Cathode Dependence of Liquid-Alloy Na–K Anodes - ACS Publications

Feb 11, 2018 - INTRODUCTION. Alkali-metal rechargeable batteries have a metallic Li, Na, or K anode. A solid alkali-metal anode forms dendrites during...
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Cathode Dependence of Liquid-Alloy Na-K Anodes Leigang Xue, Hongcai Gao, Yutao Li, and John B. Goodenough J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12267 • Publication Date (Web): 11 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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Journal of the American Chemical Society

Cathode Dependence of Liquid-Alloy Na-K Anodes Leigang Xue,† Hongcai Gao,† Yutao Li, and John B. Goodenough* Materials Science and Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA ABSTRACT: Alkali ions can be plated dendrite-free into a liquid alkali-metal anode. Commercialized Na-S battery technology operates above 300°C. A low-cost Na-K alloy is liquid at 25°C from 9.2 to 58.2 wt% of sodium; sodium and/or potassium can be plated dendrite-free in the liquid range at room temperature. The co-existence of two alkali metals in an anode raises a question: whether the liquid Na-K alloy acts as a Na or a K anode. Here we show the alkali-metal that is stripped from the liquid Na-K anode is dependent on the preference of the cathode host. It acts as the anode of a sodium rechargeable cell if the cathode host structure selectively accepts only Na+ ions; as the anode of a potassium rechargeable cell if the cathode accepts K+ ions in preference to Na+ ions. This dual-anode behavior means the liquid Na-K alkali-alloy can be applied as a dendrite-free anode in Na-metal batteries as well as K-metal batteries.

resolves the dendrite problem while providing fast charge/discharge at room temperature.

INTRODUCTION Alkali-metal rechargeable batteries have a metallic Li, Na or K anode. A solid alkali-metal anode forms dendrite during charge.1-3 The dendrites can grow across the electrolyte to the cathode to create an internal shortcircuit with incendiary consequences. This problem has limited commercial lithium-metal batteries to disposable primary cells that only allow discharge; they are found in watches, cameras and implantable medical devices.

As two neighbors in Group one of the periodic table, Na and K have similar physical and chemical properties, which raises the question: Does the Na-K alkali-alloy act as a sodium or a potassium anode? We show the answer depends on the cathode. A previous study of a cell having a liquid Na-K anode, a standard liquid-carbonate Na+ electrolyte, and a Na2MnFe(CN)6 cathode showed the cathode progressively transforms to K2MnFe(CN)6 with cycling, indicating the Na-K alloy was working as a potassium anode.15 It met the normal expectation because potassium is more electropositive than sodium (K+/K = 2.93 V, Na+/Na = -2.71 V). However, here we report that the liquid Na-K alloy acts as a sodium anode when it works with cathodes that only accept Na+ ions, such as the layered Na2/3Ni1/3Mn2/3O2 and NASICON-structured Na3V2(PO4)3. It acts as a potassium anode when the cathode host prefers K+ over Na+ guest ions, such as Na2MnFe(CN)6, MnFe(CN)6 and K2MnFe(CN)6. The determinant of whether the liquid Na-K is a Na or a K anode is the energy gained by insertion of K+ versus Na+; K+ is preferred over Na+ if K+ version is more stable, but Na+ if the K+ can not be inserted.

So far, rechargeable alkali-metal batteries can only be commercially realized by using a molten alkali-metal anode at high temperature in a Na-S battery; no dendrites form on a liquid anode.4-7 A solid electrolyte separates the two liquid electrodes of a Na-S cell. Although the melting point of sodium is only 97.8 °C, the liquid-sodium batteries operate over 300°C due to the low conductivity of the solid electrolytes. The high operating temperature gives rise to problems of thermal management, corrosion, safety, and also imposes stringent requirements on the rest of the battery components.6, 8 Alkali metals are not liquid at room temperature, but a Na-K alloy is liquid from 9.2 to 58.2 wt% of sodium at 25°C;9-11 both Na and K are earth-abundant and inexpensive. The liquid range is much wider than that of other room-temperature liquid alloys containing alkali metals (see the different phase diagrams in Figure S 1);12-15 the alloy provides a high specific capacity of 1366 mAh g−1 as a liquid sodium anode at 25°C (see the calculation in Supporting Information). Solid electrolytes that suffer from a low ionic conductivity are not necessary because the Na-K liquid alloy is immiscible with the standard liquidcarbonate electrolyte. Therefore, the liquid Na-K alloy

