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A high discharge voltage dual-ion rechargeable battery using pure (DMPI+)(AlCl4−) ionic liquid electrolyte
Journal of Power Sources 418 (2019) 233–240
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A high discharge voltage dual-ion rechargeable battery using pure (DMPI+) (AlCl4−) ionic liquid electrolyte
T
Zichuan Lv, Mei Han, Junhui Sun, Lixue Hou, Hui Chen, Yuxia Li∗∗, Meng-Chang Lin∗ College of Electrical Engineering and Automation, Shandong University of Science and Technology, Qingdao, 266590, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
battery (DIB) containing pure • Dual-ion ionic liquid as an electrolyte was prepared.
Natural graphite flakes were used as • both anode and cathode materials. DIB functioned under high-rate • This charge/discharge, bending, and burning conditions.
non-flammability/stability • Electrolyte allowed for enhanced safety and performance.
prepared DIB was well suited for • The grid energy storage.
A R T I C LE I N FO
A B S T R A C T
Keywords: Ionic liquid Imidazolium Graphite Intercalation/deintercalation Self-discharge
Dual-ion batteries (DIBs) are widely studied as novel electrochemical energy storage devices in view of their low cost, environmental friendliness, and high working voltage. However, the widespread application of these batteries is hindered by organic electrolyte decomposition at high working voltages and the absence of electrode materials allowing for high capacity and long cycle lifetime, which necessitates further investigations toward performance improvement. Herein, we use 1,2-dimethyl-3-propylimidazolium chloroaluminate (DMPI+) (AlCl4−) as an ionic liquid electrolyte and graphite rods as electrodes to fabricate a DIB with excellent electrochemical performance, revealing that the redox amphotericity of carbon allows the intercalation/deintercalation of both cations and anions into/from the graphite electrodes. The developed DIB is shown to exhibit high discharge voltage plateaus of 4.2–4.0 and 3.6–3.1 V, a reversible specific capacity of ∼80 mAh g−1 at a current density of 300 mA g−1, and a high coulombic efficiency of ∼97% over 300 cycles. Moreover, the non-flammability and electrochemical stability of the employed electrolyte result in improved safety and performance, allowing the fabricated DIB to function under various critical conditions such as high C-rate charge/discharge, continuous bending, and even burning. Thus, the developed battery is concluded to hold great promise for grid energy storage and other applications.
1. Introduction Compared to conventional Li-ion batteries (LIBs), dual-ion batteries (DIBs) feature higher working voltages [1–3], increased operation
∗
safety, and enhanced environmental friendliness [4,5]. The main mechanistic difference between DIBs and LIBs is that only Li+ ions intercalate/deintercalate into/from the electrode during charging/discharging in the latter case, while both cations (such as Li+) and anions (such
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (M.-C. Lin).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.02.035 Received 16 November 2018; Accepted 9 February 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Operation principle of the DIB containing (DMPI+)(AlCl4−) as an IL electrolyte. (b) Galvanostatic charge/discharge curves of DIB cells comprising SP−1 NGF and HPSGG powder electrodes. Insets show SEM images of SP−1 NGF and HPSGG. (c) Raman spectra and (d) XRD patterns of NGF and bulk/powdered HPSGG. Galvanostatic charge/discharge curves of NGF DIB cells with (e) different AlCl3:DMPIC mole ratios and (f) (DMPI+)(AlCl4−) and (EMI+)(AlCl4−) IL electrolytes.
as PF6−) intercalate/deintercalate in the former case [6–9]. A number of metal salt–based electrolytes have been reported for the intercalation/deposition of metal cations such as Li+, Na+, and K+ in DIBs [10–22], which typically exhibit high discharge voltages (5.2–2.0 V), reasonable specific capacities (50–120 mAh g−1), and sufficiently high average charge/discharge cycle lifetimes (∼400 cycles). However, the grid-scale energy storage applications of DIBs are hindered by the high cost of constituent metal elements [23,24] and the flammability of employed organic solvents [25,26]. Ionic liquids (ILs) exhibit the advantages of low flammability, high thermal stability, low volatility, and wide electrochemical stability window, and are consequently intensively investigated for potential electrochemical applications [27,28]. Very recently, several studies described DIBs comprising IL electrolytes and revealed that (i) the non-metallic cations of these ILs (e.g., 1-Ethyl-3-methylimidazolium cation (EMI+), N-butyl-N+ Methylpiperidinium cation (PP14 ), and N-methyl-N-butylpyrrolidinium cation (Pyr14+)) can be intercalated into negative graphite electrodes [23,24,29,30] and (ii) the wide electrochemical stability window
of ILs allows for high cut-off charging voltages that can reach 4.0–5.0 V [23,24,29,30]. However, the thus fabricated cells suffer from short cycle lifetime and low discharge capacity at high C-rate [23,24,29,30] and are therefore of limited practical applicability. In the early 1990s, Carlin et al. reported a DIB containing 1,2-dimethyl-3-propylimidazolium chloroaluminate [(DMPI+)(AlCl4−)] as an electrolyte and high-purity spectroscopic-grade graphite (HPSGG) rods as both negative and positive electrodes [31], realizing a discharge voltage plateau of 3.2–2.3 V and a Coulombic efficiency (CE) of 85%. Moreover, the above DIB performed better than analogous cells comprising other ILs such as (EMI+)(AlCl4−), (EMI+)(BF4−), or (EMI+) (PF6−) [31]. However, despite the importance of exploring the longterm stability of the (DMPI+)(AlCl4−) electrolyte-based DIB for realizing cost-effective batteries, no further work has been performed on this topic for the last two decades. Herein, we studied (DMPI+)(AlCl4−) as a DIB electrolyte, using natural graphite flakes (NGF) and HPSGG as host materials for both positive and negative electrodes and further probing the structure and defect content of these two graphite materials by several instrumental 234
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fabrication. Moreover, electrodes were also fabricated using SP–1 NGF, a graphite material that is commonly used in Al-ion batteries as a positive electrode and exhibits good electrochemical performance [32,33] but has not been extensively explored as a DIB electrode material. After the two kinds of graphite electrodes were assembled into battery cells, a galvanostatic current density of 25 mA g−1 was applied to record charge/discharge curves (Fig. 1b), and the NGF battery was shown to exhibit a higher specific discharge capacity and discharge voltage plateau than the HPSGG one. Scanning electron microscopy (SEM) imaging (insets in Fig. 1b), Raman spectroscopy (Fig. 1c), and Xray diffraction (XRD; Fig. 1d) revealed that powdered HPSGG featured a less homogeneous particle size distribution and higher defect density (ID/IG = 0.45) than NGF (ID/IG = 0.05). The presence of defects is known to hinder the intercalation of AlCl4− between graphene layers and thus decreases the specific capacity of the graphite electrode [34]. Consequently, the NGF DIB delivered a specific capacity two times higher than that of the powdered HPSGG DIB (Fig. 1b). On the other hand, ion intercalation can also be affected by graphite morphology [5,35], e.g., the flake-like structure and high particle uniformity of NGF might facilitate ion intercalation, whereas the irregular shape of HPSGG particles might hinder this process. At this point, it is worth re-stating that HPSGG particles were prepared by milling and sieving, which could result in graphene layer wrinkling and partial closure of graphite interlayer gaps at the surface [5,35]. In the developed DIB system, the IL electrolyte acts not only as a charge carrier but also as an active material and therefore strongly impacts DIB operation [5], which inspired us to probe the influence of the AlCl3:DMPIC mole ratio on battery performance. First, we used CV to determine the passivation potential of the Ni current collector in electrolytes with AlCl3:DMPIC ratios of 0.83, 1.00, and 1.50 (mol/mol), showing that the contribution of the corresponding redox reaction to current density (i.e., specific capacity) was negligible compared to that of the active material (graphite) (Fig. S2). Therefore, the redox capacity contribution of the Ni current collector in electrolytes with various AlCl3:DMPIC ratios was ignored, which allowed us to directly compare the electrochemical performances of DIBs with various AlCl3:DMPIC mole ratios. As the above ratio decreased from 1.0 to 0.83, the discharge capacity dropped from 27 to 6 mAh g−1, and the discharge voltage exhibited a rapid concomitant decline (Fig. 1e). When the above ratio was increased to 1.5, the galvanostatic charge/discharge voltages and capacity exhibited marked changes, i.e., the discharge capacity increased more than two-fold (up to 73 mAh g−1), while the working voltage decreased to ∼2.0 V. This behavior was ascribed to the fact that at AlCl3:IL mole ratios above 1.0, the DIB cell is converted into an Al-graphite cell, since under these conditions, the electrolyte contains both AlCl4− and Al2Cl7− anions [36]. Therefore, the positiveelectrode reaction does not change, whereas the negative-electrode reaction changes to [36].
