Research Article www.acsami.org
Safety-Reinforced Succinonitrile-Based Electrolyte with Interfacial Stability for High-Performance Lithium Batteries Qingqing Zhang,†,‡ Kai Liu,† Fei Ding,*,‡ Wei Li,† Xingjiang Liu,‡ and Jinli Zhang*,† †
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, P. R. China National Key Laboratory of Science and Technology on Power Sources, Tianjin Institute of Power Sources, Tianjin 300384, P. R. China
‡
S Supporting Information *
ABSTRACT: Different contents of fluoroethylene carbonate (FEC) as cosolvent is added into succinonitrile (SN) solution to form a novel electrolyte for lithium batteries. The SN-based electrolyte with 20 wt % FEC exhibits the most favorable properties involving the good thermal stability, wide electrochemical window and high ionic conductivity. Comparing with the commercial electrolyte, the 20% FECSN electrolyte demonstrates the advantage of high safety and excellent interfacial compatibility with lithium due to the form of compact and smooth solid electrolyte interphase layer on the anode. LiCoO2/Li cells using the SN-based electrolyte behave a high reversible discharge capacity of 122.4 mAh g−1 and keep an outstanding capacity retention of 91% (122.1 mAh g−1) at 0.5 C after 100 cycles at 25 °C, 50 °C, respectively. More importantly, the soft-package cells with the SNbased electrolyte can withstand harsh surroundings at 120 °C for 30 min without gas emitted, and can still keep the capacity retention of 77% compared to that before heat treatment, significantly higher than traditional commercial electrolyte (0%). All above results indicate the novel SNbased electrolyte can be an excellent alternative electrolyte in a practical lithium battery. KEYWORDS: succinonitrile, fluoroethylene carbonate, interfacial compatibility, high temperature, lithium battery, soft-package
1. INTRODUCTION Safety issues of lithium-ion rechargeable batteries are becoming increasingly prominent alongside battery performance, with the growing demands of the power source products.1,2 Electrolytes play an important role on the batteries’ safety characteristics, capacity, and cycle performance.3,4 For instance, the current most widely used components of electrolytes include LiPF6 salt and carbonate solvents involving ethylene carbonate, dimethyl carbonate, etc.,5,6 which are faced up with the safety problem due to the high volatility, flammable properties that can trigger fire and explosions.7−11 In addition, at the interfaces between electrolytes and the electrode may form lithium dendrites that can pierce the separator and probably cause the short circuit and explosion with the continuous heat-evolution.12−14 To achieve good safety of lithium batteries, alternative electrolytes have been studied including ionic liquids,15,16 nitrile17−19 and fluorinated compounds,20,21 etc. For instance, Simonetti et al. reported that using the ionic liquid solvents including Nmethyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI) and the N-methyl-Npropylpyrrolidiniumbis(fluorosulfonyl)imide (PYR13FSI) obtained the excellent performance at high temperature of 50 °C and wide electrochemical window.22 Isken et al. adopted adiponitrile as the cosolvent of linear ethylene carbonate with LiBF4 as the conducting salt to increase the flash point of the solvents and consequently improve the safety of the electrolyte.23 In addition, Zhang et al. studied the effect of LiBF4 © 2017 American Chemical Society
additive on the performance of Li-ion cells using the ethylene carbonate/ethylmethy carbonate mixed solvent, and found that adding LiBF4 into the electrolytes resulted in a lower capacity fading rate at elevated temperature up to 50 °C, comparing with those using the moisture-sensitive LiPF6-based electrolytes.24 Xu also stated that LiBF4-based electrolytes can improve the performance at temperature of 50 °C but also at low temperature under high discharge rates, which is attributed to its relatively higher conductivity and smaller charge-transfer resistance under such harsh circumstance comparing with LiPF6.25,26 However, no reports have been found so far on such modified electrolytes displaying the charge and discharge process higher than 50 cycles at elevated temperatures. It is still a challenge to explore the high-safety electrolyte alternative with favorable interfacial compatibility to sustain long life span for lithium batteries in views of substantial applications. Succinonitrile (SN, NC−CH2−CH2−CN), a representative molecular nonionic plastic crystal with the plastic behavior from −40 to 60 °C, has the advantage of high solubility for most lithium salts, faster transmission of lithium ion conductive phase at 25 °C, high boiling point (267 °C) with negligible vapor pressure and lower flammability.27−33 Thus, SN has recently been adopted as the additive in commercial carbonateReceived: June 25, 2017 Accepted: August 14, 2017 Published: August 14, 2017 29820
DOI: 10.1021/acsami.7b09119 ACS Appl. Mater. Interfaces 2017, 9, 29820−29828
Research Article
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Figure 1. (a) TGA curves of FEC, SN, and SN-based electrolytes with different contents of FEC; (b) LSV performance of FEC and the SN-based electrolytes with different contents of FEC at a scan rate of 0.5 mV s−1.
