A High-voltage and Cycle-stable Aqueous Rechargeable Na-ion

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A High-voltage and Cycle-stable Aqueous Rechargeable Na-ion Battery Based on Na2Zn3[Fe(CN)6]2 –NaTi2(PO4)3 Intercalation Chemistry Miaomiao Shao, Bo Wang, Mengchuang Liu, Chen Wu, Fusheng Ke, Xinping Ai, Hanxi Yang, and Jiangfeng Qian ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00935 • Publication Date (Web): 11 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Energy Materials

A High-voltage and Cycle-stable Aqueous Rechargeable Na-ion Battery Based on Na2Zn3[Fe(CN)6]2-NaTi2(PO4)3 Intercalation Chemistry Miaomiao Shao,† Bo Wang,‡ Mengchuang Liu,† Chen Wu,† Fusheng Ke,* † § Xinping Ai,† Hanxi Yang,† Jiangfeng Qian*†

College of Chemistry and Molecular Sciences, Hubei Key Laboratory of

†

Electrochemical Power Sources, Wuhan University, Wuhan 430072, China E-mail: [email protected]; Sauvage Center for Molecular Sciences, Wuhan University, Wuhan 430072, China

§

E-mail: [email protected]; Global Energy Interconnection Research Institute North America, San Jose, CA 95134,

‡

USA KEYWORDS:

aqueous

Na-ion

battery,

Prussian

Blue,

dissolution

issue,

electrochemical corrosion, high-concentrated electrolyte

ABSTRACT: Aqueous rechargeable Na-ion batteries (ARNBs) hold great promise for grid-scale electric energy storage owing to their outstanding merits of low cost and resource abundance, however, their low energy density and poor cycling stability limit practical application. In this work, we reported a Prussian Blue (PB) analogue

ACS Paragon Plus Environment

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Na2Zn3[Fe(CN)6]2 as a high-voltage aqueous cathode for ARNBs and achieved its stable cycling at a high operation potential of 1.13 V (vs. SHE) by using of a highly concentrated NaClO4 electrolyte. Raman spectroscopy, in-situ XRD and DFT calculations have been utilized to study the underlying mechanism of electrode performance as a function of electrolyte concentration. It was revealed that in the concentrated 17 m NaClO4 electrolyte, almost all the water molecules are coordinated with Na+ ions and the solvation energy of PB materials increases considerably with increasing salt concentrations, which broadens the electrochemical stability window of the electrolyte and greatly alleviates the dissolution of the materials. An aqueous rechargeable Na-ion battery was constructed by using Na2Zn3[Fe(CN)6]2 cathode, NaTi2(PO4)3 anode and 17 m NaClO4 electrolyte. This full cell demonstrates a high voltage output of 1.6 V and an energy density of 55 Wh kg-1 (based on the total mass of the electrode-active materials), offering a viable alternative to commercial aqueous batteries for large-scale EES applications.


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1. INTRODUCTION Low-cost and efficient electric energy storage (EES) technologies are greatly demanded in the development of clean and sustainable energy applications such as electric vehicles, renewable power stations and smart grids. In the pursuit of these technologies, almost all battery systems, from conventional lead-acid batteries to lithium ion batteries and redox flow batteries have been tested, but few of them can meet the strict cost and safety requirements for large-scale EES applications.1-2 To surpass this difficulty, aqueous rechargeable Na-ion batteries (ARNBs)3-6 have been proposed as a promising alternative technology to existing storage batteries in recent years due to their low cost and the natural abundance of Na resources.7-9 Particularly, aqueous electrolytes offer a number of intrinsic advantages such as high ionic conductivity, high safety, and low manufacturing cost,10 all of which enable ARNB to be an attractive candidate for large-scale ESS applications. However, unlike their organic counterparts, the electrochemical insertion/extraction processes of Na+ ions in aqueous electrolytes always involve the oxidative or reductive decomposition reaction of water,10 which leads to a great impact on the selectivity of electrode materials. Firstly, the electrochemically stable window of aqueous solution is only 1.23 V, beyond which the electrochemical decomposition of H2O may occur with severe H2/O2 evolution,11-12 thus limiting the maximum cell voltage and consequently the energy density. Secondly, most Na-insertion compounds, such as Na3V2(PO4)3,13-14 Na2VTi(PO4)3,15 Na2MnFe(CN)6,16-17 and Na2Zn3[Fe(CN)6]218 etc. are reported to suffer from issues of severe structural instability in aqueous electrolytes, especially



