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graphite batteries
Electrochimica Acta 317 (2019) 146e154
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) as an ionic liquid-type electrolyte additive to enhance the low-temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite batteries Wenlian Wang a, Tianxiang Yang a, Shuai Li a, Weizhen Fan c, Xiaoyang Zhao a, b, Chaojun Fan c, Le Yu c, Shaoyun Zhou c, Xiaoxi Zuo a, **, Ronghua Zeng a, Junmin Nan a, * a b c
School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, PR China Department of Environmental Engineering, Henan Polytechnic Institute, Nanyang, 473009, PR China Guangzhou Tinci Materials Technology Co., Ltd., Guangzhou, 510760, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2019 Received in revised form 5 May 2019 Accepted 6 May 2019 Available online 9 May 2019
A 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIeBF4) ionic liquid is introduced into an electrolyte as a functional additive to enhance the low-temperature performance of LiNi0.5Co0.2Mn0.3O2 (NCM523)/ graphite batteries. Linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests show that EMIeBF4 can be oxidized in advance and produce low impedance solid-electrolyte interphase (SEI) films on the electrode surfaces. Compared with the batteries without the EMIeBF4 additive, the capacity retention of the NCM523/graphite batteries with 1% EMIeBF4 additive is enhanced from 82.3% to 93.8% after 150 cycles at a low temperature of 10 C. In addition, at 30 C, the discharge capacity of the batteries with 1% EMIeBF4 additive is nearly doubled. And at the room temperature of 25 C and after 400 cycles, the capacity retention also increases from 80.3% to 85.9%, and the coulomb efficiency remains at approximately 100%. These results demonstrate that EMIeBF4 used as a functional electrolyte additive has promising prospects for application to improve the lowtemperature performance of NCM523-based lithium-ion batteries. © 2019 Elsevier Ltd. All rights reserved.
Keywords: 1-ethyl-3-methylimidazolium tetrafluoroborate Electrolyte additive LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite batteries Low-temperature performance
1. Introduction Compared with the traditional lithium cobalt oxide (LiCoO2) cathode materials, layered LiNixCoyMnzO2 (0 < x, y, Z < 1, NCM) materials have lower cost, which can improve the sustainable development of lithium ion batteries (LIBs). These materials have good prospects for applications in next-generation energy storage devices and electric vehicles, and in other fields [1e6]. However, when NCM/graphite LIBs operate at low temperatures, their capacities rapidly decrease and lithium dendrite growth is observed, leading to safety risks and a rapid decline of battery performance; this has become the core issue that restricts the applications of NCM/graphite batteries [7e13]. Under low-temperature conditions, the mobility of Liþ in the electrolyte is reduced, resulting in the weakening of the Liþ migration ability and the decrease of the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zuo), [email protected] (J. Nan). https://doi.org/10.1016/j.electacta.2019.05.027 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
reaction ability in the electrode interface film that give rise to a rapid degradation of battery performance [9,14]. Thus, when the composition of the cathode and anode active materials remains unchanged, the development of practical strategies for reducing the sensitivity of batteries to low temperature has become a key issue for improving the low-temperature performance of the NCM-based LIBs [15e17]. In the practical application of LIBs, in order to improve their poor low-temperature performance, there are two main approaches, i.e. to keep a suitable working environment and to improve the performance of the batteries themselves at low temperatures. The use of an external or a built-in heater for the battery packs can enable LIBs to operate in room temperature conditions when the battery packs are in low-temperature environments. Wang et al. [18,19] designed a new type of “All Climate Batteries TM” that used the ohmic heat generated by nickel sheets at low temperatures to realize the self-heating of LIBs. This method had successfully improved the performance of LIBs subjected to an external low temperature. At an ambient temperature of 30 C,
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the battery power was increased by more than 10 times, while the performance and cycling lifetime of the batteries at room temperature were not weakened. Further improvements enabled fast charging at low temperatures, with full charging in 15 min even in an extreme environment of 50 C. Hao et al. [20] utilized a memory alloy based on a thermal regulator to regulate the temperature of the battery pack. The demonstration of a battery pack consisting of commercial 18650 batteries showed that this thermal regulator effectively blocked the heat leak from the battery pack. In a temperature environment of 20 C, the temperature of battery pack rose rapidly to 20 C and the available capacity of the batteries was increased by a factor of greater than three. These approaches for increasing the temperature regulation effectively alleviate the impact of low ambient temperature, but they also increase the manufacturing cost and reduce the volume-specific energy of battery pack. As an alternative approach to external intervention, it is also possible to improve the low-temperature performance of LIBs themselves by optimizing the electrolyte composition. Using a suitable electrolyte composition in the LIBs, lithium ion conductivity can be increased and a low-impedance SEI film can also be generated on the electrode surfaces, improving the LIB performance characteristics [7,14,21,22]. It is well-known that at low temperatures, the Liþ conductivity of the electrolyte decreases and the impedance of the SEI increases, resulting in slow reaction kinetics and rapid capacity decay. In addition to optimize the solvent composition, the simplest approach for optimizing the electrolyte is to add functional additives to change the electrolyte properties. Usually, the amounts of functional additives in the electrolyte are small, but their effects on the LIB performance characteristics are very significant. Compared to external heaters, the addition of functional additives is undoubtedly a simple and economical strategy. Some organic molecules and lithium salts have been demonstrated to exert excellent low temperature effects when used as the functional electrolyte additives in LIBs [23]. Yang et al. [24] reported that lithium difluorophosphate (LiPO2F2) formed a stable low-impedance SEI film on the electrode surface. At 20 C and 30 C and a discharge rate of 0.2C, the capacity of LiNi0.5Co0.2Mn0.3O2 (NCM523)-based LIBs containing 1% LiPO2F2 was 71.9% and 57.93% (the room temperature capacity value), respectively. Whereas under the same conditions, the batteries without LiPO2F2 released only 49.41% and 9.6% of the capacity. Li et al. [25] reported another lithium salt additive, namely, lithium difluorobis (oxalato) phosphate (LiDFBOP). It was found that LiDFBOP could simultaneously form a stable SEI film on the cathode and anode surfaces, which effectively improved the electrochemical performance of the NCM523/graphite full cell at low temperature. At 20 C, the NCM523/graphite full cell was charged and discharged at a rate of 0.5C, and the capacity retentions of the batteries with 1% LiDFBOP and without LiDFBOP after 50 cycles were 93% and 73%, respectively. Jurng et al. [26] proposed the use of allyl sulfide (AS) derivatives as the low-temperature additives. The addition of 2% AS to a Li/graphite half-cell inhibited the formation of lithium dendrites at 30 C and increased the reversible capacity by more than three times. These results indicate that the addition of additives may effectively promote the low-temperature performance of the NCM-based LIBs and decrease the manufacturing cost. In addition to the traditional organic functional molecule additives, ionic liquids have also been recognized as environmentally friendly electrolytes and potential electrolyte additives in LIBs. This is mainly due to their unique properties such as low vapor pressure, high thermal stability, chemical stability, excellent solubility, polarity and good conductivity [27,28]. Ionic liquids have been developed as new solvents to replace the current solvents or as functional additives
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to optimize the existing solvents in the carbonate-based systems. Dong et al. [29] used the safe ionic liquid-sulfolane/LiDFOB electrolytes in Li/Li1.15(Ni0.36Mn0.64)0.85O2 cells, at 55 C and 70 C, after cycling for 50 cycles at a rate of 0.5 C in the range of 2.0-4.6 V, the capacities reached 172.5 and 238.8 mAh g-1, respectively. Kim et al. [30] prepared a N-allyl-2-methoxypyrroliniumbis(fluorosulfonyl) imide (A(OMe)Pyrl-FSI) ionic liquid to replace the current carbonate electrolyte. The initial discharge capacity of the Li/LiFePO4 cells was 153.7 mAh g-1 at 0.1C, the capacity retention rate was 81.2% (124.8 mAh g-1) at 5C, and high thermal stability and nonflammability were obtained. Pan et al. [31] used N-methyl-Nmethoxycarbonylpiperidinebis (trifluoromethanesulfonyl)imide as the electrolyte. At room temperature, after 20 cycle at 0.1C, the discharge capacity of the Li/LiFePO4 cells was 109.7 mAh g-1, and the capacity retention was 98.7%. These results indicate that more ionic liquids should be evaluated to forward their use in the LIBs. Besides the performances, the high-cost of the ionic liquids is also a key factor hindered their commercial application in the LIBs. Therefore, an appropriate choice at current stage is to evaluate ionic liquids as electrolyte additives. In our previous work [32], it was shown that BF 4 anions formed low-impedance interface films on the surfaces of the NCM-based cathode and the graphite anode. In this study, based on the low viscosity and high conductivity of 1ethyl-3-methylimidazole cations, 1-ethyl-3-methylimidazole tetrafluoroborate (EMI-BF4) was evaluated as a low-temperature electrolyte additive in the NCM-based LIBs. It was found that the advantages of EMIþ and BF 4 could be integrated to improve the low-temperature performance of the NCM-based LIBs. The introduction of ionic liquids into the existing solvent systems offers a new method to design functional electrolyte additives. 