based electrolytes34−36 or the plasticizer37−39 in polymer electrolyte. For example, Chen et al. added 1 wt % SN into a commercial electrolyte to improve the cycling performance and thermal stability of the Li1.2Ni0.2Mn0.6O2/Li system at a voltage range of 2.0−5.0 V.40 Zhou et al. used SN as a plasticizer to prepare all solid state electrolyte via in situ polymerization with cyanoethyl poly(vinyl alcohol) (PVA-CN), which exhibited excellent mechanical strength and high security.41 Besides, in previous work, we prepared a cross-linking composite polymer electrolyte with SN as the plasticizer by an ultraviolet curing method, which showed high ionic conductivity, excellent cycle performance and good rate capabilities.42 However, there is a side-reaction between lithium and SN due to the polymerization of nitriles catalyzed by lithium metal,43 resulting in high interface resistance and low capacity retention during cycling. It is essential to study an effective method to inhibit the interfacial reaction between SN and lithium, although the physicochemical property of SN is promising to promote the safety performance of lithium batteries. Fluoroethylene carbonate (FEC) is an electrolyte filmforming material in lithium ion batteries. It is well-known that FEC can readily decompose to form a protective film on the surface of lithium prior to the reductive decomposition of carbonate-based electrolytes, and consequently FEC has a positive influence on the interface stability between electrodes and electrolytes of batteries.44,45 In particular, Liu et al. mixed 3-(2-methoxyethoxy) propanenitrile, FEC and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether to produce a safe electrolyte with the electrochemical window up to 4.8 V and obtained the graphite/LiMn2O4 cell with excellent rate and cycle performances.46 Ogumi and his colleagues used FEC as an additive to form the solid electrolyte interphase on the surface of anode during the deposition and dissolution of lithium.47 Jung et al. also demonstrated that FEC-derived solid electrolyte interface layer could significantly improve the lifetime and the capacity retention of batteries.48 Therefore, it is suggested that FEC plays an important role in stabilizing the interface between electrolytes and electrodes. In this article, we adopted a novel SN-based electrolyte including SN and FEC, with LiBF4 as the lithium salt, to constitute lithium batteries and studied the effect of FEC content on the thermal behavior, electrochemical performance and interface compatibility of batteries. It demonstrates that the SN-based electrolyte shows good thermal stability up to 220 °C. FEC can effectively suppress the side reaction between the electrolyte and lithium, and improve the interfacial compatibility. Then, the performance of the SN-based electrolyte was
studied at elevated temperature. Adopting the SN-based electrolyte, the cycling performance of LiCoO2/Li cells is superior to that with the commercial electrolyte, especially at 50 °C. Importantly, the soft-package cells can withstand harsh surroundings at 120 °C for 30 min with the capacity retention of 77%. It is illustrated that the SN-based electrolyte has promising applications in a practical lithium battery.
