CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201301036
Energetic Aqueous Rechargeable Sodium-Ion Battery Based on Na2CuFe(CN)6–NaTi2(PO4)3 Intercalation Chemistry Xian-yong Wu, Meng-ying Sun, Yi-fei Shen, Jiang-feng Qian,* Yu-liang Cao, Xin-ping Ai, and Han-xi Yang*[a] Aqueous rechargeable sodium-ion batteries have the potential to meet growing demand for grid-scale electric energy storage because of the widespread availability and low cost of sodium resources. In this study, we synthesized a Na-rich copper hexacyanoferrate(II) Na2CuFe(CN)6 as a high potential cathode and used NaTi2(PO4)3 as a Na-deficient anode to assemble an aqueous sodium ion battery. This battery works very well with a high average discharge voltage of 1.4 V, a specific energy of 48 Wh kg1, and an excellent high-rate cycle stability with approximately 90 % capacity retention over 1000 cycles, achieving a new record in the electrochemical performance of aqueous Na-ion batteries. Moreover, all the anode, cathode, and electrolyte materials are low cost and naturally abundant and are affordable for widespread applications.
With increasing penetration of renewable energy sources such as solar and wind electricity into the global energy markets, electric energy storage technologies are greatly needed to suppress the impact of intermittent power and integrate renewable energy sources into the electric grid.[1–3] Although different types of electrochemical batteries, from conventional lead-acid to vanadium-flow batteries and advanced Li-ion batteries, have been proposed as a possible strategy for grid-scale energy storage, none of them have been recognized as an enabling technology for grid-scale electric storage because of their cost and resource restrictions. Aqueous rechargeable Na-ion batteries[4–8] seem to be a good choice of storage battery for widespread electric storage applications due to their low-cost, material abundance, and safety. Recently, considerable attention has been devoted to exploring viable Na host materials for aqueous Na-ion batteries. Whitacre et al. firstly reported a tunnel-structured Na4Mn9O18 with reversible cathodic Na insertion,[6] but its gravimetric capacity is only 45 mAh g1 through a wide voltage range of 0.5 V. Later, Yamaki et al. revealed that Na ions can be reversibly inserted into NaTi2(PO4)3 in aqueous electrolytes.[9] [a] X.-y. Wu, M.-y. Sun, Y.-f. Shen, Dr. J.-f. Qian, Dr. Y.-l. Cao, Dr. X.-p. Ai, Prof. H.-x. Yang College of Chemistry and Molecular Sciences Wuhan University Wuhan 430072 (PR China) E-mail: [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301036.
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Its redox potential is 0.6 V versus the standard hydrogen electrode (NHE), indicating that NaTi2(PO4)3 can be utilized as a suitable anode for aqueous sodium-ion batteries. Based on these materials, Chiang et al. constructed an aqueous “rockingchair” type Na-ion battery using NaTi2(PO4)3 anode (negative electrode) and Na0.44MnO2 cathode (positive electrode).[10] This battery exhibits an average discharge voltage of approximately 1.1 V and an energy density of 33 Wh kg1 in terms of electroactive materials. Recently, our group built an aqueous Na-ion battery based on Na2NiFe(CN)6–NaTi2(PO4)3 intercalation chemistry that shows a higher output voltage of 1.27 V with an improved energy density of 42.5 Wh kg1.[11] In this system, Prussian blue analogue Na2NiFe(CN)6 serves as the cathode; it has an open framework crystal structure containing large interstitial sites that allows for fast insertion and extraction of Na + with little crystallographic lattice strain. However, a deficiency of these aqueous systems is their low voltage, which is a major cause for the low specific energy of the aqueous batteries. A feasible strategy to elevate the specific energy of the aqueous Na-ion battery is to use a cathode with a higher potential and larger reversible capacity. This can be done by adjusting the transition-metal cations at the M site in the Prussian blue compounds of NaxMyFe(CN)6 (M = Fe, Co, Ni, Cu, etc.) type.[12, 13] Cui et al. recently demonstrated that copper hexacyanoferrate(III) CuHCF exhibits a higher potential than nickel hexacyanoferrate(III) NiHCF for a variety of insertion ions (K + , Na + , and NH4 + ) in aqueous electrolytes.[14–16] Therefore, it is likely that CuHCF will allow development of higher voltage aqueous Na-ion full cells than NiHCF. Unfortunately, these compounds exist initially in a charged state that cannot be coupled with conventional Na-deficient anodes for battery use.[17] In general, a Na-insertion cathode should be designed in a Na-rich state (a discharged state), so as to act as a Na + reservoir to provide removable Na + ions for the Na + -deficient anode, thus enabling a rocking chair Na-ion battery. In this work, we synthesized a Na-rich copper hexacyanoferrate(II) (Na2CuFe(CN)6, denoted as NaCuHCF) as a high potential cathode and used NaTi2(PO4)3 as a Na-deficient anode to assemble an aqueous Na-ion battery. This battery demonstrated a high average discharge voltage of 1.4 V and a specific energy of 48 Wh kg1 based on the total weight of the electrode-active materials, achieving a new record in the energy density of aqueous Na-ion batteries. NaCuHCF is a typical Prussian blue compound with a nominal formula of Na2CuFe(CN)6 with Cu/Fe molar ratio of 1:1. In fact, as in most Prussian blue compounds, NaCuHCF exists ChemSusChem 2014, 7, 407 – 411
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CHEMSUSCHEM COMMUNICATIONS in a nonstoichiometric form with a Cu/Fe ratio > 1, that is, a large number of Fe(CN)6 vacancies exist in the lattice. Based on inductively coupled plasma atomic emission spectroscopy (ICP-AES) and thermogravimetric analysis (Figure S1), the chemical composition of the as-prepared NaCuHCF was determined to be Na1.4Cu1.3Fe(CN)6·8 H2O. The crystalline structure and morphology of the as-prepared NaCuHCF particles can be seen in Figure 1. As shown in Figure 1 a, all the XRD lines can be indexed to a face-centered cubic lattice (FCC, space group Fm3m) with a lattice parameter of a = 10.01 . The crystal structure is analogous to that of the ABX3 perovskite. The CuII and FeII ions are in ordered arrangement on the B sites. The CuII ions are 6-fold coordinated to the nitrogen atoms of the CN ligands, and FeII ions are octahedrally neighboured with the carbon atoms of the CN ligands, forming a three-dimensional (3D) polymeric framework with large interstitial spaces. The guests (Na + ions and zeolitic water) are ac-
