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Cite This: ACS Appl. Energy Mater. 2019, 2, 4936−4942
3 V Cu−Al Rechargeable Battery Enabled by Highly Concentrated Aprotic Electrolyte Huimin Wang† and Denis Y. W. Yu*,†,‡ †
School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon, Hong Kong
‡
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S Supporting Information *
ABSTRACT: Cu and Al foils are commonly used in lithium-ion batteries as current collectors. They are inactive and do not participate in the charge−discharge reactions but take up resources and space within a cell. Herein, we demonstrate a proof of concept to turn the Cu and Al foils into active materials by constructing a Cu−Al full cell, with Cu undergoing stripping/deposition reactions at the cathode and Al alloying with lithium at the anode. Stable cycle performance is possible with the use of a highly concentrated electrolyte; a 3 V cell exhibits excellent cycle stability for more than 200 cycles in 6 M LiTFSI DMC electrolyte. Our Cu−Al battery can give a volumetric energy density in the range 79−156 Wh L−1, comparable to that of state-of-the-art all-vanadium redox-flow batteries. The use of inexpensive Cu and Al as active materials can also potentially reduce the cost of energy storage. KEYWORDS: Cu stripping, Cu deposition, Cu−Al battery, aprotic electrolyte, anion-exchange membrane negative electrodes, respectively,28 though LiCl possesses low solubility in carbonate-based electrolyte and can potentially corrode cell components. Here, we are successful in extending the battery voltage to 3 V without the use of LiCl by coupling a Cu foil as the positive electrode with an Al foil as the negative electrode in LiTFSIbased aprotic electrolyte. Figure 1 shows the schematic of our
1. INTRODUCTION Metals are attractive materials for electrochemical devices as they can undergo oxidation and reduction reactions with high specific capacities. For example, Zn metal is widely used as a negative electrode in energy storage applications such as Zn− air,1−5 Zn−Ni6−10 and other Zn-based redox-flow batteries,11−13 whereas Li metal has been extensively studied for next-generation Li−S and Li−O2 batteries.14−20 Apart from being excellent negative electrodes, metals such as Cu and Fe can also be promising candidates as high-capacity positive electrodes. For instance, Zhang et al.21 and Dong et al.22 demonstrated rechargeable Daniell cells with a Cu positive electrode and a Li-ion conductive membrane in aqueous electrolyte, but the voltage of these cells is only about 1 V because of the use of Zn as the negative electrode. Ying et al.23 also couples Cu-CNFs composite film with Li4Ti5O12 to construct a 2 V full cell in aprotic electrolyte, though the Cu loading is only 0.5 mg cm−2. To increase energy density, a higher cell voltage is desired. By replacing the Zn electrode with Li, such as that in Li− NiOOH24 and Li−AgO25 using a NASICON solid electrolyte to separate the aprotic and aqueous electrolytes, cell voltage can be increased to more than 2 V. However, the use of lithium metal in the presence of an aqueous electrolyte, even though separated, poses safety concerns, especially since ceramic separators tend to be fragile.26,27 The risk can be reduced by using a single aprotic electrolyte, and we have previously demonstrated a 2.5 V stainless steel−graphite battery with LiCl-mediated stripping/plating of stainless steel and intercalation/deintercalation of Li+ in graphite at the positive and © 2019 American Chemical Society
Figure 1. Schematic of a Cu−Al full cell during charging. Received: March 25, 2019 Accepted: June 11, 2019 Published: June 24, 2019 4936
DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
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ACS Applied Energy Materials
Figure 2. Electrochemical performance of Cu−Al full cell. (a) Charge−discharge curves and (b) cycle performance of Cu−Al full cells in 1, 3, and 6 M LiTFSI DMC, respectively, at a current of 0.045 mA and a capacity limitation of 0.45 mAh. (c) XRD patterns of fresh Al and Al anode in 3 and 6 M LiTFSI DMC after 15 galvanostatic cycles in Cu−Al full cells (at charged state). (d) Profiles of Cu−Al full cell in 6 M LiTFSI DMC with a prolonged capacity limitation of 4 mAh (0.1 mA for 40 h).