techniques. The possibility of achieving high specific capacity, discharge voltage, and C-rate capability of the corresponding DIB was explored in detail, with much attention directed at analyzing the phenomenon of DIB self-discharge. 2. Experimental The electrode comprised 95 wt.% SP−1 NGF (> 99%, Ted Pella, Inc., USA) and 5 wt.% polyacrylic latex binder (LA132, Chengdu Institute of Organic Chemistry, China). A homogenous slurry with a viscosity of 1400 cP was obtained by blending graphite with a dilute binder solution in deionized water at ∼25 °C and subsequently pasted onto a Ni foil current collector (> 99%, Jiangsu Jiangneng New Material Technology, Co., Ltd., China) and dried at 80 °C for 12 h under vacuum to achieve an electrode mass loading of 5 mg cm−2. The IL electrolyte was prepared by mixing 1,2-dimethyl-3-propylimidazolium chloride (DMPIC) or 1-ethyl-3-methylimidazolium chloride (EMIC) (> 99%, in-house-made and vacuum-dried at 60 °C for 12 h before use) and anhydrous AlCl3 (99%, Acros Organics, USA) in a 1:1 mol/mol ratio in an Ar-filled glove box (water and oxygen contents less than 1 ppm). The sandwich-structured battery cell was assembled in the glove box using graphite electrodes as both the cathode and anode (sample size = 3 × 3 cm2) and glass fiber paper (GF/A, Whatman, USA) as the separator. A proper amount of electrolyte was injected into the pouch battery inside the glove box. A three-electrode setup with graphite as the working electrode and Al foil (> 99%, Chinalco Aluminum Foil Co., Ltd., China) as the reference/auxiliary electrode was used. Galvanostatic charge/discharge measurements were performed using a battery testing system (CT-4008-5V1A-S1, Shenzhen Neware, China). Cyclic voltammetry (CV) measurements were performed using a potentiostat/galvanostat (VMP3, Biologic Science Instrument, France). Ex situ XRD was performed with Cu Kα radiation (λ = 1.54056 Å) on a Bruker D8 X-ray diffractometer, Germany. Raman spectra were recorded on a LabRAM HR Evolution Raman spectrometer (Horiba Jobin Yvon, France) with a 532 nm laser. The microstructural characterization of graphite was carried out by using a scanning electron microscopy (SEM) system (JEOL JXA-8230, Japan). 3. Results and discussion The operation principle of the DIB cell is schematically illustrated in Fig. 1a. Upon charging, DMPI+ cations and AlCl4− anions are intercalated into the graphite anode and cathode, respectively, while deintercalation of these ions and their diffusion back into the electrolyte are observed during discharge. Notably, both DMPI+ cations and AlCl4− anions exhibit structural symmetry, as demonstrated in Video S1. The electrochemical reactions occurring in the fabricated cell can be expressed as follows [31]. Positive electrode: AlCl4− + yC ↔ Cy(AlCl4) + e−
(1)
Negative electrode: DMPI+ + xC + e− ↔ (DMPI)Cx
(2)
4Al2Cl7− + 3e− ↔ Al + 7AlCl4−.
(4)
Thus, the charge/discharge profiles of the DIB with an AlCl3:DMPIC mole ratio of 1.5 were similar to those of previously reported Al-graphite cells [32,33,36,39]. The electrolyte with an AlCl3:EMIC mole ratio of unity was also studied (Fig. 1f), but the obtained capacity and CE were lower than those achieved for the corresponding AlCl3/DMPIC system. This performance decrease was ascribed to the lack of an alkyl substituent in the 2-position of the EMI+ cation, which resulted in poor anode charge efficiency, since imidazolium cations with protons in the 2-position are inherently unstable in the presence of reduced graphite [37]. Overall, although the DIB with AlCl3:DMPIC = 1.5 showed a high discharge capacity of 73 mAh g−1, we were still interested in studying the DIB with AlCl3:DMPIC = 1, because its higher discharge voltage (close to that of LIBs (3.7 V)) allowed one to potentially realize a higher energy density. Fig. 2 shows CV curves of the DIB cell recorded between 1 and
Overall: DMPI+ + AlCl4− + (x + y)C ↔ (DMPI)Cx + Cy(AlCl4) (3) Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.02.035. First, following Carlin's work [31], we used 5-mm-diameter HPSGG rods as the anode and cathode and applied a charge/discharge current density of 10 mA cm−2 to the DIB cell. Fig. S1 shows the galvanostatic charge/discharge curves obtained for AlCl3:DMPIC = 1:1 (mol/mol), demonstrating that although a well-defined voltage plateau (3.3–2.4 V) was observed, the specific capacity was estimated to be low. To enhance electrochemical performance and precisely calculate the specific capacity of the HPSGG DIB cell, powdered HPSGG (prepared by knife milling (kitchen blender) followed by mesh filtration to a particle size of < 74 μm) was used to prepare a graphite slurry for electrode 235
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Fig. 2. CV curve of the DIB cell recorded at a scan rate of 1 mV s−1, with inset showing the corresponding dQ/dV curve.