2. Experimental 2.1. Chemicals and materials Battery grade ethylene carbonate (EC), ethyl methyl carbonate (EMC) solvent, dimethyl carbonate (DMC), and lithium hexafluorophosphate (LiPF6) were supplied by Guangzhou Tinci Materials Technology Co., Ltd., China. The electrolyte additive EMIeBF4 (>99%) was purchased from Lanzhou Zhongke Kate Industry & Trade Co., Ltd., China. All of the chemicals were not purified further. The blank electrolyte was 1 mol L-1 LiPF6/ECþEMC (with a mass ratio of 1:2). The electrolyte was prepared in an argon-filled glove box (MBraun, Germany) with both oxygen and water contents controlled below 0.1 ppm. The ionic liquid functional electrolyte was obtained by adding appropriate amounts of EMIeBF4 into the blank electrolyte. The moisture and acid content (HF) of the electrolyte were determined using a Karl-Fisher 831 Coulometer (Metrohm, Switzerland) and a Karl-Fisher 798 GPT Titrino (Metrohm, Switzerland), and the values were controlled below 20 and 50 ppm, respectively. 2.2. Preparation of the batteries NCM523 active material (Shenzhen Tianjiao Tech. Co., Ltd., China), polyvinylidene difluoride (PVDF) binder and Super-P conductive carbon with a mass ratio of 96.8:1.2:2 were uniformly dispersed in N-methylpyrrolidone, and then the slurry was coated onto an Al current collector. After the subsequent drying, rolling, and cutting treatment, NCM523 cathodes were finally obtained. The thickness and compaction density of the prepared cathode were 121 mm and 3.289 g cm-3, respectively. Commercial graphite (Shenzhen BTR Co., Ltd., China), CMC, Super-P and polymerized styrene-butadiene rubber SBR with a mass ratio of 95:1.5:1.5:2
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were uniformly dispersed in the diluted water to prepare the anode slurry. Then, the slurry was coated onto a Cu current collector and subjected to drying, rolling, and cutting treatment to obtain the graphite anodes. The thickness and compaction density of the prepared graphite anodes were 126 mm and 1.515 g cm3, respectively. Afterwards, the pouch NCM523/graphite full cells with 6.2 g electrolyte per cell and normal capacity of 1950 mAh were assembled using the general manufacturing process at Guangzhou Tinci Materials Technology Co., Ltd., China. A Celgard 2400 microporous membrane was used as the separator, and an Al plastic film was used as the outer shell. In addition, the 2032 type Li/NCM523 half-cell and Li/graphite half-cell were also assembled to evaluate the effects of the EMI-BF4 additive on the NCM523 cathode and the graphite anode, respectively. 2.3. Electrochemical measurements The cycling performance of the NCM523/graphite full cells in the voltage range of 2.75e4.2 V was evaluated using a multichannel battery cycler (CT-3008W, Neware, China). Prior to the cycling test, the NCM523/graphite full cells were first charged at a rate of 0.1C for 6.5 h, aged at 45 C under a pressure of 500 kg for 24 h, and were successively discharge-charged at the rates of 0.1, 0.2, and 0.5C. During the room temperature (25 C) test, the NCM523/graphite full cells were discharge-charged at a rate of 1C. For the lowtemperature test, the NCM523/graphite full cells were tested at a rate of 0.2C in a test chamber (GDW-100L, SUOYATE, China) with the temperature set to 10 C. The NCM523/graphite full cell was first cycled at room temperature for 3 cycles at 1C rate and then was subjected to low-temperature testing. After the fully charged batteries, i.e. state of charge (SOC) ¼ 100%, were placed in the lowtemperature test chamber for 4 h, the discharge test was performed at a rate of 0.5C. The cyclic voltammetry (CV) measurements were carried out using an electrochemical workstation (CHI660E, Chenhua, Shanghai) at a scanning rate of 0.1 mV s-1, and the voltage ranges of the Li/NCM523 and the Li/graphite half-cells were 3e4.5 V and 0.01e3 V, respectively. A linear sweep voltammetry (LSV) test was performed in the voltage range of 0e7 V, with Pt as the working electrode and Li as the counter and reference electrodes at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) results were obtained using a Solartron 1470 (England) instrument in a frequency range from 100 kHz to 0.01 Hz and with a voltage amplitude of 5 mV. 2.4. Characterization of the electrolyte and cycled electrodes The electrolytes containing 0, 0.5, 1, 2 and 5% EMI-BF4 were prepared and stored in a chamber (GDW-100L, SUOYATE, China) with a controlled temperature for 4 h, respectively. After the temperatures of these electrolytes stabilized, their ionic conductivities were measured using a conductivity meter (DDS-307, Shanghai INESA Scientific Instrument Co., Ltd.). All of the batteries that required physical characterization were disassembled in an argonfilled glove box after discharging to 2.75 V. The electrodes disassembled from the batteries were randomly selected and washed with DMC solvent, dried and removed for the subsequent characterizations. The obtained electrodes were cut into small pieces and attached to a conductive paste, and then after a gold spray treatment, the surface morphologies of the cycled electrodes were observed using a scanning electron microscope (SEM, HITACHI SU8010, Japan). The elemental composition at the surface of the cycled electrode was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA) using an Al Ka wire as the X-ray source.
3. Results and discussion 3.1. Effects of the EMIeBF4 additive on the electrolyte properties It is well-known that appropriate additives can change the properties of the electrolyte and subsequently affect the electrochemical performance of the LIBs. The influence of the EMI-BF4 additive on the electrochemical window of the electrolyte was first explored. As shown in Fig. 1a, the anode current of the electrolyte containing 1% EMI-BF4 began to increase at 3.5 V, indicating that the EMI-BF4 oxidation reaction occurred. This means that the oxidative decomposition potential of EMI-BF4 is lower than that of other carbonate components, and EMI-BF4 is pre-oxidized and decomposed. These results indicate that the EMI-BF4 additive may oxidize in advance to form a protective film on the electrode surface [33]. Generally, the ionic conductivity of the electrolyte decreases with decreasing temperature, which is one of the key reasons for the rapid decline in the low-temperature performance of LIBs. To investigate the effect of the EMIeBF4 addition on the ionic conductivity, the ionic conductivities of the electrolytes containing 0, 0.5, 1, 2, and 5% EMIeBF4 were measured at different temperatures, as shown in Fig. 1b. Overall, it can be seen that the ionic conductivity is related to the temperature, and the drop in the temperature will lead to a decrease in the ionic conductivity. Above 0 C, EMI-BF4 has almost no observable effect on the ionic conductivity of the electrolyte due to the limited addition amount. By contrast, the addition of EMIeBF4 has a positive influence on the ionic conductivity when the temperature drops to between 0 C and 40 C. At temperatures lower than 0 C, the ionic conductivity of the electrolyte is improved after adding 0.5 and 1% EMI-BF4, and is better than that obtained with the blank electrolyte. When the temperature drops below 40 C, the viscosity of the electrolyte will increase, resulting in a decrease in ionic conductivity. However, when the additive content is continuously increased, the ionic conductivity of the electrolyte decreases, which is possibly due to the decrease in the fluidity of the electrolyte caused by the addition of an excessive amount of the additive. The addition of 0.5% and 1% EMI-BF4 improves the ionic conductivity of the electrolyte at low temperature and may alleviate the problematic performance degradation of the LIBs at low temperatures based on the electrolyte itself. The oxidation of functional additives prior to their addition can be used to construct a low-impedance SEI on the electrode surface that may be beneficial to improving the performance of LIBs at low temperatures. To evaluate the electrochemical redox behaviors of EMIeBF4 on the electrode surfaces, CV tests were performed using a Li/NCM523 half cell and a Li/graphite half cell at a scan rate of 0.1 mV s-1 and in the voltage ranges of 3.0e4.5 V and 0.01e3 V, respectively. As shown in Fig. 2a and b, a pair of significant redox peaks can be observed in the CV curves of the Li/NCM523 half-cell that correspond to the insertion/extraction of the Li ions, respectively. In Fig. 2b, the EMI-BF4 redox peaks overlap and cannot be identified because of the strong insertion/extraction peaks of Liþ. Compared with the blank electrolyte, the CV curves of the additivecontaining electrolyte almost completely overlap with the other two scanning curves except for the first cycle, and the peak current is stronger. It was observed that there was a little difference appeared in the obtained CV curves when the different test cells were used, but the differences between the CV curves were within the normal error range, and these differences would not affect the qualitative interpretation of the additives. The above results indicate that the addition of 1% EMI-BF4 increases the cyclic reversibility of the cathode and improves the rate of the interfacial reaction. Based on this result combined with the previous LSV
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Fig. 1. a) LSV curves of the platinum electrode in the electrolytes with and without 1% EMIeBF4 additive; b) Ionic conductivity curves of the electrolytes containing 0, 0.5, 1, 2, and 5% EMIeBF4 from 40 C to 25 C.