2. EXPERIMENTAL SECTION 2.1. Materials. The commercial electrolyte solution with 1.0 mol L−1 LiPF6 in a mixture of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethylene carbonate (EC) (DMC/DEC/EC, volume ratio 1:1:1) was supplied by Zhangjiagang Guotai Huarong Chemical New Material Co. Ltd. (named commercial electrolyte). SN (Aldrich Industrial Inc.) and FEC (Aladdin Co. Ltd.) were used as received without further treatment. LiBF4 purchased from Aldrich Industrial Inc. was dried at 50 °C for 24 h before used. 2.2. Preparation of SN-Based Electrolytes. SN-based electrolytes were prepared simply by mixing SN and 1.0 mol L−1 LiBF4 in the presence of different contents of FEC additive in an argon-filled glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm, Mikrouna) and stirring at 60 °C for 4 h. 2.3. Materials Characterizations. The thermal properties involving the transition temperature of Tcp (from normal crystal to plastic crystal) and the melting temperature of Tm for the samples of SN electrolytes were tested by TG-DSC analysis from −60 to 350 °C with a heating rate of 5 °C min−1 under nitrogen atmosphere. Moreover, the thermal stability of the electrolytes was inspected by TGA analysis from 25 to 400 °C at a heating speed of 10 °C min−1 under argon gas atmosphere. The electrolyte samples were analyzed from 200 to 400 nm by UV−Vis (Hitachi 330). After cycling tests, the coin cells were dissembled to harvest lithium electrodes for characterizations with a scanning electron microscope (SEM, Phenom) in a glovebox filled with argon gas. 2.4. Electrochemical Measurements. Ionic conductivity of the SN-based electrolytes was tested on the conductivity meter (Mettler Toledo). The electrochemical stability was determined by linear sweep voltammetry (LSV) on a Princeton applied research electrochemical workstation (PARSTAT 2273) with a scanning rate of 0.5 mV s−1 from open circuit to 6 V at room temperature, using the stainless steel as the blocking working electrode, the lithium foils as both the counter and the reference electrode. The interface chemistry between the electrolyte and electrode was analyzed by electrochemical impedance spectra (EIS). EIS were measured over the frequency range of 1 × 10−2 to 1 × 106 Hz, with AC amplitude of 10 mV on the electrochemical workstation. The interfacial stability of the lithium and electrolyte under both dynamic conditions and during different storage period was investigated on a symmetrical Li/electrolyte/Li cell. The lithium stripping and plating process was cycled using a land battery test system (Wuhan Land Electronic Co., Ltd. China) with a current density from 0.05 to 1.0 mA cm−2. 29821
DOI: 10.1021/acsami.7b09119 ACS Appl. Mater. Interfaces 2017, 9, 29820−29828
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Figure 2. (a) Cycle performance of LiCoO2/Li cells assembled with SN-based electrolytes with 20% FEC and without FEC at the current density of 0.5 C at 50 °C. (b) CV curves of batteries assembled with FEC/LiBF4/SN at a scan rate of 0.5 mV s−1 at 50 °C. Comparison of AC impedance spectra of cells assembled with different electrolytes, measured (c) before and (d) after 100 cycles at 50 °C. The insets are the corresponding details with enlarged scale.
stability without weight loss until 220 °C and achieve an ionic conductivity of 3.54 mS cm−1 at room temperature (Table S1), outperforming the electrolytes with 30% and 50% FEC. Importantly, the thermal behavior of the SN-based electrolyte with 20% FEC is much better than that of commercial electrolytes which begins to lose weight since 45 °C, as shown in Figure S2, indicating that it can meet the practical demands of lithium batteries in terms of the thermal safety. DSC profiles were measured to investigate the change of plastic crystal behavior of SN-based electrolytes in the absence and the presence