a poor interface stability. This 1 M LiPF6-DMC/EC electrolyte is thus not adequate to be used for Li metal anodes. To probe the effects of the electrolyte composition on
4. Conclusions This study demonstrates that highly concentrated LiFSI-DEC/EC and LiFSI-DMC/EC can be utilized as high-performance and stable electro lytes for Li batteries. A series of experiments with various Li salt types, concentrations, and solvent types of the electrolytes were conducted to assess their influences on the performance of Li metal and graphite an odes. It was shown that the application of 5.5 M LiFSI-DMC/EC or 5.5 M LiFSI-DEC/EC electrolyte (instead of 1 M LiPF6-DEC/EC electrolyte) effectively improved the coulombic efficiency and rate capability of Li metal anodes. In the former electrolyte, the coulombic efficiency values were 97.0% at 0.4 mA cm 2 and 94.5% at 6 mA cm 2. Such an improvement is related to the reduce solvent reactivity (e.g., the free solvent amount), better SEI stability, and the knot-type Li morphology. This study also demonstrated that the highly concentrated LiFSI elec trolyte, especially 5.5 M LiFSI-DMC/EC, can lead to a high specific ca pacity and great rate capability of a graphite anode. Moreover, the low flammability of this electrolyte was also confirmed. The proposed electrolyte has shown great potential for high-performance and highsafety battery applications. Declaration of competing interest
Fig. 6. (a) Voltage profile of Li/Li symmetric cell with 5.5 M LiFSI-DMC/EC electrolyte cycled at 1 mA cm 2 for 500 times. (b) variation of EIS spectra of above cell upon cycling.
The authors declare that they have no known competing financial 6
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Journal of Power Sources 450 (2020) 227657
Fig. 7. Charge-discharge curves of graphite anodes measured in (a) 1 M LiPF6-DEC/EC, (b) 5.5 M LiFSI-DEC/EC, (c) 5.5 M LiFSI-DMC, and (d) 5.5 M LiFSI-DMC/EC electroelytes. (e) Rate capability comparison of various cells.
interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The financial support provided for this work by the Ministry of Sci ence and Technology (MOST) of Taiwan is gratefully appreciated. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227657. References [1] Z.S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.M. Cheng, ACS Nano 4 (2010) 3187–3194. [2] P. Guo, H. Song, X. Chen, Electrochem. Commun. 11 (2009) 1320–1324. [3] P. Lian, X. Zhu, H. Xiang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 56 (2010) 834–840. [4] G. Wang, X. Shen, J. Yao, J. Park, Carbon 47 (2009) 2049–2053. [5] M.S. Whittingham, Proc. IEEE 100 (2012) 1518–1534. [6] W.A. Henderson, J. Phys. Chem. B 110 (2006) 13177–13183. [7] J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J. G. Zhang, Nat. Commun. 6 (2015) 6362 1–9. [8] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Nat. Mater. 11 (2012) 19–29. [9] E. Mengeritsky, P. Dan, I. Weissman, A. Zaban, D. Aurbach, J. Electrochem. Soc. 143 (1996) 2110–2116.
Fig. 8. Flammability tests of glass fiber membranes containing (a) 1 M LiPF6DEC/EC and (b) 5.5 M LiFSI-DMC/EC electrolytes.
