A high-voltage concept with sodium-ion conducting β-alumina for magnesium-sodium dual-ion batteries

ABSTRACT Rechargeable magnesium-sodium dual-ion batteries that use dendrite-free magnesium metal as an anode, magnesium-sodium dual-ion electrolyte and sodium-ion cathode are appealing as

safe, low-cost systems for large-scale stationary electricity storage. Although substantial advances have been made on magnesium-sodium dual-ion batteries with various sodium-ion cathodes,

further development of such batteries is inherently limited by the low electrochemical oxidative stability of known dual-ion electrolytes (2–2.6 V vs. Mg2+/Mg). Here, we examine a

magnesium-sodium dual-ion concept, which allows for higher operating voltages of magnesium-sodium dual-ion batteries by using oxidatively stable sodium-ion electrolytes along with a

sodium-ion conducting β-alumina membrane on the cathode side. The proposed concept is assessed using a high-voltage Na1.5VPO4.8F0.7 cathode demonstrating a high average discharge voltage of

3.0 V, a cathodic capacity of 110 mAh g−1 and an energy efficiency of 90%. SIMILAR CONTENT BEING VIEWED BY OTHERS HIGH-CAPACITY, FAST-CHARGING AND LONG-LIFE MAGNESIUM/BLACK PHOSPHOROUS

COMPOSITE NEGATIVE ELECTRODE FOR NON-AQUEOUS MAGNESIUM BATTERY Article Open access 07 October 2024 LOW-SOLVATION ELECTROLYTES FOR HIGH-VOLTAGE SODIUM-ION BATTERIES Article 16 June 2022

ENHANCED RATE CAPABILITIES IN A GLASS-CERAMIC-DERIVED SODIUM ALL-SOLID-STATE BATTERY Article Open access 11 June 2020 INTRODUCTION The urgent need to shift from fossil fuels to renewable

energy sources — and thus CO2 emission-free energy production — is considerably impeded by the lack of efficient, low-cost solutions for large-scale storage of electricity. Whereas

lithium-ion batteries (LIBs) dominate the market of portable electronics due to their high energy density, whether they are an economically viable option for large-scale stationary storage

due to the limited and uneven global distribution of known natural reserves of lithium and the high cost of manufacturing is questionable1,2. In this regard, magnesium-ion batteries (MIBs)

might evolve as an inexpensive alternative because of the much higher natural abundance of magnesium (2.3 wt% in the Earth’s crust, ~1100 times higher than that of Li)3 and hence

significantly lower cost (4 USD kg−1, 15 times lower in comparison with that of Li for pure metals). Notably, both the gravimetric charge storage capacity of metallic magnesium (3833 mAh 

cm−3, 2205 mAh g−1) and its standard electrode potential (−2.35 V vs. SHE) compare favorably with those of lithium (2062 mAh cm−3, 3861 mAh g−1, and −3.04 V vs. SHE, respectively) and sodium

(1128 mAh cm−3, 1166 mAh g−1, and -2.7 V vs. SHE, respectively)4,5,6,7,8,9. Importantly, in contrast to lithium10,11 and sodium12, metallic magnesium can be safely employed as an anode in

MIBs due to dendrite-free electrodeposition4,5,13,14,15,16. The development of practical MIBs is, however, hampered by several factors. First, contrary to the facile intercalation of

monovalent Li+ and Na+ ions, the intercalation of Mg2+ ions, which are as small as Li+ ions but have a higher charge density, is found to be sluggish due to strong coulombic interactions

with negatively charged anionic lattices in cathodic materials17. The second problem relates to the desolvation of Mg2+ ions at the electrolyte–cathode interface, where the strong

coordination of Mg2+ ions with solvent hampers their insertion into a cathode18,19,20,21,22. Consequently, most of the reported magnesium cathode materials are characterized by poor rate

