Nonflammable Fluorinated Carbonate Electrolyte with High Salt-to

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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

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Nonflammable Fluorinated Carbonate Electrolyte with High Salt-toSolvent Ratios Enables Stable Silicon-Based Anode for NextGeneration Lithium-Ion Batteries Guifang Zeng,† Yongling An,† Shenglin Xiong,‡ and Jinkui Feng*,† †

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SDU & Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid Solid Structural, Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China ‡ School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: High energy density and safety are two key factors for the development of next-generation lithium-ion batteries. Recently, silicon (Si) has attracted tremendous interest owing to its high theoretical capacity. However, the fast capacity decay triggered by huge volume change restricts its practical application. Moreover, higher energy density brings about more serious safety issues. To solve these problems, here we propose a safer high salt-to-solvent electrolyte that consisted of nonflammable mixture solvents of di-2,2,2-trifluoroethyl carbonate and fluoroethylene carbonate. It is revealed that this electrolyte could not only enhance the cycling stability toward the silicon nanoparticle (SiNPs) anode but also solve the safety hazards. A high initial reversible capacity of 2644 mAh g−1 and a low capacity fading rate (only 0.064% per cycle) after 300 cycles are delivered. The performance enhancement mechanism is further explored by electrochemical impedance spectroscopy, Fourier transform infrared, and scanning electron microscopy. This study may shed an inspiring light on the development of next-generation high-energy-density batteries. KEYWORDS: concentrated electrolyte, fluoroethylene carbonate (FEC), safe lithium batteries, di-2,2,2-trifluoroethyl carbonate (TFEC), nonflammable, silicon

1. INTRODUCTION Lithium-ion batteries (LIBs) have been widely used in our daily life and may dominate the electric vehicles.1−4 Nevertheless, the energy density of current LIBs is insufficient to fulfill the expectation of electric vehicles.3−6 Moreover, the state-of-the-art electrolytes for LIBs make use of flammable carbonates as the main solvents, which may bring about serious safety problems.5,7 Both the energy density and safety issues have been the key hindrances for the further application of LIBs.1−11 To enhance the high-energy density, one practical approach is to utilize cathode or anode materials with higher capacity.12 Recently, Si is considered as a promising anode candidate due to its high theoretical capacity (∼3700 mAh g−1), abundant resource, and low working platform.13−18 Unfortunately, its electrochemical performance is inhibited by several key factors.14,15,19,20 The main challenge is the huge volume change (about 300%) during the alloying and dealloying process, which may cause cracking and crumbing of active material, leading to partial and/or full disconnection with current collectors and exposing new surfaces to cost an extra solid electrolyte interphase (SEI) layer, which results in active material loss, low Coulombic efficiency (CE), and fast capacity decay.14,20−22 To settle these issues, many strategies have been © 2019 American Chemical Society

proposed such as nanostructure designing, carbon coating, binder optimizing, the use of SEI additives, etc.14,23 However, the reports on developing a new electrolyte system for silicon based anodes are rare.14 Fluorinated carbonates have been proved to possess excellent physicochemical and electrochemical properties compared with their hydrogenated counterparts such as lower melting point, more stable SEI film, and higher oxidation stability.5,24,25 Moreover, recently it is found that certain fluorinated organic carbonates are nonflammable.26−28 Thus, utilizing fluorinated carbonates as electrolyte solvents may be a new strategy to enhance the safety of lithium batteries.26,27 In another aspect, concentrated electrolytes recently have attracted much attention due to their unique high salt-tosolvent characteristics, which could enhance the compatibility of electrolytes to electrodes.10,29−37 Different from the common dilute electrolyte, the highly concentrated solvents contain less free solvent molecules, which are more reactive.32 Benefiting from the special electrolyte, a robust passivation film can be formed on the surface of electrodes, which may alleviate Received: March 29, 2019 Accepted: June 5, 2019 Published: June 5, 2019 23229

DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical molecular structures of EC, EMC, LiPF6, FEC, TFEC, and LiFSI. (b) XRD patterns of SiNPs and the SiNP electrode. (c) The ionic conductivities of different electrolytes. (d) Concentration dependences of the ionic conductivities of the LiFSI/FEC−TFEC electrolyte at room temperature. Flammability tests of (e) 1 M LiPF6/EC−EMC and (f) 3.5 M LiFSI/FEC−TFEC electrolytes.

the degradation of electrodes and electrolytes.30,32−34,38 It can be expected that the combination of nonflammable fluorinated carbonates with high concentrated salts may provide an attractive multifunctional electrolyte system for silicon-based anodes. Here, we demonstrate an electrolyte composed of mixed fluorinated carbonate solvent of fluoroethylene carbonate (FEC) and di-2,2,2-trifluoroethyl carbonate (TFEC) with 3.5 M lithium bis(fluorosulfonyl)imide (LiFSI) as the salt. It is found that this electrolyte system is nonflammable; moreover, an Si-based anode shows a significantly more stable cycling performance (0.064% per cycle capacity decay over 300 cycles) in this electrolyte. The performance enhancing mechanism is further probed, and it is attributed to a more robust SEI layer generated on the silicon-based anode.

3. RESULTS AND DISCUSSION Figure 1a summarizes the chemical molecule structures of all of the solvents investigated in this study, including EC, EMC, LiPF6, FEC, TFEC, and LiFSI.12 FEC is proved to be an effective anode-filming additive for Si-based electrodes in previous reports.20,39 FEC is less flammable than other common cyclic carbonates, such as EC and propylene carbonate (PC), due to the introduction of an F atom.5,24 Above all, the solubility of LiFSI salt in FEC is much higher than that in TFEC. TFEC is a nonflammable solvent;26 however, according to our study, the solubility of LiFSI salt in TFEC is limited (less than 1 mol L−1). Crystal phase structures of SiNPs and the SiNP anode were characterized by XRD. Figure 1b displays the characteristic peaks of Si and copper, which confirmed that the silicon is pure phase.13,23 In addition, the morphology of SiNPs with a typical size of about 50 nm was verified by the scanning electron microscopy (SEM) image (Figure S1). The ionic conductivity of different electrolytes was also measured. Figure 1c shows the ionic conductivity of 1 M LiPF6/EC−EMC at room temperature reaching a value of 9.60 mS cm−1. Figure 1d manifests the ionic conductivity of LiFSI/FEC−TFEC electrolytes at diverse concentrations. As we can see, the conductivity slowly increases with the LiFSI content increase, then slowly decreases. The ionic conductivity of 3.5 M is 1.21 mS cm−1, and the low conductivity may be ascribed to the high viscosity in a concentrated salt electrolyte.10 To solve this issue without sacrificing the advantages of the concentrated electrolyte, it is universal to add some inert solvents with low viscosity into the concentrated electrolyte.40 The flammability of 3.5 M LiFSI/ FEC−TFEC and 1 M LiPF6/EC−EMC was examined by a direct ignition test. It can be seen that the common carbonatebased electrolytes rapidly caught fire after ignition, whereas the hybrid fluorinated solvents can’t be ignited (Figure 1e,f). These results proved that the 3.5 M LiFSI/FEC−TFEC electrolyte has an inner nonflammable character, which could greatly enhance the safety of batteries.30,32 CV curves of SiNPs in different electrolytes are also explored (Figure 2). All of the CV curves are similar in shape, suggesting