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Journal of Power Sources 450 (2020) 227657 [25] X.B. Cheng, R. Zhang, C.Z. Zhao, F. Wei, J.G. Zhang, Q. Zhang, Adv. Sci. 3 (2016) 1–20, 1500213. [26] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.G. Zhang, Energy Environ. Sci. 7 (2014) 513–537. [27] X.B. Cheng, Q. Zhang, J. Mater. Chem. 3 (2015) 7207–7209. [28] N. Togasaki, T. Momma, T. Osaka, J. Power Sources 294 (2015) 588–592. [29] L.E.C. Forero, T.W. Smith, P.B. Balbuena, J. Phys. Chem. C 121 (2017) 182–194. [30] H.B. Han, S.S. Zhou, D.J. Zhang, S.W. Feng, L.F. Li, K. Liu, W.F. Feng, J. Nie, H. Li, X.J. Huang, M. Armand, J. Power Sources 196 (2011) 3623–3632. [31] K. Kanamura, H. Tamura, S. Shiraishi, Z.I. Takehara, J. Electrochem. Soc. 142 (1995) 340–347. [32] Y. Lu, Z. Tu, J. Shu, L.A. Archer, J. Power Sources 279 (2015) 413–418. [33] C.R. Yang, Y.Y. Wang, C.C. Wan, J. Power Sources 72 (1998) 66–70. [34] Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc. 136 (2014) 5039–5046. [35] C. Wan, S. Xu, M.Y. Hu, R. Cao, J. Qian, Z. Qin, J. Liu, K.T. Mueller, J.G. Zhang, J. Z. Hu, ACS Appl. Mater. Interfaces 9 (2017) 14741–14748. [36] B. Duan, W. Wang, H. Zhao, A. Wang, M. Wang, K. Yuan, Z. Yu, Y. Yang, ECS Electrochem. Lett. 2 (2013) A47–A51. [37] Q. Ma, Z. Fang, P. Liu, J. Ma, X. Qi, W. Feng, J. Nie, Y.S. Hu, H. Li, X. Huang, L. Chen, ChemElectroChem 3 (2016) 531–536. [38] X. Fan, L. Chen, X. Ji, T. Deng, S. Hou, J. Chen, J. Zheng, F. Wang, J. Jiang, K. Xu, C. Wang, Chem 4 (2018) 174–185. [39] G.M.A. Girard, M. Hilder, D. Nucciarone, K. Whitbread, S. Zavorine, M. Moser, M. Forsyth, D.R. MacFarlane, P.C. Howlett, J. Phys. Chem. C 121 (2017) 21087–21095. [40] Y. Yamada, M. Yaegashi, T. Abe, A. Yamada, Chem. Commun. 49 (2013) 11194–11196.
[10] M. Gauthier, T.J. Carney, A. Grimaud, L. Giordano, N. Pour, H.H. Chang, D. P. Fenning, S.F. Lux, O. Paschos, C. Bauer, F. Maglia, J. Phys. Chem. Lett. 6 (2015) 4653–4672. [11] K. Xu, Chem. Rev. 104 (2004) 4303–4417. [12] S.K. Jeong, H.Y. Seo, D.H. Kim, H.K. Han, J.G. Kim, Y.B. Lee, Y. Iriyama, T. Abe, Z. Ogumi, Electrochem. Commun. 10 (2008) 635–638. [13] T. Abe, F. Sagane, M. Ohtsuka, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 152 (2005) A2151–A2154. [14] T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 151 (2004) A1120–A1123. [15] Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc. 136 (2014) 5039–5046. [16] Y. Yamada, K. Usui, C.H. Chiang, K. Kikuchi, K. Furukawa, A. Yamada, ACS Appl. Mater. Interfaces 6 (2014) 10892–10899. [17] D. Aurbach, J. Electrochem. Soc. 134 (1987) 1611–1620. [18] D. Aurbach, J. Electrochem. Soc. 142 (1995) 2873–2882. [19] G.V. Zhuang, K. Xu, H. Yang, T.R. Jow, P.N. Ross, J. Phys. Chem. B 109 (2005) 17567–17573. [20] J. Wang, Y. Yamada, K. Sodeyama, C.H. Chiang, Y. Tateyama, A. Yamada, Nat. Commun. 7 (2016) 12032 1–9. [21] S.T. Myung, Y. Sasaki, S. Sakurada, Y.K. Sun, H. Yashiro, Electrochim. Acta 55 (2009) 288–297. [22] X. Zhang, T.M. Devine, J. Electrochem. Soc. 153 (2006) B344–B351. [23] A.A. Assegie, C.C. Chung, M.C. Tsai, W.N. Su, C.W. Chen, B.J. Hwang, Nanoscale 11 (2019) 2710–2720. [24] C. Arbizzani, G. Gabrielli, M. Mastragostino, J. Power Sources 196 (2011) 4801–4805.
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