capability and cycling performance and high overpotentials between charge and discharge, reducing the energy efficiency4,6,23,24,25. Chevrel-phase Mo6S8 developed by Aurbach et al.14 in the

early 2000s remains the benchmark cathode material with an average discharge voltage of ~1.1 V (vs. Mg2+/Mg) and practical room temperature capacities of 80–100 mAh g−1 (the theoretical

capacity is 128 mAh g−1). To circumvent the issues associated with Mg-ion cathode materials, novel Mg/Na or Mg/Li dual-ion (hybrid) battery concepts have been recently

proposed26,27,28,29,30,31,32,33. Mg/Na(Li) dual-ion batteries consist of a dendrite-free magnesium metal as an anode, a Na-ion32,34,35,36 or Li-ion27,28,30,33,37,38,39,40,41,42,43,44,45

cathode and Mg/Na or Li/Mg dual-ion electrolytes that support reversible plating of Mg on the anode and provide a suitable voltage range for the concomitant insertion/removal of Na+(Li+)

ions at the cathode. While recent research efforts on Mg/Na dual-ion batteries have been mainly focused on testing various Na-ion cathodes, the further development of such batteries is

inherently limited by the low oxidative stability of dual-ion electrolytes. Specifically, typical Mg/Na dual-ion electrolytes are based on highly reductive Grignard reagents or borohydrides,

needed for Mg-electroplating, which thus have a limited operational voltage range of 2–2.6 V. This choice of electrolytes imposes severe restrictions on the selection of cathode materials.

We note that Mg electrolytes composed of more oxidatively stable magnesium salts (e.g., perchlorates) and solvents (e.g., carbonates) do not support reversible electroplating of Mg due to

the formation of an ionically insulating layer of MgO46. To produce Mg/Na dual-ion batteries with higher working voltages, here we show an alternative cell employing oxidatively stable

Na-ion electrolytes and Na-ion conductive membranes (_β_-alumina) on the cathode side of the battery (Fig. 1). This approach is tested with a high-voltage Na1.5VPO4.8F0.7 cathode and 1 M

NaClO4 in propylene carbonate (PC) as a Na-ion electrolyte. The resulting Mg/Na1.5VPO4.8F0.7 cells exhibited a high average discharge voltage of 3.0 V, cathodic capacity of 110 mAh g−1, and

energy efficiency of 90%. RESULTS SYNTHESIS AND ELECTROCHEMICAL PERFORMANCE OF NA1.5VPO4.8F0.7 CATHODE Na1.5VPO4.8F0.7 was synthesized according to the reported procedure of Park et al.47 by

a solid-state reaction of VPO4, VOPO4, NaF, and Na2CO3. Prior to electrode preparation, Na1.5VPO4.8F0.7 powder was mixed with carbon black (20 wt%) by ball-milling and annealed at _T_ = 450

 °C for 12 h to enhance its electrical conductivity and crystallinity (see Supplementary Fig. 2)48. As illustrated in Fig. 2a, the X-ray diffraction (XRD) pattern of the resulting material

points to a pure Na1.5VPO4.8F0.7 phase with particle sizes <5 μm (see inset, Fig. 2a). For the electrochemical measurements, electrodes were prepared by mixing a powder of

Na1.5VPO4.8F0.7/carbon with poly(vinylidene fluoride) binder in N-methyl-pyrrolidone as a solvent, and the resulting slurries were cast onto tungsten foil as a current collector (for details

see Methods section). Prior to carrying out Mg/Na1.5VPO4.8F0.7 dual-ion battery tests, the electrochemical performance of the Na1.5VPO4.8F0.7 cathode was individually assessed using

elemental sodium as both the counter and reference electrode and 1 M NaBF4 in a mixture of ethylene carbonate (EC) and PC (1:1 by vol.) as the electrolyte. As shown in Fig. 2b, the

Na1.5VPO4.8F0.7 cathode exhibited a high capacity of 112 mAh g−1, close to the theoretical value of 129.7 mAh g−1. Two reversible redox processes—V3.8+/V4.3+ and V4.3+/V4.8+—occur at ca.

