NH4 Topotactic Insertion in Berlin Green: an Exceptionally Long

ACS Applied Energy Materials. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16 ... Abstract: Aqueous batteries represent promising solutions for ...
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Letter

NH4+ Topotactic Insertion in Berlin Green: an Exceptionally Long-Cycling Cathode in Aqueous Ammonium Ion Batteries Xianyong Wu, Yunkai Xu, Heng Jiang, Zhixuan Wei, Jessica Hong, Alexandre Hernandez, Fei Du, and Xiulei Ji ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00789 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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NH4+ Topotactic Insertion in Berlin Green: an Exceptionally Long-Cycling Cathode in Aqueous Ammonium Ion Batteries Xianyong Wu,a Yunkai Xu,a Heng Jiang,a Zhixuan Wei,a,b Jessica J. Hong,a Alexandre S. Hernandez,a Fei Du,b and Xiulei Ji*a

a

Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA

b

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education),

College of Physics, Jilin University, Changchun 130012, People’s Republic of China Corresponding Author *E-mail: [email protected]

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Abstract: Aqueous batteries represent promising solutions for large-scale energy storage considering the cost, safety, and performance. Despite the tremendous efforts devoted to the metal cations as charge carriers for batteries, scarce attention has been paid to the non-metal cations such as proton or ammonium. In this study, we discovered that a Berlin green framework exhibits a much higher structural compatibility for NH4+ (de)insertion than Na+ and K+. Ex situ structural studies reveal that the topochemistry of NH4+ in Berlin green is of nearly zero strain. The NH4+ topotactic performance gives rise to a higher operation potential and an ultra-long cycling performance of 50,000 cycles with 78% capacity retention, far superior to Na+ and K+ (de)insertion. Furthermore, we propose a double-ion battery, where the Berlin green cathode hosts NH4+ and sodium titanium phosphate NaTi2(PO4)3 accommodates Na+ during operation. Such a new system exhibits promising results in capacity and cycling life. Our results point to a new direction of expanding the battery chemistry with NH4+ as a charge carrier.

Keywords: Berlin green, ammonium (de)insertion, zero-strain insertion, long cycling, double-ion battery

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Recently aqueous batteries are emerging as promising solutions for large-scale energy storage, benefiting from the inexpensive, safe, and environmentally friendly aqueous electrolytes.1,2 Toward the design of advanced batteries, it is the choice of charge carriers that dominates the nature of the battery chemistry.3,4 To date, extensive efforts have been devoted to metal ions as charge carriers such as Li+, Na+, K+, Mg2+, Al3+, and Zn2+ etc.5-10 In stark contrast, non-metal cations, such as proton, hydronium, and ammonium, have received little attention from the battery community.11-15 Nevertheless, non-metal cations may afford distinct properties and unique insertion mechanisms that are yet to be explored to boost the electrochemical performance of aqueous batteries.11-15 Compared to proton or hydronium, NH4+ is less corrosive and suffers a less extent of hydrogen evolution. To date, some pioneering work of NH4+ topochemistry has been reported on Prussian blue analogues (PBAs).16-18 Cui et al. investigated the relationship between the various cation species and electrochemical performance of PBA electrodes of KM[Fe(CN)6] (M=Ni and Cu).17,18 Most recently, our group proposed a “rocking-chair” NH4+-ion battery, in which ammonium Prussian white (PW) analogue serves as the NH4+-insertion cathode.14 Despite these early studies, the NH4+ topochemistry in solid electrodes remains to be further investigated, particularly on the long-term cycling stability and structural evolution of PBA electrodes. Berlin green (BG) represents the fully oxidized PBA compound with a nominal formula of FeIII[FeIII(CN)6].19,20 Benefiting from its absence of guest ions, BG is a good model compound to investigate the insertion chemistry of various alkali ions such as Li+, Na+, and K+.21-25 Herein, we first investigate NH4+ topochemistry in the BG lattice in a comparative study with Na+ and K+. We discovered that BG framework exhibits much greater structural compatibility toward hosting NH4+ than Na+ and K+ ions, which is reflected by the higher reaction potential and the nearly