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upon repeated electrochemical cycling. For instance, Na2Zn3[Fe(CN)6]2 (ZnHCF) has large ion channels of ~6.2 Å and a very high operating potential of ~1.1 V vs. SHE, possibly serving as a high-voltage and high-rate Na-storage cathode.19 However, Liu et al. found that ZnHCF exhibits rapid capacity decay in a few cycles due to its severe dissolution in 1 M Na2SO4 electrolyte.18 Recent studies by Zhou et al. have revealed that the capacity fading of ZnHCF material is closely dependent on its phase structure, where cubic-phase ZnHCF is found to be easily dissolved in aqueous electrolyte, whereas

rhombic

ZnHCF

exhibits

slightly

better

stability

for

long-term

charge/discharge cycles.20 Using highly concentrated electrolytes appears to be a facile and efficient way to extend the electrochemical stability window of aqueous electrolyte4,

11, 21-24

suppress the electrochemical dissolution of electrode-active materials.17,

25-26

and For

example, Suo et al.11 first reported a wide stability window of >2.5 V for an aqueous electrolyte solution by use of 9.26 m sodium trifluoromethane sulfonate (NaOTF) electrolyte. Very recently, Nakamoto et al.12,

17

revealed that increasing the salt

concentration of NaClO4 aqueous solutions can effectively reduce the electrochemical corrosion of Na2MnFe(CN)6 due to a common-ion effect, which restricts the dissolution of MnFe(CN)62- anions into the aqueous electrolyte and therefore dramatically enhances the cycling stability of the electrode. Inspired by these new findings, we tried to construct a high-voltage and cycle-stable ARNB by use of rhombohedral Prussian blue analogue Na2Zn3[Fe(CN)6]2 cathode and a concentrated 17 m (mol-salt in kg-solvent) NaClO4 electrolyte, and we attempted to ACS Paragon Plus Environment

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reveal the underlying mechanism of how electrolyte concentration impacts on the electrochemical performance of the Na2Zn3[Fe(CN)6]2 cathode. It was found that there are practically no free solvent molecules in the 17 m NaClO4 electrolyte, which can not only suppress the decomposition reaction of water but also greatly elevate the solvation energy and inhibit the phase transformation of active electrode materials, thus enabling stable

cycling

of

the

Na2Zn3[Fe(CN)6]2

cathode.

As

a

result,

the

Na2Zn3[Fe(CN)6]2/NaTi2(PO4)3 full cell demonstrated a high voltage output of 1.6 V, a specific energy density of 55 Wh kg-1 based on the total weight of the electrode-active materials and a superior cyclability with indiscernible capacity losses over 1000 cycles. 2. EXPERIMENTAL SECTION 2.1 Materials Synthesis The Prussian Blue analogue Na2Zn3[Fe(CN)6]2 was prepared by a controlled crystallization reaction as described in our previous work.27 The specific experimental operation was as follows: 60 mL ZnSO4/EDTA∙2Na (0.06 M) solution and 60 mL Na4Fe(CN)6 (0.04 M) solution were slowly added to 60 mL deionized water at the same time and stirred for 20 hours at room temperature. The resulting precipitate was washed three times with water and then dried in a vacuum oven at 60 °C for 24 h. The NaTi2(PO4)3/C composite was synthesized by a simple soft-template method. Firstly, 6 mmol CH3COONa, 18 mmol NH4H2PO4 and 2.42 g cetyltrimethylammonium bromide (CTAB) were dissolved in 80 mL equivoluminal deionized water and isopropanol under stirring. After dissolved completely, 12 mmol tetrabutyl titanate (C16H36O4Ti)