3.725 V at a scan rate of 1 mV s−1. The corresponding dQ/dV curve is presented in the inset of the above figure and is in full agreement with the CV curve. The oxidation peak between 3.3 and 3.725 V (vs. Al) was clearly responsible for the charging process and capacity accumulation, whereas the reduction peak between 3 and 3.6 V was held responsible for discharge. In the next step, the DIB cell was cycled at charge/discharge current densities of 27–100 mA g−1 in a voltage window of 1.0–3.725 V, with the obtained charge/discharge capacities and CEs presented in Fig. 3a. At current densities of 27, 50, 75, and 100 mA g−1, the discharge capacities were stable for 10 cycles and equaled 27.5, 27, 26.7, and 26 mAh g−1, respectively. Moreover, an increase of current density from 27 to 100 mA g−1 triggered a CE increase from 98.6 to 99.3%. The discharge capacity recovered to ∼27 mAh g−1 when the current density was decreased back to 27 mA g−1, i.e., the fabricated cell featured good reversibility. Fig. 3b presents typical charge/discharge curves obtained at current densities of 27 and 100 mA g−1, showing that only a slight decrease of specific capacity was observed
Fig. 4. (a) CV curves of the DIB cell recorded at cut-off charging voltages of 3.725–4.4 V and a scan rate of 1 mV s−1. (b) CV curves of NGF positive (red) and negative (black) electrodes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
with increasing current density. Fig. 3c presents the results of long-term stability testing that was performed at a current density of 27 mA g−1 for 100 cycles, demonstrating the absence of obvious capacity decay after 100 cycles and showing that the tested cell exhibited a high
Fig. 3. (a) Rate performance and (b) charge/discharge profiles of the DIB cell at various current densities. (c) Long-term cycling performance of the DIB cell. 236
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The mechanism of DMPI+ and AlCl4− intercalation/deintercalation was further probed by ex situ XRD. Fig. 6a shows the XRD patterns of the negative NGF electrode in pristine, charged, and discharged states, demonstrating that the pristine electrode exhibited a typical graphite structure with a sharp (002) peak at 2θ ≈ 26.4°. Upon charging to 4.3 V, the above peak shifted to 2θ ≈ 26.0°, which was attributed to the increased spacing between adjacent graphene layers caused by DMPI+ intercalation. Upon discharge to 1.0 V, the peak shifted back to higher angles, although the original value of 26.4° was not reached, which was ascribed to the trapping of a small amount of DMPI+ in graphite [23,41]. Moreover, peak broadening observed upon intercalation suggested the concomitant formation of highly strained graphene stacks [38]. In the case of the positive electrode (Fig. 6b), the (002) peak vanished and was split into two peaks with 2θ ≈ 23.7° and 28.5° upon charging, which was ascribed to the intercalation of AlCl4− [36]. Upon discharge, the two peaks merged into one, and the original peak position was fully restored, which implied that AlCl4− intercalation was highly reversible [36]. The increase of cut-off charging voltage was accompanied by a decrease of CE (Fig. 5a) [15,23], which was ascribed to self-discharge rather than to electrolyte decomposition (i.e., no gas evolution was observed after long-term cycling, see Fig. S5). In view of the importance of self-discharge reactions for IL electrolyte–based DIBs, these reactions were further investigated in detail at a constant charge current density of 50 mA g−1 and variable discharge current densities (50–500 mA g−1). Fig. 7a shows that CE was positively correlated with discharge current density, with further details provided in Table S1. At the lowest current density (50 mA g−1), CE equaled 92.3%, increasing to 96.1% at 1000 mA g−1. The above increase was accompanied by a slight decrease of discharge capacity, which indicated that the selfdischarge reaction leads to the reduction of CE in the discharge process [5]. Previous works [32,33] on AlCl4−-intercalated graphite revealed that the investigated cells (e.g., Al/EMIC-AlCl3/graphite ones) featured high CEs (> 99%). Therefore, the self-discharge of the (DMPI+) (AlCl4−) cell might be caused by the formation of unstable DMPI+graphite intercalation compounds, which, however, requires further investigation. Fig. 7b shows the voltage profile of a DIB cell subjected to 12-h rest after charging to a cut-off charging voltage of 4.3 V at a constant current density of 100 mA g−1, revealing that the cut-off voltages dropped to 4.06, 3.91, and 3.88 V after three rest periods. Finally, the DIB was discharged at a constant current density of 100 mA g−1 and delivered specific discharge capacities of 50.5, 46.6, and 42.5 mAh g−1. The rate of self-discharge (X) was calculated as 4.2%/h using Eq. (5) [42]:
capacity retention of ∼100%. Importantly, CE was maintained at ∼94% except in the first three cycles. The CE gradually increased with cycle number, becoming stable after the fourth cycle. This phenomenon indicated that the first few cycles were required for battery activation, as is commonly observed for other DIBs [23,24,29,30]. Although the probed cell featured a high CE, its specific discharge capacity at a cut-off voltage of 3.725 V was low (∼27 mAh g−1), and we therefore investigated the influence of a higher cut-off charging voltage on specific discharge capacity and CE to realize better performance. Previously, the AlCl3:DMPIC mole ratio of unity was reported to provide an electrochemical window of ∼4.8 V [40], which suggested that higher DIB charge/discharge voltages can be achieved. However, the electrochemical performance of DIBs at such high working voltages has not been explored. Fig. 4a shows the CV curves of the DIB cell recorded at cut-off charging voltages of 3.725–4.4 V, revealing that no electrolyte decomposition was observed up to 4.4 V, i.e., both cations and anions could intercalate/deintercalate into/from the graphite electrodes within this voltage window. When a cut-off charging voltage of 4.0 V was applied, an additional intercalation peak appeared at 3.9 V, gaining intensity and becoming broader with increasing voltage and eventually affording a broad peak between 4.0 and 4.4 V, which might reflect the intercalation of a large amount of ions and enhanced specific capacity. Furthermore, two additional deintercalation peaks were observed at 4 and 3.7 V, indicating that an enhanced discharge voltage plateau and increased discharge capacity can be expected. Fig. 4b shows the CV curves of positive and negative graphite electrodes recorded using an Al reference electrode. The positive-electrode curve in the range of 0–2.45 V indicated the occurrence of an obvious redox reaction, which suggested that the intercalation of AlCl4− into graphite was initiated at 2.2 V and continued up to 2.45 V (vs. Al), while deintercalation occurred in the range of 2.2–1.6 V (vs. Al). Conversely, the CV curve of the negative electrode in the range of 0 to −1.85 V (vs. Al) indicated the occurrence of an asymmetric redox reaction. Fig. 5a shows the cycling performances of the DIB cell at cut-off charging voltages of 3.8–4.4 V, demonstrating that discharge capacity increased with increasing voltage, e.g., at a current density of 100 mA g−1, the discharge capacity increased from 32 mAh g−1 at 3.8 V to 90 mAh g−1 at 4.4 V. However, an opposite trend was observed for CE, e.g., a CE exceeding 96% that was maintained at 4.2 V declined to ∼91% at 4.4 V. Therefore, a cut-off charging voltage of 4.3 V was selected to keep the balance between discharge capacity and CE. Fig. 5b shows charge/discharge curves recorded at cut-off charging voltages of 3.8–4.4 V, revealing the presence of a higher charging curve plateau between 4.25 and 4.4 V. Additionally, two additional plateaus (4.0–4.2 and 3.6–3.1 V) were observed in discharge curves, in agreement with the results of prior CV analysis (Fig. 4). The DIB rate performance and charge/discharge curves obtained at various current densities are shown in Fig. S3, which reveals that a CE of ∼98% and a discharge capacity of 82 mAh g−1 were observed at a current density of 300 mA g−1. Fig. 5c presents the results of long-term stability testing performed at a current density of 300 mA g−1 and cut-off voltages of 3.0–4.3 V for 300 charge/discharge cycles. During this testing, the DIB delivered an initial capacity of 77 mAh g−1 that declined to 71 mAh g−1 after 300 cycles, which corresponded to a capacity retention of 92%, while a CE of approximately 97% was achieved after 300 cycles. Moreover, the employed battery retained its long-term stability and achieved a high CE of ∼99% when cycling was performed at a high current density of 1000 mA g−1 (Fig. 5d). The corresponding charge/ discharge curves (Fig. S4) revealed that a discharge capacity of ∼41 mAh g−1 was maintained even after 1000 cycles with a capacity retention of 99% (as compared to the sixth cycle). To the best of our knowledge, this is first report of a pure IL electrolyte–based DIB exhibiting a long cycle lifetime at such high current density. Notably, no obvious battery pouch swelling was observed after cycling (Fig. S5), and the (DMPI+)(AlCl4−) electrolyte was concluded to be stable at high cut-off charging voltages of up to 4.3 V.