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zero-strain structural characteristics of NH4+ insertion. As a result, the BG electrode demonstrates an excellent cyclability of 50,000 cycles with ~78% capacity retention. BG was prepared through a slow crystallization reaction between FeCl3 and K3Fe(CN)6 as precursors at 60 °C, as described in Supporting Information. Due to the strong oxidation capability of FeIII[FeIII(CN)6], the as-prepared BG contains a minor percentage of FeII ions that are coordinated with carbon atoms of the cyanide ions, which gives rise to a stoichiometry of FeIII[FeIII/II(CN)6]x·□y·zH2O (□ represents Fe(CN)6 vacancies).21-25 According to the inductively coupled plasma optical emission spectrometry (ICP-OES) on Fe, the elemental analyses on C and N (Table S1), and the thermogravimetric analysis (TGA) on H2O (Figure S1a), the BG sample in this study exhibits a chemical formula of Fe[Fe(CN)6]0.88·□0.12·2.8H2O. Fourier transform infrared spectroscopy (FTIR) demonstrates two peaks at 2168 and 2070 cm-1 (Figure S1b), which can be attributed to the vibration of the cyanide groups in FeIII-CN-FeIII and FeIICN-FeIII bonds, respectively.21-23 X-ray photoelectron spectra (XPS) also confirm that the nitrogen-coordinated Fe-ions exist as FeIII, whereas the carbon-coordinated Fe-ions are in the mixed states of FeII and FeIII (Figure S1d).21-23

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Figure 1. Material characterization of the BG material. a, XRD pattern and Rietveld refinement. The inset is an SEM image; b, The schematic of an intact Berlin green crystal structure. Figure 1a shows the X-ray diffraction (XRD) pattern and Rietveld refinement of the BG sample. As shown, all the XRD peaks can be well indexed to a face-centered cubic (FCC) structure with a space group of Fm-3m. Rietveld refinement reveals the lattice parameter a as 10.22 Å, which is in accord with previous reports.26 Figure 1b illustrates the crystal structure of an intact Fe2III[Fe1III(CN)6], where the low-spin Fe1 and high-spin Fe2 ions are six-coordinated to the carbon and nitrogen atoms, respectively, thus constituting a three-dimensional (3D) zeolitic structure.19,20 Scanning electron microscopy (SEM) in Figure 1a inset reveals that the BG sample mainly comprises sub-micron cubes. Energy dispersive X-ray spectroscopy (EDX) elemental mappings in Figure S2 reveal the homogenous distribution of Fe, N, and O elements in the BG sample, which suggests a high chemical purity.

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Figure 2. Electrochemical characterization of the BG electrode. a, Typical CV curves for Na+, K+, and NH4+ insertion at a scan rate of 1 mV s-1; b, The relation between the cation radius and the formal potential of the N-FeIII/FeII redox couple; c, GCD profiles for NH4+ (de)insertion at a current rate of 100 mA g-1; d, GCD profiles for K+ (de)insertion at a current rate of 100 mA g-1.

To better understand the NH4+ topochemistry in the BG lattice, we conducted comparative studies with its counterparts of Na+ and K+ ions. We employed three-electrode Swagelok cells, where BG, activated carbon, and Ag/AgCl serve as working, counter, and reference electrode, respectively. All the potentials in this work are referenced to the standard hydrogen electrode (SHE). Figure 2a shows the CV curves of the BG electrode in the electrolytes containing Na+,

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K+, and NH4+ ions. As well known, BG exhibits two pairs of well-defined redox peaks for Na+ and K+ (de)insertion, which are attributed to the redox couples of C-FeIII/FeII (at high potentials) in forming, e.g., KFe[Fe(CN)6] known as Prussian blue and N-FeIII/FeII (at low potentials) in forming K2Fe[Fe(CN)6] known as Prussian white (PW).19-25 The CV peaks at the high potential are weaker than the low-potential ones, which is resulted from the presence of [Fe(CN)6] vacancies in the BG structure.19-23 Interestingly, NH4+ does not deliver the high-potential redox peaks but only the low potential ones.27 Even scanning to +1.5 V does not lead to any cathodic current in the following discharge for the high-potential redox couple, where the anodic current should be attributed to the irreversible oxygen evolution reaction (OER) (Figure S3). As the C-FeIII/FeII redox couple is inactive in the NH4+ electrolyte, we focus on the formal potential of the N-FeIII/FeII redox couple as a function of the cation radii, as displayed in Figure 2b. Here, the formal reaction potential, Ef, is defined as the arithmetic average of the anodic (Epa) and the cathodic peak potentials (Epc).28 As shown, the formal potential increases with the cation size in an order of NH4+ > K+ > Na+ (0.46 > 0.44 > 0.33 V), which is in good accord with prior reports[17,18,29] The bulkier NH4+ fits the voids in BG better, which gives rise to a lower Gibbs free energy of (NH4+)xFe[Fe(CN)6], thereby leading to a higher insertion potential.29 Figure 2c shows the galvanostatic charge/discharge (GCD) profiles of the BG electrode for NH4+ at a current rate of 100 mA g-1, in comparison with the K+ (de)insertion (Figure 2d). As shown, the initial discharge capacity of ~115 mAh g-1 is almost identical for NH4+, K+, and Na+ (Figure S4), which corresponds to the reduction of both C-FeIII and N-FeIII in the BG lattice. However, in the subsequent charge process, only a capacity of ~91.5 mAh g-1 is reversible for the NH4+ de-insertion. This is in sharp contrast to the behavior with Na+ and K+, where in the first charge process, the BG lattice gets further oxidized than its pristine electrode, including