Fig. 2. CV curves of the Li/NCM532 half cells with a) the blank electrolyte and b) the electrolyte containing 1% EMIeBF4 at a scan rate of 0.1 mV s-1 over a voltage range of 3.0e4.5 V. CV curves of Li/graphite half cells with c) the blank electrolyte and d) the electrolyte containing 1% EMIeBF4 at a scan rate of 0.1 mV s-1 over a voltage range of 0.01e3 V.
results, it is speculated that EMIeBF4 can be pre-oxidized to participate in the SEI formation. The possible participation of f EMIeBF4 in the construction of the SEI on the anode surface was also evaluated, as shown in Fig. 2c and d. The redox peak below 0.3 V corresponds to the insertion/ extraction of Liþ, and the reduction peaks at approximately 0.75 V and 1.3 V correspond to the EC and EMC reductive decomposition, respectively [34]. Compared with the blank electrolyte, the reduction peak near 0.24 V of the Li/graphite half-cell with the electrolyte containing EMIeBF4 became much weaker, indicating that the amount of lithium intercalation was reduced after the addition of additives. In Fig. 2d, significant irreversible reduction peaks occur at approximately 0.95 V and 1.75 V in the first cycle, which may correspond to the preferential reduction of EMI and BFþ 4 in the
additive, respectively [32]. In addition, the formation of the SEI film causes irreversible capacity loss that in turn suppresses further interfacial reactivity and reduces peak current [35]. As shown in Fig. 2d, after the addition of the additive, the anode electrode interface reacted much less, resulting in a stable CV curve and excellent original cycle performance. These results indicate that the added EMIeBF4 can simultaneously decompose on the surfaces of the anode and cathode to participate in the construction of a stable low-impedance SEI. 3.2. Effects of the EMIeBF4 additive on the batteries As shown in Fig. 3a, the dQ/dV curves obtained during the precharging process were used to evaluate the performance of the
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Fig. 3. Electrochemical performance characteristics of the NCM523/graphite full cells with different amounts of the EMI-BF4 additive. a) dQ/dV curves of the NCM523/graphite full cells. b) Cycle stability and Coulombic efficiency of the NCM523/graphite full cell at a voltage range of 2.75e4.2 V at room temperature. c) Discharge curves of the NCM523/graphite full cells at the temperatures of 30 C, 20 C, and 25 C at 0.5C. d) The cycle stability and coulombic efficiency of the NCM523/graphite full cells at a voltage range of 2.75e4.2 V at 10 C.
NCM523/graphite full cells containing 0, 0.5, 1, 2, and 5% EMI-BF4 additive. Compared with the blank electrolyte, the dQ/dV curves of the electrolyte with EMIeBF4 show a distinct peak at approximately 2.5 V. This peak corresponds to the reduction peak near 0.95 V of the CV measurements presented in Fig. 2d and implies means the formation of SEI and the reductive decomposition of EMIþ. As the EMIeBF4 content increases, the peak intensity near 2.5 V gradually increases, indicating that more EMIeBF4 is decomposed. This may cause an overgrowth of the electrode interface film to and correspondingly increase the interface impedance. Fig. 3b shows the cycle performance and coulombic efficiency of the NCM523/graphite full cells at room temperature. The coulomb efficiency of the NCM523/graphite full cells with the blank electrolyte is 82.7% for the first cycle. As the number of cycles increases, the capacity decreases rapidly, and the capacity retention approaches 80.0% after 400 cycles. By contrast, the first coulomb efficiencies of the NCM523/graphite full cells containing 0.5, 1, 2, and 5% EMI-BF4 were 85.2, 84.3, 80.4, and 65.9%, and the capacity retention values were 80.4%, 85.9, 80.4, and ~0%, respectively, after 400 cycles. In particular, with the addition of 1% EMIeBF4, the coulombic efficiency of the NCM523/graphite full cells is close to ~100% during the charging/discharging process. According to these results, the addition of EMIeBF4 is beneficial for improving the first coulombic efficiency and cycle stability of the NCM523/graphite full cells due to an expected SEI being generated on the electrode surfaces. By contrast, the SEI formed in the blank electrolyte is thought to be unstable which eventually leads to a more rapid decline in the battery performances. The amount of added EMIeBF4 must be controlled, only a suitable amount of EMIeBF4 could produce a lowimpedance and robust SEI on the electrode surface, improving the performance of the NCM523/graphite full cells. The performance improvement of NCM523/graphite full cells with 1% EMIeBF4 at room temperature is the most obvious, thus, the electrolyte with 1% EMIeBF4 will be investigated in the following experiments.
Fig. 3c shows the effect of 1% EMIeBF4 on the discharge capacity of NCM523/graphite full cells at 30, 20, and 25 C. The discharge capacities of the cells with the blank electrolyte at 30, 20, and 25 C were 303.7, 1092.4, and 1863.5 mAh, respectively. Upon addition of 1% EMI-BF4, the discharge capacity was increased to 878.7, 1372.2 and 1919.0 mAh, respectively. The addition of EMIeBF4 increased the discharge capacity of the NCM523/graphite full cells at low temperatures. However, the excellent low-temperature discharge capacity only is far from meeting the requirements for LIBs to operate at low temperatures. Therefore, the effect of EMIeBF4 on the cycle stability of the NCM523/graphite full battery at low temperature was further verified, as shown in Fig. 3d. At 10 C and after 150 cycles, the capacity retention of the cells with the blank electrolyte and the EMI-BF4 containing electrolyte were 78.2% and 89.4%, respectively. Compared with the blank electrolyte, the capacity retention of the cells with EMIeBF4 is increased by more than ten percentage points, and the Coulomb efficiency is always close to 100%. The addition of EMIeBF4 not only increases the ionic conductivity of the electrolyte at low temperatures but also facilitates the formation of a low-impedance SEI on the electrode surface, which may be the main reason for the improvement in the low temperature performance. The EIS spectra of the NCM523/graphite full cells after the precharge at room temperature are shown in Fig. 4a. Compared to the cell with the blank electrolyte, the Rs and RSEI values of the cell decreased slightly upon addition of 1% EMI-BF4, and the decrease was more pronounced after 400 cycles. Both Rs and RSEI of the cells with 2 and 5% EMI-BF4 after the pre-charge and recycling increased, and maximal enhancement was obtained for the cell with 5% EMIBF4. Surprisingly, the cells with 0.5 and 1% EMI-BF4 electrolytes had similar impedances at the pre-charge. However, after 400 cycles, the RSEI of the cell with 0.5% EMI-BF4 electrolyte increased significantly, indicating that an unstable SEI was generated and is also in
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full cells. 3.3. Surface analysis of the cycled electrodes
Fig. 4. EIS spectra of the NCM523/graphite full cells using different electrolytes. a) after the pre-charge and b) after 400 cycles at room temperature, c) after the precharge and d) after 150 cycles at 10 C.