Cu−Al battery during charging. The charge transfer on the Cu electrode is achieved by stripping and plating of Cu/Cu+, while that on the Al electrode is carried out by the alloying and dealloying reactions between Li and Al. TFSI− anions migrate through an anion-exchange membrane (AEM) to balance the charge. The difficulties are to ensure reversible reaction and maintain stable capacity during cycling, which we overcame by increasing the concentration of the electrolyte. Our novel system is attractive for the following reasons: (1) the cell voltage is as high as 3 V; (2) both Cu and Al are commonplace inexpensive materials with well-developed production processes; (3) there is no need to use other high-cost active materials such as LiCoO2; (4) there is no need to use Li metal, thus lowering the battery cost and making it easier to handle; (5) the cell only consists of metal foils with separator, simplifying the battery production process. The volumetric energy density of the cell is estimated to be between 79 and 156 Wh L−1, comparable to that of state-of-the-art allvanadium redox-flow batteries.29,30
Figure 3. Charge−discharge curves of Cu foil−Li and Al foil−Li half cells in 3 M LiTFSI DMC with AEM at a current of 0.045 mA and a capacity limitation of 0.45 mAh.
consistent with the alloying of Al with Li. On the other hand, the Cu−Li gives only one charge−discharge plateau at 3.4 V vs Li/Li+ because of the capacity limitation we imposed during the test. We attribute this to the Cu/Cu+ reaction as the Cu foil used here contains a large reserve of Cu. During charge and discharge, Cu is being stripped and redeposited on the positive electrode. This is verified by inductively coupled plasma mass spectroscopy (ICP-MS) measurements. Cu−Li cells with Cu foils were charged and discharged to different states of charge (Supporting Information Figure S3), and the Cu content in each electrolyte was measured (Table 1). We observe that the Cu content increases with increasing charging time (7.21 ppm for 5 h charging and 14.02 ppm for 10 h charging), demonstrating the dissolution of Cu ions from the electrode into the electrolyte during the charging process. The measured concentrations of Cu ions agree well with the applied capacity assuming only one electron transfer per Cu atom (i.e., Cu/Cu+ reaction), which is also consistent with the results from galvanostatic charge− discharge tests. During the discharge process to 2 V, the Cu content in the electrolyte is decreased from 14.02 to 5.92 ppm, indicating the redeposition of Cu ions onto the electrode
2. RESULTS AND DISCUSSION 2.1. Charge−Discharge Mechanism of a Cu−Al Full Cell. A Cu−Al full cell (Supporting Information Figure S1) was assembled with 16 mm diameter Cu foil as the positive electrode and Al foil as the negative electrode, separated by an anion-exchange membrane (Supporting Information Figure S2). LiTFSI, 1, 3, and 6 M in dimethyl carbonate (DMC), was used as the electrolyte, and the cells were charged with a capacity limitation of 0.45 mAh and discharged to 2 V. Figure 2a shows the charge/discharge profiles of the full cells. We first note that the operating voltage of all the cells (Ecell) is 3 V with a single charge and a single discharge plateau, and the reversible capacity increases with salt concentration. To deconvolute the reactions on the two sides of the electrodes, we constructed Al−Li and Cu−Li half cells and tested them separately. As shown in Figure 3, the Al−Li cell gives a charge−discharge potential of 0.4 V vs Li/Li+, which is 4937
DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
Article
ACS Applied Energy Materials Table 1. Variation of Cu Contents in Electrolytea state as-purchased electrolyte (A) charged for 5 h (B) charged for 10 h (C) charged for 10 h, followed by discharge to 2 V
cumulative capacity (mAh)
measured Cu content (ppm)
estimated M+ in electrolyte (ppm)
0 0.225 0.45 0.27
0 7.21 14.02 5.92
0 8.40 16.80 6.72