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some Fe(II) in the original structure. Therefore, the following 2nd discharge—Na+ or K+ insertion reveals a larger capacity of ~130 mAh g-1 than the first discharge. Such a capacity increase from the 1st to the 2nd cycle did not occur to the NH4+ topochemical reactions. A high-potential polarization could not oxidize the C-FeII sites, next to which NH4+ ions reside. We hypothesize that NH4+ ions that compensate the redox reactions of C-FeIII/FeII sites are trapped within the lattice after the 1st discharge (to be further discussed later with XRD results). Similar phenomena were reported that some bulky cations such as NH4+, Rb+, or Tl+ would be trapped in the PB lattice, thus inhibiting the reversible redox behavior of the C-FeIII/FeII couple.30,31,32 FTIR analysis on the charged electrode confirms the presence of trapped NH4+ ions in the BG lattice, as shown in Figure S5. Notably, the reversible NH4+ (de)insertion capacity of ~90 mAh g-1 corresponds to full utilization of the N-FeIII/FeII redox couple in the subsequent cycles. Interestingly, NH4+ storage exhibits a much smaller extent of self-discharge than Na+ and K+. After 12 hours’ rest at open circuit voltage (OCV), the capacity retention is 73%, 69%, and 86% for Na+, K+, and NH4+, respectively, as shown in Figure S6. We attribute the faster selfdischarge of BG with Na+ and K+ to the C-FeIII/FeII redox couple that is of a high-enough potential to oxidize water (at pH = 7, the standard potential of OER is 0.817 V).7

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Figure 3. Rate and cycling performance of the BG electrode. a, Rate performance for NH4+ (de)insertion; b, Capacity retention for Na+, K+, and NH4+ (de)insertion at the same current rate of 1 A g-1, the potential range is from -0.2 to +1.2 V; c, Cycling performance for NH4+ insertion at a low current rate of 0.2 A g-1; d, Cycling performance for NH4+ (de)insertion at a high current rate of 5 A g-1.

We further characterized the rate and cycling performance of NH4+ (de)insertion in BG. As shown in Figure 3a, the discharge capacity is 90, 85, 81, 78, 68, 59, and 48 mAh g-1 at current rates of 0.1, 1, 5, 10, 30, 50, and 80 A g-1, respectively. Even at a very high rate of 100 A g-1 (~1100 C), the discharge capacity is still 41 mAh g-1, which is nearly half the capacity obtained at a low rate of 1.1C (0.1 A g-1). Such remarkable rate performance is likely due to the roomy

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voids of the BG lattice, 19-25 which facilitates the fast kinetics of NH4+ ions. Similar high-rate behavior is also observed for Na+ and K+, as shown in Figure S7. The most outstanding advantage for NH4+ (de)insertion is the exceptional cycling stability in the BG framework. Figure 3b shows the comparison of the cycling performance for Na+, K+, and NH4+ in the same potential range of -0.2~1.2 V at the same current rate of 1 A g-1. The capacity retention is 87% over 4000 cycles for NH4+ much higher than K+ (48% over 1000 cycles) and Na+ (72% over 1000 cycles), which implies a better structural compatibility between the BG framework and NH4+ ions. SEM analysis on the cycled electrodes also reveals that the NH4+ insertion can maintain BG’s morphology (Figure S8). Note that, the better cycling of Na+ insertion than K+ seems counterintuitive; however, it can be rationalized by lower capacity utilization of the C-FeIII/FeII redox for Na+ insertion at such a current rate (Figure S7), which may alleviate the lattice change of the BG framework. We further tested NH4+ cycling performance at various current rates. As shown in Figure 3c, when cycled at 0.2 A g-1, the discharge capacity fades from 90 to 79 mA g-1 after 450 cycles, which corresponds to a capacity retention of ~88% and an average Coulombic efficiency of ~99.4%. When cycled at a high rate of 5 A g-1, the capacity retention is as high as 78% even after 50,000 cycles (Figure 3d), which rivals the most stable topotactic battery electrodes reported. For a better comparison, Table S2 summarizes some reported battery electrodes with long cycling stability for various cation insertion, where the NH4+ topochemistry in BG framework is competitive with those common metal ions including Li+, Na+, and K+. Additionally, the BG electrode exhibits a lower reaction potential, higher capacity, and superior rate and cycling performance in comparison with to the KM[Fe(CN)6] (M=Ni, Cu) cathodes.17