agreement with the results presented in Fig. 3b. These results further demonstrate that a stable low-impedance SEI is generated on the electrode surface and therefore, the cycle stability of the NCM523/graphite full cells can be improved by adding 1% EMI-BF4 to the electrolyte. Fig. 4 c and 4d show the EIS results for the NCM523/graphite full cells after a pre-charge and 150 cycles at a low temperature of 10 C. Compared to the blank electrolyte, the Rs and RSEI values of the NCM523/graphite full cells with 1% EMIeBF4 electrolyte are reduced before and after the cycling operation, which is the same as the low temperature cycle and can be used to explain the results presented in Fig. 3d. The impedance reduction at 10 C can be attributed to the increase in the ionic conductivity due to the addition of EMI-BF4. More importantly, EMIeBF4 participated in the construction of a low-impedance SEI, which may play a decisive role in the low-temperature performance of the NCM523/graphite
The SEM images of the electrodes disassembled from the cycled batteries were obtained to reveal the effect of EMI-BF4 on the surface morphology of the electrodes. It is observed from Fig. 5a that fresh NCM523 particles exhibit smooth appearance. After 150 cycles in the blank electrolyte, a large amount of the electrolyte decomposition products was left on the surface of the NCM523 particles, and cracks were also formed on some particles (Fig. 5b). The organic decomposition products remaining on the electrode surface may give rise to an increase in the interface impedance. The cracks indicate that the particle structure was destroyed to a certain degree and causes the capacity decay. By contrast, as shown in Fig. 5c, after 150 cycles in the electrolyte containing 1% EMIeBF4, the surfaces of NCM523 particles are still smooth, and no obvious organic decomposition products and cracks were observed. The presence of EMIeBF4 in the electrolyte facilitates the formation of a stable SEI film on the NCM523 particles, thereby avoiding the continuous decomposition of the electrolyte and improving the structural stability of the NCM523 active material. Similarly, the layered structure and smooth surface of the fresh graphite can be clearly observed, as shown in Fig. 5d. After 150 cycles in the blank electrolyte, the graphite particles are covered by the dense SEI film formed by the decomposition resultants of the electrolyte (Fig. 5e). Some cracks were also observed on the graphite surfaces, indicating that the generated SEI is unstable and may cause continuous decomposition of the electrolyte. When EMIBF4 was added into the electrolyte, dense SEI films were formed on the graphite surfaces, but compared with that with the blank electrolyte, the SEI constructed by the additives was smoother and had no cracks. This indicated that the electrolyte with EMIeBF4 can form a stable SEI on the anode surface, avoiding the continuous decomposition of the electrolyte and maintaining the stability of the electrode structure. These results once again proved that the added EMIeBF4 molecules were involved in the construction of the interface films, and the electrochemical performance of batteries was improved upon the addition of the additive. In order to understand the effect of EMI-BF4 additive on SEI
Fig. 5. SEM images of a) fresh NCM523 cathode, b) NCM523 cathode after 150 cycles at 10 C using the blank electrolyte, c) NCM523 cathode after 150 cycles at 10 C using the electrolyte containing 1% EMI-BF4, d) fresh graphite anode, e) graphite anode after 150 cycles at 10 C using blank electrolyte, and f) graphite anode after 150 cycles at 10 C using the electrolyte containing 1% EMI-BF4.
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Fig. 6. XPS spectra of the NCM523 cathode after 150 charge-discharge cycles in the blank electrolyte and in the electrolyte containing 1% EMI-BF4 (10 C) compared with those of the fresh NCM523 cathode.
formation, analysis was performed by XPS [36]. Fig. 6 shows the XPS spectra of the fresh NCM523 cathode and the cycled NCM523 cathode after 150 charge-discharge cycles in different electrolytes at 10 C. In the C1s spectrum of the NCM523 cathode, the peak near 284.77 eV corresponds to the CeC bond and represents the conductive carbon component. The peaks at 285.78 and 290.87 eV correspond to the CeH and CeF bonds that are associated with the PVDF binder, and the corresponding peaks appear at 688.08 eV in the F1s spectrum. After 150 cycles, the peaks related to alkyl carbonate (R-CH2OCO2-Li) CeO and Li2CO3 appeared at 288.07 and 290.13 eV in the NCM523 cathode spectrum and are found at 533.8 and 531.84 eV in the O1s spectrum. It is generally considered that CeO and C]O are the decomposition products of the electrolyte [37]. Compared with the blank electrolyte, the peak intensities of CeO and C]O decrease after the addition of EMI-BF4. This means that on the cathode side, the addition of EMI-BF4 inhibits the decomposition of the carbonate solvents in the electrolyte. In the
F1s spectrum, the peak at 688.08 eV corresponds to the PeF bond in PVDF, and a new peak appears at 685.07 eV after the cycling. This is generally considered to be due to LiF that is one of the most important components of the SEI. After the addition of EMI-BF4, the LiF peak intensity increases. Combining this finding with the previous results, it is concluded that EMI-BF4 participates in the construction of a stable SEI. The peaks at 136.42 eV and 133.68 eV in the P2p spectrum are due to the LixPFy and LixPOyFz decomposition products of LiPF6. Compared with the blank electrolyte, the peak intensity of LixPOyFz is weakened after adding the additive, and the LixPFy peak intensity is increased along with the peak intensity of LiF in F1s. Compared with the blank electrolyte, new peaks appear at 400.53 and 191.60 eV in N1s and B1s spectra after adding the additives. Therefore, these results prove that after the addition of EMI-BF4, and the EMI-BF4 decomposes and participates in the construction of the SEI. This is also confirmed by the CV and LSV results shown above.
Fig. 7. XPS spectra of the graphite anode after 150 charge-discharge cycles in the blank electrolyte and the electrolyte containing 1% EMI-BF4 (10 C) compared with that of the fresh graphite anode.
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Fig. 7 shows the XPS spectra of the fresh graphite anode and the graphite anode obtained after 150 cycles that were chargedischarged in different electrolytes at 10 C. In the C1s spectrum, the peak at 284.8 eV represents the CeC bond of graphite, and the peak near 285.88 eV represents the CeOeC bond of the CMC binder. After 150 cycles, a new peak at 289.80 eV in the C1s spectrum corresponds to lithium alkyl carbonates and polycarbonates that are the decomposition products of the electrolyte. At the same time, the peak intensities of the CeC and CeOeC bonds are significantly reduced because electrolyte decomposition deposits and EMIeBF4 preferentially reduce the SEI, causing these peaks to be weakened after the cycling [24]. In the O1s spectrum, the intensities of the CeO and C]O peaks at 533.91 and 532.36 eV increase significantly after the cycling similar to the C1s spectra, and this is associated with the decomposition of the electrolyte to form the SEI. In the F1s spectrum, the peak intensity of LiF increases after the cycling in EMI-BF4-containing electrolyte, indicating that the LiF content in the SEI increases after adding the additives. Previous results have shown that an appropriate increase in LiF reduces the interface impedance and improves the low temperature performance of LIBs [38]. The increase in LiF content also means an increase in LixPFy and LixPOyFz contents. The results in the P2p spectrum are similar to those for the F1s spectrum, indicating a significant increase in the intensities of the LixPFy and LixPOyFz peaks. Compared to the blank electrolyte, new peaks appear at 400.21 and 193.87 eV in the N1s and B1s spectra after the addition of the additive. Therefore, the results show that the added EMI-BF4 also decomposes on the surface of the graphite anode and participates in the construction of the SEI. At the same time, this results in more LiF participating in the construction of SEI, reducing the interface impedance and improving the low temperature performance of the NCM532/graphite battery. 4. Conclusions In summary, an ionic liquid EMIeBF4 was developed as an electrolyte additive to participate in the construction of a stable low-impedance SEI on the cathode and anode surfaces of the NCM532/graphite batteries. While improving the cycle performance of the NCM532/graphite full cells at low temperatures, it also improves the cycle stability of the cells at room temperature. The addition of EMIeBF4 additives in the electrolyte with a mass ratio of 1% significantly improved the electrochemical performance characteristics of the NCM532/graphite batteries and proved the feasibility of using ionic liquids in LIBs. In addition, it is possible to screen out a large number of groups that contribute to the performance improvement of NCM532/graphite batteries and then use the expected adjustability of the ionic liquids to design a series of new additives and functional electrolytes to match the requirements of LIBs.