of 20% FEC, as shown in Figure S3. In the absence of FEC, there are characteristic endothermic peaks for LiBF4/SN, corresponding respectively to the transition from rigid crystal to plastic crystal phase (Tcp) at −31 °C, the melting peak (Tm) at 42 °C and the thermal decomposition at 276 °C, in accord with the previous literature.29 While in the presence of 20% FEC, there appear a significant decrease in Tcp for FEC/ LiBF4/SN (no sharp peak existing in the test temperature range) and a lower Tm of 13 °C, demonstrating that the electrolytes with FEC exhibits a liquid eutectic at room temperature. The noticeable reduction of Tm suggests a decrease of structural regularity of the SN-based electrolytes, which is favorable to enhance the ionic conductivity of the electrolytes. Wide electrochemical window of electrolytes is another crucial factor to estimate its practical application in lithium batteries. Figure 1b displays the LSV curves of the LiBF4/FEC, and the SN-based electrolytes with different contents of FEC. In the SN-based electrolytes with FEC, no decomposition of any components takes place below 4.75 V vs Li+/Li, at which FEC becomes anodic reactivity. Above 5 V, SN in the composite electrolytes starts to decompose. However, the electrolyte with 1 M LiBF4 in FEC appears to be unstable in the voltage range above 4.25 V, which is in agreement with the
The charge/discharge tests of the LiCoO2/Li coin cells were carried out using a land battery test system. The electrode formulation consisted of 85 wt % LiCoO2 (Dongguan Shanshan Battery Materials Co., Ltd.), 10 wt % carbon black, and 5 wt % PVDF. The active material loading of the electrode is 10 mg cm−2 on averages. The fabrication of test cells (2430) was carried out in an argon-filled glovebox. The charge−discharge cycling was conducted from 2.5 to 4.2 V at a stable testing temperature of 25 °C. The cyclic voltammetry (CV) analysis was implemented on a cell of LiCoO2/Li system at a scan rate of 0.5 mV s−1 to explore the electrochemical stability of electrolyte samples. The C-rate performances of LiCoO2/Li coin cells with different electrolytes were galvanostatically measured in the voltage range of 2.5−4.2 V at various charge/discharge current densities ranging from 0.1 to 2 C. Besides, the electrochemical tests of cells assembled by LiBF4/SN electrolytes were performed at 50 °C because of its solid state at room temperature. For comparison, LiCoO2/Li cells with commercial electrolyte were also assembled.
3. RESULTS AND DISCUSSION 3.1. Performance of SN-Based Electrolytes with Different Contents of FEC. Figure 1a shows the TGA curves of FEC, SN and LiBF4/SN with different contents of FEC. It can be noticed that FEC is negligibly volatile until the temperature of 210 °C except for a slight loss due to the presence of trace water, and SN shows the best thermal stability up to 280 °C. Meanwhile, the thermal decomposing temperature of the SN-based electrolytes decrease after the incorporation of FEC with the first weight loss slope of FEC as well as the second of SN. With the increase in FEC concentration, the thermal behavior of the composite electrolytes deteriorates at inert atmosphere. Although the SN-based electrolytes with 5 and 10% FEC exhibit similarly excellent thermostability, they are not suitable for battery assembly at 25 °C because of their nonfluid property at room temperature (Figure S1). Among the other composite electrolytes, the one containing 20% FEC is favorable, because it can maintain its 29822
DOI: 10.1021/acsami.7b09119 ACS Appl. Mater. Interfaces 2017, 9, 29820−29828
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Figure 3. AC impedance spectra of symmetrical cells (Li/electrolyte/Li) with different electrolyte after different storage time at 50 °C. (a) SN-based electrolyte without FEC; (b) SN-based electrolyte with 20% FEC.