2. EXPERIMENTAL SECTION The control electrolyte was composed of 1 M lithium hexafluorophosphate (LiPF6) salt, ethyl methyl carbonate (EMC), and ethylene carbonate (EC) (EMC/EC, 7:3 v/v), purchased from Nanjing Mojiesi Energy Technology Co. Ltd. (China). The fluorinated electrolytes were prepared by dissolving 1, 3.5, and 4.3 M LiFSI in an FEC/TFEC mixture solvent (3:7 by volume), respectively. All of these electrolytes were confected in a glovebox. The glovebox is full of nitrogen (H2O, O2 < 0.1 ppm). All of these reagents were applied directly without further purification. The anode was prepared with SiNP, sodium carboxymethyl cellulose binder, and carbon black (70:15:15, by weight). The cathode electrode was prepared with LiFePO4, polyvinylidene fluoride, and carbon black (80:10:10, by weight). A galvanostatic charger was used to evaluate the electrochemical performance of SiNPs. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were tested by an electrochemical workstation. Crystal structures of SiNPs and the SiNPs electrode were performed via X-ray diffraction (XRD) with a Rigaku D/Max-RC diffractometer. Surface morphologies and size of cycled electrodes were obtained via an SU-70 SEM. Fourier transform infrared (FTIR) analysis of SiNPs was performed by a NICOLET AVATAR 360 FTIR spectrometer. 23230

DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

Research Article

ACS Applied Materials & Interfaces

Figure 2. CV plots of SiNPs in (a) 1 M LiPF6/EC−EMC and (b) 1, (c) 3.5, and (d) 4.3 M LiFSI/FEC−TFEC electrolytes. The scanning speed is 0.1 mV s−1 (0.01−3 V).

Figure 3. Electrochemical performance of the SiNP anode at 500 mA g−1 (0.01−3 V). (a) Initial discharge/charge profiles. (b) Cycling performance. (c) Discharge/charge curves in the 3.5 M LiFSI/FEC−TFEC electrolyte. (d) Coulombic efficiencies.

3611, 3480, and 2482 mAh g−1, respectively, corresponding to 62, 66, 76, and 73% CEs, which suggest that CE could also be improved by the fluorinated electrolyte.23,39 The irreversible capacity is well ascribed to the formation of an SEI film.13,39 Furthermore, a decline of discharge capacity in the 4.3 M LiFSI/FEC−TFEC electrolyte can be assigned to the lower conductivity and higher viscosity in the concentrated electrolyte.23,32,34,39 The charge−discharge profile of the SiPN anode in the 3.5 M LiFSI/FEC−TFEC electrolyte is overlapped well (Figures 3c and S2−S4), indicating a stable cycling performance.13 Figure 3b shows a comparison of the discharge capacities in different electrolytes. After 50 cycles, the capacity

the same delithiation/lithiation mechanism.13 The wide peaks (0.5−2.5 V) in the first cycle are assigned to the SEI layer formation.13,19,41 The strong peaks located at (0.01−0.18 V) and (∼0.3 and 0.5 V) are related to the lithium alloying and dealloying with Si.19,23,42 The electrochemical performance of the SiNP anode in different electrolytes was estimated using a half-cell test (Li|| SiNPs). The first discharge−charge electrochemical performance in 1 M LiPF6/EC−EMC and in 1, 3.5, and 4.3 M LiFSI/ FEC−TFEC electrolytes was presented (Figure 3a). The initial discharge capacities in the common carbonate electrolyte and in 1, 3.5, and 4.3 M LiFSI/FEC−TFEC electrolyte are 3400, 23231

DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

Research Article

ACS Applied Materials & Interfaces

Figure 4. Electrochemical performance of the SiNP anode in different electrolytes (0.01−3 V). Cycling capability at (a) 1000 mA g−1, (b) 2000 mA g−1 (in the first three cycles at 500 mA g−1), and (c) 5000 mA g−1 (in the first three cycles at 500 mA g−1). (d) Rate property.