and 12 mmol acetylacetone (C5H8O2) were added to the above solution with strong stirring for 12 h. The solution was then dried at 80 °C in an oven to obtain a white powder. Finally, the obtained white powder was calcined at 700 ℃ for 12 hours under argon atmosphere to obtain the NaTi2(PO4)3/C composite. 2.2 Structural characterizations Ex situ X-ray diffraction (XRD) analysis of the Na2Zn3[Fe(CN)6]2 powder was performed on a Smartlab (Rigaku 2500) using filtered Cu Kα radiation at a scan of 2 ° min-1 from 10 ° to 50 °. In situ XRD experiments were conducted in a home-made cell using beryllium (Be) as the X-ray transmissive window coupled with a LAND cycling data recording system(Wuhan Kingnuo Electronic Co., China). In situ XRD data were collected by sequential scans, with each scan tested between 10o and 50o at a scanning rate of 20o min-1. The morphological features of the Na2Zn3[Fe(CN)6]2 sample were observed by scanning electron microscope (SEM, Ultra/Plus, Zeiss) and transmission electron microscope (TEM, JEM-2010 FEF). Water content in the Na2Zn3[Fe(CN)6]2 sample was determined by a thermogravimetric analyzer (TGA Q500, USA) in N2 atmosphere at a scan rate of 10 °C min-1. The ionic conductivities of the electrolytes were characterized by a DDS-307 meter (INESA Scientific Instrument Co., Ltd, Shanghai, China). 2.3 Electrochemical measurements The electrochemical performance of the Na2Zn3[Fe(CN)6]2 cathode was characterized by using three-electrode beaker cells, in which a Ag/AgCl electrode (0.197 V vs. SHE)


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and excess activated carbon were used as reference and counter electrodes, respectively. The working electrodes were prepared by pressing a diameter of 1.2 cm thin film on Ti mesh, and the thin film was composed of 70 wt.% active materials, 20 wt.% Ketjen black (KB) and 10 wt.% polytetrafluoroethylene (PTFE). The mass loading of active material in electrode films is about 3~5 mg cm-2. The electrolytes were 1 m, 5 m, 10 m, and 17 m NaClO4 aqueous solution (pH = 7). The aqueous Na-ion full cells were assembled in 2016 coin-type cells, in which Na2Zn3[Fe(CN)6]2 and NaTi2(PO4)3 served as cathode and anode, respectively. Cyclic voltammetry (CV) and galvanostatic charging/discharging experiments were tested by CHI 600C electrochemical workstation (ChenHua Instruments Co.) and LAND cycler (Wuhan Kingnuo Electronic Co., China), respectively. 3. RESULTS AND DISCUSSION The Prussian Blue analogue Na2Zn3[Fe(CN)6]2 was prepared by a controlled crystallization reaction as described in our previous work.27 As shown in Figure 1a, the powder X-ray diffraction peaks of Na2Zn3[Fe(CN)6]2 sample can be all indexed to rhombohedral Na2Zn3[Fe(CN)6]2∙9H2O (JCPDS No. 36-0539), which indicates that the synthesized sample is well-crystallized without any impurities. Figure 1b displays the unit cell structure of sodium zinc hexacyanoferrate, where ZnN4 tetrahedra and FeC6 octahedra are linked together by the C≡N ligands, thus building up a 3D framework to accommodate the Na+ ions. Unlike the conventional cubic Prussian Blue lattice, the rhombohedral Na2Zn3[Fe(CN)6]2 has larger ion channels of ~6.2 Å, which facilitates reversible insertion of large-sized Na ions. SEM and TEM images in Figure 1c and d



reveal that the Na2Zn3[Fe(CN)6]2 sample appears as uniform polyhedral shape with an average diameter of 300 ~ 400 nm. Selected area electron diffraction (SAED) pattern (Figure 1e) from a single particle demonstrates its single-crystalline nature with a rhombohedral lattice. Thermogravimetric analysis (TGA, Figure S1) reveals a 17 % water content in the sample, corresponding to approximately 7.3 H2O molecules per Na2Zn3[Fe(CN)6]2 unit.

