Mechanical Energy

Communication

pubs.acs.org/JACS

Magnesium Antimony Liquid Metal Battery for Stationary

Energy Storage

David J. Bradwell, Hojong Kim,* Aislinn H. C. Sirk, and Donald R. Sadoway*

Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,

Massachusetts 02139-4307, United States

* Supporting Information

S

ABSTRACT: Batteries are an attractive option for grid-

scale energy storage applications because of their small

footprint and flexible siting. A high-temperature (700 C)

magnesium antimony (MgSb) liquid metal battery

comprising a negative electrode of Mg, a molten salt

electrolyte (MgCl2 KCl NaCl), and a positive electrode

of Sb is proposed and characterized. Because of the

immiscibility of the contiguous salt and metal phases, they

stratify by density into three distinct layers. Cells were

cycled at rates ranging from 50 to 200 mA/cm2 and

demonstrated up to 69% DC DC energy efficiency. The

self-segregating nature of the battery components and the

use of low-cost materials results in a promising technology

for stationary energy storage applications.

L arge-scale energy storage is poised to play a critical role in

enhancing the stability, security, and reliability of

tomorrow s electrical power grid, including the support of

intermittent renewable resources.1 Batteries are appealing because Figure 1. Sectioned MgSb liquid metal battery operated at 700 C

showing the three stratified liquid phases upon cooling to room

of their small footprint and flexible siting; however, conventional

temperature. The cell was filled with epoxy prior to sectioning.

battery technologies are unable to meet the demanding low-cost

and long-lifespan requirements of this application.

where R is the gas constant, T is temperature in Kelvins, F is the

A high-temperature (700 C) magnesium antimony (MgSb)

Faraday constant, aMg(in Sb) is the activity of Mg dissolved in Sb,

liquid metal battery comprising a negative electrode of Mg, a

and aMg is the activity of pure Mg.

molten salt electrolyte (MgCl2 KCl NaCl), and a positive

Recent work on self-healing Li Ga electrodes for lithium ion

electrode of Sb is proposed (Figure 1). Because of density batteries has demonstrated the appeal of liquid components.2

differences and immiscibility, the salt and metal phases stratify into While solid electrodes are susceptible to mechanical failure by

mechanisms such as electrode particle cracking,3 these are

three distinct layers. During discharge, at the negative electrode Mg

is oxidized to Mg2+ (Mg Mg2+ + 2e ), which dissolves into the inoperative in liquid electrodes, potentially endowing cells with

unprecedented lifespans. The self-segregating nature of liquid

electrolyte while the electrons are released into the external circuit.

Simultaneously, at the positive electrode Mg2+ ions in the electro- electrodes and electrolytes could also facilitate inexpensive

lyte are reduced to Mg (Mg2+ + 2e MgSb), which is deposited manufacturing of a battery so constructed. However, there do

not appear to be economical materials options that exist as

into the Sb electrode to form a liquid metal alloy (Mg Sb) with

liquids at or near room temperature.

attendant electron consumption from the external circuit (Figure 2).

Previous work with elevated-temperature liquid batteries demon-

The reverse reactions occur when the battery is charged. Charging

strated impressive current density capabilities (>1000 mA/cm2

and discharging of the battery are accompanied by volumetric when discharged at 0 V) with a variety of chemistries.4 7 However,

changes in the liquid electrodes. The difference in the chemical that work generally used prohibitively expensive metalloids (such as

potentials of pure Mg ( Mg) and Mg dissolved in Sb [ Mg(in Sb)] Bi and Te) as the positive electrode. The resulting cells exhibited

self-discharge current densities of 40 mA/cm2, attributed to the

generates a voltage that can be expressed as

solubility of the negative electrode metal (i.e., Na) in the

RT aMg(in Sb)

Ecell = ln Received: October 17, 2011

2F aMg

Published: January 6, 2012

dx.doi.org/10.1021/ja209759s J. Am. Chem.Soc. 2012, 134, 1895 1897

1895

2012 American Chemical Society

Journal of the American Chemical Society Communication

Figure 2. Schematic of a MgSb liquid metal battery comprising three liquid layers that operates at 700 C. During charging, Mg is electrochemically

extracted from the Mg Sb alloy electrode and deposited as liquid Mg on the top (negative) electrode. During discharging, the Mg electrode is

consumed, and Mg is deposited into the Mg Sb liquid bottom (positive) electrode. During charging, the battery consumes energy; upon discharge,

the battery supplies energy.

electrolyte.5 These systems failed to achieve commercial Further electrochemical characterization was performed.