Figure 5. Electrochemical performance of the LiFePO4 cathode in the 3.5 M LiFSI/FEC−TFEC electrolyte (2−3.8 V). (a) CV profiles at 0.1 mV s−1. (b) Charge/discharge curve. (c) Cycling capability. The current density is 20 mA g−1. (d) Rate property.

capacity in the 3.5 M LiFSI/FEC−TFEC electrolyte still remains 1981 mAh g−1. Notably, the most impressive feature of SiNPs in the 3.5 M LiFSI/FEC−TFEC electrolyte is its cycling stability. After 300 cycles (Figures 4c and S5), no significant capacity fade is observed (0.064% capacity loss rate per cycle and 99.8% CE). As a result, the SiNPs in the 3.5 M LiFSI/FEC−TFEC electrolyte exhibits the best cycling performance among these electrolytes. Figure 4d shows a comparison of capacities of SiNPs in the common carbonate electrolyte and 1, 3.5, and 4.3 M LiFSI/FEC−TFEC electrolytes at different rates. It is obviously seen that rate performance in 1 M LiFSI/FEC−TFEC electrolyte is the best. Capacities of 3300, 2828, 2359, 1912, 1703, 1312, and 2802 mAh g−1 at 500, 1000, 2000, 4000, 5000, and 8000 mA g−1 are

in the common carbonate electrolyte decreases to 1327 mAh g−1 (39% capacity retention) due to the huge volume expansion.39 In contrast, the discharge capacities in 1, 3.5, and 4.3 M LiFSI/FEC−TFEC electrolytes remain 1989, 2105, and 1578 mAh g−1 (55, 60, and 63% capacity retention). In addition, CE of SiNPs in the 3.5 M LiFSI/FEC−TFEC electrolyte grows rapidly to nearly 100% (Figure 3d), which illustrates the irreversible lithium alloying and dealloying reactions, which may be ascribed to a stable SEI film.10 To further compare the electrochemical charge−discharge performances of SiNPs in different electrolytes, rate tests (1000, 2000, and 5000 mA g−1) were performed (Figure 4a−c). Capacity in the common carbonate electrolyte fades rapidly from 3084 to 658 mAh g−1 after 60 cycles, whereas the 23232

DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

Research Article

ACS Applied Materials & Interfaces delivered in the 1 M LiFSI/FEC−TFEC electrolyte, respectively. The excellent rete performance may be ascribed to the higher ionic conductivity and the stable F-rich SEI film.7,10,30,31 Furthermore, to validate the feasibility of 3.5 M LiFSI/ FEC−TFEC electrolyte in practical applications, the LiFePO4 cathode is also tested using a half cell (Li||LiFePO4). The CV plots of LiFePO4 are displayed in Figure 5a. The peaks located at ∼3.2 and 3.6 V are well ascribed to the reversible transformation of Fe2+/Fe3+, respectively.43 Additionally, perfect overlap manifests the excellent capacity retention of LiFePO4 in the 3.5 M LiFSI/FEC−TFEC electrolyte.43 The capacity-potential graphs are shown in Figure 5b. In the first scan, it delivers charge and discharge capacities of 133 and 136 mAh g−1 with a high CE of 97%. LiFePO4 retains 95% capacity retention even after 130 cycles (Figure 5c). Except for the excellent cycling performance, an outstanding rate property is also obtained (Figure 5d). From the above results, it can be concluded that the 3.5 M LiFSI/FEC−TFEC electrolyte could be used as a promising candidate electrolyte for silicon-based batteries. The outstanding electrochemical performance of SiNPs in the 3.5 M LiFSI/FEC−TFEC electrolyte is further probed. The morphology of the initial SiNP electrode is provided in Figure S6, which was homogeneously distributed. However, after 50 cycles, severe shedding of active materials and large cracks can be seen in SiNPs with the common carbonate-based electrolyte, which is ascribed to the huge volume expansion and shrink during the lithiation/delithiation process (Figure 6a).31,33 In contrast, a smooth morphology was observed with the 3.5 M LiFSI/FEC−TFEC electrolyte (Figure 6b), revealing a stable electrode framework.31 which may effectively prevent the SiNPs from losing contact with the current collector and extra cost SEI layer.33 The EIS measurement was also investigated for the SiNP anode in the carbonate electrolyte and the 3.5 M LiFSI/FEC−TFEC electrolyte after 10 and 50 cycles. The semicircle in the high-frequency region is related to the composite resistance of the SEI film and charge transfer (Rct). The slope line in the low-frequency region is consistent with the Warburg resistance. As shown in Figure 6c, compared to the cells cycled in carbonate solvent, the fluorinated electrolyte demonstrated a higher R ct impedance in the 10th cycle due to higher impedance of LiF in the SEI film and the lower electrolyte conductivity.24 However, the Rct resistance in carbonate solvent rapidly increases after 50 cycles, whereas no obvious resistance changes in the fluorinated electrolyte (Figure 6d). The lower resistance change in the fluorinated electrolyte can be ascribed to the stable F-rich SEI film, which can prevent the continuous electrolyte decomposition. In addition, the concentrated LiFSI salt could reduce the free electrolyte molecules, which also help in reducing electrolyte decomposition. The synthetic advantages enable the stable cycling performance of the silicon anode. The lower impedance indicates that the extra growth of SEI is restrained,31 which agrees with the results of SEM images. Based on these results, we can deduce that the SEI features can preserve the initial morphology of SiNPs and effectively self-limit the extra growth of the SEI. In other words, it can mitigate the crack propagation and improve the cycling stability of SiNPs.29 The structure of SEI layer components was also explored with FTIR. The SiNP anodes after 50 cycles obtained from the carbonate electrolyte and 3.5 M LiFSI/FEC−TFEC electrolyte