www.chemsuschem.org almost unchanged during successive scans, suggesting a cycling stability of the material in aqueous Na2SO4 solution. In the light of the well-documented intercalation chemistry of Prussian blue compounds,[18–20] this pair of CV peaks can be attributed to the reversible redox reactions of the FeII/FeIII couple in the NaCuHCF lattice along with the insertion/extraction of Na + ions for charge counterbalance: Na2 CuFeII ðCNÞ6 -Naþ -e $ NaCuFeIII ðCNÞ6
The charge/discharge performance of the NaCuHCF electrode was evaluated at a current density of 60 mA g1. As shown in Figure 2 b, the NaCuHCF electrode displays charge/discharge plateaus at approximately 0.61 V (vs. Ag/ AgCl), which agrees well with the peak positions in the CV curves (Figure 2 a). It should be noted that the mean potential of NaCuHCF (0.61 V vs. Ag/AgCl) is approximately 130 mV higher than that of NaNiHCF (0.48 V vs. Ag/AgCl) as reported previously,[11] implying an effective enhancement in the working voltage of the aqueous Naion batteries if the NaCuHCF is used as a cathode. The initial charge and discharge capacities of the NaCuHCF cathode reach 71 and 59 mAh g1, respectively, corresponding to a coulombic efficiency of 83 %. This irreversiFigure 1. a) XRD pattern of the as-prepared NaCuHCF, the inset illustrates the lattice structure; b) TEM image of ble capacity at the aqueous the NaCuHCF sample. cathode usually results from oxygen evolution at the high pocommodated at the cubic nanopores, as schematically shown in the inset of Figure 1 a. This open framework structure of the Prussian blue compound offers a fast and reversible intercalation process for alkali ions Na + along the < 100 > directions. The TEM image in Figure 1 b reveals that the NaCuHCF samples appear as aggregated particles with an average diameter of approximately 50 nm. Figure 2 displays the electrochemical properties of the NaCuHCF electrode in aqueous Na2SO4 electrolyte. As shown in Figure 2 a, the main feature in the cyclic voltammetry (CV) curves appears as a pair of very symmetric redox peaks at the high potential region of 0.55– 0.80 V (vs. Ag/AgCl), within Figure 2. Electrochemical performance of the NaCuHCF cathode: a) CV curves measured at a scan rate of which the peak shapes and po- 5 mV s1; b) charge/discharge profiles at 60 mA g1; c) reversible capacities cycled at changing rates tential positions remained (1C = 60 mAh g1); d) cycling stability at a constant current of 5C. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM COMMUNICATIONS tential in the late stage of charge. After the second cycle, the reversible discharge capacity remains stable at 58.5 mAh g1, and the coulombic efficiency increases to 95 % at the 5th cycle. This redox capacity corresponds to a nearly 100 % utilization of the single Na-ion insertion capacity of the NaCuHCF material, based on the chemical composition of Na1.4Cu1.3Fe(CN)6·8 H2O. The NaCuHCF electrode also exhibits a remarkably high rate performance as shown in Figure 2 c. The electrode delivers a discharge capacity of 57, 52, 50, and 43 mAh g1 at current rates of 5C, 20C, 50C, and 80C (1C = 60 mA g1). Even at a high rate of 100C, the reversible capacity remains at 38 mAh g1, corresponding to a 65 % utilization of its reversible capacity. In comparison, the Na0.44MnO2 cathode can realize half of its reversible capacity at a 18C rate in aqueous electrolyte due to the inhibited transport of large Na ions in the oxide lattices.[6] Such an extraordinary high rate capability of the NaCuHCF material can be ascribed to its open framework structure that allows fast diffusion of Na + within the Prussian blue lattice. Also, it can be seen from Figure 2 c that the coulombic efficiency remains at 100 % even on increasing the current rate from 5C to a very high value of 100C, suggesting that the NaCuHCF cathode has a large overpotential of oxygen evolution, thus suppressing the oxygen evolution at the charged cathode even at high rates. In addition to its excellent high rate capability, the NaCuHCF electrode also demonstrates a superior cyclability. As displayed in Figure 2 d, the reversible discharge capacity decreases slightly from 57 to 53 mAh g1 over 500 cycles at the 5C rate, corresponding to 93 % capacity retention and a 100 % coulombic efficiency. This excellent cyclability is clearly due to the stable and large-channeled framework of the Prussian blue structure that facilitates reversible Na-ion insertion without structural change (Figure S2). Apparently, our NaCuHCF cathode sample has quite similar electrochemical performances to the CuHCF sample reported recently by Cui et al.[15] However, our NaCuHCF material is designed initially in a Na-rich state (a discharged FeII(CN)64 lattice), which can be directly applied to assemble Na-ion batteries with commonly used Na-deficient anodes. In contrast, the Na-deficient CuHCF compound (a charged FeIII(CN)63 lattice) has to react chemically with reductive Na salts to first form a Na-rich state[17] and is then paired with Na-deficient anodes for battery applications. In principle, many Na-deficient hosts having sufficiently low redox potential and high Na-storage capacity can serve as promising anodes for aqueous Na-ion batteries.[21, 22] However, an anode suitable for practical applications must have a potential for Na-ion insertion above the reversible potential of hydrogen (RHE) in the electrolyte. In the aqueous Na2SO4 electrolyte, the hydrogen evolution reaction takes place at 1.10 V vs. Ag/AgCl (see Figure 3), above which the Na-insertion reaction can proceed without much interference. In the search for an aqueous Na-storage anode, NaTi2(PO4)3 appears to be a suitable choice of the Na-deficient hosts because of its appropriately low potential of 0.82 V (vs. Ag/AgCl) and a theoretical Na-storage capacity of two Na ions.