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This work was financially supported by the National Natural Science Foundation of China (No. 21875077 and 21875076), and the Scientific and Technological Plan Projects of Guangzhou City (No. 2019-01-01-12-1006-0020) and Zhuhai City (No. ZH01084702180037HJL), P.R. China.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) as an ionic liquid-type electrolyte additive to enhance the low-temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite batteries Wenlian Wang a, Tianxiang Yang a, Shuai Li a, Weizhen Fan c, Xiaoyang Zhao a, b, Chaojun Fan c, Le Yu c, Shaoyun Zhou c, Xiaoxi Zuo a, **, Ronghua Zeng a, Junmin Nan a, * a b c
School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, PR China Department of Environmental Engineering, Henan Polytechnic Institute, Nanyang, 473009, PR China Guangzhou Tinci Materials Technology Co., Ltd., Guangzhou, 510760, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2019 Received in revised form 5 May 2019 Accepted 6 May 2019 Available online 9 May 2019
A 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIeBF4) ionic liquid is introduced into an electrolyte as a functional additive to enhance the low-temperature performance of LiNi0.5Co0.2Mn0.3O2 (NCM523)/ graphite batteries. Linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) tests show that EMIeBF4 can be oxidized in advance and produce low impedance solid-electrolyte interphase (SEI) films on the electrode surfaces. Compared with the batteries without the EMIeBF4 additive, the capacity retention of the NCM523/graphite batteries with 1% EMIeBF4 additive is enhanced from 82.3% to 93.8% after 150 cycles at a low temperature of 10 C. In addition, at 30 C, the discharge capacity of the batteries with 1% EMIeBF4 additive is nearly doubled. And at the room temperature of 25 C and after 400 cycles, the capacity retention also increases from 80.3% to 85.9%, and the coulomb efficiency remains at approximately 100%. These results demonstrate that EMIeBF4 used as a functional electrolyte additive has promising prospects for application to improve the lowtemperature performance of NCM523-based lithium-ion batteries. © 2019 Elsevier Ltd. All rights reserved.
Keywords: 1-ethyl-3-methylimidazolium tetrafluoroborate Electrolyte additive LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite batteries Low-temperature performance
1. Introduction Compared with the traditional lithium cobalt oxide (LiCoO2) cathode materials, layered LiNixCoyMnzO2 (0 < x, y, Z < 1, NCM) materials have lower cost, which can improve the sustainable development of lithium ion batteries (LIBs). These materials have good prospects for applications in next-generation energy storage devices and electric vehicles, and in other fields [1e6]. However, when NCM/graphite LIBs operate at low temperatures, their capacities rapidly decrease and lithium dendrite growth is observed, leading to safety risks and a rapid decline of battery performance; this has become the core issue that restricts the applications of NCM/graphite batteries [7e13]. Under low-temperature conditions, the mobility of Liþ in the electrolyte is reduced, resulting in the weakening of the Liþ migration ability and the decrease of the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zuo), [email protected] (J. Nan). https://doi.org/10.1016/j.electacta.2019.05.027 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
reaction ability in the electrode interface film that give rise to a rapid degradation of battery performance [9,14]. Thus, when the composition of the cathode and anode active materials remains unchanged, the development of practical strategies for reducing the sensitivity of batteries to low temperature has become a key issue for improving the low-temperature performance of the NCM-based LIBs [15e17]. In the practical application of LIBs, in order to improve their poor low-temperature performance, there are two main approaches, i.e. to keep a suitable working environment and to improve the performance of the batteries themselves at low temperatures. The use of an external or a built-in heater for the battery packs can enable LIBs to operate in room temperature conditions when the battery packs are in low-temperature environments. Wang et al. [18,19] designed a new type of “All Climate Batteries TM” that used the ohmic heat generated by nickel sheets at low temperatures to realize the self-heating of LIBs. This method had successfully improved the performance of LIBs subjected to an external low temperature. At an ambient temperature of 30 C,
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the battery power was increased by more than 10 times, while the performance and cycling lifetime of the batteries at room temperature were not weakened. Further improvements enabled fast charging at low temperatures, with full charging in 15 min even in an extreme environment of 50 C. Hao et al. [20] utilized a memory alloy based on a thermal regulator to regulate the temperature of the battery pack. The demonstration of a battery pack consisting of commercial 18650 batteries showed that this thermal regulator effectively blocked the heat leak from the battery pack. In a temperature environment of 20 C, the temperature of battery pack rose rapidly to 20 C and the available capacity of the batteries was increased by a factor of greater than three. These approaches for increasing the temperature regulation effectively alleviate the impact of low ambient temperature, but they also increase the manufacturing cost and reduce the volume-specific energy of battery pack. As an alternative approach to external intervention, it is also possible to improve the low-temperature performance of LIBs themselves by optimizing the electrolyte composition. Using a suitable electrolyte composition in the LIBs, lithium ion conductivity can be increased and a low-impedance SEI film can also be generated on the electrode surfaces, improving the LIB performance characteristics [7,14,21,22]. It is well-known that at low temperatures, the Liþ conductivity of the electrolyte decreases and the impedance of the SEI increases, resulting in slow reaction kinetics and rapid capacity decay. In addition to optimize the solvent composition, the simplest approach for optimizing the electrolyte is to add functional additives to change the electrolyte properties. Usually, the amounts of functional additives in the electrolyte are small, but their effects on the LIB performance characteristics are very significant. Compared to external heaters, the addition of functional additives is undoubtedly a simple and economical strategy. Some organic molecules and lithium salts have been demonstrated to exert excellent low temperature effects when used as the functional electrolyte additives in LIBs [23]. Yang et al. [24] reported that lithium difluorophosphate (LiPO2F2) formed a stable low-impedance SEI film on the electrode surface. At 20 C and 30 C and a discharge rate of 0.2C, the capacity of LiNi0.5Co0.2Mn0.3O2 (NCM523)-based LIBs containing 1% LiPO2F2 was 71.9% and 57.93% (the room temperature capacity value), respectively. Whereas under the same conditions, the batteries without LiPO2F2 released only 49.41% and 9.6% of the capacity. Li et al. [25] reported another lithium salt additive, namely, lithium difluorobis (oxalato) phosphate (LiDFBOP). It was found that LiDFBOP could simultaneously form a stable SEI film on the cathode and anode surfaces, which effectively improved the electrochemical performance of the NCM523/graphite full cell at low temperature. At 20 C, the NCM523/graphite full cell was charged and discharged at a rate of 0.5C, and the capacity retentions of the batteries with 1% LiDFBOP and without LiDFBOP after 50 cycles were 93% and 73%, respectively. Jurng et al. [26] proposed the use of allyl sulfide (AS) derivatives as the low-temperature additives. The addition of 2% AS to a Li/graphite half-cell inhibited the formation of lithium dendrites at 30 C and increased the reversible capacity by more than three times. These results indicate that the addition of additives may effectively promote the low-temperature performance of the NCM-based LIBs and decrease the manufacturing cost. In addition to the traditional organic functional molecule additives, ionic liquids have also been recognized as environmentally friendly electrolytes and potential electrolyte additives in LIBs. This is mainly due to their unique properties such as low vapor pressure, high thermal stability, chemical stability, excellent solubility, polarity and good conductivity [27,28]. Ionic liquids have been developed as new solvents to replace the current solvents or as functional additives