X = (C1 − C2)/C1T × 100%,
(5)
where C1 is the discharge capacity without resting, C2 is the discharge capacity after 12-h resting, and T is the rest time (12 h). Therefore, the further study of DIB self-discharge was concluded to be a matter of high practical significance. Fig. 8 compares the performance of our DIB with those of previously reported ones (including batteries comprising metal-salt-based and pure IL electrolytes (highlighted in yellow)). Notably, although the (DMPI+) (AlCl4−) DIB does not show superior electrochemical performance compared to IA metal-salt-based electrolyte DIBs, it still exhibits certain advantages, e.g., high C-rate capability and long charge/discharge cycle lifetime. In particular, our battery exhibited a high energy density of ∼270 Wh kg−1 (based on the weight of graphite material only) at 1090 W kg−1 that decreased to 125 Wh kg−1 at 3027 W kg−1. For grid storage applications, the high C-rate capability of the fabricated DIB might be superior for frequency regulation, which requires high-current-density input/output, i.e., the occurrence of self-discharge might not affect grid storage applications. The developed DIB was superior to other pure-IL DIBs (e.g., those containing PP14TFSI, Pyr14TFSI, and PP14NTF2) in terms of nominal discharge voltage (∼3.7 V), current 237
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Fig. 5. (a) Cycling performances and (b) charge/discharge curves of the DIB cell at cut-off charging voltages of 3.8–4.4 V. (c) Results of long-term (300 cycles) DIB cell cycling at a current density of 300 mA g−1. (d) Results of long-term (1000 cycles) DIB cell cycling at a current density of 1000 mA g−1.
Fig. 6. Ex situ XRD patterns of (a) negative and (b) positive electrodes.
density (up to 1000 mA g−1), specific capacity (∼80 mAh g−1 at 300 mA g−1) and energy efficiency (> 88%) (Fig. S6 and Table S2). Moreover, electrode and separator material flexibility allowed for uninterrupted power delivery at different bending angles and directions (Video S2), while the high thermal stability of the employed IL allowed for successful operation even when the cell was exposed to an open flame (Video S3). Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.02.035.
4. Conclusions In summary, we successfully fabricated and tested a rechargeable DIB containing (DMPI+)(AlCl4−) as an IL electrolyte and low-cost NGF as both the anode and cathode, achieving superior discharge voltage, specific capacity, and energy efficiency compared to those obtained for other pure-IL DIBs. The fabricated battery pouch cell continued to function under various harsh conditions such as high power drainage (up to 1000 mA g−1), continuous bending (Video S2), or even burning (Video S3), which was ascribed to the high stability of (DMPI+) (AlCl4−). Although the problem of self-discharge needs to be further 238
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Fig. 7. (a) Dependence of CE on current density and (b) self-discharge behavior of a DIB cell subjected to 12-h rest after constant-current charging at 100 mA g−1.
DMPIC 1,2-dimethyl-3-propylimidazolium chloride (DMPI+)(AlCl4−) 1,2-dimethyl-3-propylimidazolium chloroaluminate EMIC 1-ethyl-3-methylimidazolium chloride HPSGG high-purity spectroscopic-grade graphite IL ionic liquid LIB Li-ion battery NGF natural graphite flakes SEM scanning electron microscopy XRD X-ray diffraction References [1] X. Tong, F. Zhang, B. Ji, M. Sheng, Y. Tang, Dual-ion batteries: carbon-coated porous aluminum foil anode for high-rate, long-term cycling stability, and high energy density dual-ion batteries, Adv. Mater. 28 (2016) 99789978 https://doi.org/ 10.1002/adma.201670317. [2] M. Wang, C. Jiang, S. Zhang, X. Song, Y. Tang, H.-M. Cheng, Reversible calcium alloying enables a practical room-temperature rechargeable calcium-ion battery with a high discharge voltage, Nat. Chem. 10 (2018) 667–672 https://doi.org/10. 1038/s41557-018-0045-4. [3] P. Han, X. Han, J. Yao, L. Yue, J. Zhao, X. Zhou, G. Cui, Mesocarbon microbead based dual-carbon batteries towards low cost energy storage devices, J. Power Sources 393 (2018) 145–151 https://doi.org/10.1016/j.jpowsour.2018.05.021. [4] J.A. Read, A.V. Cresce, M.H. Ervin, K. Xu, Dual-graphite chemistry enabled by a high voltage electrolyte, Energy Environ. Sci. 7 (2014) 617–620 https://doi.org/10. 1039/C3EE43333A. [5] T. Placke, O. Fromm, S.F. Lux, P. Bieker, S. Rothermel, H.-W. Meyer, S. Passerini, M. Winter, Reversible intercalation of bis(trifluoromethanesulfonyl)imide anions from an ionic liquid electrolyte into graphite for high performance dual-ion cells, J. Electrochem. Soc. 159 (2012) A1755–A1765 https://doi.org/10.1149/2.011211jes. [6] T. Ishihara, M. Koga, H. Matsumoto, M. Yoshio, Electrochemical intercalation of hexafluorophosphate anion into various carbons for cathode of dual-carbon rechargeable battery, Electrochem. Solid State Lett. 10 (2007) A74–A76 https://doi. org/10.1149/1.2424263. [7] J.R. Dahn, J.A. Seel, Energy and capacity projections for practical dual-graphite cells, J. Electrochem. Soc. 147 (2000) 899–901 https://doi.org/10.1149/1. 1393289. [8] J.A. Seel, J.R. Dahn, Electrochemical intercalation of PF6 into graphite, J. Electrochem. Soc. 147 (2000) 892–898 https://doi.org/10.1149/1.1393288. [9] T. Ishihara, Y. Yokoyama, F. Kozono, H. Hayashi, Intercalation of PF6− anion into graphitic carbon with nano pore for dual carbon cell with high capacity, J. Power Sources 196 (2011) 6956–6959 https://doi.org/10.1016/j.jpowsour.2010.11.075. [10] X. Zhang, Y. Tang, F. Zhang, C.-S. Lee, A novel aluminum–graphite dual-ion battery, Adv. Energy Mater. 