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Figure 3. CV curves of the NaCuHCF cathode (positive electrode), the NaTi2(PO4)3 anode (negative electrode), and the current collector (Ti mesh) measured at a scan rate of 5 mV s1.
The synthesis and physical and electrochemical characterizations of the NaTi2(PO4)3 anode are given in detail in our previous article[11] and in the Supporting Information (Figures S3 and S4). Figure 3 compares the CV curves of the NaCuHCF cathode, NaTi2(PO4)3 anode and the Ti current collector in 1 mol L1 Na2SO4 aqueous solution. It can be seen from Figure 3 that the oxygen and hydrogen evolutions are clearly indicated by a steeply rising oxidation current at + 1.30 V vs. Ag/AgCl and a reduction current at 1.10 V vs. Ag/AgCl, respectively, indicating that the electrochemical window of this aqueous system could extend to a wider voltage range of > 2 V. In this voltage range, NaTi2(PO4)3 shows a pair of symmetric redox peaks at 0.96 and 0.68 V, which is slightly higher than the hydrogen-evolving potential (1.10 V vs. Ag/ AgCl), suggesting that the anodic Na-ion insertion/extraction reactions could proceed without the interference of hydrogen evolution. Similarly, the NaCuHCF cathode gives a pair of redox peaks at + 0.55 and + 0.63 V, which is far below the potential of oxygen evolution (+ 1.30 V vs. Ag/AgCl), preventing the cathodic Na + -insertion reaction from being disturbed by oxygen evolution. Since Na + -insertion reactions on the NaCuHCF cathode and on the NaTi2(PO4)3 anode occur within the electrochemical window of this aqueous electrolyte, it is thus expected that the Na-ion battery with NaTi2(PO4)3 anode and NaCuHCF cathode would have a high capacity utilization and a cycling stability in the aqueous electrolyte. Furthermore, it can also be concluded from the CV curves in Figure 3 that the NaCuHCF/NaTi2(PO4)3 couple would give an output voltage of approximately 1.4 V, which is probably the highest voltage in aqueous Na-ion batteries reported so far. To reveal the battery performance of this NaCuHCF/NaTi2(PO4)3 couple, we assembled a rocking chair-type aqueous Naion battery with an optimized mass ratio of NaCuHCF/NaTi2(PO4)3 = 1.7:1. Figure 4 a shows the charge/discharge curves of this Na-ion battery at a constant current of 2C (for battery test, 1C = 100 mA g1, based on anode mass). As expected, the NaCuHCF/NaTi2(PO4)3 battery shows a discharge plateau at 1.4 V and a slightly higher charge voltage without significant voltage hysteresis. Based on the anode mass, the cell can deliver a reversible capacity of 104 mAh g1 at 2C, corresponding to approximately 100 % utilization of the realizable capacity of the ChemSusChem 2014, 7, 407 – 411
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Figure 4. Electrochemical performance of the NaCuHCF-NaTi2(PO4)3 full cell: a) charge/discharge curves at a current density of 2C (1C = 100 mA g1, based on anode mass), the capacity was calculated on the mass of anode; b) rate performance from 2C to 100C; c) discharge curves at different rates 2C to 100C; d) long-term cycle performance of the full cell at various C rates.