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to optimize the existing solvents in the carbonate-based systems. Dong et al. [29] used the safe ionic liquid-sulfolane/LiDFOB electrolytes in Li/Li1.15(Ni0.36Mn0.64)0.85O2 cells, at 55 C and 70 C, after cycling for 50 cycles at a rate of 0.5 C in the range of 2.0-4.6 V, the capacities reached 172.5 and 238.8 mAh g-1, respectively. Kim et al. [30] prepared a N-allyl-2-methoxypyrroliniumbis(fluorosulfonyl) imide (A(OMe)Pyrl-FSI) ionic liquid to replace the current carbonate electrolyte. The initial discharge capacity of the Li/LiFePO4 cells was 153.7 mAh g-1 at 0.1C, the capacity retention rate was 81.2% (124.8 mAh g-1) at 5C, and high thermal stability and nonflammability were obtained. Pan et al. [31] used N-methyl-Nmethoxycarbonylpiperidinebis (trifluoromethanesulfonyl)imide as the electrolyte. At room temperature, after 20 cycle at 0.1C, the discharge capacity of the Li/LiFePO4 cells was 109.7 mAh g-1, and the capacity retention was 98.7%. These results indicate that more ionic liquids should be evaluated to forward their use in the LIBs. Besides the performances, the high-cost of the ionic liquids is also a key factor hindered their commercial application in the LIBs. Therefore, an appropriate choice at current stage is to evaluate ionic liquids as electrolyte additives. In our previous work [32], it was shown that BF 4 anions formed low-impedance interface films on the surfaces of the NCM-based cathode and the graphite anode. In this study, based on the low viscosity and high conductivity of 1ethyl-3-methylimidazole cations, 1-ethyl-3-methylimidazole tetrafluoroborate (EMI-BF4) was evaluated as a low-temperature electrolyte additive in the NCM-based LIBs. It was found that the advantages of EMIþ and BF 4 could be integrated to improve the low-temperature performance of the NCM-based LIBs. The introduction of ionic liquids into the existing solvent systems offers a new method to design functional electrolyte additives. 2. Experimental 2.1. Chemicals and materials Battery grade ethylene carbonate (EC), ethyl methyl carbonate (EMC) solvent, dimethyl carbonate (DMC), and lithium hexafluorophosphate (LiPF6) were supplied by Guangzhou Tinci Materials Technology Co., Ltd., China. The electrolyte additive EMIeBF4 (>99%) was purchased from Lanzhou Zhongke Kate Industry & Trade Co., Ltd., China. All of the chemicals were not purified further. The blank electrolyte was 1 mol L-1 LiPF6/ECþEMC (with a mass ratio of 1:2). The electrolyte was prepared in an argon-filled glove box (MBraun, Germany) with both oxygen and water contents controlled below 0.1 ppm. The ionic liquid functional electrolyte was obtained by adding appropriate amounts of EMIeBF4 into the blank electrolyte. The moisture and acid content (HF) of the electrolyte were determined using a Karl-Fisher 831 Coulometer (Metrohm, Switzerland) and a Karl-Fisher 798 GPT Titrino (Metrohm, Switzerland), and the values were controlled below 20 and 50 ppm, respectively. 2.2. Preparation of the batteries NCM523 active material (Shenzhen Tianjiao Tech. Co., Ltd., China), polyvinylidene difluoride (PVDF) binder and Super-P conductive carbon with a mass ratio of 96.8:1.2:2 were uniformly dispersed in N-methylpyrrolidone, and then the slurry was coated onto an Al current collector. After the subsequent drying, rolling, and cutting treatment, NCM523 cathodes were finally obtained. The thickness and compaction density of the prepared cathode were 121 mm and 3.289 g cm-3, respectively. Commercial graphite (Shenzhen BTR Co., Ltd., China), CMC, Super-P and polymerized styrene-butadiene rubber SBR with a mass ratio of 95:1.5:1.5:2
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were uniformly dispersed in the diluted water to prepare the anode slurry. Then, the slurry was coated onto a Cu current collector and subjected to drying, rolling, and cutting treatment to obtain the graphite anodes. The thickness and compaction density of the prepared graphite anodes were 126 mm and 1.515 g cm3, respectively. Afterwards, the pouch NCM523/graphite full cells with 6.2 g electrolyte per cell and normal capacity of 1950 mAh were assembled using the general manufacturing process at Guangzhou Tinci Materials Technology Co., Ltd., China. A Celgard 2400 microporous membrane was used as the separator, and an Al plastic film was used as the outer shell. In addition, the 2032 type Li/NCM523 half-cell and Li/graphite half-cell were also assembled to evaluate the effects of the EMI-BF4 additive on the NCM523 cathode and the graphite anode, respectively. 2.3. Electrochemical measurements The cycling performance of the NCM523/graphite full cells in the voltage range of 2.75e4.2 V was evaluated using a multichannel battery cycler (CT-3008W, Neware, China). Prior to the cycling test, the NCM523/graphite full cells were first charged at a rate of 0.1C for 6.5 h, aged at 45 C under a pressure of 500 kg for 24 h, and were successively discharge-charged at the rates of 0.1, 0.2, and 0.5C. During the room temperature (25 C) test, the NCM523/graphite full cells were discharge-charged at a rate of 1C. For the lowtemperature test, the NCM523/graphite full cells were tested at a rate of 0.2C in a test chamber (GDW-100L, SUOYATE, China) with the temperature set to 10 C. The NCM523/graphite full cell was first cycled at room temperature for 3 cycles at 1C rate and then was subjected to low-temperature testing. After the fully charged batteries, i.e. state of charge (SOC) ¼ 100%, were placed in the lowtemperature test chamber for 4 h, the discharge test was performed at a rate of 0.5C. The cyclic voltammetry (CV) measurements were carried out using an electrochemical workstation (CHI660E, Chenhua, Shanghai) at a scanning rate of 0.1 mV s-1, and the voltage ranges of the Li/NCM523 and the Li/graphite half-cells were 3e4.5 V and 0.01e3 V, respectively. A linear sweep voltammetry (LSV) test was performed in the voltage range of 0e7 V, with Pt as the working electrode and Li as the counter and reference electrodes at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) results were obtained using a Solartron 1470 (England) instrument in a frequency range from 100 kHz to 0.01 Hz and with a voltage amplitude of 5 mV. 2.4. Characterization of the electrolyte and cycled electrodes The electrolytes containing 0, 0.5, 1, 2 and 5% EMI-BF4 were prepared and stored in a chamber (GDW-100L, SUOYATE, China) with a controlled temperature for 4 h, respectively. After the temperatures of these electrolytes stabilized, their ionic conductivities were measured using a conductivity meter (DDS-307, Shanghai INESA Scientific Instrument Co., Ltd.). All of the batteries that required physical characterization were disassembled in an argonfilled glove box after discharging to 2.75 V. The electrodes disassembled from the batteries were randomly selected and washed with DMC solvent, dried and removed for the subsequent characterizations. The obtained electrodes were cut into small pieces and attached to a conductive paste, and then after a gold spray treatment, the surface morphologies of the cycled electrodes were observed using a scanning electron microscope (SEM, HITACHI SU8010, Japan). The elemental composition at the surface of the cycled electrode was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA) using an Al Ka wire as the X-ray source.
3. Results and discussion 3.1. Effects of the EMIeBF4 additive on the electrolyte properties It is well-known that appropriate additives can change the properties of the electrolyte and subsequently affect the electrochemical performance of the LIBs. The influence of the EMI-BF4 additive on the electrochemical window of the electrolyte was first explored. As shown in Fig. 1a, the anode current of the electrolyte containing 1% EMI-BF4 began to increase at 3.5 V, indicating that the EMI-BF4 oxidation reaction occurred. This means that the oxidative decomposition potential of EMI-BF4 is lower than that of other carbonate components, and EMI-BF4 is pre-oxidized and decomposed. These results indicate that the EMI-BF4 additive may oxidize in advance to form a protective film on the electrode surface [33]. Generally, the ionic conductivity of the electrolyte decreases with decreasing temperature, which is one of the key reasons for the rapid decline in the low-temperature performance of LIBs. To investigate the effect of the EMIeBF4 addition on the ionic conductivity, the ionic conductivities of the electrolytes containing 0, 0.5, 1, 2, and 5% EMIeBF4 were measured at different temperatures, as shown in Fig. 1b. Overall, it can be seen that the ionic conductivity is related to the temperature, and the drop in the temperature will lead to a decrease in the ionic conductivity. Above 0 C, EMI-BF4 has almost no observable effect on the ionic conductivity of the electrolyte due to the limited addition amount. By contrast, the addition of EMIeBF4 has a positive influence on the ionic conductivity when the temperature drops to between 0 C and 40 C. At temperatures lower than 0 C, the ionic conductivity of the electrolyte is improved after adding 0.5 and 1% EMI-BF4, and is better than that obtained with the blank electrolyte. When the temperature drops below 40 C, the viscosity of the electrolyte will increase, resulting in a decrease in ionic conductivity. However, when the additive content is continuously increased, the ionic conductivity of the electrolyte decreases, which is possibly due to the decrease in the fluidity of the electrolyte caused by the addition of an excessive amount of the additive. The addition of 0.5% and 1% EMI-BF4 improves the ionic conductivity of the electrolyte at low temperature and may alleviate the problematic performance degradation of the LIBs at low temperatures based on the electrolyte itself. The oxidation of functional additives prior to their addition can be used to construct a low-impedance SEI on the electrode surface that may be beneficial to improving the performance of LIBs at low temperatures. To evaluate the electrochemical redox behaviors of EMIeBF4 on the electrode surfaces, CV tests were performed using a Li/NCM523 half cell and a Li/graphite half cell at a scan rate of 0.1 mV s-1 and in the voltage ranges of 3.0e4.5 V and 0.01e3 V, respectively. As shown in Fig. 2a and b, a pair of significant redox peaks can be observed in the CV curves of the Li/NCM523 half-cell that correspond to the insertion/extraction of the Li ions, respectively. In Fig. 2b, the EMI-BF4 redox peaks overlap and cannot be identified because of the strong insertion/extraction peaks of Liþ. Compared with the blank electrolyte, the CV curves of the additivecontaining electrolyte almost completely overlap with the other two scanning curves except for the first cycle, and the peak current is stronger. It was observed that there was a little difference appeared in the obtained CV curves when the different test cells were used, but the differences between the CV curves were within the normal error range, and these differences would not affect the qualitative interpretation of the additives. The above results indicate that the addition of 1% EMI-BF4 increases the cyclic reversibility of the cathode and improves the rate of the interfacial reaction. Based on this result combined with the previous LSV
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Fig. 1. a) LSV curves of the platinum electrode in the electrolytes with and without 1% EMIeBF4 additive; b) Ionic conductivity curves of the electrolytes containing 0, 0.5, 1, 2, and 5% EMIeBF4 from 40 C to 25 C.