EXPERIMENTAL SECTION Liquid Na-K anode preparation. The Na-K liquid alloy was prepared by mixing 33.7 wt% sodium and 66.3 wt% potassium and shaking the container as previously reported.15 The liquid Na-K alloy was absorbed into a carbon paper at 420°C and it remains an immobilized liquid in the paper down to room temperature. 1 g carbon could absorb about 5 g Na-K liquid. Carbon paper (GDL

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20BA, areal weight = 85 g m-2) was bought from Ion Power. The carbon paper containing the Na-K liquid alloy was cut into disks for coin-cell tests. The liquid Na-K anode is excessive in all cells to make sure both Na and K are enough for the cathode hosts.

Electrolyte preparations. The composition of the Na+ electrolyte is 1 mol L-1 NaClO4 in propylene carbonate (PC) containing 10 wt% fluoroethylene carbonate (FEC). The composition of the K+ electrolyte was saturated KClO4 in PC containing 10 wt% FEC.

Cathode preparations. The cathode powders, 17 Na3V2(PO4)3,18 including Na2/3Ni1/3Mn2/3O2,16, 19 20 Na2MnFe(CN)6, and K2MnFe(CN)6 , were prepared as reported previously. MnFe(CN)6 was prepared by removal of Na+ from Na2MnFe(CN)6 by charging; then it was assembled into new cells with fresh electrolyte and anode. The Na2/3Ni1/3Mn2/3O2 cathode contained 70% of cathode powder, 20% of carbon black, and 10% of CMC binder. Distilled water was used as solvent and the resultant slurry was thoroughly mixed and coated onto carbon-coated Al foil; the coating was dried under vacuum at 100 °C overnight. The Na3V2(PO4)3 cathodes consisted of 70% of cathode powder, 20% of carbon black, and 10% of PVDF binder. NMP was used as solvent for the slurry coating onto carbon-coated Al foil and the coating was dried under vacuum at 100 °C for 8 hours. The Na2MnFe(CN)6 and K2MnFe(CN)6 cathode electrodes consisted of 60% cathode powder, 30% carbon black, and 10% CMC binder. Distilled water was used as solvent to create a slurry that was thoroughly mixed and coated onto carbon-coated Al foil; the coating was dried under air overnight.

Battery assembly and tests. CR2032 coin cells (Figure 1a) composed of a liquid Na-K anode, a liquid-carbonate electrolyte containing a NaClO4 or KClO4 salt, a glass-fiber separator, and different kinds of cathodes were assembled in an Ar-filled glovebox. The cells were galvanostatically cycled at room temperature with a LAND battery testing system. EDS and XRD. After cycling (from the discharged state), the cells were disassembled in an argon-filled glovebox. The cycled cathode was rinsed thoroughly with DEC for EDS (Hitachi S5500 SEM/STEM equipped with an Oxford EDS system) and XRD (Rigaku MiniFlex 600 II) analysis. The cycled electrolyte salt in the glass fiber separator was extracted with DEC solvent and then transferred to a grid of a scanning transmission electron microscope (STEM). After the solvent was evaporated, the electrolyte salt stuck to the holey carbon film of the STEM grid and was ready for elemental analysis. The initial electrolyte before cycling was directly dropped onto a STEM grid for EDS analysis as the control sample.

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Figure 1. Configuration and performance of a cell composed of a liquid Na-K anode, a liquid-carbonate Na electrolyte, and a Na2/3Ni1/3Mn2/3O2 cathode. (a) schematic of the cell structure. (b) Charge/discharge curves at different cycles; (c) differential

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Journal of the American Chemical Society capacity vs. voltage; (d) coulombic efficiency; and (e) cycling stability. The specific capacities were calculated based on the mass of cathode material.

RESULTS AND DISCUSSION A cell composed of a liquid Na-K anode, a standard liquid-carbonate electrolyte of 1 mol L-1 NaClO4 in PC containing 10 wt% FEC, and a layered-structured Na2/3Ni1/3Mn2/3O2 cathode was assembled and tested. By taking advantage of the immiscibility between the liquid Na-K alloy and the organic liquid-carbonate electrolyte,15 a coin cell can be assembled with a traditional “sandwich” structure as shown in Figure 1a. The liquid alloy is immobilized in a carbon paper and the immiscible organic electrolyte is immobilized in a glass-fiber separator. Figure 1b shows charge/discharge curves of the cell over 500 cycles. Only a slight decrease of storage efficiency was observed due to an increase of polarization. Between 2.4 and 3.9V, four pairs of peaks in dQ/dV were observed; their central positions were at 3.15, 3.31, 3.60, and 3.66 V, respectively (Figure 1c), which are almost identical to those obtained with sodium half-cells.16 The cell exhibited a high