anode material (Figure S4). However, it should be pointed out that the coulombic efficiency in the first cycle is relatively low at only 75.6 %, but it increases up to 91 % from the fifth cycle. The lower coulombic efficiency in the initial cycles might be attributed to the onset of O2/H2 evolution occurring during the tail end of charge due to H2O decomposition. Figure 4 b displays the rate performance of this aqueous Naion battery at changing rates from 2C to 100C. This battery can deliver a discharge capacity of 93, 86, 83, 70, and 60 mAh g1 at high rates of 5C, 10C, 20C, 50C, and 80C, respectively. Even at a very high rate of 100C, the reversible capacity can still reach 50 mAh g1, corresponding to a 50 % capacity utilization in 36 seconds. As can also be seen from Figure 4 c, the battery exhibits well-defined discharge plateaus with a slight decrease in the discharge voltage from 1.4 V to 1.1 V when the current rate increases drastically from 2C to 100C. Such small voltage drops with increased discharge rates are rarely observed in the existing battery systems, suggesting a very facile kinetics in all steps of this battery reaction. The long-term cycle stability of this battery was examined at high rate cycling and the results are given in Figure 4 d. Cycled at a 2C rate, the battery shows indiscernible capacity decay from initial 100 mAh g1 to 97 mAh g1 over 100 cycles, corresponding to 97 % capacity retention. On continuously cycling at a very high rate of 10C, the reversible capacity drops slightly to 85 mAh g1 and then slowly decreases to 74 mAh g1 at the 1000th cycle with high capacity retention of 88 %. This longterm cyclability is very impressive, as most aqueous and nonaqueous Na-ion batteries are found to suffer from their poor cycling life.[23–26]
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Taking into account the total mass weight of the electroactive materials, we constructed a Ragone plot to show the dependence of the energy densities on the power densities of the Na-ion system, which were calculated from the discharge profiles of the coin cells at various current densities from 1C to 100C rates. As shown in Figure 5, the specific energy of the battery is 48 Wh kg1 at a power density of 91 W kg1, and still remains at 16.6 Wh kg1 at a very high power of 3500 W kg1. This specific energy along with higher power density and longer cycling life is competitive with conventional aqueous rechargeable batteries such as lead acid, vanadium redox flow and aqueous lithium ion batteries,[27–29] enabling the Naion technology to serve as an inexpensive and clean alternative for large scale electric storage.
Figure 5. Ragone plots of the NaTi2(PO4)3/NaCuHCF aqueous Na-ion battery. The energy and power densities of the cell are calculated based on the total mass of active cathode and anode.
In summary, we have constructed an aqueous Na-ion battery by use of NaCuHCF cathode, NaTi2(PO4)3 anode, and 1 mol L1 Na2SO4 aqueous electrolyte. The as-fabricated NaCuHCF/NaTi2(PO4)3 cell shows a high voltage of 1.4 V and a specific energy of 48 Wh kg1 based on the total weight of the active electrode materials, which is much better than other aqueous sodiumion batteries reported so far. In addition, the aqueous Na-ion battery exhibits excellent high-rate discharge capability and cycle capability with 88 % capacity retention over 1000 cycles at 10C rate. The strategy in this work also implies that the electrochemical performance of this battery system can be further enhanced by adopting anode and cathode materials with high capacity, large potential difference, and cycle stability. Particularly, both the NaCuHCF and NaTi2(PO4)3 materials are low cost ChemSusChem 2014, 7, 407 – 411
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and environmentally benign, making the Na-ion battery promising for large scale electric storage applications.
Research Fund for the Doctoral Program of Higher Education (No. 20130141120007) for financial Support.
Experimental Section
Keywords: aqueous electrolyte · electrochemistry · energy storage · intercalation chemistry · sodium-ion batteries
Synthesis of cathode and anode materials: Na-rich copper hexacyanoferrate (NaCuHCF) was prepared by mixing 0.1 mol L1 CuCl2·6 H2O and 0.1 mol L1 Na4Fe(CN)6·10 H2O aqueous solutions under continuous stirring for 6 h, and then aging for another 24 h. The resulting precipitate was filtered, washed with water and ethanol for three times, and then dried in vacuum at 60 8C. Elemental analysis of the precipitate was conducted by using ICP-AES. Carbon-coated sodium titanium phosphate (NaTi2(PO4)3/C) was prepared by a simple high-temperature calcination method as reported in our previous paper.[11] For this purpose, stoichiometric amounts of CH3COONa·3 H2O, NH4H2PO4 and citric acid were dissolved in deionized water. Then, a diluted solution of tetrabutoxytitanium in ethanol was poured into the solution to reach the final stoichiometry under vigorously stirring. After the solvent evaporation, the solid precursor was annealed at 700 8C for 12 h under an argon atmosphere. The decomposition of citrate precursor in inert atmosphere yielded a coating layer of pyrolytic carbon. The carbon content in the NaTi2(PO4)3/C composite was about 3.3 % by mass, as determined by elemental analysis using an elemental analyser VarioELIII. X-ray powder diffraction patterns of the as-prepared samples were collected on a Bruker D8 diffractometer using CuKa radiation operating at 40 kV–40 mA in the range of 108–808 with a step size of 0.02. The morphologies of the samples were observed by using SEM (Sirion 2000, FEI) and TEM (JEM-2010FEF). Electrochemical measurements: Electrochemical characterization of the NaCuHCF cathode was carried out using a three-electrode cell. The working electrodes were made by pressing a 1 cm2 thin film (~10 mg cm2, containing 70 wt % the as-prepared materials, 15 wt % carbon black, 5 wt % conducting graphite, and 10 wt % polytetrafluoroethylene) onto a Ti mesh. Ti mesh was selected as the current collector because of its higher oxygen evolution overpotential, which is essential for avoiding the interference of oxygen evolution with the cathodic Na-insertion reaction. The counter electrode was a large piece of activated carbon and the electrolyte was a 1 mol L1 Na2SO4 aqueous solution (pH 7) purged with N2 flow for 1 h before use. A Ag/AgCl electrode saturated with aqueous NaCl solution (0.197 V vs. NHE) was used as the reference electrode. The full Na-ion batteries using the NaCuHCF cathode and NaTi2(PO4)3 anode was assembled in 2032-type coin cells. CV was carried out at a scan rate of 5 mV s1 on a CHI 600c electrochemical workstation (ChenHua Instruments Co., China). Galvanostatic charge/discharge experiments were conducted by on LAND cycler (Wuhan Kingnuo Electronic Co., China) at various rates at RT.