shows crystalline phases corresponding to AlLi and Al only, the charged Al electrode in 3 M LiTFSI DMC shows additional peaks corresponding to metallic Cu. The presence of Cu on the negative electrode side suggests that the part of the low CE and poor cycle performance for the cell with dilute electrolyte is due to Cu crossover through the AEM. Increasing salt concentration is able to suppress Cu crossover. Salt concentration is known to affect solvation structure of the electrolyte, morphology of the deposits, as well as surface reactions on the electrodes. We therefore conducted more detailed characterizations with Raman spectroscopy, electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) on the Cu−Al cells and electrodes to further understand the reasons for the improved performance in the highly concentrated electrolyte. 2.3. Effect of Salt Concentration on Solvation Structure of Electrolyte. Our results suggest that Cu crossover can be suppressed in the highly concentrated electrolyte. To identify the reason for this, the evolution of solvation structures for LiTFSI DMC electrolyte at various concentrations was studied with Raman vibrational spectroscopy between 650 and 980 cm−1 (Figure 4a). Pure DMC shows a band centered at 905 cm−1. With the addition of LiTFSI, a newly emerged peak appears at 924 cm−1 owing to the coordination of DMC with Li+ ions. The Raman spectra as shown in Figure 4b in the range 720−740 cm−1 (enclosed by black dashed box) are associated with anion TFSI−, which can be deconvoluted into three Gaussian−Lorentzian bands centered at 729, 733, and 736 cm−131,32 corresponding to
a
Cu contents obtained from ICP-MS in electrolyte of Cu−Li cells at different states of charge.
during discharge. The change in the Cu content is also consistent with the measured discharge capacity of the electrode. 2.2. Stability of Cu−Al Full Cell. Even though the Cu−Al full cell can be charged and discharged, we observe that Coulombic efficiency (CE) and stability are highly influenced by the salt concentration. The CE values of Cu−Al full cells with 1, 3, and 6 M TFSI DMC electrolyte are 19%, 80%, and 88%, respectively (see Figure 2a). Cycle performance of the cells is shown in Figure 2b. A cell with 1 M TFSI DMC fails after eight cycles (Supporting Information Figure S4), while a cell with 3 M LiTFSI DMC can be cycled up to 100 cycles. When a much more concentrated electrolyte 6 M LiTFSI DMC was utilized, the stability of the Cu−Al full cell is further improved to 200 cycles. The cells were disassembled after cycling, and the Al electrodes were analyzed with X-ray diffraction (XRD) (Figure 2c). While the charged Al electrode in 6 M LiTFSI DMC
Figure 4. Characterizations of electrolyte with different salt concentrations. (a) Raman vibrational spectroscopies of DMC solvent; fresh LiTFSI DMC electrolyte at 1, 3, 5, and 6 M; as well as electrolyte in charged Cu−Li cells, after charging for 10 and 20 h in 6 M LiTFSI DMC and after charging for 20 h in 3 M LiTFSI DMC (at 0.045 mA). (b) The deconvolution of the Raman profiles corresponding to TFSI (black dots and solid lines represent the original spectra and the fitted results, respectively). (c) EIS of symmetric cells with stainless steel as blocking electrodes at two ends, separated by an AEM F310 membrane. (d) Ionic conductivity of the DMC electrolyte with LiTFSI at 25 °C. 4938
DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
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ACS Applied Energy Materials
Figure 5. Surface morphology of Cu and Al electrodes in 3 and 6 M electrolytes. Surface morphology of the pristine Cu foil (a) and Cu electrodes in 3 M (b) and 6 M (c) LiTFSI DMC after 15 galvanostatic cycles in Cu−Al full cells (discharged state); surface morphology of the pristine Al foil (d) and Al electrode in 3 M (e) and 6 M (f) LiTFSI DMC after 15 galvanostatic cycles in Cu−Al full cells (charged state). EDX mappings of element O (g, j), element Al (h, k), and element Cu (i) on Al electrodes of (e) and (f); (l) EDX spectra of Al electrodes in (e) and (f).