reported data.49 This demonstrates that the synergistic effect by combining the advantages of SN and FEC, i.e., the antioxidization of SN and the antireduction of FEC, provides the SN-based electrolytes with an enhanced electrochemical stability.50 This can also be proven by the UV−vis spectra in Figure S4, showing that the characteristic peak around 300 nm is due to the π−π* transition of carbonyl group. With the content increase of FEC, the absorption intensity enhances, together with the red-shifts from 229 to 233 nm. It is probably due to the formation of intermolecular hydrogen bondings between FEC and SN. All these results suggest that the SNbased electrolytes present a high anodic stability and thus could be potentially applied to the lithium batteries. Compared to the electrolytes with 30 and 50% FEC, in the presence of 20% FEC obtains the optimal electrochemical stability due to the declined response of the current. According to Figure 1a, b, the optimal content of FEC is 20% in the SN solvent, which is the most favorable to meet the demand of practical lithium batteries. The cycling stability, CV performance together with EIS characterizations were examined for LiCoO2/Li cells without and with 20% FEC as the electrolytes at 50 °C, in order to study whether FEC can suppress the side-reaction of SN catalyzed by lithium metal. Figure 2a presents the discharge capacity as a function of cycle number of the cells at 0.5 C. It is clear that the cell is capable of achieving a discharge capacity approximately 130.9 mAh g−1 after 30 cycles, and still retains a discharge capacity of 122.0 mAh g−1 after 100 cycles, indicating high discharge capacity retention and remarkable cycle performance. However, the capacity rapidly declines to zero after only 30 cycles for rechargeable batteries with the SNbased electrolytes without FEC, which is probably associated with the unstable interfacial layer generated by the irreversible reaction between lithium and SN.23 The reaction exposes fresh lithium metal surface to the electrolyte and severely consume the electrolyte solution, which leads to continuous increase in additional resistive layers until the cell fails. Furthermore, the reversibility of LiCoO2/Li batteries with the SN-electrolyte containing 20% FEC is investigated by CV, as shown in Figure 2b. The first cycle is a little different from other cycles due to the irreversible reaction and generation of solid electrolyte interfacial films. Afterward, the curves almost coincide, displaying a favorable cycling stability, which is obviously better than that of the electrolyte without FEC (Figure S5). This result suggests that adding FEC into the LiBF4/SN electrolytes can help to form a stable electrode/ electrolyte interface to depress the side-reaction of SN and lithium, and facilitate the reversible intercalation and
deintercalation of lithium ions. At the same time, a stable electrode/electrolyte interface would also help to enhance the structural stability of the electrode, possibly by affecting the inner solid electrochemical reaction of electrodes. EIS measurements were performed to probe the effect of FEC on the cycling performance of cells at 50 °C and to understand the electrochemical process occurring at the electrode/electrolyte interface in the lithium batteries. The measured impedance spectra before and after 100 cycles are presented in Figure 2c, d and the inset graphs show the corresponding details with enlarged scale. In general, a highfrequency semicircle is related to the passivating surface film. The intermediate-frequency semicircle can be ascribed to the charge-transfer process resistance (Rct) in the electrode/ electrolyte interface, reflecting the electrodes corrosion rate.51 The low-frequency tail is associated with the lithium ion diffusion process in the working electrode.52 The ohmic resistance (Rs), which can be estimated by the starting point of the high frequency semicircle with real axis, represents the resistance of electrolyte, electrode, and separator. Before charge and discharge process, the Rs value of the electrolyte with 20% FEC is about 10 ohm, and it changes little after 100 cycles. While the value for that without FEC increases to 35 ohm probably due to the exhaustion of the electrolyte. The Rct value of electrolytes without FEC increases apparently before and after 100 cycles, whereas in the presence of FEC the value shows almost no change. It implies that the electrolyte without FEC may form a thick layer leading to a higher Rct value due to the side-reaction between lithium and the electrolyte. Remarkably, after the long-term cycling, two semicircles assigned to the passivating surface film and charge-transfer process can be found for the electrolyte after the addition of FEC. These results confirm that adding FEC into the SN-based electrolyte can lead to the formation of a stable solid/ electrolyte interface film that will depress aggressive side reactions, in good accordance with the above cycle and CV performance. To further investigate the effect of FEC on the interfacial stability, we assessed the EIS of the symmetrical cell of Li/ electrolyte/Li containing the SN-based electrolytes with and without FEC after different storage days at 50 °C, as depicted in Figure 3. The diameter of capacitive arcs in AC impedance spectra is a representation of interfacial resistance of the electrolyte with electrodes. Figure 3a indicates that without FEC the diameter of capacitive arcs increases with improvement of the storage time constantly due to the interfacial instability between the SN-based electrolyte and lithium 29823
DOI: 10.1021/acsami.7b09119 ACS Appl. Mater. Interfaces 2017, 9, 29820−29828
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Figure 4. Comparison of the (a) cycle performance and (b) efficiency of cells assembled with the SN-based electrolyte with 20% FEC and commercial electrolyte at the current density of 0.5 C at 25 °C.