Acknowledgements We gratefully thank the National Natural Science Foundation of China (Grant No. 21303125 & No. 21333007) and the Specialized
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Energetic Aqueous Rechargeable Sodium-Ion Battery Based on Na2CuFe(CN)6–NaTi2(PO4)3 Intercalation Chemistry Xian-yong Wu, Meng-ying Sun, Yi-fei Shen, Jiang-feng Qian,* Yu-liang Cao, Xin-ping Ai, and Han-xi Yang*[a] Aqueous rechargeable sodium-ion batteries have the potential to meet growing demand for grid-scale electric energy storage because of the widespread availability and low cost of sodium resources. In this study, we synthesized a Na-rich copper hexacyanoferrate(II) Na2CuFe(CN)6 as a high potential cathode and used NaTi2(PO4)3 as a Na-deficient anode to assemble an aqueous sodium ion battery. This battery works very well with a high average discharge voltage of 1.4 V, a specific energy of 48 Wh kg1, and an excellent high-rate cycle stability with approximately 90 % capacity retention over 1000 cycles, achieving a new record in the electrochemical performance of aqueous Na-ion batteries. Moreover, all the anode, cathode, and electrolyte materials are low cost and naturally abundant and are affordable for widespread applications.
With increasing penetration of renewable energy sources such as solar and wind electricity into the global energy markets, electric energy storage technologies are greatly needed to suppress the impact of intermittent power and integrate renewable energy sources into the electric grid.[1–3] Although different types of electrochemical batteries, from conventional lead-acid to vanadium-flow batteries and advanced Li-ion batteries, have been proposed as a possible strategy for grid-scale energy storage, none of them have been recognized as an enabling technology for grid-scale electric storage because of their cost and resource restrictions. Aqueous rechargeable Na-ion batteries[4–8] seem to be a good choice of storage battery for widespread electric storage applications due to their low-cost, material abundance, and safety. Recently, considerable attention has been devoted to exploring viable Na host materials for aqueous Na-ion batteries. Whitacre et al. firstly reported a tunnel-structured Na4Mn9O18 with reversible cathodic Na insertion,[6] but its gravimetric capacity is only 45 mAh g1 through a wide voltage range of 0.5 V. Later, Yamaki et al. revealed that Na ions can be reversibly inserted into NaTi2(PO4)3 in aqueous electrolytes.[9] [a] X.-y. Wu, M.-y. Sun, Y.-f. Shen, Dr. J.-f. Qian, Dr. Y.-l. Cao, Dr. X.-p. Ai, Prof. H.-x. Yang College of Chemistry and Molecular Sciences Wuhan University Wuhan 430072 (PR China) E-mail: [email protected] [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201301036.
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Its redox potential is 0.6 V versus the standard hydrogen electrode (NHE), indicating that NaTi2(PO4)3 can be utilized as a suitable anode for aqueous sodium-ion batteries. Based on these materials, Chiang et al. constructed an aqueous “rockingchair” type Na-ion battery using NaTi2(PO4)3 anode (negative electrode) and Na0.44MnO2 cathode (positive electrode).[10] This battery exhibits an average discharge voltage of approximately 1.1 V and an energy density of 33 Wh kg1 in terms of electroactive materials. Recently, our group built an aqueous Na-ion battery based on Na2NiFe(CN)6–NaTi2(PO4)3 intercalation chemistry that shows a higher output voltage of 1.27 V with an improved energy density of 42.5 Wh kg1.[11] In this system, Prussian blue analogue Na2NiFe(CN)6 serves as the cathode; it has an open framework crystal structure containing large interstitial sites that allows for fast insertion and extraction of Na + with little crystallographic lattice strain. However, a deficiency of these aqueous systems is their low voltage, which is a major cause for the low specific energy of the aqueous batteries. A feasible strategy to elevate the specific energy of the aqueous Na-ion battery is to use a cathode with a higher potential and larger reversible capacity. This can be done by adjusting the transition-metal cations at the M site in the Prussian blue compounds of NaxMyFe(CN)6 (M = Fe, Co, Ni, Cu, etc.) type.[12, 13] Cui et al. recently demonstrated that copper hexacyanoferrate(III) CuHCF exhibits a higher potential than nickel hexacyanoferrate(III) NiHCF for a variety of insertion ions (K + , Na + , and NH4 + ) in aqueous electrolytes.[14–16] Therefore, it is likely that CuHCF will allow development of higher voltage aqueous Na-ion full cells than NiHCF. Unfortunately, these compounds exist initially in a charged state that cannot be coupled with conventional Na-deficient anodes for battery use.[17] In general, a Na-insertion cathode should be designed in a Na-rich state (a discharged state), so as to act as a Na + reservoir to provide removable Na + ions for the Na + -deficient anode, thus enabling a rocking chair Na-ion battery. In this work, we synthesized a Na-rich copper hexacyanoferrate(II) (Na2CuFe(CN)6, denoted as NaCuHCF) as a high potential cathode and used NaTi2(PO4)3 as a Na-deficient anode to assemble an aqueous Na-ion battery. This battery demonstrated a high average discharge voltage of 1.4 V and a specific energy of 48 Wh kg1 based on the total weight of the electrode-active materials, achieving a new record in the energy density of aqueous Na-ion batteries. NaCuHCF is a typical Prussian blue compound with a nominal formula of Na2CuFe(CN)6 with Cu/Fe molar ratio of 1:1. In fact, as in most Prussian blue compounds, NaCuHCF exists ChemSusChem 2014, 7, 407 – 411
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CHEMSUSCHEM COMMUNICATIONS in a nonstoichiometric form with a Cu/Fe ratio > 1, that is, a large number of Fe(CN)6 vacancies exist in the lattice. Based on inductively coupled plasma atomic emission spectroscopy (ICP-AES) and thermogravimetric analysis (Figure S1), the chemical composition of the as-prepared NaCuHCF was determined to be Na1.4Cu1.3Fe(CN)6·8 H2O. The crystalline structure and morphology of the as-prepared NaCuHCF particles can be seen in Figure 1. As shown in Figure 1 a, all the XRD lines can be indexed to a face-centered cubic lattice (FCC, space group Fm3m) with a lattice parameter of a = 10.01 . The crystal structure is analogous to that of the ABX3 perovskite. The CuII and FeII ions are in ordered arrangement on the B sites. The CuII ions are 6-fold coordinated to the nitrogen atoms of the CN ligands, and FeII ions are octahedrally neighboured with the carbon atoms of the CN ligands, forming a three-dimensional (3D) polymeric framework with large interstitial spaces. The guests (Na + ions and zeolitic water) are ac-