free ion, contact ion pairs (CIPs), and aggregates (AGGs), respectively. In dilute electrolyte (1−3 M), TFSI− exists as free anions with a small number of CIPs. So, Li+ and Cu+ in electrolyte tend to coordinate with the free DMC and TFSI− to form small complexes which can move swiftly in the electrolyte but can also penetrate more easily through the AEM, leading to a low CE. An increasing salt concentration (3−6 M) decreases the amount of free DMC and free TFSI− anions as they coordinate with Li+ cations to form big CIPs and AGGs.33−35 The dissolved Cu+ ions are more likely to coordinate with the active O sites in CO bonds and SO units within the Li+ solvation shell36 to form large Cu complexes, as suggested by the presence of two newly emerged Raman peaks centered at 799 and 832 cm−1 (enclosed with a black solid frame) from electrolytes obtained from charged Cu−Li cells in 3 and 6 M LiTFSI DMC (Figure 4a), where the peak intensities grow with charging time and salt concentration of electrolyte. In addition, the increase in salt concentration reduces the ionic conductivity of the electrolyte and increases
the impedance of the system, as observed from EIS of Cu−Li half cells in various electrolytes at open circuit voltage (OCV) (Figure 4c,d). The large solvation structure in the Cu−Al full cell with 6 M LiTFSI DMC most likely diffuses sluggishly through the AEM, leading to a smaller amount of Cu+ crossover and higher CE, but larger polarization (Figure 2a,b). 2.4. Effect of Salt Concentration on Surfaces of Cu and Al Electrodes. Apart from the change in solvation structure of the electrolyte, salt concentration also influences the Cu plating and AlLi alloying processes and affects the stability of the Cu−Al battery. FE-SEM images of the discharged Cu electrodes tested with 3 and 6 M LiTFSI DMC are shown in Figure 5. While the pristine Cu foil is roughened (Figure 5a), a rough surface consisting of grains with diameter smaller than 5 μm can still be seen on the Cu electrode cycled in 3 M LiTFSI DMC (Figure 5b). In contrast, the Cu electrode tested in the 6 M electrolyte shows a smooth and dense film (Figure 5c). The EDX mapping of the Cu electrodes shows that both surfaces are composed of Cu 4939
DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
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ACS Applied Energy Materials
Figure 6. SEI analysis of Al electrode in 3 and 6 M electrolytes. XPS spectra of Al electrode from discharged Cu−Al full cells in 3 M LiTFSI DMC (a) and 6 M LiTFSI DMC (b) after 15 galvanostatic cycles. The black lines and red lines represent the raw and fitted data, respectively; the red line fitting was done via Origin using a Gaussian peak type. Peaks filled with colors represent deconvoluted peaks with different binding energies.
OSO in TFSI−, SO42−, and SO32−, respectively.41 For the 3 M LiTFSI DMC electrolyte, the percentage of LiF at 687.4 eV in F 1s is small, and other carbonate species (294−285 eV in C 1s) make up most of the SEI (solid electrolyte interphase) layer.43 The 6 M LiTFSI DMC electrolyte shows larger intensities of the C−F bond in F 1s at about 691.3 eV and C 1s at about 295.5 eV (Figure 6b), demonstrating incremental TFSI− components in the surface layers with increasing salt concentrations.40 This is also consistent with the S 2p spectra, where the peak intensity of OSO on TFSI− increases with increasing salt concentration, along with incremental sulfate species at 171.1 and 169.5 eV in S 2p, indicating that some anions are deposited on the surface and decomposed partially to become part of the SEI layer.40 Such an SEI composed of organic and inorganic compounds is known to effectively passivate the surface of the Al electrode during initial cycling, leading to an excellent capacity retention.40,44 In addition, the peak at about 934.5 eV which can be assigned to Cu 2p is observed only on the surface of the Al electrode with 3 M LiTFSI DMC. This again verifies that the highly concentrated electrolyte prevents crossover of Cu+ species to the negative electrode. 2.5. Energy Density Comparison. Here we have constructed a Cu−Al full cell in aprotic electrolyte with stripping/plating of Cu at the cathode and alloy/dealloy of Al at the anode. The system is similar to a redox-flow battery, where the active ingredients are stored inside the electrolyte. The overall volumetric energy density will depend on the maximum solubility of the Cu species in the electrolyte. If we take a typical solubility of salt in aprotic solvent of 1−2 M for calculations, the volumetric energy density of the Cu−Al battery is estimated to be between 79 and 156 Wh L−1 (see Supporting Information). This is comparable or even better than vanadium redox-flow cells (20−60 Wh L−1).29,30 More work can be done to increase the energy density of the battery system by extending the reaction to the Cu+/Cu2+ reaction and increasing the solubility of the cations in the electrolyte by optimizing the type of solvent, corresponding anion, and also additives.
