Aqueous Mg-Ion Battery Based on Polyimide Anode and Prussian Blue Cathode L. Chen,†,§ J. L. Bao,‡ X. Dong,† D. G. Truhlar,‡ Y. Wang,*,† C. Wang,§ and Y. Xia*,† †
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEM(Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433, China ‡ Department of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States § Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *
ABSTRACT: The magnesium-metal battery, which consists of a cathode, a Mg-metal anode, and a nonaqueous electrolyte, is a safer and less expensive alternative to the popular Li-ion battery. However, the performance of Mg batteries is greatly limited by the low electrochemical oxidative stability of nonaqueous electrolytes, the slow Mg2+ diffusion into the cathode, and the irreversibility of Mg striping and plating on the Mg metal anode. Here, we report the first Mg-ion battery using a Mg2+ aqueous electrolyte, nickel hexacyanoferrate cathode, and polyimide anode. The operation depends on Mg2+ intercalation−deintercalation at the cathode and reversible enolization at the anode, accompanied by Mg2+ transport between cathode and anode. The cell exhibits a maximum cell voltage of 1.5 V and a supercapacitor-like high power, and it can be cycled 5000 times. This system points the way to improved Mg-based rechargeable batteries. cost, high safety, and long cycling life.21−23 Compared with aqueous Li-ion batteries, the aqueous Mg-ion battery has a lower cost, and as mentioned above, the Mg has high abundance and environment-friendly characteristics. Moreover, being a divalent Mg2+ ion, the electrolyte salts of aqueous Mg-ion battery will be only half that of aqueous Li- and Na-ion batteries which have monovalent ions of Li and Na, respectively. Here, we report an original aqueous electrolyte Mg-ion battery system that involves a reversible Mg2+ intercalation−deintercalation in a nickel hexacyanoferrate cathode and a reversible enolization at a polyimide anode, accompanied by Mg2+ transfer between cathode and anode. The electrode reactions do not involve dissolution or deposition of Mg metal electrode, and we demonstrate that the kinetics of both cathode and anode electrodes is not limited by ion diffusion or phase conversion. As a consequence of these features, this cell has the promise to achieve a long cycle life and high power density as supercapacitors. Polyimides have been used as electrode materials for both Li- and Na-ion batteries.24−28 In the present work, the anode is an aromatic polyimide formed by condensation of 1,4,5,8naphthalenetetracarboxylic dianhydride and ethylene diamine. The polyimide is shown in Figure 1a, where the monomer is denoted as P; each monomer contains two imide groups.
M
agnesium (Mg) metal is an attractive anode material for rechargeable batteries because it has a low reduction potential of −2.37 V (vs NHE), a volumetric capacity that is higher than that of lithium, low cost, and high abundance; is environmentally benign; and never forms dendrites during plating−stripping cycles.1 Accordingly, the Mg-metal rechargeable battery has long been considered as a safe and inexpensive alternative to the popular Li-ion battery. To date, many efforts have been devoted to improving the performance of Mg-metal batteries using nonaqueous electrolytes.2−20 However, the cell performance of the current Mg-metal batteries is still impeded by several factors such as limited electrochemical oxidative stability of nonaqueous electrolytes, slow Mg-ion diffusion into the cathode, and irreversible Mg2+/Mg conversion at the anode.8 Most of the reported Mg-metal rechargeable batteries display limited power density, insufficient energy density, and poor cycle life. Indeed, the working voltage of current Mg-based rechargeable batteries is close to that of aqueous electrolyte-based batteries (Table S1). Moreover, the use of unsafe organic electrolytes runs counter to the inherently safe and environmentally benign characteristics of Mg. These considerations motivated us to shift from current organic electrolyte-based Mg-metal batteries to aqueous Mg-ion batteries (Figure S1a,b). This enables the elimination of the passivation problem and replacing toxic and flammable organic electrolytes (such as acetonitrile or tetrahydrofuran) by aqueous electrolytes. Recently, aqueous Li-ion and Na-ion batteries have been developed as promising alternatives for organic Li-ion and Na-ion batteries for large-scale applications because of their low © XXXX American Chemical Society
Received: January 15, 2017 Accepted: April 18, 2017
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Figure 1. Electrochemical redox mechanism of polyimides. (a) Electrochemical redox mechanism (insertion and extraction) of polyimide with alkali ions. (b) Possible electrochemical redox mechanism of polyimide with Mg ion insertion and extraction. (c) Optimized molecular structures at M06/6-31+G(d,p) level of theory in aqueous phase and binding energy of 2Li−P, 2Na−P, and Mg−P.