process. When the rate returned to 0.1 C, the discharge capacity of the cells can be restored, indicating that the SNbased electrolyte is structurally and electrochemically stable as well as the sufficiently high ionic conductivity (Table S1). To give insight into the positive effect of the 20% FEC-SN electrolyte on efficiently improving the electrode/electrolyte interface stability, SEM analysis was performed for the lithium anode electrode after 100 cycles. As shown in Figure 5, the
electrodes. Whereas in the presence of 20% FEC, the diameter of capacitive arcs increases with rising of storage time and tends to a stable value gradually after 12 days (Figure 3b), demonstrating that the compatibility of the SN-based electrolyte containing FEC with lithium electrodes is favorable and a layer of passive film on the electrode side is formed when electrolyte contacts with lithium metal anode. Furthermore, under dynamic conditions, the interface stability is evaluated with galvanostatic charge and discharge cycles at a current density of 1.0 mA cm−2, and the EIS measurements after 50 and 100 cylces, as shown in Figure S6 and S7. The cell with the electrolyte in the absence of FEC exhibits significantly increasing polarization compared to the cell employing the SN-based electrolyte with FEC. What’s more, from the EIS results tested after 50 and 100 cycles (Figure S7), the impedance resistance of the electrolyte without FEC increases significantly after the second 50 cycles, however, in the presence of 20% FEC the impedance spectra remains almost no change for the cycles larger than 50. These results indicate that FEC has a vital impact on the interfacial compatibility between the electrolytes and lithium electrodes under the condition of either passive or active storage. 3.2. Performance of the SN-Based Electrolyte with 20% FEC at 25 °C. We also studied the performance of the SN-based electrolyte with 20% FEC (abbr. as 20% FEC-SN) at room temperature, comparing with the commercial electrolyte. Figure 4a illustrates the discharge capacities as a function of cycle numbers for the electrolyte 20% FEC-SN and the commercial electrolyte at a charge/discharge rate of 0.5 C at 25 °C. It is noted that the discharge capacity of the commercial electrolyte is slightly higher than that of 20% FEC-SN during the first 40 cycles; however, it has nearly the same discharge capacity after 100 cycles. It demonstrates that the 20% FEC-SN electrolyte displays the excellent capacity retention up to 93% versus 91% for the commercial electrolyte. In addition, Figure 4b reveals the comparison of efficiency for the cells with different electrolytes. The Coulombic efficiency of the 20% FEC-SN electrolyte exceeds 99% during the long-term cycles, maintaining 99.5% in the end. However, the cell with the commercial electrolyte only keeps 98.2% after 100 cycles. These results indicate the superior interfacial compatibility of the electrode with 20% FEC-SN electrolyte.53 The rate performance of cells is also preliminarily investigated, and the results are depicted in Figure S8. After the cell was cycled for 10 cycles at an initial rate of 0.1 C, the current density is gradually increased in stages until 2 C. The cells can deliver discharge capacities of 112 and 85 mAh g−1 at 1 and 2 C, respectively. With ongoing cycling for each current rate, the Coulombic efficiency approaches 99% during the charge/discharge rate
Figure 5. SEM images of lithium electrodes obtained after 100 cycles at 0.5 C in different electrolytes at 25 °C: (a) SN-based electrolyte, (b) commercial electrolyte.