(Supporting Information Figure S5). Note that the fibers on the electrodes are not part of the Cu but are the remains of the glass separator (SiO2) that was put between the AEM and the electrode. The result demonstrates that concentrated electrolyte can promote smooth and nondendritic Cu metal plating, enhancing the long-term stability of the cell. A similar effect is also observed for Li plating in a Li metal electrode in concentrated electrolyte.37−39 The highly concentrated electrolyte also affects the alloying process of the Al electrode. In particular, the concentrated electrolyte promotes a smooth alloy reaction between Li and Al. This is verified by FE-SEM images of the Al electrodes in charged Cu−Al full cells in 3 and 6 M LiTFSI DMC electrolytes after cycling (Figure 5e,f). We observe a much denser surface layer for the Al electrode tested in 6 M LiTFSI DMC, as opposed to that in 3 M electrolyte. There is a small amount of oxygen detected on the electrodes from EDX (Figure 5g,j), which is attributed to the oxidation of the lithiated film during the short period of air exposure when the electrodes are transferred into the chamber of the FE-SEM. In this case, the oxygen element in fact helps us to indirectly distinguish the morphology of the lithiated Al from the Al substrate, as oxygen will only be present at the location where there is Li. Apart from oxygen, EDX mapping of the Al electrodes also shows a significant amount of Cu on its surface when cycled in 3 M LiTFSI DMC (Figure 5e,i,l and Supporting Information Figure S6), while no Cu was detected when cycled in the 6 M counterpart (Figure 5f,l, Supporting Information Figure S7). This result is consistent with that from XRD as shown in Figure 2c, indicating Cu crossover only in dilute electrolyte. The effect of the electrolyte on the surface components of the Al electrode is further analyzed by XPS measurements. Cu−Al full cells were discharged, disassembled, and measured. As shown in Figure 6a, when the cell is cycled in 3 M LiTFSI DMC electrolyte, peaks at about 295.5, 292.4, 291.3, 288.3, and 287.3 eV in C 1s can be seen. They can be assigned to −CF3 in TFSI−, polycarbonates (poly(CO3)), O−CO, C− O, and C−C,40−42 respectively. Peaks at 691.3 and 687.4 eV in F 1s are related to C−F in TFSI− and lithium fluoride (LiF), and those at 172.2, 171.1, and 169.5 eV in S 2p correspond to 4940
DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
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ACS Applied Energy Materials
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3. CONCLUSIONS We have demonstrated that Cu and Al foils, free from any active material coatings, can be used as positive and negative electrodes directly to store energy in an aprotic system with the stripping/plating of Cu and alloy/dealloy of Al with Li at the positive and negative electrodes, respectively. A highly concentrated electrolyte is used to enable stable cycle performance: A Cu−Al full cell in 6 M LiTFSI DMC can be cycled for more than 200 cycles. We show that the concentrated electrolyte stabilizes the Cu+ in the electrolyte, reduces crossover, as well as facilitates more efficient redeposition and alloying on the Cu and Al electrodes, respectively. This is the first demonstration of a 3 V battery that can be easily fabricated by putting two metal foils together, in particular with Cu and Al which are commonly used as inactive current collectors for lithium-ion batteries. Apart from copper, other common metals such as Ni and Fe can also be used as the cathode. Our work will initiate a new category of metal−metal batteries that may be competitive with other emerging energy storage systems such as redox-flow batteries. In addition, it simplifies the manufacturing process of the batteries and can possibly reduce cost.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00627. Brief experimental details, more characterization results, and volumetric energy density calculation (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Denis Y. W. Yu: 0000-0002-5883-7087 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Strategic Research Grants (Project 7004925) from City University of Hong Kong. REFERENCES
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DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
Article
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DOI: 10.1021/acsaem.9b00627 ACS Appl. Energy Mater. 2019, 2, 4936−4942