Fourier transform infrared spectroscopy (FT-IR) was used to confirm the successful preparation of this polyimide (Figure S2). In contrast to other electrodes, the common mechanisms of intercalation or doping and dedoping of polyimides involves an enolization process of the imide monomers when combining with a cation. In aromatic polyimides this is accompanied by charge redistribution within the aromatic moiety.24,27 When a polyimide is used as an electrode material for a rechargeable Li- or Na-ion battery, its reduction and oxidation are accompanied by the association and dissociation reactions of Li+ or Na+ with the imide. As shown in Figure 1a, every monomer unit of the polymer can combine with two monovalent ions,24 producing the stoichiometry of 2Li−P or 2Na−P. For Mg2+, oxygen from adjacent monomers can ligate a single Mg2+ (Figure 1b), leading to the stoichiometry of Mg−P. To confirm this hypothesis, we have calculated the binding energies of Li+, Na+, and Mg2+ associated with polyimide and the charge−discharge potential of polyimide in the aqueous phase. In real situations, the metal cations can be thermally distributed in the polymer chain, and their definite positions relative to the monomers are not fixed. For the qualitative purposes of this work and consideration of computational cost, we truncated the polymer chain into monomers. The ratio of M cation and P anion is determined by charge balance, which is an assumption that charge balance can be achieved in the real case. The location of the M cation is then determined by calculation via minimizing the electronic energy of the whole system with respect to coordinates (see computational details in Experimental Section in the Supporting Information). The binding energies (Ebind) of 2Li−P, 2Na−P, and Mg−P are calculated to be 1.68, 1.29, and 1.31 eV, respectively (Figure 1c). For 2Li−P and 2Na−P, we have used the lowest-energy structure, by which the two metal cations are in trans positions. To compute the binding energies, the two cis isomers of 2Na−P are energetically higher than the lowest-energy isomer (Figure S3). The Cartesian coordinates (in angstroms) for optimized structures in water
are given in Table S2. The charge−discharge potential of the P|P2− pair can be computed as φ(P|P2 −) = φ°(P|P2 −) +
RT ln Kbind 2F
(1)
where R is ideal gas constant, T temperature, F the Faraday constant, φ°(P|P2−) the standard electrode potential of a P|P2− pair, and Kbind the equilibrium constant of the binding 2 reaction n Mn + + P2− ⇌ M2/nP. As already mentioned, the computed binding energies for Li+, Na+, and Mg2+ are very similar, which indicates that the Kbind for these three metal cations is very close. This is because the stronger the binding interaction, generally the more negative the binding Gibbs free energy. This implies that the charge− discharge potentials for P|P2− are almost the same in 2 M Li+, 2 M Na+, or 1 M Mg2+ aqueous solutions. Galvanostatic charge−discharge cycling of the polyimide in 1 M MgSO4 aqueous solution is shown in Figure 2a. The potential profiles show a slope plateau from −0.3 to −0.7 V. In the voltage window of 0 to −0.8 V, the polyimide shows a reversible capacity of 140 mAh g−1 in 1 M MgSO4 aqueous solution at a current density of 1 A g−1 (Figure 2a). As expected from the calculations, this is similar to the performance of polyimide in aqueous alkali ion solutions.27,28 It confirms that, like monovalent ions, the Mg divalent ions can be reversibly inserted and extracted from the polyimide. In addition, the polyimide also shows excellent cycling stability with 90% capacity retained after 2000 cycles (Figure 2b) at a current density of 1 A g−1. Furthermore, the polyimide shows a high rate performance: 120 mAh g−1 of capacity has been maintained even when the current rate was increased to 10 A g−1 (at this rate, only 43 s are needed to complete the discharge process). This rate performance is much better than that of conventional intercalation electrode materials. To cast light on this finding, we investigated the polyimide electrode kinetics by cyclic voltammetry (CV). The ideal voltammetric response of an 1116
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Figure 2. Electrochemical performance of polyimide in 1 M MgSO4 aqueous electrolyte: (a) galvanostatic charge−discharge curves at several current densities; (b) capacity of the polyimide as a function of cycle number at a current of 1 A g−1 between 0 and −0.8 V; (c) CV curves at different sweep rates (v) of polyimide-based electrode; (d) corresponding log ip vs log v of polyimide-based electrode.