Figure 1. Structure and morphologies of the Na2Zn3[Fe(CN)6]2 sample. (a) XRD patterns of as-prepared Na2Zn3[Fe(CN)6]2 and standard diffractions of Na2Zn3[Fe (CN)6]∙9H2O. (b) Unit cell structure; (c) SEM image; (d) TEM image; (e) SAED pattern. The electrochemical performances of Na2Zn3[Fe(CN)6]2 in NaClO4 aqueous electrolytes with concentrations varying from 1 m to 17 m are shown in Figure 2. Figure 2a displays the cyclic voltammogram of a Na2Zn3[Fe(CN)6]2 electrode in 1 m NaClO4 aqueous electrolyte at a scan rate of 2 mV s−1. When first scanned positively from 0.2 to 1.4 V, the electrode presents a large anodic peak at 1.30 V and then gives a cathodic


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peak at 1.01 V on the reversed scan, representing desodiation/sodiation reactions (Na2Zn3[FeII(CN)6]2 ↔2Na+ + 2e- + Zn3[FeIII(CN)6]2) from/into the Na2Zn3[Fe(CN)6]2 lattice. During subsequent scans, the CV peaks decreased their intensity seriously, probably due to a structural deterioration of the electrode. Nevertheless, in the 17 m NaClO4 aqueous electrolyte (Figure 2b), the CV feature appears as a pair of very symmetric redox peaks at the potential of 1.06/1.17 V (vs. SHE), the peak shapes and potential positions of which remain almost unchanged during the subsequent scans, suggesting a structural stability of the material. In accord with the CV features, the charge/discharge performance also confirms the improved cycling stability of the Na2Zn3[Fe(CN)6]2 electrode by increasing the NaClO4 concentration in the electrolyte solution. As displayed in Figure 2c and d, the reversible capacity of the Na2Zn3[Fe(CN)6]2 electrode decreases from 48 to 41 mAh g-1 in the first three cycles in the 1 m NaClO4 electrolyte at a current density of 120 mAh g-1, while the reversible capacity of this material keeps almost unchanged at 61 mAh g-1 when cycled in 17 m NaClO4 electrolyte. This improved reversible capacity of 61 mAhg−1 implies ~90% utilization of the theoretical 2 Na storage capacity of Na2Zn3[Fe(CN)6]2. The initial coulombic efficiency of the Na2Zn3[Fe(CN)6]2 electrode also increases from 60% to 80% for 1 m and 17 m NaClO4 electrolytes, respectively, suggesting suppressed side reactions in the concentrated electrolyte. In addition, either the CV peaks or the charge/discharge plateaus of the Na2Zn3[Fe(CN)6]2 electrode show a much larger voltage hysteresis in 1 m NaClO4 than in 17 m NaClO4, implying a higher polarization of the electrode in the diluted electrolyte. This larger electrochemical polarization most



likely results from the corrosion and dissolution of the electrode material, which leads to a poor electrical connection of the active particles. Moreover, a low-voltage discharge plateau at 0.75 V appears in the charge-discharge curve of the Na2Zn3[Fe(CN)6]2 electrode in 1 m NaClO4 electrolyte, possibly due to the redox reactions of the dissolved intermediate, as will be discussed in later section. The cycle performances of the Na2Zn3[Fe(CN)6]2 electrode in various concentrated electrolytes at a constant current of 120 mA g-1 are also shown in Figure 2e. The cycling stability of the material increases remarkably with increasing the concentration of electrolyte. When cycled in 17 m NaClO4 solution, this material demonstrated the best cycling performance with negligible capacity decay over 50 cycles at a 2 C rate. In order to get a deep insight into the capacity decay mechanism in dilute electrolyte, we first analyzed the electrode materials and electrolytes after cycling. After 3 cycles of Na2Zn3[Fe(CN)6]2 cathode in 1 m NaClO4 and 17 m NaClO4 electrolyte, the color of 1 m NaClO4 electrolyte changed to light yellow, while 17 m NaClO4 electrolyte still remained colorless (Figure 2f), suggesting that the electrode material was more or less dissolved in dilute electrolyte. As evidenced from the ICP-AES data in Figure S2, the concentrations of dissolved Zn2+ and Fe3+ ions decrease from 36.00 mg L-1 and 17.25 mg L-1 in 1 m NaClO4 electrolyte to 10.67 mg L-1 and 3.17 mg L-1 in 5 m NaClO4 electrolyte and then to indiscernible values in 17 m NaClO4 electrolyte, demonstrating that increasing the salt concentration in aqueous solutions can effectively reduce the dissolution of the PB compounds. This phenomenon can also be observed from the scanning electron microscopy images (Figure 2g, h). In the low-concentration