average coulombic efficiency of 99.64% (Figure 1d) with little capacity fade over 500 cycles at 0.5 C (Figure 1e)， both the coulombic efficiency and stability are highly improved compared with the previous reports.17, 21, 22 It also exhibited an excellent rate performance (Figure S2). After cycling, the cell was disassembled and the cycled cathode was rinsed with diethyl carbonate (DEC) and then analyzed by powder X-ray diffraction (XRD) and by energy dispersive X-ray spectroscopy (EDS). The XRD spectra of Figure 2a indicate the cycled cathode retains the P2-type layered structure and there is little structural change of the cathode particles after cycling. The EDS profile of the cathode after cycling shows no evidence of K+ insertion or K+ at the particle surface (Figure 2b). Both the XRD and EDS reveal K+ did not intercalate into the desodiated Na1/3Ni1/3Mn2/3O2 host lattice.

Figure 2. Cycled Na2/3Ni1/3Mn2/3O2 cathode and electrolyte. (a) XRD and (b) EDS patterns of the layered-structured Na2/3Ni1/3Mn2/3O2 cathode before and after the cycling with a liquid Na-K anode and a NaClO4 electrolyte. (c) STEM image of cycled electrolyte salt crystallized on a STEM grid, and (d) its EDS spectroscopy. (e, f, g) show the elemental mapping of Na, Cl, O from the selected area in (c).

The cycled liquid electrolyte was also analyzed by EDS. In order to avoid interference from the complex

components of the glass-fiber separator, which contained both Na and K (Figure S3), the cycled electrolyte salt in the

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salt after cycling is still NaClO4 with no evidence of K+ ions; the elemental distribution of Na, Cl, O shown in Figure 2e, 2f, 2g is consistent with the selected area in Figure 2c. EDS peaks of C, Cu, Al were observed in all the electrolytes before and after cycling (compare Figure 3d and Figure S5b), they come from the STEM grid; F was only observed after cycling, indicating some side reactions such as SEI formation happens with the electrolyte additive FEC.

glass fiber separator was extracted with DEC solvent and then transferred to a grid of a scanning transmission electron microscope (STEM) for elemental analysis (see the procedure illustration in Figure S4). The initial electrolyte before cycling was directly dropped onto a STEM grid for EDS analysis as the control sample. After the solvent was evaporated, the electrolyte salt crystallized and stuck to the holey carbon film of the STEM grid, as shown in Figure 2c. Compared with the initial electrolyte before cycling (Figure S5), the EDS profile of Figure 2d shows the

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Figure 3. A liquid Na-K alkali-alloy anode acts as a sodium anode when working with cathodes that only allow Na insertion. (a) + Layered-structured Na2/3Ni1/3Mn2/3O2 cathode; (b) NASICON-structured Na3V2(PO4)3 cathode. Only Na insertion into the cathode host suppresses the K dissolution but promotes the Na dissolution from the liquid Na-K anode. The lattice structures of 23, 24 Na2/3Ni1/3Mn2/3O2 and Na3V2(PO4)3 are illustrated based on their crystallographic parameters in previous reports.

These data show that the cell with the layered Na2/3Ni1/3Mn2/3O2 cathode acted like a simple sodium cell with a good cycle life; there was no evidence of K+ extraction from the liquid Na-K alloy anode even though potassium is more electropositive than sodium; the standard potential difference between Na+/Na and K+/K has been reported as 0.3 V in PC.25 If K+ had been preferentially removed from the liquid Na-K anode on discharge, but cannot intercalate into the cathode, analysis of the electrolyte after cycling should have shown evidence of K+ in the electrolyte. The charge/discharge process is

illustrated schematically in Figure 3a. During charge, only Na+ can be removed from the Na2/3Ni1/3Mn2/3O2 cathode to the electrolyte and Na+ from the electrolyte is introduced into the liquid Na-K anode. On discharge, since only Na+ can be intercalated into the layered Na1/3Ni1/3Mn2/3O2 cathode host, K+ dissolution from the anode is suppressed and Na+ dissolution from the liquid Na-K anode is promoted; only Na+ is removed from the anode to the electrolyte and Na+ returned to the cathode. The liquid NaK acts as a pure sodium anode when working with the