6 (2016) 1502588 https://doi.org/10.1002/aenm.201502588. [11] F. Zhang, B. Ji, X. Tong, M. Sheng, X. Zhang, C.-S. Lee, Y. Tang, A dual-ion battery constructed with aluminum foil anode and mesocarbon microbead cathode via an alloying/intercalation process in an ionic liquid electrolyte, Adv. Mater. Interfaces 3 (2016) 1600605 https://doi.org/10.1002/admi.201600605. [12] B. Ji, F. Zhang, N. Wu, Y. Tang, A dual-carbon battery based on potassium-ion electrolyte, Adv. Energy Mater. 7 (2017) 1700920 https://doi.org/10.1002/aenm. 201700920. [13] L. Fan, Q. Liu, S. Chen, K. Lin, Z. Xu, B. Lu, Potassium-based dual ion battery with dual-graphite electrode, Small 13 (2017) 1701011 https://doi.org/10.1002/smll. 201701011. [14] B. Ji, F. Zhang, X. Song, Y. Tang, A novel potassium-ion-based dual-ion battery, Adv. Mater. 29 (2017) 1700519 https://doi.org/10.1002/adma.201700519. [15] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.-W. Meyer, S. Nowak, M. Winter, T. Placke, Dual-graphite cells based on the reversible intercalation of bis (trifluoromethanesulfonyl)imide anions from an ionic liquid electrolyte, Energy Environ. Sci. 7 (2014) 3412–3423 https://doi.org/10.1039/C4EE01873G. [16] S. Aladinli, F. Bordet, K. Ahlbrecht, J. Tübke, M. Holzapfel, Compositional graphitic
Fig. 8. Electrochemical performance of our DIB compared to those of previously reported DIBs.
studied and addressed, the developed metal ion–free battery is expected to be well suited for grid energy storage applications because of its low cost and high safety. Acknowledgement M.-C. L. acknowledges support from the Qingdao scientific and technological innovation high-level talents project−Aluminum-ion power and energy storage battery (No.17-2-1-1-zhc), the Taishan Scholar Project of the Shandong Province of China (No. tsqn20161025) and the Qingdao Entrepreneurial Innovation Leaders Plan (No. 16-8-31-zhc). Y. L. acknowledges support from the National Natural Science Foundation of China (No. 91848206) and the Taishan Scholar Project of the Shandong Province of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpowsour.2019.02.035. Glossary CE CV DIB
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A high discharge voltage dual-ion rechargeable battery using pure (DMPI+) (AlCl4−) ionic liquid electrolyte
T
Zichuan Lv, Mei Han, Junhui Sun, Lixue Hou, Hui Chen, Yuxia Li∗∗, Meng-Chang Lin∗ College of Electrical Engineering and Automation, Shandong University of Science and Technology, Qingdao, 266590, China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
battery (DIB) containing pure • Dual-ion ionic liquid as an electrolyte was prepared.
Natural graphite flakes were used as • both anode and cathode materials. DIB functioned under high-rate • This charge/discharge, bending, and burning conditions.
non-flammability/stability • Electrolyte allowed for enhanced safety and performance.
prepared DIB was well suited for • The grid energy storage.
A R T I C LE I N FO
A B S T R A C T
Keywords: Ionic liquid Imidazolium Graphite Intercalation/deintercalation Self-discharge
Dual-ion batteries (DIBs) are widely studied as novel electrochemical energy storage devices in view of their low cost, environmental friendliness, and high working voltage. However, the widespread application of these batteries is hindered by organic electrolyte decomposition at high working voltages and the absence of electrode materials allowing for high capacity and long cycle lifetime, which necessitates further investigations toward performance improvement. Herein, we use 1,2-dimethyl-3-propylimidazolium chloroaluminate (DMPI+) (AlCl4−) as an ionic liquid electrolyte and graphite rods as electrodes to fabricate a DIB with excellent electrochemical performance, revealing that the redox amphotericity of carbon allows the intercalation/deintercalation of both cations and anions into/from the graphite electrodes. The developed DIB is shown to exhibit high discharge voltage plateaus of 4.2–4.0 and 3.6–3.1 V, a reversible specific capacity of ∼80 mAh g−1 at a current density of 300 mA g−1, and a high coulombic efficiency of ∼97% over 300 cycles. Moreover, the non-flammability and electrochemical stability of the employed electrolyte result in improved safety and performance, allowing the fabricated DIB to function under various critical conditions such as high C-rate charge/discharge, continuous bending, and even burning. Thus, the developed battery is concluded to hold great promise for grid energy storage and other applications.
1. Introduction Compared to conventional Li-ion batteries (LIBs), dual-ion batteries (DIBs) feature higher working voltages [1–3], increased operation
∗
safety, and enhanced environmental friendliness [4,5]. The main mechanistic difference between DIBs and LIBs is that only Li+ ions intercalate/deintercalate into/from the electrode during charging/discharging in the latter case, while both cations (such as Li+) and anions (such
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (M.-C. Lin).
∗∗
https://doi.org/10.1016/j.jpowsour.2019.02.035 Received 16 November 2018; Accepted 9 February 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Operation principle of the DIB containing (DMPI+)(AlCl4−) as an IL electrolyte. (b) Galvanostatic charge/discharge curves of DIB cells comprising SP−1 NGF and HPSGG powder electrodes. Insets show SEM images of SP−1 NGF and HPSGG. (c) Raman spectra and (d) XRD patterns of NGF and bulk/powdered HPSGG. Galvanostatic charge/discharge curves of NGF DIB cells with (e) different AlCl3:DMPIC mole ratios and (f) (DMPI+)(AlCl4−) and (EMI+)(AlCl4−) IL electrolytes.