Fig. 2. CV curves of the Li/NCM532 half cells with a) the blank electrolyte and b) the electrolyte containing 1% EMIeBF4 at a scan rate of 0.1 mV s-1 over a voltage range of 3.0e4.5 V. CV curves of Li/graphite half cells with c) the blank electrolyte and d) the electrolyte containing 1% EMIeBF4 at a scan rate of 0.1 mV s-1 over a voltage range of 0.01e3 V.
results, it is speculated that EMIeBF4 can be pre-oxidized to participate in the SEI formation. The possible participation of f EMIeBF4 in the construction of the SEI on the anode surface was also evaluated, as shown in Fig. 2c and d. The redox peak below 0.3 V corresponds to the insertion/ extraction of Liþ, and the reduction peaks at approximately 0.75 V and 1.3 V correspond to the EC and EMC reductive decomposition, respectively [34]. Compared with the blank electrolyte, the reduction peak near 0.24 V of the Li/graphite half-cell with the electrolyte containing EMIeBF4 became much weaker, indicating that the amount of lithium intercalation was reduced after the addition of additives. In Fig. 2d, significant irreversible reduction peaks occur at approximately 0.95 V and 1.75 V in the first cycle, which may correspond to the preferential reduction of EMI and BFþ 4 in the
additive, respectively [32]. In addition, the formation of the SEI film causes irreversible capacity loss that in turn suppresses further interfacial reactivity and reduces peak current [35]. As shown in Fig. 2d, after the addition of the additive, the anode electrode interface reacted much less, resulting in a stable CV curve and excellent original cycle performance. These results indicate that the added EMIeBF4 can simultaneously decompose on the surfaces of the anode and cathode to participate in the construction of a stable low-impedance SEI. 3.2. Effects of the EMIeBF4 additive on the batteries As shown in Fig. 3a, the dQ/dV curves obtained during the precharging process were used to evaluate the performance of the
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Fig. 3. Electrochemical performance characteristics of the NCM523/graphite full cells with different amounts of the EMI-BF4 additive. a) dQ/dV curves of the NCM523/graphite full cells. b) Cycle stability and Coulombic efficiency of the NCM523/graphite full cell at a voltage range of 2.75e4.2 V at room temperature. c) Discharge curves of the NCM523/graphite full cells at the temperatures of 30 C, 20 C, and 25 C at 0.5C. d) The cycle stability and coulombic efficiency of the NCM523/graphite full cells at a voltage range of 2.75e4.2 V at 10 C.
NCM523/graphite full cells containing 0, 0.5, 1, 2, and 5% EMI-BF4 additive. Compared with the blank electrolyte, the dQ/dV curves of the electrolyte with EMIeBF4 show a distinct peak at approximately 2.5 V. This peak corresponds to the reduction peak near 0.95 V of the CV measurements presented in Fig. 2d and implies means the formation of SEI and the reductive decomposition of EMIþ. As the EMIeBF4 content increases, the peak intensity near 2.5 V gradually increases, indicating that more EMIeBF4 is decomposed. This may cause an overgrowth of the electrode interface film to and correspondingly increase the interface impedance. Fig. 3b shows the cycle performance and coulombic efficiency of the NCM523/graphite full cells at room temperature. The coulomb efficiency of the NCM523/graphite full cells with the blank electrolyte is 82.7% for the first cycle. As the number of cycles increases, the capacity decreases rapidly, and the capacity retention approaches 80.0% after 400 cycles. By contrast, the first coulomb efficiencies of the NCM523/graphite full cells containing 0.5, 1, 2, and 5% EMI-BF4 were 85.2, 84.3, 80.4, and 65.9%, and the capacity retention values were 80.4%, 85.9, 80.4, and ~0%, respectively, after 400 cycles. In particular, with the addition of 1% EMIeBF4, the coulombic efficiency of the NCM523/graphite full cells is close to ~100% during the charging/discharging process. According to these results, the addition of EMIeBF4 is beneficial for improving the first coulombic efficiency and cycle stability of the NCM523/graphite full cells due to an expected SEI being generated on the electrode surfaces. By contrast, the SEI formed in the blank electrolyte is thought to be unstable which eventually leads to a more rapid decline in the battery performances. The amount of added EMIeBF4 must be controlled, only a suitable amount of EMIeBF4 could produce a lowimpedance and robust SEI on the electrode surface, improving the performance of the NCM523/graphite full cells. The performance improvement of NCM523/graphite full cells with 1% EMIeBF4 at room temperature is the most obvious, thus, the electrolyte with 1% EMIeBF4 will be investigated in the following experiments.
Fig. 3c shows the effect of 1% EMIeBF4 on the discharge capacity of NCM523/graphite full cells at 30, 20, and 25 C. The discharge capacities of the cells with the blank electrolyte at 30, 20, and 25 C were 303.7, 1092.4, and 1863.5 mAh, respectively. Upon addition of 1% EMI-BF4, the discharge capacity was increased to 878.7, 1372.2 and 1919.0 mAh, respectively. The addition of EMIeBF4 increased the discharge capacity of the NCM523/graphite full cells at low temperatures. However, the excellent low-temperature discharge capacity only is far from meeting the requirements for LIBs to operate at low temperatures. Therefore, the effect of EMIeBF4 on the cycle stability of the NCM523/graphite full battery at low temperature was further verified, as shown in Fig. 3d. At 10 C and after 150 cycles, the capacity retention of the cells with the blank electrolyte and the EMI-BF4 containing electrolyte were 78.2% and 89.4%, respectively. Compared with the blank electrolyte, the capacity retention of the cells with EMIeBF4 is increased by more than ten percentage points, and the Coulomb efficiency is always close to 100%. The addition of EMIeBF4 not only increases the ionic conductivity of the electrolyte at low temperatures but also facilitates the formation of a low-impedance SEI on the electrode surface, which may be the main reason for the improvement in the low temperature performance. The EIS spectra of the NCM523/graphite full cells after the precharge at room temperature are shown in Fig. 4a. Compared to the cell with the blank electrolyte, the Rs and RSEI values of the cell decreased slightly upon addition of 1% EMI-BF4, and the decrease was more pronounced after 400 cycles. Both Rs and RSEI of the cells with 2 and 5% EMI-BF4 after the pre-charge and recycling increased, and maximal enhancement was obtained for the cell with 5% EMIBF4. Surprisingly, the cells with 0.5 and 1% EMI-BF4 electrolytes had similar impedances at the pre-charge. However, after 400 cycles, the RSEI of the cell with 0.5% EMI-BF4 electrolyte increased significantly, indicating that an unstable SEI was generated and is also in
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full cells. 3.3. Surface analysis of the cycled electrodes
Fig. 4. EIS spectra of the NCM523/graphite full cells using different electrolytes. a) after the pre-charge and b) after 400 cycles at room temperature, c) after the precharge and d) after 150 cycles at 10 C.