surface morphology of lithium electrode show significant differences due to the types of electrolyte. The lithium electrode cycled in the commercial electrolyte has a large amount of dendritic features with loose accumulation of particles. In contrast, lithium faced with the 20% FEC-SN electrolyte exhibits rather smooth and flat morphology, indicating that the electrolytes are spread uniformly over the entire lithium surface and no obvious sign of lithium dendrite. Growth of the lithium dendrites is strongly suppressed in the 20% FEC-SN electrolyte due to the excellent interface compatibility. These results suggest that the SN-based electrolyte is conducive to the uniform deposition of lithium ion and can depress lithium dendrite growth. In addition, to further investigate the interfacial behavior between lithium electrode and the electrolyte under dynamic conditions, the symmetrical Li/electrolyte/Li cells with different electrolytes are subjected to galvanostatic charge and discharge cycles at current densities from 0.05 to 1.0 mA cm−2. Figure 6 shows the voltage profiles of the galvano-static lithium stripping and deposition tests performed using the two individual electrolytes in the symmetrical cells. The cell with the commercial electrolyte exhibits an unstable and high voltage profile due to high resistance and increased polarization caused by uneven deposition and stripping of lithium during charge 29824
DOI: 10.1021/acsami.7b09119 ACS Appl. Mater. Interfaces 2017, 9, 29820−29828
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3.3. Performance of the 20% FEC-SN Electrolyte at Elevated Temperature. 3.3.1. Coin Cells. Figure 7 shows the cycling performance of the coin cells with 20% FEC-SN at 50 °C under the charge/discharge rate of 0.5 C/0.5 C, comparing with that with the commercial electrolyte. The specific capacity of the cell with 20% FEC-SN electrolyte is 137 mAh g−1 at the first cycle, and remains 130 mAh g−1 after 5 cycles and 122.4 mAh g−1 after 100 cycles. On the contrary, the initial capacity for the commercial electrolyte reaches 136 mAh g−1, then decreases to 126 mAh g−1, 112.4 mAh g−1 after 5 and 100 cycles, respectively. From the plot of the voltage profiles based on different electrolytes as shown in Figure 7a, b, it is noted that the cell using 20% FEC-SN electrolyte depicts a voltage plateaus at first cycle, similar to that of the commercial electrolyte, and then shows no obvious change of the discharge plateaus with cycling, demonstrating a high reversibility and the improved electrochemical performance with the 20% FEC-SN electrolyte, whereas for the commercial electrolyte, raising the cycle number leads to a drop I the discharge plateau voltage. As displayed in Figure 7c, the capacity retention of the cell with 20% FEC-SN is 91% after 100 cycles, compared to 82% of the commercial electrolyte. Besides, the SN-based electrolyte presents the Coulombic efficiency above 98.5%, much better than that of commercial electrolyte (Figure S10). EIS were also carried out to examine the change of impedance spectra for different electrolytes, as shown in Figure S11. The SN-based electrolyte exhibits relatively small impedance resistance before cycling and after 100 cycles compared to that of commercial electrolyte. It is ascribed to the better thermal stability and electrochemically interfacial compatibility of the SN-based electrolyte even at elevated temperature of 50 °C, in agreement with the above discussion (Figures 1 and 3). Furthermore, the cycling performance at temperatures ranging from 50 to 70 °C is depicted in Figure S12. At 60 °C, the discharge capacity can reach 118 mAh g−1 after 10 cycles. Even at 70 °C, the cell still
Figure 6. Voltage profiles of the Li/electrolyte/Li cells with increasing current density from 0.05 to 1.0 mA cm−2 for different electrolytes.
and discharge cycles, particularly at high current density. In contrast, the cell employing the 20% FEC-SN electrolyte shows lower and more stable voltage profiles. It is suggested that the SN-based electrolytes with FEC have an expectedly good interfacial compatibility with lithium compared with the commercial electrolytes. To further illustrate the phenomenon, the variation of the impedance resistance between the lithium electrode and electrolytes with storage time at room temperature are investigated, as shown in Figure S9. For commercial electrolytes (Figure S9a), the resistance increases gradually during 15 days. In the case of 20% FEC-SN, the impedance spectra at 13 days coincides with that at 15 days, showing the resistance much smaller than those using the commercial electrolyte. In combination with the results in Figures 3 and 6, it can be reasonably deduced that the 20% FEC-SN electrolyte can reduce the interfacial resistance and provide a favorable charge transfer reaction between the lithium electrode and the electrolyte through the formation of stable protective interfacial film derived by FEC.48,54