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Journal of the American Chemical Society layered Na2/3Ni1/3Mn2/3O2 cathode that accepts only Na+ ions.

cycling as the cell with the Na2/3Ni1/3Mn2/3O2 cathode (Figure S6c and S6d), a capacity retention of 85% in 100 cycles were observed. The performance difference should be attributed to the cathodes themselves, Na3V2(PO4)3 should be further optimized to get better performance. The XRD (Figure S7a) and EDS data (Figure S7b) of the cycled Na3V2(PO4)3 indicated K+ was not inserted into its framework as expected. As in the cell with the layered Na2/3Ni1/3Mn2/3O2 cathode, only EDS peaks of NaClO4 and no peak of K were observed in the salt (Figure S8). The liquid Na-K acts as a sodium anode when working with the NASICON-structured Na3V2(PO4)3 cathode, which also accepts only Na+ ions, as is illustrated in Figure 3b.

In order to check the generality of this result, we tested a cell with another cathode Na3V2(PO4)3 that does not allow K+ insertion because of its small interstitial bottleneck of atomic separation of 2.50 Å (see its framework structure in Figure 3b). In this cell, also, the liquid Na-K anode acted like a simple Na-metal anode (Figure S6a) and the central position of the peaks in dQ/dV were located at 3.38 V (Figure S6b), which is identical to that obtained with sodium half-cells.18 However, the asprepared Na3V2(PO4)3 cathode did not show as excellent a

Figure 4. All cells with a MnFe(CN)6-based cathode and a Na-K anode transform into potassium batteries regardless of the initial + cathodes and electrolytes. (a) For a (-) Na-K｜Na ｜Na2MnFe(CN)6 (+) cell, it behaves as a sodium battery at the beginning, but progressively transforms into a potassium battery after decades of cycles. The charge/discharge voltage increase with cycling is a + sign of this transformation. (b) For a (-) Na-K｜K ｜Na2MnFe(CN)6 (+) cell, it transforms into a potassium battery just after the + first charge. (c) For a (-) Na-K｜K ｜MnFe(CN)6 (+) cell, it is a potassium battery at the beginning.

alloy also acts as a potassium anode with this cathode.20 These data indicate that no matter whether the initial cathode is Na2MnFe(CN)6, MnFe(CN)6 or K2MnFe(CN)6 and the initial electrolyte contains Na+ or K+, the cells with the Na-K anode finally transform to potassium cells.

On the other hand, Na2MnFe(CN)6 is transformed into K2MnFe(CN)6 after a few cycles with Na-K alloy as the anode and NaClO4 as the initial electrolyte salt; the Na+ electrolyte is transformed to a K+ electrolyte on cycling; once the cathode is transformed to K2MnFe(CN)6, it remains unchanged after cycling.15 This transformation can be monitored by the voltage increase with cycling, as shown in the charge/discharge curves of Figure 4a and dQ/dV in Figure S9. The two pairs of peaks at low and high potentials can be assigned to low-spin FeIII/FeII and highspin MnIII/MnII redox couples, respectively. With charge, a phase transformation from a monoclinic phase to a cubic one over the first peak, and from cubic to tetragonal over the second peak.20, 26, 27 When KClO4 was used as the initial electrolyte salt, the transformation of Na2MnFe(CN)6 to K2MnFe(CN)6 was accelerated. Figure 4b shows that the cell with a Na2MnFe(CN)6 cathode and a KClO4 liquid electrolyte acts as a potassium cell after the first charge, it also shows a good cycling stability (Figure S10). MnFe(CN)6 is not chemically prepared because of the instability of Mn3+ in aqueous solution, but it can be formed by charging Na2MnFe(CN)6; with a K+ electrolyte, MnFe(CN)6 behaves as a potassium cell from the beginning (Figure 4c). K2MnFe(CN)6 has been prepared chemically and the Na-K