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Figure 4. Structural characterization of the BG electrode. a, XRD comparison of the pristine state, Na+ inserted, K+ inserted, and the NH4+ inserted BG electrodes; b, Cycling comparison of Na+, K+, and NH4+ insertion cycled at a narrower potential range of -0.2~0.8 V, which only associates with the N-FeIII/FeII redox center for Na+ and K+ insertion.

To understand the disparity of different ions on the cycling performance, we carried out ex situ XRD to investigate the structural evolution of the BG framework for Na+, K+, and NH4+ insertion. Figure 4a shows the XRD comparison between the pristine state, Na+-inserted, K+inserted, and the NH4+-inserted states. We choose the (400) peak to investigate the lattice change due to its higher sensitivity against peak shift. As shown, the NH4+-inserted BG electrode exhibits negligible peak shift from the pristine state. Close examination on the entire XRD patterns for NH4+ insertion reveals that the lattice parameter a merely increases from 10.22 to 10.24 Å at 50% state of charge (Figure S9a), which gives rise to a minor volume change of 0.6%. The BG framework is nearly a zero-strain material for NH4+ insertion, which is responsible for the extraordinary cycling performance. Similar zero-strain insertion has been

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reported on Li4Ti5O12 for Li+ insertion,33 KNi[Fe(CN)6] for Na+ insertion,34 and P2Na0.66[Li0.22Ti0.78]O2 for Na+ insertion,35 all of which exhibited superior cycling performance. On the contrary, the BG framework undergoes noticeable structural change upon Na+ and K+ insertion. As shown in Figure 4a and Figure S9b, there is a dramatic lattice expansion from 10.22 to 10.40 Å for the BG framework when fully sodiated. On the other hand, the K+ insertion may result in a phase distortion from the cubic lattice to rhombohedral or monoclinic,36,37 due to the relative intensity change between (200) and (220) peaks and the emergence of new peaks at 25~35°, as shown in Figure S9c. Additionally, the very flat discharge plateau of K+ insertion at ~0.4 V is also characteristic of a two-phase reaction. Even in a narrower potential range of 0.2~0.8 V, which only associates with the N-FeIII/FeII redox center, the (de)insertion of Na+ and K+ still leads to the inferior cycling stability compared to NH4+ (see Figure 4b and Figure S10). We postulate that the distinct lattice change and cycling performance of Na+, K+, and NH4+ insertion also correlates with the ion solvation conditions in the BG structure, i.e. being hydrated or naked cations.38 In light of previous electrochemical quartz crystal microbalance (EQCM) studies,39,40 it is suggested that Na+ insertion accompanies with partial water co-insertion, which gives rise to considerable lattice expansion when fully discharged. On the other hand, the bulky sizes of K+ and NH4+ not only result in a complete de-hydration, but also expels some lattice water out of the structure,39,40 which may explain the initial lattice expansion at early stage of K+ or NH4+ insertion but subsequent lattice shrinkage upon further insertion (Figure S9).

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Figure 5. About an aqueous double-ion Na+/NH4+ battery. a, A schematic of the working mechanism; b, CV curves of the PW cathode and NTP anode at a scan rate of 1 mV s-1; c, Rate performance, the capacity, and current density are based on the active mas of the cathode; d, Long-term cycling performance at a moderate current density of 0.5 A g-1.