3.55 V and 4.0 V vs. Na+/Na, respectively47. We note that low initial coloumbic efficiency of Na1.5VPO4.8F0.7 cathode (Fig. 2c, Supplementary Fig. 4) could be related to the formation of the

solid electrolyte interface formed on sodium metallic anode during first charge. ELECTROCHEMICAL PERFORMANCE OF MG/NA1.5VPO4.8F0.7 DUAL-ION BATTERIES Next, we assembled Mg/Na1.5VPO4.8F0.7

dual-ion cells using a Na1.5VPO4.8F0.7 cathode and magnesium metal as the anode. On the anode and cathode side of the battery, Mg/Na dual-ion (2 M NaBH4 + 0.2 M Mg(BH4)2 in tetraglyme) and

Na-ion (1 M NaClO4 in PC) electrolytes were used, respectively. The _β_-alumina membrane was placed between the two electrolytes, providing charge transfer between the anode and cathode of

the battery through Na-ion transport49,50,51,52. We note that the Mg/Na dual-ion borohydride electrolyte was chosen based on our previous study of Mg/Na dual-ion batteries with FeS2

nanocrystals as a cathode material32. One important advantage of borohydride electrolytes compared to frequently used Grignard-based electrolytes is the absence of Cl− ions, which would

otherwise significantly limit the concentration of Na+ ions in solution due to the limited solubility of NaCl in the used solvents. The electrochemical properties of the Mg/Na dual-ion

battery are summarized in Fig. 3. Similar to Na-ion half-cell tests, Na1.5VPO4.8F0.7 measured in the Mg/Na1.5VPO4.8F0.7 configuration delivers a stable discharge capacity of 98 mAh g−1 on

average with a coulombic efficiency of ~99.0% at a rate of 1C in the potential range of 2.4–3.8 V vs. Mg2+/Mg (see Supplementary Fig. 5 for details). Figure 3c shows the rate capability of

Na1.5VPO4.8F0.7 at rates of 0.2C, 1C, and 2C, yielding capacities of 110–80 mAh g−1. The galvanostatic charge/discharge curves for Na1.5VPO4.8F0.7 in the Mg/Na dual-ion cell match those

observed for the Na-ion half-cell tests (see Figs. 2b and 3a), indicating that the same electrochemical cathodic processes occur. It should be noted that the potentials shifted by ~0.7 V,

which is ~0.35 V higher than the value expected based on the difference in the standard potentials of Na+/Na and Mg2+/Mg. This mismatch and slight hysteresis between charged and discharged

galvanostatic curves are most likely caused by the overpotential for Mg plating/stripping on the anode, as previously reported in the literature32. The resulting average discharge voltage of

Mg/Na1.5VPO4.8F0.7 with the _β_-alumina membrane is 3.0 V at a rate of 1C. To the best of our knowledge, this is the highest voltage demonstrated thus far for Mg/Na dual-ion batteries (see

Supplementary Table 2 for a detailed comparison). From the non-rocking-chair operation principle of the dual-ion concept, the large mass/volume of the electrolyte, which is a reservoir of

all the ions needed for battery operation, must be factored into energy density calculations. Specifically, in the case of Mg/Na dual-ion batteries utilizing Mg/Na dual-ion electrolytes, the

anodic capacity associated with reversible magnesium electrodeposition/stripping depends on the concentration of Mg2+ ions in the electrolyte. Thus, the solubility of the respective Mg salt

is a limiting factor. Considering the whole mass of the liquid electrolyte, the theoretical charge storage capacity of the Mg/Na dual-ion battery at the cell level can be determined from

the standard relationship \(C_{{\mathrm{total}}} = \frac{{C_{\mathrm{A}}C_{\mathrm{C}}}}{{C_{\mathrm{A}} + C_{\mathrm{C}}}}\)53, as follows (see Supplementary Note 1 for details):

$${\mathrm{Gravimetric}}\,C_{{\mathrm{cell}}} = \frac{{Fx(M_{\mathrm{d}} - M_{\mathrm{c}})C_{\mathrm{c}}}}{{Fx(M_{\mathrm{d}} - M_{\mathrm{c}}) + C_{\mathrm{c}}\rho \cdot