MnFe(CN)6 has a large octahedral interstitial space interconnected by open faces bordered by four C≡N anions (see the Na and K versions of MnFe(CN)6 structure in Figure 5a and 5b); the narrowest C≡N interatomic distance is 4.77 Å in Na2MnFe(CN)6, which has been experimentally proven big enough for both Na+ and K+ reversible insertion/extraction.15, 20, 27, 28 The recent observation that only K2MnFe(CN)6 is precipitated from a mixture of Na+, K+, and Fe(CN)64- in aqueous solution on the addition of Mn2+ shows that K2MnFe(CN)6 is more stable than Na2MnFe(CN)6.20 Their lattice structures show K+ tends to form stronger bonds with C≡N than Na+ (see the interatomic distance comparison in Figure 5). It reveals the higher energy gain from inserting K+ into the MnFe(CN)6 host structure compared to inserting Na+. That’s why the K2MnFe(CN)6 has a higher energy density and discharge voltage than that of Na2MnFe(CN)6.15, 20, 27 These experimental observations are supported by a first principles study of FeFe(CN)6, that concluded the insertion

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of cations with larger ionic radius happens at higher voltages, and thus provides higher energy density29.

into the cathode host, transforming Na2MnFe(CN)6 into K2MnFe(CN)6. If K2MnFe(CN)6 is used as the initial cathode, K+ is always the working ion, as shown in Figure 5b. The liquid Na-K acts as a pure potassium anode when working with the MnFe(CN)6 host that prefer K+ ions. If the cathode host cannot accept K+ ions, the sodiated cathode is the more stable discharged cathode and the Na-K alloy acts as a Na anode. Where both Na+ and K+ can be inserted into the cathode, whether the liquid Na-K alloy acts as a Na or a K anode depends on the anode-cathode reaction that gives the highest reactive energy.

The charge/discharge processes of MnFe(CN)6-based cathodes are illustrated schematically in Figure 5. During charge, only Na+ can be removed from the Na2MnFe(CN)6 cathode to the electrolyte and Na+ from the electrolyte is introduced into the liquid Na-K anode (Figure 5a). On discharge, since the MnFe(CN)6 host prefers K+, K+ dissolution from the liquid Na-K anode is promoted while the Na+ dissolution from the anode is suppressed; K+ is removed from the anode to the electrolyte and inserted

Figure 5. A liquid Na-K alkali-alloy anode acts as a potassium anode when working with a MnFe(CN)6–based cathode host. (a) + With an initial Na2MnFe(CN)6 cathode, K stripped and is inserted into the cathode, transforming Na2MnFe(CN)6 into K2MnFe(CN)6 cathode, the cell reaction transforms from (a) to (b). The major cause of this transformation is the thermodynamic + priority of the K2MnFe(CN)6. (b) With an initial K2MnFe(CN)6 cathode and initial K electrolyte, it is always a potassium battery.

CONCLUSIONS

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Journal of the American Chemical Society (1)

The Na-K alloy molten at room temperature is immiscible with the flammable liquid-carbonate electrolyte of the alkali-ion batteries. However, the alloy can be immobilized in a porous membrane that it wets. Therefore, it provides a way to plate dendrite-free sodium or potassium with the alloy at room-temperature from a flammable liquid electrolyte to provide a safe, fast charge/discharge Nametal or K-metal anode for a rechargeable battery. Whether the alloy acts as a pure sodium or pure potassium anode depends on the relative stability of a discharged cathode into which K+ versus Na+ ions are inserted on discharge.

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SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Phase diagrams of room-temperature liquid alloys containing alkali metals；Capacity calculation； Rate capability；EDS spectroscopy of the glass fiber separator ； The procedure illustration used to extract the cycled electrolyte salt for element analysis ； EDS of the initial NaClO4 salt before cycling；Performance of a cell composed of a liquid Na-K + anode, a standard liquid-carbonate Na electrolyte, and a NASICON-structured Na3V2(PO4)3 cathode ； Cycled Na3V2(PO4)3 cathode；Cycled electrolyte salt in the cell with a NASICON-structured Na3V2(PO4)3 cathode； Evolution of + dQ/dV with cycle number for a (-) Na-K ｜ Na ｜ Na2MnFe(CN)6 (+) cell；Performance of a cell composed of a + liquid Na-K anode, a standard liquid-carbonate K electrolyte, and a Na2MnFe(CN)6 cathode. (PDF)

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AUTHOR INFORMATION

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Corresponding Author *[email protected]

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Author Contributions †

These authors contributed equally. (21)

NOTES The authors declare no competing financial interests.

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ACKNOWLEDGEMENTS (24)

Cell design and characterization were supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) award number DE-EE0007762. Cathode material preparation was supported by NSF CBET-1438007. We thank J. Wozniak of the Texas Advanced Computing Center for the art design on Figure 1a, 5 and TOC.

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