Stepped-potential experiments indicated low leakage current

success, possibly because of a lack of interest in grid-scale

densities of

storage at that time or the use of high-cost metalloids.

systems. This was attributed to the complexation of Mg2+ by

Sb is less costly ($7/kg average commodity price over the

ligand donors from the supporting electrolyte (NaCl, KCl)13

past 5 years) and more earth-abundant than Bi ($24/kg) and

Te ($150/kg).8 When costs are compared on a per-mole basis and the attendant suppression of metal solubility in its halide

salts.14

(which is more relevant when considering the cost per unit of

Cells cycled at 50 mA/cm2 for a predefined discharge period

energy storage capacity), Sb ($0.74/mol) appears even more

of 10 h to a cutoff charging voltage limit of 0.85 V achieved a

appealing than Bi ($4.40/mol) and Te ($19.19/mol). Interestingly,

round-trip Coulombic efficiency of 97% and a voltage efficiency

the use of Sb had not, until now, been demonstrated in a liquid

of 71%, resulting in an overall energy efficiency of 69% (Figure 3a).

metal battery.

Mg was selected as the negative electrode material on the

basis of its low cost ($5.15/kg, $0.125/mol), high earth

abundance, low electronegativity, and overlapping liquid range

with both Sb and candidate electrolytes. The electrolyte was

MgCl2:NaCl:KCl (50:30:20 mol %), which was selected on the

basis of its sufficiently low melting point (396 C9) and the

greater electrochemical stability of NaCl and KCl in

comparison with MgCl2.10

MgSb single cell batteries were assembled in the fully

charged state in an Ar-filled glovebox, placed inside a sealed test

vessel, and heated in a vertical tube furnace to 700 C. When

the cell was heated above the melting point of the molten salt,

cell open-circuit voltages were found to stabilize at 0.44 V,

consistent with thermodynamic data.11

The cells were electrochemically characterized by cyclic

voltammetry (CV) and electrochemical impedance spectrosco-

py (EIS) using a two-electrode electrochemical setup with the

negative electrode (Mg) as the counter electrode/reference

electrode and the positive electrode (Sb) as the working

electrode. The cells exhibited negligible charge-transfer over-

potentials, as demonstrated by the linearity of the current voltage

relationship in the CV scans and the absence of an obvious

semicircle in the EIS scans. The slope of the CV was consistent

with the area-normalized solution resistance as measured through Figure 3. Electrochemical performance of a MgSb liquid metal

EIS (typically 1.1 cm2), further demonstrating IR voltage loss to battery operated at 700 C. (a) Variation of the cell voltage with the

be the dominant overpotential. state of charge over one cycle. The current was set at 50 mA/cm2. (b)

There were, however, indications of mass-transport limi- Deep discharge results at different current rates. The theoretical cell

EMF was calculated from data in the literature.11

tations under certain conditions. The cells exhibited increased

cell impedance at lower EIS scan frequencies, suggesting that at

At full discharge, the composition of the positive (bottom) liquid

long time periods the reaction rates might be limited by

diffusion.12 Mass-transport limitations could arise from local electrode was estimated to be 12 mol % Mg and 88 mol % Sb.

depletion of Mg2+ ions in the electrolyte at either of the Cells were fully discharged at various rates ranging from 50

electrode electrolyte interfaces or Mg mass-transport limi- to 200 mA/cm2 with 0.05 V as the discharge cutoff limit

tations in the Mg Sb electrode at the Mg Sb electrode (Figure 3b). Operation at higher current density resulted in

electrolyte interface. increased IR voltage loss and decreased capacity, consistent

dx.doi.org/10.1021/ja209759s J. Am. Chem.Soc. 2012, 134, 1895 1897

1896

Journal of the American Chemical Society Communication

with the measured solution resistance and observed mass- Future work will include long-term corrosion testing of solid-state

components, current collector optimization, and investigation of

transport limitations. The operating efficiency could be

alternative sheath materials. While the initial cell performance

improved by reducing the thickness of the electrolyte or

results are promising, exploration of other metal metalloid

operating at lower current density. The cell performance could

couples with still greater cell voltages and lower operating

be optimized by changes in the current collector design and in

temperatures is warranted. If a low-cost, high-voltage system

the electrolyte composition to increase the cell conductivity.

with sufficiently low levels of corrosion were discovered, it would

Cells were cycled more than 30 times for periods of up to 2

find utility in a wide array of stationary storage applications.

weeks and did not exhibit obvious signs of corrosion of the

solid-state cell components (current collectors and walls), as

ASSOCIATED CONTENT

determined through optical imaging and scanning electron

* Supporting Information

S

microscopy (SEM)/energy-dispersive spectroscopy (EDS)

Experimental procedures, cell design details, heating profile,

analysis. Analysis of the positive electrodes of cells that were

materials selection, and additional electrochemical results. This

cooled in a discharged state revealed the presence of Mg

material is available free of charge via the Internet at http://

platelets, consistent with the formation of Mg3Sb2 (Figure 4).

pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

abqq6g@r.postjobfree.com; abqq6g@r.postjobfree.com

Present Address

Department of Law, University of Victoria, Victoria, BC,

Canada.

