Magnesium battery

Magnesium batteries are batteries with magnesium ion as the working on of an electrochemical cell. Both non-rechargeable primary cell and rechargeable secondary cell chemistries have been investigated. Magnesium primary cell batteries have been commercialized and have found use as reserve and general use batteries.

Rechargeable magnesium batteries are an active research topic, specifically as an alternative to lithium ion based battery chemistries. Since magnesium ion is doubly charged and still has a low molecular weight, a solid magnesium metal anode provides a higher volumetric energy density than lithium metal (3833 mAh cm⁻³ vs. 2036 mAh cm⁻³ for Li metal,^[1]), although its specific energy is under half that of lithium (18.8 MJ/kg vs. 42.3 MJ/kg).^{[note 1][note 2]} In comparison to metallic lithium anodes, magnesium anodes have not exhibited dendrite formation during charging,^[2] which may allow magnesium metal to be used without an intercalation compound at the anode;^{[note 3]} eliminating the intercalation layer raises the theoretical maximum relative volumetric energy density to around 5 times that of a lithium ion cell.^[4] Magnesium batteries may have an added cost advantage over lithium due to the higher abundance of magnesium on earth.^[2]

Potential uses of Mg-based batteries were recognised as early as the 1990s and the first prototype of a magnesium-ion rechargeable cell was reported in 2000, based on a Chevrel-type Mo₆S₈ cathode with a magnesium organohaloaluminate/THF electrolyte.^[5] However, such battery operated at relatively low voltage (approx. 1.2V) and provided low energy density (110mAh/g), unable to compete with lithium-ion battery systems.

Primary cells

Primary magnesium cells have been developed since the early 20th century. Cathode materials include silver chloride, copper(I) chloride, palladium(II) chloride, copper(I) iodide, copper(I) thiocyanate, manganese dioxide and air (oxygen).^[6] For example, a water-activated silver chloride/magnesium reserve battery became commercially available by 1943.^[7]

The magnesium dry battery type BA-4386 was fully commercialised, with costs per unit approaching that of zinc batteries - in comparison to equivalent zinc-carbon cells the batteries had greater capacity by volume, and longer shelf life. The BA-4386 was widely used by the US military from 1968 until c.1984 when it was replaced by a lithium thionyl chloride battery.^[8][9]

A magnesium-air fuel cell has theoretical operating voltages of 3.1V and energy densities of 6.8 kWh/kg. General Electric produced a magnesium air fuel cell operating in neutral NaCl solution as early as the 1960s. The magnesium air battery is a primary cell, but has the potential to be 'refuelable' by replacement of the anode and electrolyte. Magnesium air batteries have been commercialised and find use as land based backup systems as well as undersea power sources, using seawater as the electrolyte.^[10]

Secondary cells

As of 2015 the barriers to producing a commercially useful magnesium battery were the lack of practical electrolytes and cathode materials for magnesium ions.^[2]

Electrolyte

A key challenge to magnesium electrolyte development is the formation of a passivating (non-ion-conducting) layer on the magnesium metal anode surface when conventional electrolyte solutions (based on anions such as PF6⁻, ClO4⁻, BF4⁻, and solvents such as carbonates or nitriles). The passivating layer is thought to originate from decomposition of the electrolyte due to incompatibility with magnesium metal.^[11] The formation of this layer blocks further battery cycling (unlike lithium).^[12]

Another basic drawback compared to lithium ion is magnesium's higher charge (+2), which typically yields increased solution viscosity and reduce ion mobility in the electrolyte.^[13] In solution a number of species may exist depending on counter ions/complexing agents - these often include singly charged species (e.g. MgCl⁺ in the presence of chloride) - though dimers are often formed (e.g. Mg₂Cl+
3 ).^[14]

Grignard based ethereal electrolytes have been shown not to passivate magnesium metal surface.^[15] For solvent based systems, ethers have been generally used in research. Magnesium organoborates, such as Mg(BPh₂Bu₂)₂, also showed electroplating without passivation. Electrolyte solution have been also obtained by addition of Lewis acids, especially aluminium trichloride, to magnesium compounds such as Grignard reagents, amides, phenolates, alkoxides, thiolates or magnesium chloride, usually in THF solvent. Other magnesium metal compatible salts include anions such borohydrides, carborane salts. Compound Mg(BH₄)(NH₂) was reported as a solid state electrolyte. Gel polymers containing Mg(AlCl₂EtBu)₂ in tetraglyme/PVDF have also been reported.

In 2015 an electrolyte based on monocarborane, CB₁₁H₁₂⁻, displayed > 99% coulombic efficiency, high anodic stability (3.8 V vs. Mg), was non-corrosive, and produced the first halogen-free, simple-type Mg salt that is compatible with Mg metal and is oxidatively stable.^[1][16][17]

Anodes

The main advantage of magnesium battery chemistry is the usage of magnesium metal anode due to its high capacity. However, insertion anodes have also been investigated. Reversible insertion of magnesium metal into metal alloy anodes (such as Bismuth/Antinomy or Tin) have been shown to be able to prevent anode surface passivation, but suffered from anode destruction due to high volumetric changes on insertion, as well as slow kinetics.

Cathode

The main challenge of magnesium cathode development resides on the slow Mg ion solid state diffusion, yielding slow movement of the magnesium ion into cathode host lattices.

Potential cathodes include those used in magnesium primary batteries. New cathode materials investigated include zirconium disulfide, cobalt(II,III) oxide, tungsten selenide, vanadium pentoxide and vanadate compounds. Cobalt-based spinels showed inferior kinetics to insertion compared to their behaviour with lithium.^[6] In 2000 the chevrel phase form of Mo₆S₈ displayed good suitability, enduring 2000 cycles at 100% discharge with a 15% loss; drawbacks were poor low temperature performance (reduced Mg mobility, compensated by substituting Selenium), as well as a low voltage, c. 1.2V, and low energy density (110mAh/g). A molybdenum disulfide cathode showed improved voltage and energy density, 1.8V and 170mAh/g. Transition metal sulfies are considered promising candidates for magnesium ion battery cathodes.^[18] A hybrid magnesium cell using a mixed magnesium/sodium electrolyte with sodium insertion into a nanocrystalline iron(II) disulfide cathode was reported in 2015.^[19]

Manganese dioxide based cathodes have shown good properties, but deteriorated on cycling.^[20] Modified manganese based spinels ("post spinels") are an active topic of research (2014) for magnesium ion insertion cathodes.^[21] In 2014 a rechargeable magnesium battery was reported utilising an ion exchanged, olivine type MgFeSiO₄ cathode with a bis(trifluoromethylsulfonyl)imide/triglyme electrolyte. The cell showed a capacity of 300mAh/g with a voltage of 2.4V,^[22] but these results proved to be difficult to reproduce by other researchers. MgMnSiO₄ has also been investigated as a potential Mg²⁺ insertion cathode.^[23]

Other than intercalation cathodes mentioned above, conversion cathodes like sulfur and oxygen are also under investigation due to their promise of high energy density. A rechargeable magnesium/sulfur battery is able to provide an energy density of 1722 Wh/kg in theory.

As of 2014 secondary magnesium battery research had not progressed as far as producing a commercialisable battery, with specific challenges being the electrolytes and cathode materials.^[24]

Notes

References

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