The X-ray powder diffraction (XRD) patterns of PBN demonstrate the high crystallinity and phase purity of the as-prepared material (Figure S4). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show nanoparticles of PBN have a diameter of ∼50 nm (Figure S5). On the basis of inductively coupled plasma mass spectrometry and thermogravimetric analysis (Figure S6), the chemical composition of the as-prepared PBN was determined to be Na1.4Ni1.3Fe(CN)6·5H2O. Galvanostatic charge−discharge cycling was performed on PBN at different current rates between the voltages of −0.2 and 0.75 V (Figure 4a). At the current rate of 0.1 A g−1, the specific capacity was found to be 65 mAh g−1. When the current rate was increased to 10 A g−1, a specific capacity of 39 mAh g−1 was maintained. Furthermore, PBN showed excellent cyclic capacity retention, with hardly any capacity decrease after 2000 cycles (Figure 4b). We also studied the kinetics of the PBN electrode with CV at various scan rates. The b-value of the PBN-based electrode in 1 M aqueous MgSO4 electrolyte is 0.76, indicating a high rate of insertion and extraction of Mg2+ (Figure 4c,d). This high rate is attributed to the open framework facilitating Mg2+ diffusion. Moreover, the electrochemical impedance spectroscopic (EIS) measurement of PBN and polyimide (P) was performed at the open-circuit potential in the frequency range of 105−0.01 Hz with AC signal amplitude of 5 mV. The obtained Nyquist plot (−Z″ vs Z′) also confirmed that the PBN and polyimide are not controlled by the diffusion process (Figure S7). As observed in Figure S8, the first charge curves of PNB are different from the subsequent cycles, which involves extraction of Na+ from PNB. During the discharge process, as the Na+ content is very low, the Mg2+ from the electrolyte (1 M MgSO4) inserts into the open framework without destruction (Figure S8). Subsequent cycles involve deintercalation−intercalation of Mg2+. To further confirm the above reaction, we changed the
electrode active material at various sweep rates can be summarized according to i = avb
(2)
where a and b are two adjustable parameters and the measured current (i) at a fixed potential obeys a power law relationship with the potential sweep rate (v). For a redox reaction limited by semi-infinite diffusion, as in the conventional rechargeable batteries, the peak current i varies as v1/2 (i.e., b = 0.5). Whereas for a capacitive or pseudocapacitor material, it varies as v (i.e., b = 1).29 We have found that the redox reaction (i.e., the charge−discharge process) of polyimide-based electrodes in aqueous MgSO4 electrolyte has a b value of 0.92 (Figure 2c,d), indicating a pseudocapacitive characteristic. This is the key reason for the high rate performance of the polyimide-based electrode. In addition, this endows the polyimide with a power density that is much higher than that of conventional rechargeable materials limited by diffusion processes. Moreover, we measured its stability (self-discharge) in the discharged state (i.e., Mg−P). An open-circuit voltage (OCV) test was carried out for a duration of 2 days after the polyimide-based electrode was discharged until the voltage decreases to −0.8 V. Subsequently, the electrode was charged back to 0 V. Throughout the whole experiment, chronopotentiometric measurements (potential vs time) showed that about 80% capacity was retained after the 2 days of measurment, which indicates that the polyimide has a very good ability to overcome self-discharge (see Experimental Section and Figure 3). This measurement was carried out in the absence of O2 to protect the oxidation of polyimide.28 Prussian blue type nickel hexacyanoferrate (PBN) has been shown to be capable of reversibly intercalating both monovalent cations and Mg2+ in its open-framework structure.30,31 In this study, PBN has been used as the cathode material. It was synthesized by a coprecipitation method (see Experimental Section). 1117
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Figure 3. Stability investigation of Mg−P: (a) galvanostatic discharge curve of polyimide (P) electrode at the current of 1A g−1; (b) galvanostatic charge curve of polyimide (P) electrode at the current of 1A g−1; (c) open-circuit voltage (OCV) test of Mg−P for 2 days.
Figure 4. Electrochemical performance of PBN in 1 M MgSO4 aqueous electrolyte: (a) galvanostatic charge−discharge curves at several current densities; (b) cycle and coulombic performance of the PBN at the current of 1 A g−1 between −0.2 and 0.75 V; (c) CV curves at different sweep rates (v) of PBN-based electrode; (d) corresponding log ip vs log v of PBN-based electrode.