agreement with the results presented in Fig. 3b. These results further demonstrate that a stable low-impedance SEI is generated on the electrode surface and therefore, the cycle stability of the NCM523/graphite full cells can be improved by adding 1% EMI-BF4 to the electrolyte. Fig. 4 c and 4d show the EIS results for the NCM523/graphite full cells after a pre-charge and 150 cycles at a low temperature of 10 C. Compared to the blank electrolyte, the Rs and RSEI values of the NCM523/graphite full cells with 1% EMIeBF4 electrolyte are reduced before and after the cycling operation, which is the same as the low temperature cycle and can be used to explain the results presented in Fig. 3d. The impedance reduction at 10 C can be attributed to the increase in the ionic conductivity due to the addition of EMI-BF4. More importantly, EMIeBF4 participated in the construction of a low-impedance SEI, which may play a decisive role in the low-temperature performance of the NCM523/graphite
The SEM images of the electrodes disassembled from the cycled batteries were obtained to reveal the effect of EMI-BF4 on the surface morphology of the electrodes. It is observed from Fig. 5a that fresh NCM523 particles exhibit smooth appearance. After 150 cycles in the blank electrolyte, a large amount of the electrolyte decomposition products was left on the surface of the NCM523 particles, and cracks were also formed on some particles (Fig. 5b). The organic decomposition products remaining on the electrode surface may give rise to an increase in the interface impedance. The cracks indicate that the particle structure was destroyed to a certain degree and causes the capacity decay. By contrast, as shown in Fig. 5c, after 150 cycles in the electrolyte containing 1% EMIeBF4, the surfaces of NCM523 particles are still smooth, and no obvious organic decomposition products and cracks were observed. The presence of EMIeBF4 in the electrolyte facilitates the formation of a stable SEI film on the NCM523 particles, thereby avoiding the continuous decomposition of the electrolyte and improving the structural stability of the NCM523 active material. Similarly, the layered structure and smooth surface of the fresh graphite can be clearly observed, as shown in Fig. 5d. After 150 cycles in the blank electrolyte, the graphite particles are covered by the dense SEI film formed by the decomposition resultants of the electrolyte (Fig. 5e). Some cracks were also observed on the graphite surfaces, indicating that the generated SEI is unstable and may cause continuous decomposition of the electrolyte. When EMIBF4 was added into the electrolyte, dense SEI films were formed on the graphite surfaces, but compared with that with the blank electrolyte, the SEI constructed by the additives was smoother and had no cracks. This indicated that the electrolyte with EMIeBF4 can form a stable SEI on the anode surface, avoiding the continuous decomposition of the electrolyte and maintaining the stability of the electrode structure. These results once again proved that the added EMIeBF4 molecules were involved in the construction of the interface films, and the electrochemical performance of batteries was improved upon the addition of the additive. In order to understand the effect of EMI-BF4 additive on SEI
Fig. 5. SEM images of a) fresh NCM523 cathode, b) NCM523 cathode after 150 cycles at 10 C using the blank electrolyte, c) NCM523 cathode after 150 cycles at 10 C using the electrolyte containing 1% EMI-BF4, d) fresh graphite anode, e) graphite anode after 150 cycles at 10 C using blank electrolyte, and f) graphite anode after 150 cycles at 10 C using the electrolyte containing 1% EMI-BF4.
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Fig. 6. XPS spectra of the NCM523 cathode after 150 charge-discharge cycles in the blank electrolyte and in the electrolyte containing 1% EMI-BF4 (10 C) compared with those of the fresh NCM523 cathode.
formation, analysis was performed by XPS [36]. Fig. 6 shows the XPS spectra of the fresh NCM523 cathode and the cycled NCM523 cathode after 150 charge-discharge cycles in different electrolytes at 10 C. In the C1s spectrum of the NCM523 cathode, the peak near 284.77 eV corresponds to the CeC bond and represents the conductive carbon component. The peaks at 285.78 and 290.87 eV correspond to the CeH and CeF bonds that are associated with the PVDF binder, and the corresponding peaks appear at 688.08 eV in the F1s spectrum. After 150 cycles, the peaks related to alkyl carbonate (R-CH2OCO2-Li) CeO and Li2CO3 appeared at 288.07 and 290.13 eV in the NCM523 cathode spectrum and are found at 533.8 and 531.84 eV in the O1s spectrum. It is generally considered that CeO and C]O are the decomposition products of the electrolyte [37]. Compared with the blank electrolyte, the peak intensities of CeO and C]O decrease after the addition of EMI-BF4. This means that on the cathode side, the addition of EMI-BF4 inhibits the decomposition of the carbonate solvents in the electrolyte. In the
F1s spectrum, the peak at 688.08 eV corresponds to the PeF bond in PVDF, and a new peak appears at 685.07 eV after the cycling. This is generally considered to be due to LiF that is one of the most important components of the SEI. After the addition of EMI-BF4, the LiF peak intensity increases. Combining this finding with the previous results, it is concluded that EMI-BF4 participates in the construction of a stable SEI. The peaks at 136.42 eV and 133.68 eV in the P2p spectrum are due to the LixPFy and LixPOyFz decomposition products of LiPF6. Compared with the blank electrolyte, the peak intensity of LixPOyFz is weakened after adding the additive, and the LixPFy peak intensity is increased along with the peak intensity of LiF in F1s. Compared with the blank electrolyte, new peaks appear at 400.53 and 191.60 eV in N1s and B1s spectra after adding the additives. Therefore, these results prove that after the addition of EMI-BF4, and the EMI-BF4 decomposes and participates in the construction of the SEI. This is also confirmed by the CV and LSV results shown above.
Fig. 7. XPS spectra of the graphite anode after 150 charge-discharge cycles in the blank electrolyte and the electrolyte containing 1% EMI-BF4 (10 C) compared with that of the fresh graphite anode.
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Fig. 7 shows the XPS spectra of the fresh graphite anode and the graphite anode obtained after 150 cycles that were chargedischarged in different electrolytes at 10 C. In the C1s spectrum, the peak at 284.8 eV represents the CeC bond of graphite, and the peak near 285.88 eV represents the CeOeC bond of the CMC binder. After 150 cycles, a new peak at 289.80 eV in the C1s spectrum corresponds to lithium alkyl carbonates and polycarbonates that are the decomposition products of the electrolyte. At the same time, the peak intensities of the CeC and CeOeC bonds are significantly reduced because electrolyte decomposition deposits and EMIeBF4 preferentially reduce the SEI, causing these peaks to be weakened after the cycling [24]. In the O1s spectrum, the intensities of the CeO and C]O peaks at 533.91 and 532.36 eV increase significantly after the cycling similar to the C1s spectra, and this is associated with the decomposition of the electrolyte to form the SEI. In the F1s spectrum, the peak intensity of LiF increases after the cycling in EMI-BF4-containing electrolyte, indicating that the LiF content in the SEI increases after adding the additives. Previous results have shown that an appropriate increase in LiF reduces the interface impedance and improves the low temperature performance of LIBs [38]. The increase in LiF content also means an increase in LixPFy and LixPOyFz contents. The results in the P2p spectrum are similar to those for the F1s spectrum, indicating a significant increase in the intensities of the LixPFy and LixPOyFz peaks. Compared to the blank electrolyte, new peaks appear at 400.21 and 193.87 eV in the N1s and B1s spectra after the addition of the additive. Therefore, the results show that the added EMI-BF4 also decomposes on the surface of the graphite anode and participates in the construction of the SEI. At the same time, this results in more LiF participating in the construction of SEI, reducing the interface impedance and improving the low temperature performance of the NCM532/graphite battery. 4. Conclusions In summary, an ionic liquid EMIeBF4 was developed as an electrolyte additive to participate in the construction of a stable low-impedance SEI on the cathode and anode surfaces of the NCM532/graphite batteries. While improving the cycle performance of the NCM532/graphite full cells at low temperatures, it also improves the cycle stability of the cells at room temperature. The addition of EMIeBF4 additives in the electrolyte with a mass ratio of 1% significantly improved the electrochemical performance characteristics of the NCM532/graphite batteries and proved the feasibility of using ionic liquids in LIBs. In addition, it is possible to screen out a large number of groups that contribute to the performance improvement of NCM532/graphite batteries and then use the expected adjustability of the ionic liquids to design a series of new additives and functional electrolytes to match the requirements of LIBs.
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[24]
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Acknowledgments [26]
This work was financially supported by the National Natural Science Foundation of China (No. 21875077 and 21875076), and the Scientific and Technological Plan Projects of Guangzhou City (No. 2019-01-01-12-1006-0020) and Zhuhai City (No. ZH01084702180037HJL), P.R. China.
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