To take the advantage of the superior cyclability and a higher working potential of NH4+ (de)insertion in the PBA’s framework, herein, we develop a double-ion battery with a Prussian white Na2Fe[Fe(CN)6] (Na-PW) cathode preferably operating on NH4+ insertion after its desodiation and a sodium titanium phosphate NaTi2(PO4)3 (NTP) as anode preferably with Na+ as the working charge carrier in a mixed (50%/50%) aqueous electrolyte of Na2SO4 and (NH4)2SO4. We name this cell as a double-ion battery to differentiate it from the dual-ion

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batteries that typically employ both anions and cations as charge carriers for cathode and anode, respectively.41 Figure 5a schematically illustrates the working mechanism of this double-ion battery. After the first charge, Na-PW is converted to a BG electrode, which will solely operate on the NH4+ topochemistry in the following (de)insertion due to its cation preference of NH4+ over Na+. On the other hand, the NTP lattice will exclusively store Na+ due to its small ionic channel that does not fit bulky NH4+. The associated reaction equations on the cathode side are shown as follows: Initial charge: Na2Fe[Fe(CN)6] (Na-PW) → Fe[Fe(CN)6] (BG) + 2Na+ + 2eSubsequent discharge/charge: Fe[Fe(CN)6] (BG) + NH4+ + e- ↔ (NH4)Fe[Fe(CN)6] The CV results confirm the cation selectivity in the PW and NTP framework, as shown in Figure 5b and Figure S11. The well-defined CV peaks indicate the feasible uptake/release of Na+ and NH4+ ions in NTP and PW framework, respectively, while their potential difference gives rise to a working voltage of ~1.1 V for the double-ion battery. Similar cation selectivity has been employed to fabricate the so-called hybrid batteries, such as Li+/Na+ and Na+/K+ hybrid batteries.42,43 The new configuration introduced here pushes this concept to a new level with ion combination of metal and non-metal. The properties of Na-PW and NTP are shown in Figure S12 and S13, respectively. The reversible capacity values of the Na-PW cathode and NTP anode in the hybrid electrolyte are determined to be ~80 and ~80 mAh g-1, respectively (Figure S14). Here we designed the full cell as cathode limited (excess anode). Figure S15 shows the GCD profiles of this double-ion battery, which delivers an average voltage of ~1.1 V and a maximum energy density of ~38 Wh

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kg-1 based on the active mass of both cathode and anode. Such an operation voltage and energy density are comparable to some aqueous Na-ion or K-ion batteries, such as Na0.44MnO2/NaTi2(PO4)3 (~1.1 V, ~33 Wh kg-1)44 and KCu[Fe(CN)6]/K2Mn[Mn(CN)6] (~1.0 V, ~30 Wh kg-1)45 etc. In addition, this double-ion battery also exhibits good rate capability and cycling performance. As shown in Figure 5c, the discharge capacity is 77, 65, 57, 50, and 40 mAh g-1 (calculated based on the mass of cathode) at current density of 0.1, 0.2, 0.5, 1, and 3 A g-1, respectively. When cycled at a moderate current rate of 0.5 A g-1, the discharge capacity fades from 57 to 30 mAh g-1 after 600 cycles, which corresponds to a capacity retention of 53% and an average Coulombic efficiency of 99.1%, as shown in Figure 5d. With further optimization on the anode or electrolyte concentration, better cycling performance can be expected. Overall, as a proof-of-concept, this Na+/NH4+ double-ion battery may point to a new direction for utilizing non-metal ions in the full cell device. In summary, we scrutinized the NH4+ topochemistry in the BG-based framework. BG exhibits a much higher structural compatibility with NH4+ than Na+ and K+ ions, which results in a higher insertion potential. We discovered the exceptional cycling stability of NH4+ (de)insertion in stark comparison to the fast fading behavior of Na+ and K+. Ex situ XRD studies reveal the zero-strain (de)insertion behavior of BG framework with respect to NH4+. To take the advantage of the unique NH4+ topochemistry, we further proposed a double-ion battery, where BG framework favors NH4+ and the NTP lattice prefers Na+ in the mixed electrolyte, which further pushes the boundaries of using non-metal cations in energy storage applications.

ASSOCIATED CONTENT

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Supporting Information. Experimental details on materials preparation, material characterizations (ICP-AES, XPS, TGA, FTIR), structural characterizations (SEM, ex situ XRD), and electrochemical performance (CV, GCD, Rate, self-discharge). Corresponding Authors: Xiulei Ji: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS X. J. acknowledges the financial support from the U.S. National Science Foundation, Award Number:1551693. F.D. thanks the Joint Foundation between Jilin Province and Jilin University (SXGJQY2017-10). Z. W. Thanks the China Scholarship Council (CSC, No. 201706170130) for providing a scholarship for Ph.D. study at Oregon State University.

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