1 M MgSO4 aqueous electrolyte several times to remove the extracted Na ions; we then resembled the cell again (Figure S10). All of the cells have shown similar profiles. The equations of the electrode reaction are described below.
electrolyte after the PBN electrode was completely charged and compared the next charge−discharge profiles (Figure S9). In addition to all this, the following experiment has also been carried. The PBN electrode was first charged−discharged in the 1118
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voltage near 1 V. It delivers a capacity of 33 mAh g−1 based on the total weight of both active electrode materials at the current density of 1 A g−1. (The current density is calculated based on the weight of polyimide.) This corresponds to a specific energy of 33 Wh kg−1, which is close to their theoretical energy density of 48 Wh kg−1. This maximum operational voltage (∼1.5 V) and specific capacity are comparable to those of the Mg-metal battery using an organic electrolyte. However, the aqueous Mg-ion battery is much safer and faster than the organic electrolyte-based battery. We have found that the aqueous Mg-ion battery could be charged and discharged at remarkably high current densities up to 10 A g−1 (it takes about 21 s to complete a charge−discharge cycle at this current density) while maintaining a discharge capacity of 15 mAh g−1 (Figure 6a). Charge−discharge cycling at
First charge Cathode: Na1.4Ni1.3Fe(CN)6 − x e− → Na1.4 − xNi1.3Fe(CN)6 + x Na + Anode: P +
x Mg 2 + + x e− → Mgx /2P 2
Subsequent discharge and charge Cathode: Na1.4 − xNi1.3Fe(CN)6 +
x Mg 2 + + x e− 2
↔ Na1.4 − xMgx /2Ni1.3Fe(CN)6 x Mg 2 + 2 With aqueous electrolytes, we must consider possible oxygen and hydrogen evolution at the electrodes. The CV curves indicate that the Mg2+ ions can be extracted from PBN successfully before oxygen evolution occurs on the positive electrode and they can be inserted into polyimide ahead of hydrogen evolution at the negative electrode (Figure 5a). The charge−discharge voltage of Anode: Mgx/2P − x e− ↔ P +
Figure 6. Electrochemical performance of aqueous Mg-ion battery based on PBN/polyimide: (a) galvanostatic charge−discharge curves at several current densities between 0 and 1.55 V and (b, c) cycle and Coulombic performance of the aqueous Mg-ion battery at the current of 2 A g−1 between 0 and 1.55 V. The current density (A g−1) is calculated based on the weight of anode material. The capacity (mAh g−1) is calculated based on the total weight of both cathode and anode material.
Figure 5. Typical CV curves of individual electrodes and typical charge−discharge curves of aqueous Mg-ion cell based on PBN/ polyimide. (a) CV curves of PBN and polyimide at the scan rate of 1 mV s−1. (b) Typical charge−discharge curves of aqueous Mg-ion cell based on PBN/polyimide at the current density of 1 A g−1. The current density (A g−1) is calculated based on the weight of anode material. The capacity (mAh g−1) is calculated based on the total weight of both cathode and anode material.
a current density of 2 A g−1 demonstrated the highest stability of the aqueous Mg-ion battery, which nearly perfectly maintained its specific capacity over the first 100 cycles, and the capacity retention is about 60% after 5000 charge−discharge cycles. The Coulombic efficiency remained 100% during the whole charge− discharge cycling process, which indicates that no gas evolution occurred in the cycling voltage region, which is the key reason for the stability of this aqueous cell (Figure 6b,c). We also cycled the aqueous Mg-ion battery at the current of 0.02 A g−1 (0.15 C, Figure S11), and it has shown a stable cycle performance. To the best of our knowledge, it is the best cycling life of any Mg-based battery reported. In addition, we measured the self-discharge
the full aqueous Mg-ion battery based on polyimide/PBN was controlled between 0 and 1.55 V in 1 M MgSO4 aqueous solution, which is a safe voltage window without O2 and H2 evolution. The balancing weight ratio of PBN to polyimide was 3:1. It was calculated using the PBN charge capacity of 50 mAh g−1 and the polyimide discharge capacity of 148 mAh g−1 at the first charge−discharge cycle with the current density of 1 A g−1 (Figure S8 and 2b). The cell shows excellent reversibility with a sloping voltage profile from 0.8 to 1.55 V with an average 1119
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Figure 7. Self-discharge performance of aqueous Mg-ion cell based on PBN/polyimide (P): (a) galvanostatic charge−discharge and OCV test curves of Mg-ion cell based on PBN/polyimide (P); (b) galvanostatic charge curve of Mg-ion cell based on PBN/polyimide (P) at the current of 1A g−1; (c) open-circuit voltage (OCV) test of Mg-ion cell based on PBN/polyimide (P) for 20 h; (d) galvanostatic discharge curve of Mg-ion cell based on PBN/polyimide (P) at the current of 1A g−1. The current density (A g−1) is calculated based on the weight of anode material. The capacity (mAh g−1) is calculated based on the total weight of cathode and anode material.