10^3}}({\mathrm{Ah}}\,{\mathrm{kg}}^{ - 1})$$ (1) where _F_ = 26.8 × 103 mAh mol−1 (Faraday constant), _x_ is the charge of the electroactive species, (_M_d) and (_M_c) are molarities (mol 

L−1) of Mg(BH4)2 in electrolyte in discharged and charged state of battery, respectively, _C_c is the specific gravimetric capacity of the cathode in mAh g−1, and _ρ_ is the density of the

electrolyte in g ml−1. To estimate the energy density, the _C_cell value must be multiplied by the average battery voltage, \(E = C_{{\mathrm{cell}}}\cdot V\). According to Eq. (1), with an

average discharge voltage of 3.0 V, the theoretical gravimetric energy density of the Mg/Na1.5VPO4.8F0.7 dual-ion battery can be estimated at 57 Wh kg−1 with an overall energy efficiency of

90%. DISCUSSION In summary, we have demonstrated a proof-of-concept for high-voltage Mg/Na1.5VPO4.8F0.7 dual-ion batteries utilizing oxidatively stable Na-ion electrolytes separated from

Mg/Na dual-ion electrolytes by Na-ion conducting β-alumina membranes. The Mg/Na1.5VPO4.8F0.7 battery delivers a stable cathodic capacity of 98 mAh g−1 at a current rate of 129.8 mA g−1 (1 C)

with an average discharge voltage of 3.0 V. Its energy density of 57 Wh kg−1 is comparable with that of state-of-the-art lead-acid and vanadium redox-flow batteries, illustrating great

potential in large-scale energy storage applications. To further maximize its energy density, we suggest that future work in this field should focus on finding Mg/Na dual-ion electrolytes

with higher Mg2+ molarity (solubility). METHODS CHEMICALS AND BATTERY COMPONENTS Carbon black (CB, Super C65, TIMCAL), poly(vinylidene fluoride) (PVdF, Sigma-Aldrich), N-methyl-2-pyrrolidone

(NMP, 99%, Sigma-Aldrich), NaBF4 (>98%, Sigma-Aldrich), NaClO4 (98%, Alfa Aesar), NaBH4 (98%, ABCR), Mg(BH4)2 (95%, Sigma-Aldrich), propylene carbonate (PC, BASF, battery grade),

ethylene carbonate (EC, Novolyte, battery grade), tetraethylene glycol dimethyl ether (tetraglyme, ≥98%, Sigma-Aldrich, dried), sodium (Sigma-Aldrich), magnesium (99.95%, GalliumSource),

glass microfiber separator (GF/D, Whatman), and β-alumina membrane (Ionotec). ELECTRODE FABRICATION AND ELECTROCHEMICAL MEASUREMENTS Prior to electrode preparation, Na1.5VPO4.8F0.7 was

carbon coated by ball-milling with CB (20 wt%) following annealing at 450 °C for 12 h48. Electrodes were prepared by mixing the Na1.5VPO4.8F0.7/CB powder with pVdF and NMP using a Fritsch

pulverisette 7 classic planetary mill (500 rpm, 1 h). The resulting electrode composition was 72 wt% Na1.5VPO4.8F0.7, 18 wt% CB and 10 wt% pVdF. Slurries were coated onto tungsten current

collectors and dried at 80 °C for 12 h under vacuum. All electrochemical measurements were conducted on homemade, reusable, airtight two-electrode cells assembled in an Ar-filled glove box