Figure 6. SEM images of SiNP anodes cycled in (a) 1 M LiPF6/EC− EMC and (b) 3.5 M LiFSI/FEC−TFEC electrolytes. EIS spectra of SiNP anodes after (c) 10th cycle and (d) 50th cycle in 1 M LiPF6/ EC−EMC and 3.5 M LiFSI/FEC−TFEC electrolytes. (e) FTIR pattern of the SiNP electrode in 1 M LiPF6/EC−EMC and 3.5 M LiFSI/FEC−TFEC electrolytes.

are compared. Figure 6e shows the FTIR spectra of SEI layers from 600 to 2500 cm−1. For the anode cycled in common carbonate electrolytes, ROCO2Li, Li2CO3, and polycarbonates compounds are detected, which is the typical chemical composition of the SEI layer.44 For SEI formed in the fluorinated electrolyte, it contains not only the abovementioned products, but also ethylene-oxide-based polymers and sulfur-based compounds, corresponding to the peaks at 1152 cm−1 and 1108, 992, and 901 cm−1.29,44,45 The presence of ethylene-oxide-based polymers can be derived from the FEC solvent decomposition.33,45,46 FEC may decompose to obtain vinylene carbonate (VC) and vinoxyl species.45,47 Then, the presence of poly(VC) and poly(ethylene oxide) can be ascribed to the polymerization of VC and vinoxyl species.44−48 The flexible polymer may further buffer the volume change.45 In addition, organic sulfate-containing LiFSI may also help in forming a more stable SEI layer.29,49

4. CONCLUSIONS In summary, we demonstrate a nonflammable fluorinated carbonate-based concentrated electrolyte. Besides significantly improved safety, the silicon-based anode exhibits an outstanding cycling performance and improved CE in this electrolyte. A high reversible capacity and 0.064% capacity fading rate per cycle are delivered. Our results reveal that the robust SEI layer that is formed from fluorinated carbonates with high concentrated salt is the key factor for superior capacity retention. 23233

DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

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ACS Applied Materials & Interfaces

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05570.

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SEM image and electrochemical performance (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shenglin Xiong: 0000-0002-8324-4160 Jinkui Feng: 0000-0002-5683-849X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Shandong Provincial Natural Science Foundation (China, ZR2017MB001), the Shandong Provincial Science and Technology Key Project (2018GGX104002), the State Key Program of National Natural Science of China (Nos 61633015), the Young Scholars Program of Shandong University (2016WLJH03), and the Project of the Taishan Scholar (Nos tsqn201812002 and ts201511004).

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DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235

Research Article

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DOI: 10.1021/acsami.9b05570 ACS Appl. Mater. Interfaces 2019, 11, 23229−23235