www.chemsuschem.org almost unchanged during successive scans, suggesting a cycling stability of the material in aqueous Na2SO4 solution. In the light of the well-documented intercalation chemistry of Prussian blue compounds,[18–20] this pair of CV peaks can be attributed to the reversible redox reactions of the FeII/FeIII couple in the NaCuHCF lattice along with the insertion/extraction of Na + ions for charge counterbalance: Na2 CuFeII ðCNÞ6 -Naþ -e $ NaCuFeIII ðCNÞ6
The charge/discharge performance of the NaCuHCF electrode was evaluated at a current density of 60 mA g1. As shown in Figure 2 b, the NaCuHCF electrode displays charge/discharge plateaus at approximately 0.61 V (vs. Ag/ AgCl), which agrees well with the peak positions in the CV curves (Figure 2 a). It should be noted that the mean potential of NaCuHCF (0.61 V vs. Ag/AgCl) is approximately 130 mV higher than that of NaNiHCF (0.48 V vs. Ag/AgCl) as reported previously,[11] implying an effective enhancement in the working voltage of the aqueous Naion batteries if the NaCuHCF is used as a cathode. The initial charge and discharge capacities of the NaCuHCF cathode reach 71 and 59 mAh g1, respectively, corresponding to a coulombic efficiency of 83 %. This irreversiFigure 1. a) XRD pattern of the as-prepared NaCuHCF, the inset illustrates the lattice structure; b) TEM image of ble capacity at the aqueous the NaCuHCF sample. cathode usually results from oxygen evolution at the high pocommodated at the cubic nanopores, as schematically shown in the inset of Figure 1 a. This open framework structure of the Prussian blue compound offers a fast and reversible intercalation process for alkali ions Na + along the < 100 > directions. The TEM image in Figure 1 b reveals that the NaCuHCF samples appear as aggregated particles with an average diameter of approximately 50 nm. Figure 2 displays the electrochemical properties of the NaCuHCF electrode in aqueous Na2SO4 electrolyte. As shown in Figure 2 a, the main feature in the cyclic voltammetry (CV) curves appears as a pair of very symmetric redox peaks at the high potential region of 0.55– 0.80 V (vs. Ag/AgCl), within Figure 2. Electrochemical performance of the NaCuHCF cathode: a) CV curves measured at a scan rate of which the peak shapes and po- 5 mV s1; b) charge/discharge profiles at 60 mA g1; c) reversible capacities cycled at changing rates tential positions remained (1C = 60 mAh g1); d) cycling stability at a constant current of 5C. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM COMMUNICATIONS tential in the late stage of charge. After the second cycle, the reversible discharge capacity remains stable at 58.5 mAh g1, and the coulombic efficiency increases to 95 % at the 5th cycle. This redox capacity corresponds to a nearly 100 % utilization of the single Na-ion insertion capacity of the NaCuHCF material, based on the chemical composition of Na1.4Cu1.3Fe(CN)6·8 H2O. The NaCuHCF electrode also exhibits a remarkably high rate performance as shown in Figure 2 c. The electrode delivers a discharge capacity of 57, 52, 50, and 43 mAh g1 at current rates of 5C, 20C, 50C, and 80C (1C = 60 mA g1). Even at a high rate of 100C, the reversible capacity remains at 38 mAh g1, corresponding to a 65 % utilization of its reversible capacity. In comparison, the Na0.44MnO2 cathode can realize half of its reversible capacity at a 18C rate in aqueous electrolyte due to the inhibited transport of large Na ions in the oxide lattices.[6] Such an extraordinary high rate capability of the NaCuHCF material can be ascribed to its open framework structure that allows fast diffusion of Na + within the Prussian blue lattice. Also, it can be seen from Figure 2 c that the coulombic efficiency remains at 100 % even on increasing the current rate from 5C to a very high value of 100C, suggesting that the NaCuHCF cathode has a large overpotential of oxygen evolution, thus suppressing the oxygen evolution at the charged cathode even at high rates. In addition to its excellent high rate capability, the NaCuHCF electrode also demonstrates a superior cyclability. As displayed in Figure 2 d, the reversible discharge capacity decreases slightly from 57 to 53 mAh g1 over 500 cycles at the 5C rate, corresponding to 93 % capacity retention and a 100 % coulombic efficiency. This excellent cyclability is clearly due to the stable and large-channeled framework of the Prussian blue structure that facilitates reversible Na-ion insertion without structural change (Figure S2). Apparently, our NaCuHCF cathode sample has quite similar electrochemical performances to the CuHCF sample reported recently by Cui et al.[15] However, our NaCuHCF material is designed initially in a Na-rich state (a discharged FeII(CN)64 lattice), which can be directly applied to assemble Na-ion batteries with commonly used Na-deficient anodes. In contrast, the Na-deficient CuHCF compound (a charged FeIII(CN)63 lattice) has to react chemically with reductive Na salts to first form a Na-rich state[17] and is then paired with Na-deficient anodes for battery applications. In principle, many Na-deficient hosts having sufficiently low redox potential and high Na-storage capacity can serve as promising anodes for aqueous Na-ion batteries.[21, 22] However, an anode suitable for practical applications must have a potential for Na-ion insertion above the reversible potential of hydrogen (RHE) in the electrolyte. In the aqueous Na2SO4 electrolyte, the hydrogen evolution reaction takes place at 1.10 V vs. Ag/AgCl (see Figure 3), above which the Na-insertion reaction can proceed without much interference. In the search for an aqueous Na-storage anode, NaTi2(PO4)3 appears to be a suitable choice of the Na-deficient hosts because of its appropriately low potential of 0.82 V (vs. Ag/AgCl) and a theoretical Na-storage capacity of two Na ions.