electrolytes. Both the Prussian blue materials and polyimide are highly commercialized and environment-friendly materials. The rechargeable aqueous Mg-ion battery presented here would be very convenient for practical applications. The obtained energy densities (35 and 45 Wh kg−1) of the aqueous Mg-ion battery are close to those of current aqueous batteries for stationary or grid-level energy storage applications, such as Prussian blue analog aqueous battery (45/27 Wh kg−1),34,35 LiTi2(PO4)3/LiFePO4 aqueous Li-ion battery (∼50 Wh kg−1),21 NaTi 2 (PO 4 ) 3 /Na 0 . 4 4 MnO 2 aqueous Na-ion battery (∼33 Wh kg−1),36 and organic−inorganic aqueous flow battery (50 Wh kg−1).37 In addition, its stable cycle performance for 5000 cycles is also comparable or even better than these aqueous battery systems (only 1000 cycles of refs 21, 34, and 35 and 1600 cycles of ref 36). Moreover, because of its convenient preparation and the recyclability of Prussian blue and polyimide, the cost of this aqueous Mg-ion battery will be much lower than that of other aqueous battery systems. Therefore, the environment-friendly and nontoxic aqueous Mg-ion battery would be a viable alternative for stationary or grid-level energy storage applications.
performance of this aqueous Mg-ion cell, and the results have shown that the charged battery has lost only 17% capacity after a 20 h OCV measurment (see Experimental Section and Figure 7). We have also applied prussian blue type copper (PBC) hexacyanoferrate as the cathode.32,33 X-ray powder diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images indicate high-purity and nanosized particles (Figures S12 and S13). On the basis of inductively coupled plasma mass spectrometry and thermogravimetric analysis (Figure S14), the chemical composition of the as-prepared PBC was determined to be Na1.4Cu1.3Fe(CN)6· 5H2O. Using PBC we can achieve a higher Mg2+ intercalation− deintercalation potential and thus a higher operating voltage. PBC delivers a slope voltage plateau from 0.6 to 0.95 V which is higher than that of PBN (from 0.3 to 0.6 V). It has also shown excellent rate and cycle performances (Figures S15 and S16). A PBC/polyimide battery with 1 M MgSO4 aqueous electrolyte delivers a much higher average operation voltage of 1.3 V and an overall capacity of 35 mAh g−1 based on the total mass of both cathode (PBC) and anode (polyimide) active materials. The specific energy is about 45 Wh kg−1. The cell also exhibited a high rate and stable cycling performance (Figure S17). We have developed a new aqueous Mg-ion battery by using polyimide as anode, Prussian blue materials as cathode, and 1 M MgSO4 aqueous solution as electrolyte. This aqueous battery provides an operating voltage equivalent to that of Mg-metal battery using highly unstable organic electrolyte. Additionally, it has a much more stable cycling life of up to 5000 charge− discharge cycles with 60% capacity retention. Furthermore, the present aqueous Mg-ion battery affords an excellent rate performance. The energy density was calculated based on the measured capacity at a current of 1 A g−1 and on the total mass of active materials in both anode and cathode electrodes. This yields a promising energy density of ∼40 Wh kg−1 with room for improvement by developing other novel Prussian blue materials with higher redox potentials. Moreover, the aqueous electrolyte used here resolves the safety problem arising from using organic
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00040. Experimental section; characterization of materials (SEM, TEM, XRD, TGA etc.); electrochemical performance of materials and batteries (charge−discharge data, cycle and rate performance, etc.) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
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D. G. Truhlar: 0000-0002-7742-7294 Y. Wang: 0000-0002-2447-4679 C. Wang: 0000-0002-8626-6381 Y. Xia: 0000-0001-6379-9655 Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (21333002, 21622303), the National Key Research and Development Plan (2016YFB0901503). The work done by J.L.B and D.G.T. was supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences, under Grant DE-SC0008662. REFERENCES
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DOI: 10.1021/acsenergylett.7b00040 ACS Energy Lett. 2017, 2, 1115−1121