ACKNOWLEDGMENTS

Financial support from the Deshpande Center for Technological

Innovation at MIT, the Chesonis Family Foundation at MIT, the

Advanced Research Projects Agency-Energy (U.S. Department of

Energy), and Total, S.A. is gratefully acknowledged.

REFERENCES

(1) Advanced Electricity Storage Technologies Programme. Energy

Storage Technologies: A Review Paper; Australian Greenhouse Office,

Department of the Environment and Heritage: Commonwealth of

Figure 4. (a) Cross section of a MgSb cell. (b) SEM image of a

Australia, 2005.

positive electrode in the discharged state. (c) Mg EDS map showing

(2) Deshpande, R. D.; Li, J.; Cheng, Y. T.; Verbrugge, M. W.

high Mg concentrations in sectioned platelets.

J. Electrochem. Soc. 2011, 158, A845.

(3) Christensen, J.; Newman, J. J. Solid State Electrochem. 2006, 10,

The Mg Sb phase diagram indicates that a two-phase micro-

293.

structure is to be expected as a result of phase separation as the (4) Shimotake, H.; Rogers, G. L.; Cairns, E. J. Ind. Eng. Chem. Process

electrode is cooled from a Mg Sb single-phase liquid regime Des. Dev. 1969, 8, 51.

(5) Cairns, E. J.; Shimotake, H. Prepr. Pap. Am. Chem. Soc., Div.

into a two-phase Sb and Mg3Sb2 regime.

Stationary storage applications require devices to operate Fuel Chem. 1967, 11 (3), 321.

(6) Shimotake, H.; Cairns, E. J. In Proceedings of the Intersociety Energy

reliably for many years. In the present study, corrosion was not

Conversion Engineering Conference; American Society of Mechanical

an issue. However, after several weeks of cycling, the cells

Engineers: New York, 1967.

ceased to operate. The observed cause of failure was

(7) Cairns, E. J.; Shimotake, H. Science 1969, 164, 1347.

evaporation of the molten salt electrolyte into the surrounding (8) Mineral Commodity Summaries 2011; U.S. Geological Survey:

containment vessel, a mechanism that could be mitigated by Reston, VA, 2011.

alternative cell designs with reduced head space. (9) Janecke, E. Z. Anorg. Allg. Chem. 1950, 261, 218.

In summary, an all-liquid battery with Mg and Sb liquid (10) Plambeck, J. A.; Bard., A. J. Encyclopedia of Electrochemistry of the

Elements; Marcel Dekker: New York, 1976; Vol. X, pp 127 148.

metal electrodes has been proposed and its performance

(11) Eckert, C.; Irwin, R.; Smith, J. Metall. Mater. Trans. B 1983, 14,

capability demonstrated. The use of Sb as the positive electrode

451.

and the self-segregating nature of the liquid components may

(12) Bard, A.; Faulkner, L. Electrochemical Methods: Fundamentals and

enable a low-cost energy storage solution. Cells were cycled

Applications, 2nd ed.; Wiley: New York, 2001.

under constant-current conditions, demonstrating high current (13) Brooker, M.; Huang, C. Can. J. Chem. 1980, 58, 168.

density capabilities and negligible corrosion of the solid-state (14) Mulcahy, M. F. R.; Heymann, E. J. Phys. Chem. 1943, 47, 485.

cell components over the testing period. (15) Haupin, W.; Frank, W. Comprehensive Treatise of Electro-

chemistry; Plenum Press: New York, 1981; Vol. 2, pp 301 325.

Further work is required for evaluation of the long-term

performance of the proposed cells, which may require an

alternative cell design. At some larger scale, the action of

electric current flowing through the electrolyte could generate

enough Joule heat to keep the components molten, thereby

obviating the need for external heaters, as is the case with

electrolytic cells producing aluminum on a commercial scale.15

dx.doi.org/10.1021/ja209759s J. Am. Chem.Soc. 2012, 134, 1895 1897

1897

Contact this candidate