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electrolyte, the Na2Zn3Fe(CN)6 nanocrystals become obviously etched with vague and porous surfaces (Figure 2g), indicating a partial dissolution of the material into the electrolyte. The dissolution of active material into the electrolyte results in the loss of active material and a deterioration of the electrical connectivity between particles, thus leading to severe capacity decay. The ex-situ XRD patterns (Figure S3) of the cycled Na2Zn3Fe(CN)6 electrode also showed significantly reduced peak intensities in 1 m NaClO4 electrolyte, further confirming the dissolution of the material. Besides, the electrochemical stability window tests of the aqueous electrolytes (Figure S4) indicated that the onset of O2 evolution potential reaction was extended from 1.05 V to 1.22 V for 1m and 17 m NaClO4 electrolytes, thus allowing the stable operation of Na2Zn3Fe(CN)6 cathode at such a high potential of 1.13 V vs. SHE.



Figure 2. CV curves of Na2Zn3[Fe(CN)6]2measured at a scan rate of 2 mV s -1 in (a) 1 m NaClO4 and (b) 17 m NaClO4 electrolyte; Charge/discharge profiles at a current density of 120 mA g-1 in (c)1 m NaClO4 and (d) 17 m NaClO4 electrolyte; (e) The cycle performance profiles of Na2Zn3[Fe(CN)6]2 electrode with different electrolyte concentrations at a current density of 120 mA g-1;(f) Digital photographs of 1 m and 17 m NaClO4 electrolytes after three cycles; SEM of the Na2Zn3[Fe(CN)6]2 electrode after 50 charge/discharge cycles in (g) 1 m NaClO4 electrolyte and (h) 17 m NaClO4 electrolyte. The above-presented results seem to indicate that the capacity decay of Na2Zn3[Fe(CN)6]2 electrode is closely related to its electrochemical corrosion along with the decomposition reaction of water in dilute electrolyte. Increasing the salt concentration in aqueous solutions effectively reduces the content of water molecules in the electrolyte (Figure S5), therefore depressing the undesirable side reactions of water decomposition. Raman spectra gives a clear evidence for the decreased population of free water molecules in the concentrated electrolyte. As shown in Figure 3a, clusters of water molecules appeared as a broad band at 3000-3800 cm-1 in pure water and dilute solutions, whereas in 17 m NaClO4 electrolyte, almost all the water molecules are coordinated with Na+ ions and appeared as a new peak at 3565 cm-1. The lack of free water molecules in concentrated electrolyte is thereby responsible for its elevated electrochemical stability, which can decrease the O2 evolution reactions to a certain degree. In addition, increasing the salt concentration can greatly alleviate the chemical and electrochemical dissolution of the Na2Zn3[Fe(CN)6]2 electrode material. It is known that Na2Zn3[Fe(CN)6]2 was prepared by the precipitation reaction of ZnSO4 with Na4Fe(CN)6. Because of the coordination and chelating ability between Zn and N