as PF6−) intercalate/deintercalate in the former case [6–9]. A number of metal salt–based electrolytes have been reported for the intercalation/deposition of metal cations such as Li+, Na+, and K+ in DIBs [10–22], which typically exhibit high discharge voltages (5.2–2.0 V), reasonable specific capacities (50–120 mAh g−1), and sufficiently high average charge/discharge cycle lifetimes (∼400 cycles). However, the grid-scale energy storage applications of DIBs are hindered by the high cost of constituent metal elements [23,24] and the flammability of employed organic solvents [25,26]. Ionic liquids (ILs) exhibit the advantages of low flammability, high thermal stability, low volatility, and wide electrochemical stability window, and are consequently intensively investigated for potential electrochemical applications [27,28]. Very recently, several studies described DIBs comprising IL electrolytes and revealed that (i) the non-metallic cations of these ILs (e.g., 1-Ethyl-3-methylimidazolium cation (EMI+), N-butyl-N+ Methylpiperidinium cation (PP14 ), and N-methyl-N-butylpyrrolidinium cation (Pyr14+)) can be intercalated into negative graphite electrodes [23,24,29,30] and (ii) the wide electrochemical stability window
of ILs allows for high cut-off charging voltages that can reach 4.0–5.0 V [23,24,29,30]. However, the thus fabricated cells suffer from short cycle lifetime and low discharge capacity at high C-rate [23,24,29,30] and are therefore of limited practical applicability. In the early 1990s, Carlin et al. reported a DIB containing 1,2-dimethyl-3-propylimidazolium chloroaluminate [(DMPI+)(AlCl4−)] as an electrolyte and high-purity spectroscopic-grade graphite (HPSGG) rods as both negative and positive electrodes [31], realizing a discharge voltage plateau of 3.2–2.3 V and a Coulombic efficiency (CE) of 85%. Moreover, the above DIB performed better than analogous cells comprising other ILs such as (EMI+)(AlCl4−), (EMI+)(BF4−), or (EMI+) (PF6−) [31]. However, despite the importance of exploring the longterm stability of the (DMPI+)(AlCl4−) electrolyte-based DIB for realizing cost-effective batteries, no further work has been performed on this topic for the last two decades. Herein, we studied (DMPI+)(AlCl4−) as a DIB electrolyte, using natural graphite flakes (NGF) and HPSGG as host materials for both positive and negative electrodes and further probing the structure and defect content of these two graphite materials by several instrumental 234
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fabrication. Moreover, electrodes were also fabricated using SP–1 NGF, a graphite material that is commonly used in Al-ion batteries as a positive electrode and exhibits good electrochemical performance [32,33] but has not been extensively explored as a DIB electrode material. After the two kinds of graphite electrodes were assembled into battery cells, a galvanostatic current density of 25 mA g−1 was applied to record charge/discharge curves (Fig. 1b), and the NGF battery was shown to exhibit a higher specific discharge capacity and discharge voltage plateau than the HPSGG one. Scanning electron microscopy (SEM) imaging (insets in Fig. 1b), Raman spectroscopy (Fig. 1c), and Xray diffraction (XRD; Fig. 1d) revealed that powdered HPSGG featured a less homogeneous particle size distribution and higher defect density (ID/IG = 0.45) than NGF (ID/IG = 0.05). The presence of defects is known to hinder the intercalation of AlCl4− between graphene layers and thus decreases the specific capacity of the graphite electrode [34]. Consequently, the NGF DIB delivered a specific capacity two times higher than that of the powdered HPSGG DIB (Fig. 1b). On the other hand, ion intercalation can also be affected by graphite morphology [5,35], e.g., the flake-like structure and high particle uniformity of NGF might facilitate ion intercalation, whereas the irregular shape of HPSGG particles might hinder this process. At this point, it is worth re-stating that HPSGG particles were prepared by milling and sieving, which could result in graphene layer wrinkling and partial closure of graphite interlayer gaps at the surface [5,35]. In the developed DIB system, the IL electrolyte acts not only as a charge carrier but also as an active material and therefore strongly impacts DIB operation [5], which inspired us to probe the influence of the AlCl3:DMPIC mole ratio on battery performance. First, we used CV to determine the passivation potential of the Ni current collector in electrolytes with AlCl3:DMPIC ratios of 0.83, 1.00, and 1.50 (mol/mol), showing that the contribution of the corresponding redox reaction to current density (i.e., specific capacity) was negligible compared to that of the active material (graphite) (Fig. S2). Therefore, the redox capacity contribution of the Ni current collector in electrolytes with various AlCl3:DMPIC ratios was ignored, which allowed us to directly compare the electrochemical performances of DIBs with various AlCl3:DMPIC mole ratios. As the above ratio decreased from 1.0 to 0.83, the discharge capacity dropped from 27 to 6 mAh g−1, and the discharge voltage exhibited a rapid concomitant decline (Fig. 1e). When the above ratio was increased to 1.5, the galvanostatic charge/discharge voltages and capacity exhibited marked changes, i.e., the discharge capacity increased more than two-fold (up to 73 mAh g−1), while the working voltage decreased to ∼2.0 V. This behavior was ascribed to the fact that at AlCl3:IL mole ratios above 1.0, the DIB cell is converted into an Al-graphite cell, since under these conditions, the electrolyte contains both AlCl4− and Al2Cl7− anions [36]. Therefore, the positiveelectrode reaction does not change, whereas the negative-electrode reaction changes to [36].
techniques. The possibility of achieving high specific capacity, discharge voltage, and C-rate capability of the corresponding DIB was explored in detail, with much attention directed at analyzing the phenomenon of DIB self-discharge. 2. Experimental The electrode comprised 95 wt.% SP−1 NGF (> 99%, Ted Pella, Inc., USA) and 5 wt.% polyacrylic latex binder (LA132, Chengdu Institute of Organic Chemistry, China). A homogenous slurry with a viscosity of 1400 cP was obtained by blending graphite with a dilute binder solution in deionized water at ∼25 °C and subsequently pasted onto a Ni foil current collector (> 99%, Jiangsu Jiangneng New Material Technology, Co., Ltd., China) and dried at 80 °C for 12 h under vacuum to achieve an electrode mass loading of 5 mg cm−2. The IL electrolyte was prepared by mixing 1,2-dimethyl-3-propylimidazolium chloride (DMPIC) or 1-ethyl-3-methylimidazolium chloride (EMIC) (> 99%, in-house-made and vacuum-dried at 60 °C for 12 h before use) and anhydrous AlCl3 (99%, Acros Organics, USA) in a 1:1 mol/mol ratio in an Ar-filled glove box (water and oxygen contents less than 1 ppm). The sandwich-structured battery cell was assembled in the glove box using graphite electrodes as both the cathode and anode (sample size = 3 × 3 cm2) and glass fiber paper (GF/A, Whatman, USA) as the separator. A proper amount of electrolyte was injected into the pouch battery inside the glove box. A three-electrode setup with graphite as the working electrode and Al foil (> 99%, Chinalco Aluminum Foil Co., Ltd., China) as the reference/auxiliary electrode was used. Galvanostatic charge/discharge measurements were performed using a battery testing system (CT-4008-5V1A-S1, Shenzhen Neware, China). Cyclic voltammetry (CV) measurements were performed using a potentiostat/galvanostat (VMP3, Biologic Science Instrument, France). Ex situ XRD was performed with Cu Kα radiation (λ = 1.54056 Å) on a Bruker D8 X-ray diffractometer, Germany. Raman spectra were recorded on a LabRAM HR Evolution Raman spectrometer (Horiba Jobin Yvon, France) with a 532 nm laser. The microstructural characterization of graphite was carried out by using a scanning electron microscopy (SEM) system (JEOL JXA-8230, Japan). 3. Results and discussion The operation principle of the DIB cell is schematically illustrated in Fig. 1a. Upon charging, DMPI+ cations and AlCl4− anions are intercalated into the graphite anode and cathode, respectively, while deintercalation of these ions and their diffusion back into the electrolyte are observed during discharge. Notably, both DMPI+ cations and AlCl4− anions exhibit structural symmetry, as demonstrated in Video S1. The electrochemical reactions occurring in the fabricated cell can be expressed as follows [31]. Positive electrode: AlCl4− + yC ↔ Cy(AlCl4) + e−
(1)
Negative electrode: DMPI+ + xC + e− ↔ (DMPI)Cx
(2)
4Al2Cl7− + 3e− ↔ Al + 7AlCl4−.