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Figure 3. CV curves of the NaCuHCF cathode (positive electrode), the NaTi2(PO4)3 anode (negative electrode), and the current collector (Ti mesh) measured at a scan rate of 5 mV s1.
The synthesis and physical and electrochemical characterizations of the NaTi2(PO4)3 anode are given in detail in our previous article[11] and in the Supporting Information (Figures S3 and S4). Figure 3 compares the CV curves of the NaCuHCF cathode, NaTi2(PO4)3 anode and the Ti current collector in 1 mol L1 Na2SO4 aqueous solution. It can be seen from Figure 3 that the oxygen and hydrogen evolutions are clearly indicated by a steeply rising oxidation current at + 1.30 V vs. Ag/AgCl and a reduction current at 1.10 V vs. Ag/AgCl, respectively, indicating that the electrochemical window of this aqueous system could extend to a wider voltage range of > 2 V. In this voltage range, NaTi2(PO4)3 shows a pair of symmetric redox peaks at 0.96 and 0.68 V, which is slightly higher than the hydrogen-evolving potential (1.10 V vs. Ag/ AgCl), suggesting that the anodic Na-ion insertion/extraction reactions could proceed without the interference of hydrogen evolution. Similarly, the NaCuHCF cathode gives a pair of redox peaks at + 0.55 and + 0.63 V, which is far below the potential of oxygen evolution (+ 1.30 V vs. Ag/AgCl), preventing the cathodic Na + -insertion reaction from being disturbed by oxygen evolution. Since Na + -insertion reactions on the NaCuHCF cathode and on the NaTi2(PO4)3 anode occur within the electrochemical window of this aqueous electrolyte, it is thus expected that the Na-ion battery with NaTi2(PO4)3 anode and NaCuHCF cathode would have a high capacity utilization and a cycling stability in the aqueous electrolyte. Furthermore, it can also be concluded from the CV curves in Figure 3 that the NaCuHCF/NaTi2(PO4)3 couple would give an output voltage of approximately 1.4 V, which is probably the highest voltage in aqueous Na-ion batteries reported so far. To reveal the battery performance of this NaCuHCF/NaTi2(PO4)3 couple, we assembled a rocking chair-type aqueous Naion battery with an optimized mass ratio of NaCuHCF/NaTi2(PO4)3 = 1.7:1. Figure 4 a shows the charge/discharge curves of this Na-ion battery at a constant current of 2C (for battery test, 1C = 100 mA g1, based on anode mass). As expected, the NaCuHCF/NaTi2(PO4)3 battery shows a discharge plateau at 1.4 V and a slightly higher charge voltage without significant voltage hysteresis. Based on the anode mass, the cell can deliver a reversible capacity of 104 mAh g1 at 2C, corresponding to approximately 100 % utilization of the realizable capacity of the ChemSusChem 2014, 7, 407 – 411
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Figure 4. Electrochemical performance of the NaCuHCF-NaTi2(PO4)3 full cell: a) charge/discharge curves at a current density of 2C (1C = 100 mA g1, based on anode mass), the capacity was calculated on the mass of anode; b) rate performance from 2C to 100C; c) discharge curves at different rates 2C to 100C; d) long-term cycle performance of the full cell at various C rates.