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of Fe-CN, the material is precipitated in the form of a large network of Fe-C≡N-Zn units (metal organic framework). However, when the Na2Zn3[Fe(CN)6]2 electrode is immersed in aqueous solution, it encounters a dissolution equilibrium and partially leaks into H2O, as shown below: Dissolution Equilibrium: Na2Zn3[Fe(CN)6]2(s)↔2 Na+ (aq)+3 Zn2+(aq) + 2Fe(CN)64-(aq) Particularly, when the electrode is repeatedly charged and discharged, a large number of water molecules will co-insert into the lattice. As a result, the PB lattice has to expand and contract repeatedly, and the Fe-C≡N-Zn coordination networks would be attacked by the free water molecules, which in turn accelerates and promotes the dissolution of the material. By increasing of the Na+ concentration in aqueous electrolytes, the dissolution equilibrium shifts in the opposite direction due to the common ion effect (Figure 3c). Moreover, the decreased dielectric constant of the high concentration electrolyte can greatly change the solvation energy of Na+, Zn2+, and Fe(CN)64-, and thus change the solubility of the PB electrode. DFT calculations (Figure 3b, Table S1 and Supplementary Note) indicated that the solvation free energies for Na+, Zn2+, and Fe(CN)64- increase significantly when a high concentration of electrolyte is used. This increased solvation free energy implies an increased difficulty for the chemical and electrochemical dissolution of Na2Zn3[Fe(CN)6]2 when using a concentrated 17 m NaClO4 electrolyte.25, 28



Figure 3. (a) Raman spectra for the aqueous electrolytes. (b) Solvation free energy changes (kcal mol-1 per formula unit) of Na+, Zn2+, and Fe(CN)64- with respect to NaClO4 concentration. (c) Schematic illustration of Na2Zn3Fe(CN)6 before and after cycling in different electrolytes. In addition, we carried out an in situ XRD study of Na2Zn3[Fe(CN)6]2 electrodes to investigate and determine the structural changes during cycling in 1 m and 17 m NaClO4 electrolyte. When using 1 m NaClO4 as electrolyte (Figure 4a and S6), a new cubic phase20 with 2θ values of 17.10°, 24.26°, 34.60° and 38.83° attributed to Zn3[Fe(CN)6]2 was observed during Na ion extraction. As the intensity of those new peaks increased, the original peaks of the Na2Zn3[Fe(CN)6]2 phase disappeared at the end of the charging process, suggesting that the rhombohedral phase has been completely transformed into cubic phase. During the discharge process, the reflections


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for the cubic phase initially decreased, while the rhombohedral phase peaks emerged again, and similar phenomena were consistently observed in the subsequent cycles. These results suggest a two-phase reaction mechanism in 1 m NaClO4 electrolyte. Ni et al.20 also found that the electrochemical redox reaction of Zn3[Fe(CN)6]2 would cause continuous phase transition between rhombic and cubic structures, and the such-formed cubic phase is more soluble than the rhombic phase. The phase transition during the charging/discharging process can result in the dissolution and capacity decay of the materials and is the most plausible cause for the low-voltage discharge plateau appeared in the voltage profiles (Figure 2c). By stark contrast, when using 17 m NaClO4 as the electrolyte (Figure 4b and S7), all the XRD peaks are slightly shifted to lower angles (by 0.13 °) when the electrode is charged from 0.6 V to 1.8 V, corresponding to the removal of Na+ ions from the discharged state to the charged state and vice versa, indicating a single phase insertion/extraction mechanism and a reversible expansion of the lattice in 17 m NaClO4 solution. Due to the structural integrity of the lattice, this Na2Zn3[Fe(CN)6]2 framework can tolerate the repeated volumetric change during Na+ insertion/extraction cycles, leading to long term cyclability.