(4)
Thus, the charge/discharge profiles of the DIB with an AlCl3:DMPIC mole ratio of 1.5 were similar to those of previously reported Al-graphite cells [32,33,36,39]. The electrolyte with an AlCl3:EMIC mole ratio of unity was also studied (Fig. 1f), but the obtained capacity and CE were lower than those achieved for the corresponding AlCl3/DMPIC system. This performance decrease was ascribed to the lack of an alkyl substituent in the 2-position of the EMI+ cation, which resulted in poor anode charge efficiency, since imidazolium cations with protons in the 2-position are inherently unstable in the presence of reduced graphite [37]. Overall, although the DIB with AlCl3:DMPIC = 1.5 showed a high discharge capacity of 73 mAh g−1, we were still interested in studying the DIB with AlCl3:DMPIC = 1, because its higher discharge voltage (close to that of LIBs (3.7 V)) allowed one to potentially realize a higher energy density. Fig. 2 shows CV curves of the DIB cell recorded between 1 and
Overall: DMPI+ + AlCl4− + (x + y)C ↔ (DMPI)Cx + Cy(AlCl4) (3) Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.02.035. First, following Carlin's work [31], we used 5-mm-diameter HPSGG rods as the anode and cathode and applied a charge/discharge current density of 10 mA cm−2 to the DIB cell. Fig. S1 shows the galvanostatic charge/discharge curves obtained for AlCl3:DMPIC = 1:1 (mol/mol), demonstrating that although a well-defined voltage plateau (3.3–2.4 V) was observed, the specific capacity was estimated to be low. To enhance electrochemical performance and precisely calculate the specific capacity of the HPSGG DIB cell, powdered HPSGG (prepared by knife milling (kitchen blender) followed by mesh filtration to a particle size of < 74 μm) was used to prepare a graphite slurry for electrode 235
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Fig. 2. CV curve of the DIB cell recorded at a scan rate of 1 mV s−1, with inset showing the corresponding dQ/dV curve.
3.725 V at a scan rate of 1 mV s−1. The corresponding dQ/dV curve is presented in the inset of the above figure and is in full agreement with the CV curve. The oxidation peak between 3.3 and 3.725 V (vs. Al) was clearly responsible for the charging process and capacity accumulation, whereas the reduction peak between 3 and 3.6 V was held responsible for discharge. In the next step, the DIB cell was cycled at charge/discharge current densities of 27–100 mA g−1 in a voltage window of 1.0–3.725 V, with the obtained charge/discharge capacities and CEs presented in Fig. 3a. At current densities of 27, 50, 75, and 100 mA g−1, the discharge capacities were stable for 10 cycles and equaled 27.5, 27, 26.7, and 26 mAh g−1, respectively. Moreover, an increase of current density from 27 to 100 mA g−1 triggered a CE increase from 98.6 to 99.3%. The discharge capacity recovered to ∼27 mAh g−1 when the current density was decreased back to 27 mA g−1, i.e., the fabricated cell featured good reversibility. Fig. 3b presents typical charge/discharge curves obtained at current densities of 27 and 100 mA g−1, showing that only a slight decrease of specific capacity was observed
Fig. 4. (a) CV curves of the DIB cell recorded at cut-off charging voltages of 3.725–4.4 V and a scan rate of 1 mV s−1. (b) CV curves of NGF positive (red) and negative (black) electrodes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
with increasing current density. Fig. 3c presents the results of long-term stability testing that was performed at a current density of 27 mA g−1 for 100 cycles, demonstrating the absence of obvious capacity decay after 100 cycles and showing that the tested cell exhibited a high
Fig. 3. (a) Rate performance and (b) charge/discharge profiles of the DIB cell at various current densities. (c) Long-term cycling performance of the DIB cell. 236
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The mechanism of DMPI+ and AlCl4− intercalation/deintercalation was further probed by ex situ XRD. Fig. 6a shows the XRD patterns of the negative NGF electrode in pristine, charged, and discharged states, demonstrating that the pristine electrode exhibited a typical graphite structure with a sharp (002) peak at 2θ ≈ 26.4°. Upon charging to 4.3 V, the above peak shifted to 2θ ≈ 26.0°, which was attributed to the increased spacing between adjacent graphene layers caused by DMPI+ intercalation. Upon discharge to 1.0 V, the peak shifted back to higher angles, although the original value of 26.4° was not reached, which was ascribed to the trapping of a small amount of DMPI+ in graphite [23,41]. Moreover, peak broadening observed upon intercalation suggested the concomitant formation of highly strained graphene stacks [38]. In the case of the positive electrode (Fig. 6b), the (002) peak vanished and was split into two peaks with 2θ ≈ 23.7° and 28.5° upon charging, which was ascribed to the intercalation of AlCl4− [36]. Upon discharge, the two peaks merged into one, and the original peak position was fully restored, which implied that AlCl4− intercalation was highly reversible [36]. The increase of cut-off charging voltage was accompanied by a decrease of CE (Fig. 5a) [15,23], which was ascribed to self-discharge rather than to electrolyte decomposition (i.e., no gas evolution was observed after long-term cycling, see Fig. S5). In view of the importance of self-discharge reactions for IL electrolyte–based DIBs, these reactions were further investigated in detail at a constant charge current density of 50 mA g−1 and variable discharge current densities (50–500 mA g−1). Fig. 7a shows that CE was positively correlated with discharge current density, with further details provided in Table S1. At the lowest current density (50 mA g−1), CE equaled 92.3%, increasing to 96.1% at 1000 mA g−1. The above increase was accompanied by a slight decrease of discharge capacity, which indicated that the selfdischarge reaction leads to the reduction of CE in the discharge process [5]. Previous works [32,33] on AlCl4−-intercalated graphite revealed that the investigated cells (e.g., Al/EMIC-AlCl3/graphite ones) featured high CEs (> 99%). Therefore, the self-discharge of the (DMPI+) (AlCl4−) cell might be caused by the formation of unstable DMPI+graphite intercalation compounds, which, however, requires further investigation. Fig. 7b shows the voltage profile of a DIB cell subjected to 12-h rest after charging to a cut-off charging voltage of 4.3 V at a constant current density of 100 mA g−1, revealing that the cut-off voltages dropped to 4.06, 3.91, and 3.88 V after three rest periods. Finally, the DIB was discharged at a constant current density of 100 mA g−1 and delivered specific discharge capacities of 50.5, 46.6, and 42.5 mAh g−1. The rate of self-discharge (X) was calculated as 4.2%/h using Eq. (5) [42]:
capacity retention of ∼100%. Importantly, CE was maintained at ∼94% except in the first three cycles. The CE gradually increased with cycle number, becoming stable after the fourth cycle. This phenomenon indicated that the first few cycles were required for battery activation, as is commonly observed for other DIBs [23,24,29,30]. Although the probed cell featured a high CE, its specific discharge capacity at a cut-off voltage of 3.725 V was low (∼27 mAh g−1), and we therefore investigated the influence of a higher cut-off charging voltage on specific discharge capacity and CE to realize better performance. Previously, the AlCl3:DMPIC mole ratio of unity was reported to provide an electrochemical window of ∼4.8 V [40], which suggested that higher DIB charge/discharge voltages can be achieved. However, the electrochemical performance of DIBs at such high working voltages has not been explored. Fig. 4a shows the CV curves of the DIB cell recorded at cut-off charging voltages of 3.725–4.4 V, revealing that no electrolyte decomposition was observed up to 4.4 V, i.e., both cations and anions could intercalate/deintercalate into/from the graphite electrodes within this voltage window. When a cut-off charging voltage of 4.0 V was applied, an additional intercalation peak appeared at 3.9 V, gaining intensity and becoming broader with increasing voltage and eventually affording a broad peak between 4.0 and 4.4 V, which might reflect the intercalation of a large amount of ions and enhanced specific capacity. Furthermore, two additional deintercalation peaks were observed at 4 and 3.7 V, indicating that an enhanced discharge voltage plateau and increased discharge capacity can be expected. Fig. 4b shows the CV curves of positive and negative graphite electrodes recorded using an Al reference electrode. The positive-electrode curve in the range of 0–2.45 V indicated the occurrence of an obvious redox reaction, which suggested that the intercalation of AlCl4− into graphite was initiated at 2.2 V and continued up to 2.45 V (vs. Al), while deintercalation occurred in the range of 2.2–1.6 V (vs. Al). Conversely, the CV curve of the negative electrode in the range of 0 to −1.85 V (vs. Al) indicated the occurrence of an asymmetric redox reaction. Fig. 5a shows the cycling performances of the DIB cell at cut-off charging voltages of 3.8–4.4 V, demonstrating that discharge capacity increased with increasing voltage, e.g., at a current density of 100 mA g−1, the discharge capacity increased from 32 mAh g−1 at 3.8 V to 90 mAh g−1 at 4.4 V. However, an opposite trend was observed for CE, e.g., a CE exceeding 96% that was maintained at 4.2 V declined to ∼91% at 4.4 V. Therefore, a cut-off charging voltage of 4.3 V was selected to keep the balance between discharge capacity and CE. Fig. 5b shows charge/discharge curves recorded at cut-off charging voltages of 3.8–4.4 V, revealing the presence of a higher charging curve plateau between 4.25 and 4.4 V. Additionally, two additional plateaus (4.0–4.2 and 3.6–3.1 V) were observed in discharge curves, in agreement with the results of prior CV analysis (Fig. 4). The DIB rate performance and charge/discharge curves obtained at various current densities are shown in Fig. S3, which reveals that a CE of ∼98% and a discharge capacity of 82 mAh g−1 were observed at a current density of 300 mA g−1. Fig. 5c presents the results of long-term stability testing performed at a current density of 300 mA g−1 and cut-off voltages of 3.0–4.3 V for 300 charge/discharge cycles. During this testing, the DIB delivered an initial capacity of 77 mAh g−1 that declined to 71 mAh g−1 after 300 cycles, which corresponded to a capacity retention of 92%, while a CE of approximately 97% was achieved after 300 cycles. Moreover, the employed battery retained its long-term stability and achieved a high CE of ∼99% when cycling was performed at a high current density of 1000 mA g−1 (Fig. 5d). The corresponding charge/ discharge curves (Fig. S4) revealed that a discharge capacity of ∼41 mAh g−1 was maintained even after 1000 cycles with a capacity retention of 99% (as compared to the sixth cycle). To the best of our knowledge, this is first report of a pure IL electrolyte–based DIB exhibiting a long cycle lifetime at such high current density. Notably, no obvious battery pouch swelling was observed after cycling (Fig. S5), and the (DMPI+)(AlCl4−) electrolyte was concluded to be stable at high cut-off charging voltages of up to 4.3 V.
X = (C1 − C2)/C1T × 100%,
(5)
where C1 is the discharge capacity without resting, C2 is the discharge capacity after 12-h resting, and T is the rest time (12 h). Therefore, the further study of DIB self-discharge was concluded to be a matter of high practical significance. Fig. 8 compares the performance of our DIB with those of previously reported ones (including batteries comprising metal-salt-based and pure IL electrolytes (highlighted in yellow)). Notably, although the (DMPI+) (AlCl4−) DIB does not show superior electrochemical performance compared to IA metal-salt-based electrolyte DIBs, it still exhibits certain advantages, e.g., high C-rate capability and long charge/discharge cycle lifetime. In particular, our battery exhibited a high energy density of ∼270 Wh kg−1 (based on the weight of graphite material only) at 1090 W kg−1 that decreased to 125 Wh kg−1 at 3027 W kg−1. For grid storage applications, the high C-rate capability of the fabricated DIB might be superior for frequency regulation, which requires high-current-density input/output, i.e., the occurrence of self-discharge might not affect grid storage applications. The developed DIB was superior to other pure-IL DIBs (e.g., those containing PP14TFSI, Pyr14TFSI, and PP14NTF2) in terms of nominal discharge voltage (∼3.7 V), current 237
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Fig. 5. (a) Cycling performances and (b) charge/discharge curves of the DIB cell at cut-off charging voltages of 3.8–4.4 V. (c) Results of long-term (300 cycles) DIB cell cycling at a current density of 300 mA g−1. (d) Results of long-term (1000 cycles) DIB cell cycling at a current density of 1000 mA g−1.
Fig. 6. Ex situ XRD patterns of (a) negative and (b) positive electrodes.
density (up to 1000 mA g−1), specific capacity (∼80 mAh g−1 at 300 mA g−1) and energy efficiency (> 88%) (Fig. S6 and Table S2). Moreover, electrode and separator material flexibility allowed for uninterrupted power delivery at different bending angles and directions (Video S2), while the high thermal stability of the employed IL allowed for successful operation even when the cell was exposed to an open flame (Video S3). Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.02.035.
4. Conclusions In summary, we successfully fabricated and tested a rechargeable DIB containing (DMPI+)(AlCl4−) as an IL electrolyte and low-cost NGF as both the anode and cathode, achieving superior discharge voltage, specific capacity, and energy efficiency compared to those obtained for other pure-IL DIBs. The fabricated battery pouch cell continued to function under various harsh conditions such as high power drainage (up to 1000 mA g−1), continuous bending (Video S2), or even burning (Video S3), which was ascribed to the high stability of (DMPI+) (AlCl4−). Although the problem of self-discharge needs to be further 238
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Fig. 7. (a) Dependence of CE on current density and (b) self-discharge behavior of a DIB cell subjected to 12-h rest after constant-current charging at 100 mA g−1.
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Fig. 8. Electrochemical performance of our DIB compared to those of previously reported DIBs.
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