Figure 7. Charge−discharge curves of (a) SN-based electrolyte and (b) commercial electrolyte after first, fifth, 100th cycles at the current density of 0.5 C at 50 °C. (c) Comparison of the cycle performance of cells assembled with different electrolytes of SN-based electrolyte and commercial electrolyte at 50 °C. 29825
DOI: 10.1021/acsami.7b09119 ACS Appl. Mater. Interfaces 2017, 9, 29820−29828
Research Article
ACS Applied Materials & Interfaces delivers capacity up to 112 mAh g−1 at the last cycle. In view of the exciting performance, we can speculate that such SN-based electrolyte can be a promising material used in lithium batteries running at elevated temperature. 3.3.2. Soft-Package Lithium Cells. To elucidate the effect of SN-based electrolyte on safety issue of a cell, the combustion tests of the 20% FEC-SN electrolyte and the commercial electrolyte are shown in Figure 8 for safety comparison.55 In
commercial electrolyte begins to decompose at 45 °C and has a rapid weight loss at 120 °C. As a result, it generates some gas and threatens the safety of the cells. Figure S13 shows the charge−discharge curves of soft-package lithium cells assembled with 20% FEC-SN electrolyte and the commercial electrolyte before and after being held at 120 °C for 30 min. Before the heat treatment, with 20% FEC-SN electrolyte the charge and discharge curves exhibit almost the same as that of commercial electrolyte, as shown in Figure S13a, c, maintaining a capacity retention of 77% of its initiative capacity (Figure S13b). In contrast, the cell with the commercial electrolyte presents null discharge capacity after harsh treatment (Figure S13d). The results clearly indicate that the SN-based electrolyte has an ameliorating effect on improving the cyclic performance of the cells at elevated temperature. Therefore, the SN-based electrolyte is suitable to substitute the commercial electrolyte at elevated temperature to improve the safety of batteries.
4. CONCLUSION We developed the novel SN-based electrolyte containing SN, LiBF4 and FEC and assessed the performance for lithium batteries involving the thermal stability, electrochemical window, and ionic conductivity. The results demonstrated that the SN-based electrolyte with 20% FEC is favorable to constitute high efficient batteries with superior safety. Such 20% FEC-SN electrolyte leads to the formation of a stable electrode/electrolyte interface film that will depress aggressive side reactions between lithium and SN, and consequently achieve the excellent capacity retention. In addition, compared with the commercial electrolyte, the 20% FEC-SN electrolyte efficiently improves the interfacial compatibility, and exhibits the superior rate capability, the wider electrochemical window exceeding 4.7 V and the excellent electrochemical performance at room temperature. More importantly, the cells present highperformance charge−discharge behavior with no obvious feature change of the discharge plateaus after 100 cycles at 50 °C. Even at a high temperature of 120 °C, the soft-package lithium batteries using the SN-based electrolyte with no gassing behavior can still charge and discharge normally with a capacity retention of 77% of its initiative capacity.
Figure 8. Safety comparison: the combustion test of (a) SN-based electrolyte and (b) commercial electrolyte.
contrast to the highly combustible commercial electrolyte (Figure 8b), the SN-based electrolyte exhibits a distinctly low flammability that is hardly ignited with the flame source (Figure 8a). Figure 9 shows the photographs of soft-package lithium cells assembled with 20% FEC-SN and the commercial electrolyte
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09119. Figures S1−S13 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
Figure 9. Photos of soft-package lithium cells assembled with SNbased electrolyte and commercial electrolyte (1 M LiPF6 in EC/ DMC/DEC) before and after heat treatment (120 °C) for 30 min.
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before and after heat treatment (120 °C) for 30 min. The gassing behavior of the LiCoO2/Li cell with 20% FEC-SN electrolyte is obviously suppressed compared with that of the commercial electrolyte. It is indicated that in the 20% FEC-SN electrolyte there form FEC-LiBF4−SN complex at the electrode/electrolyte surface that depress undesirable interfacial reactions to hinder gas generation. As shown in Figure S2, the
Wei Li: 0000-0003-3016-7728 Jinli Zhang: 0000-0001-5805-3824 Author Contributions
Q.Q.Z. and K.L. contributed equally to this paper. Notes
The authors declare no competing financial interest. 29826
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ACKNOWLEDGMENTS This work was supported by the Foundation of National Key Laboratory of Science and Technology on Power Sources (9140C16020212-DZ2801), P. R. China.
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