Figure 4. In situ XRD patterns and the corresponding first three-cycle charge-discharge curves of Na2Zn3[Fe(CN)6]2 electrodes at 1 C rate in (a) 1 m NaClO4 electrolyte and (b) 17 m NaClO4 electrolyte. To test the practical feasibility of Na2Zn3[Fe(CN)6]2 as a workable aqueous cathode, we assembled an aqueous Na-ion full battery using the NaTi2(PO4)3 anode and Na2Zn3[Fe(CN)6]2 cathode with an optimized anode/cathode mass ratio of 1:1.8. The electrochemical performance of the NaTi2(PO4)3 anode is shown in Figure S8-S10. The charge/discharge performance of this full cell is shown in Figure 5a. As expected, the Na2Zn3[Fe(CN)6]2/NaTi2(PO4)3 battery demonstrates an average discharge plateau at 1.6 V. Based on the cathode active materials mass, the cell can deliver a reversible capacity of 55 mAh g-1 at 2 C. The rate performance of this battery is shown in Figure


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5b, the discharge capacity is 55 mAh g-1 at 2 C rate and 39 mAh g-1 at 40 C rate, showing an excellent rate performance. Such superior rate performance of the full cell most likely arises from the high ionic conductivity of the aqueous electrolyte6 as well as the rapid transport of Na ion in PB lattice27. The long-term cycle stability of this full cell is examined at 10 C rate (Figure 5c), the battery demonstrates a superior cyclability with indiscernible capacity losses over 1000 cycles. As shown in Figure 5d, the specific energy of the full cell is 55 Wh kg-1 (based on the total active materials mass) at a power density of 120 W kg-1, and still remains 38 Wh kg-1 at a high power density of 2400 W kg-1. Table 1 gives a detailed comparison of our work with other aqueous rechargeable Na-ion batteries. The energy density of our work (55 Wh kg-1) is superior to the aqueous rechargeable Na ion batteries reported previously (33 Wh kg-1 for Na0.44MnO2NaTi2(PO4)3,29 42.5 Wh kg-1 for Na2NiFe(CN)6-NaTi2(PO4)3,30 48 Wh kg-1 for Na2CuFe(CN)6-NaTi2(PO4)3,31

and

31

Wh

kg-1

for

Na0.66[Mn0.66Ti0.34]O2-

NaTi2(PO4)311). What’s more, our work (100% capacity retention after 1000 cycles) also has better cycle performance than other ARNBs based on the Prussian Blue electrode (88% capacity retention after 250 cycles for Na2NiFe(CN)6-NaTi2(PO4)3,30 81% capacity retention after 50 cycles for Na2MnFe(CN)6-NaTi2(PO4)317).



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Figure 5. Electrochemical characterization of the Na2Zn3[Fe(CN)6]2-NaTi2(PO4)3/C full cell in 17 m NaClO4 solution. (a) Charge/discharge curves at a current density of 2 C. (b) Rate performance from 2 C to 40 C. (c) Long-term cycling performance at a rate of 10 C. (1 C = 60 mAh g-1, the capacity was calculated on the mass of cathode active materials.) (d) Ragone plot of the aqueous battery. The energy and power densities of the full cell are calculated based on the total mass of active materials.

Table 1. A comparison of the electrochemical performance of various aqueous rechargeable Na ion batteries. The energy densities are based on the total mass of the cathode and anode active materials. ARNB

Na0.44MnO2-NaTi2(PO4)3

Electrolyte

1 M Na2SO4

Average

Capacity

Capacity

Energy

voltage

retention

(mAh g-1)

density

(V)

(cycle)

(cathode)

(Wh kg-1)

1.1

60% (700)

40

33


refs

29

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Na2NiFe(CN)6-NaTi2(PO4)3

1 M Na2SO4

1.27

88% (250)

50

42.5

30

Na2CuFe(CN)6-NaTi2(PO4)3

1 M Na2SO4

1.4

90% (1000)

61

48

31

Na2MnFe(CN)6-NaTi2(PO4)3

17 m NaClO4

1.3/1.8

81% (50)

117

~60

17

Na0.66[Mn0.66Ti0.34]O2- NaTi2(PO4)3

9.26 m NaOTF

1.1

92.8 (1200)

62

31

11

Na2Zn3Fe(CN)6-NaTi2(PO4)3

17 m NaClO4

1.6

100% (1000)