anode material (Figure S4). However, it should be pointed out that the coulombic efficiency in the first cycle is relatively low at only 75.6 %, but it increases up to 91 % from the fifth cycle. The lower coulombic efficiency in the initial cycles might be attributed to the onset of O2/H2 evolution occurring during the tail end of charge due to H2O decomposition. Figure 4 b displays the rate performance of this aqueous Naion battery at changing rates from 2C to 100C. This battery can deliver a discharge capacity of 93, 86, 83, 70, and 60 mAh g1 at high rates of 5C, 10C, 20C, 50C, and 80C, respectively. Even at a very high rate of 100C, the reversible capacity can still reach 50 mAh g1, corresponding to a 50 % capacity utilization in 36 seconds. As can also be seen from Figure 4 c, the battery exhibits well-defined discharge plateaus with a slight decrease in the discharge voltage from 1.4 V to 1.1 V when the current rate increases drastically from 2C to 100C. Such small voltage drops with increased discharge rates are rarely observed in the existing battery systems, suggesting a very facile kinetics in all steps of this battery reaction. The long-term cycle stability of this battery was examined at high rate cycling and the results are given in Figure 4 d. Cycled at a 2C rate, the battery shows indiscernible capacity decay from initial 100 mAh g1 to 97 mAh g1 over 100 cycles, corresponding to 97 % capacity retention. On continuously cycling at a very high rate of 10C, the reversible capacity drops slightly to 85 mAh g1 and then slowly decreases to 74 mAh g1 at the 1000th cycle with high capacity retention of 88 %. This longterm cyclability is very impressive, as most aqueous and nonaqueous Na-ion batteries are found to suffer from their poor cycling life.[23–26]
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Taking into account the total mass weight of the electroactive materials, we constructed a Ragone plot to show the dependence of the energy densities on the power densities of the Na-ion system, which were calculated from the discharge profiles of the coin cells at various current densities from 1C to 100C rates. As shown in Figure 5, the specific energy of the battery is 48 Wh kg1 at a power density of 91 W kg1, and still remains at 16.6 Wh kg1 at a very high power of 3500 W kg1. This specific energy along with higher power density and longer cycling life is competitive with conventional aqueous rechargeable batteries such as lead acid, vanadium redox flow and aqueous lithium ion batteries,[27–29] enabling the Naion technology to serve as an inexpensive and clean alternative for large scale electric storage.
Figure 5. Ragone plots of the NaTi2(PO4)3/NaCuHCF aqueous Na-ion battery. The energy and power densities of the cell are calculated based on the total mass of active cathode and anode.
In summary, we have constructed an aqueous Na-ion battery by use of NaCuHCF cathode, NaTi2(PO4)3 anode, and 1 mol L1 Na2SO4 aqueous electrolyte. The as-fabricated NaCuHCF/NaTi2(PO4)3 cell shows a high voltage of 1.4 V and a specific energy of 48 Wh kg1 based on the total weight of the active electrode materials, which is much better than other aqueous sodiumion batteries reported so far. In addition, the aqueous Na-ion battery exhibits excellent high-rate discharge capability and cycle capability with 88 % capacity retention over 1000 cycles at 10C rate. The strategy in this work also implies that the electrochemical performance of this battery system can be further enhanced by adopting anode and cathode materials with high capacity, large potential difference, and cycle stability. Particularly, both the NaCuHCF and NaTi2(PO4)3 materials are low cost ChemSusChem 2014, 7, 407 – 411
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and environmentally benign, making the Na-ion battery promising for large scale electric storage applications.
Research Fund for the Doctoral Program of Higher Education (No. 20130141120007) for financial Support.
Experimental Section
Keywords: aqueous electrolyte · electrochemistry · energy storage · intercalation chemistry · sodium-ion batteries
Synthesis of cathode and anode materials: Na-rich copper hexacyanoferrate (NaCuHCF) was prepared by mixing 0.1 mol L1 CuCl2·6 H2O and 0.1 mol L1 Na4Fe(CN)6·10 H2O aqueous solutions under continuous stirring for 6 h, and then aging for another 24 h. The resulting precipitate was filtered, washed with water and ethanol for three times, and then dried in vacuum at 60 8C. Elemental analysis of the precipitate was conducted by using ICP-AES. Carbon-coated sodium titanium phosphate (NaTi2(PO4)3/C) was prepared by a simple high-temperature calcination method as reported in our previous paper.[11] For this purpose, stoichiometric amounts of CH3COONa·3 H2O, NH4H2PO4 and citric acid were dissolved in deionized water. Then, a diluted solution of tetrabutoxytitanium in ethanol was poured into the solution to reach the final stoichiometry under vigorously stirring. After the solvent evaporation, the solid precursor was annealed at 700 8C for 12 h under an argon atmosphere. The decomposition of citrate precursor in inert atmosphere yielded a coating layer of pyrolytic carbon. The carbon content in the NaTi2(PO4)3/C composite was about 3.3 % by mass, as determined by elemental analysis using an elemental analyser VarioELIII. X-ray powder diffraction patterns of the as-prepared samples were collected on a Bruker D8 diffractometer using CuKa radiation operating at 40 kV–40 mA in the range of 108–808 with a step size of 0.02. The morphologies of the samples were observed by using SEM (Sirion 2000, FEI) and TEM (JEM-2010FEF). Electrochemical measurements: Electrochemical characterization of the NaCuHCF cathode was carried out using a three-electrode cell. The working electrodes were made by pressing a 1 cm2 thin film (~10 mg cm2, containing 70 wt % the as-prepared materials, 15 wt % carbon black, 5 wt % conducting graphite, and 10 wt % polytetrafluoroethylene) onto a Ti mesh. Ti mesh was selected as the current collector because of its higher oxygen evolution overpotential, which is essential for avoiding the interference of oxygen evolution with the cathodic Na-insertion reaction. The counter electrode was a large piece of activated carbon and the electrolyte was a 1 mol L1 Na2SO4 aqueous solution (pH 7) purged with N2 flow for 1 h before use. A Ag/AgCl electrode saturated with aqueous NaCl solution (0.197 V vs. NHE) was used as the reference electrode. The full Na-ion batteries using the NaCuHCF cathode and NaTi2(PO4)3 anode was assembled in 2032-type coin cells. CV was carried out at a scan rate of 5 mV s1 on a CHI 600c electrochemical workstation (ChenHua Instruments Co., China). Galvanostatic charge/discharge experiments were conducted by on LAND cycler (Wuhan Kingnuo Electronic Co., China) at various rates at RT.
Acknowledgements We gratefully thank the National Natural Science Foundation of China (Grant No. 21303125 & No. 21333007) and the Specialized
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