(O2 < 0.1 ppm, H2O < 0.1 ppm). Elemental sodium and magnesium were employed as both reference and counter electrodes in Na-ion half-cells and Mg/Na dual-ion cells, respectively. For

Na-ion half-cell tests, 1 M NaBF4 in EC:PC (1:1 by vol.) served as the electrolyte. For Mg/Na dual-ion batteries, 2 M NaBH4 + 0.2 M Mg(BH4)2 in tetraglyme and 1 M NaClO4 in PC were used as

Mg/Na dual-ion and Na-ion electrolytes on the anode and cathode side of the battery, respectively. Absence of dendritic Mg plating/stripping on the anode upon cycling in 2 M NaBH4 + 0.2 M

Mg(BH4)2 in tetraglyme had been confirmed by SEM measurements (see Supplementary Fig. 6). We note that despite of relatively small difference between Na+/Na (~−2.7 V vs. SHE) and Mg2+/Mg (~

−2.4 V vs. SHE) redox potentials, only Mg is plated in 2 M NaBH4 + 0.2 M Mg(BH4)2 in tetraglyme (see Supplementary Fig. 7 for EDX data of plated electrode). A piece of glass fiber served as

a separator for the Na-ion half-cell tests, whereas an ion-selective β-alumina membrane (thickness: 0.5 mm; weight: 5 g) was used for the Mg/Na dual-ion batteries. Galvanostatic cycling

tests were carried out at room temperature on an MPG2 multichannel workstation (BioLogic). Capacities were normalized by the mass of Na1.5VPO4.8F0.7. MATERIALS CHARACTERIZATION Powder X-ray

diffraction was conducted by STOE STADI P powder X-ray diffractometer (Cu-Kα1 irradiation, _λ_ = 1.540598 Å). Scanning electron microscopy imaging was carried out using a NanoSEM 230

instrument. DATA AVAILABILITY The data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request.

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research is part of the activities of SCCER HaE, which is financially supported by the Innosuisse—Swiss Innovation Agency. We thank Dr. Frank Krumeich for SEM/EDX measurements. The authors

are grateful to the research facilities of ETH Zürich (ETH Electron Microscopy Center, Department of Chemistry and Applied Biosciences) and Empa (Empa Electron Microscopy Center and

Laboratory for Mechanics of Materials & Nanostructures) for access to instruments and for technical assistance. AUTHOR INFORMATION Author notes * These authors contributed equally:

Kostiantyn V. Kravchyk, Marc Walter. AUTHORS AND AFFILIATIONS * Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093, Zürich, Switzerland

Kostiantyn V. Kravchyk, Marc Walter & Maksym V. Kovalenko * Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, CH-8600,

Dübendorf, Switzerland Kostiantyn V. Kravchyk, Marc Walter & Maksym V. Kovalenko Authors * Kostiantyn V. Kravchyk View author publications You can also search for this author inPubMed 

Google Scholar * Marc Walter View author publications You can also search for this author inPubMed Google Scholar * Maksym V. Kovalenko View author publications You can also search for this

author inPubMed Google Scholar CONTRIBUTIONS K.V.K., M.W., and M.V.K. designed the experimental work. K.V.K., and M.W. conducted all experiments reported in the paper. K.V.K., M.W., and

M.V.K. wrote the paper. CORRESPONDING AUTHOR Correspondence to Maksym V. Kovalenko. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION

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ARTICLE Kravchyk, K.V., Walter, M. & Kovalenko, M.V. A high-voltage concept with sodium-ion conducting β-alumina for magnesium-sodium dual-ion batteries. _Commun Chem_ 2, 84 (2019).

https://doi.org/10.1038/s42004-019-0186-4 Download citation * Received: 18 April 2019 * Accepted: 21 June 2019 * Published: 19 July 2019 * DOI: https://doi.org/10.1038/s42004-019-0186-4

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