55

55

This work

4. CONCLUSION In summary, we successfully synthesized a rhombohedral Prussian Blue analogue Na2Zn3[Fe(CN)6]2 and investigated its electrochemical performance as a feasible positive electrode for aqueous sodium ion batteries. The electrochemical stability of the Na2Zn3[Fe(CN)6]2 cathode during cycles is observed to be greatly influenced by the NaClO4 concentration in the electrolyte. In a dilute electrolyte of 1 m NaClO4, the PB electrode shows a rapid and significant capacity decay during cycling, while the decay of the electrode is greatly suppressed in 17 m NaClO4. This performance enhancement phenomenon was found to be due to the reduction of free water molecules in highly concentrated electrolytes; correspondingly, the hydrogen/oxygen evolution side reaction was suppressed, and the problem of material dissolution was also eliminated. Therefore, the Na2Zn3[Fe(CN)6]2 electrode exhibited very stable structural stability and cycle performance with a high voltage of 1.13 V vs. SHE. In addition, we also constructed a rechargeable aqueous Na-ion battery by using Na2Zn3[Fe(CN)6]2 cathode, NaTi2(PO4)3 anode and 17 m NaClO4 electrolyte. This full cell demonstrates a high voltage output of 1.6 V and an energy density of 55 Wh kg-1, offering a viable alternative to commercial aqueous batteries for grid-scale EES applications. Supporting Information



TG curve of the Na2Zn3[Fe(CN)6]2 sample; ICP-AES analysis of aqueous electrolytes after 50 cycles; Ex situ and in situ XRD patterns of the cycled cathodes in different concentration electrolytes; linear voltammetry curves and ionic conductivity of aqueous electrolytes; electrochemical performance of the NaTi2(PO4)3 anode; DFT calculations of the solvation energy of various ions. Notes The authors declare no competing ﬁnancial interest. Acknowledgements We gratefully thank the National Natural Science Foundation of China (grant no. 21773177), the Fundamental Research Funds for the Central Universities (grant no. 2042018kf0019), and the Science and Technology Project of State Grid Corporation of China (grant no. SGRIDGKJ[2017]841) for ﬁnancial support. References (1) Armand, M.; Tarascon, J.-M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359-366. (2) Wan, F.; Zhang, L.; Dai, X.; Wang, X.; Niu, Z.; Chen, J. Aqueous rechargeable zinc/sodium vanadate batteries with enhanced performance from simultaneous insertion of dual carriers. Nat. Commun. 2018, 9, 1656. (3) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical energy storage for the grid: a battery of choices. Science 2011, 334 (6058), 928-935. (4) Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science 2015, 350 (6263), 938-943. (5) Kim, H.; Hong, J.; Park, K. Y.; Kim, H.; Kim, S. W.; Kang, K. Aqueous rechargeable Li and Na ion batteries. Chem. Rev. 2014, 114 (23), 11788-827. (6) Wang, G.; Fu, L.; Zhao, N.; Yang, L.; Wu, Y.; Wu, H. An Aqueous Rechargeable Lithium Battery with Good Cycling Performance. Angew. Chem. Int. Ed. 2007, 119 (12), 299-301. (7) Wang, Y.; Mu, L.; Liu, J.; Yang, Z.; Yu, X.; Gu, L.; Hu, Y. S.; Li, H.; Yang, X. Q.;


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Entry for the Table of Contents A high-voltage and cycle-stable aqueous rechargeable Na-ion battery was constructed by using Na2Zn3[Fe(CN)6]2 cathode and NaTi2(PO4)3 anode, and 17 m NaClO4 aqueous solution electrolyte, respectively. This battery demonstrates excellent electrochemical performances with a high voltage of 1.6 V, an energy density of 55 Wh kg-1, and 100% capacity retention over 1000 cycles, possibly serving for energy storage applications.

Keywords: aqueous Na-ion battery, Prussian Blue, dissolution issue, electrochemical corrosion, high-concentrated electrolyte

A High-voltage and Cycle-stable Aqueous Rechargeable Na-ion Battery Based on Na2Zn3[Fe(CN)6]2 –NaTi2(PO4)3 Intercalation Chemistry TOC figure


A High-voltage and Cycle-stable Aqueous Rechargeable Na-ion

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