Magnesium battery

Magnesium batteries are batteries with magnesium as the active element at the anode of an electrochemical cell. Both non-rechargeable primary cell and rechargeable secondary cell chemistries have been investigated. Magnesium primary cell batteries have been commercialised and have found use as reserve and general use batteries.

Magnesium secondary cell batteries are an active topic of research, specifically as a replacement for or improvement on lithium ion based battery chemistries - as a Li-ion replacement magnesium cells may be possible with a solid magnesium anode, allowing a higher energy density than that with lithium, which requires an intercalated lithium anode. Insertion type anodes ('magnesium ion') have also been researched.

Primary cells

Primary magnesium cells have been developed since the early 20th century. A number of chemistries for reserve battery types have been researched, with cathode materials including silver chloride, copper(I) chloride, palladium(II) chloride, copper(I) iodide, copper(I) thiocyanate, manganese dioxide and air (oxygen).^[1] For example a water activated silver chloride/magnesium reserve battery became commercially available by 1943.^[2]

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.^[3][4]

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 generally 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.^[5]

Secondary cells

Magnesium is under research as a basis for the replacement or improvement on lithium ion batterys: In comparison to lithium as an anode material magnesium has a (theoretical) energy density per unit mass under half that of lithium ( 18.8 MJ/kg vs. 42.3 MJ/kg ), but a volumetric energy density around 50% higher (32.731 GJ/m3 vs. 22.569 GJ/m3 ).^{[note 1][note 2]} In comparison to metallic lithium anodes, magnesium anodes have not exhibited dendrite formation during charging,^[6] which may allow magnesium metal to be used without an intercalation compound at the anode;^{[note 3]} Magnesium based batteries may have a cost advantage over lithium due to the abundance of magnesium on earth.^[6]

As of 2015 the barriers to producing a commercially useful magnesium battery were the lack of demonstrated practical electrolytes and cathode materials for magnesium ions.^[6]

A key drawback to using a metallic magnesium anode is the tendency to form a passivating (non conducting) layer when recharging, blocking further charging (in contrast to lithium's behaviour);^[8] The passivating layers were thought to originate from decomposition of the electrolyte during magnesium ion reduction. Common counter ions such as perchlorate and tetrafluoroborate were found to contribute to passivation, as were some common polar aprotic solvents such as carbonates and nitriles.^[9] Grignard based ethereal electrolytes have been shown not to passivate;^[10] Magnesium organoborates also showed electroplating without passivation. The compound Mg(BPh₂Bu₂)₂ was used in the first demonstrated rechargeable magnesium battery, its usefulness was limited by electrochemical oxidation.^[11] Other electrolytes researched include mixed grignard/aluminium trichloride compounds, borohydrides, phenolates, alkoxides, mixed magnesium chloride/aluminium chloride systems in THF, amido based complexes (eg based on hexamethyldisilazane), carborane salts, a Mg(BH₄)(NH₂) solid state electrolyte, and gel polymers containing Mg(AlCl₂EtBu)₂ in tetraglyme/PVDF.^[12] For solvent based systems ethers have been generally used in research (2014).^[13]

'Magnesium insertion electrodes', based on reversible insertion of magnesium metal into metal alloy anode (such as Bismuth/Antinomy or Tin) have been shown to be able to prevent anode surface passivation, but suffered from anode destruction due to volumetric changes on insertion, as well as slow kinetics of insertion.^[14]

Another basic drawback compared to lithium is magnesium's higher charge (+2) in solution, which tends to result in increased viscosity and reduced mobility in the electrolyte.^[15] In solution a number of species may exist depending on counter ions/complexing agents - these often include singly charged species (eg MgCl⁺ in the presence of chloride) - though dimers are often formed (eg Mg₂Cl₃⁺ ).^[16] The movement of the magnesium ion into cathode host lattices is also (as of 2014) problematically slow.^[17]

For cathode materials a number of different compounds have been researched for suitability, including those used in magnesium primary batteries. New cathode materials investigated or proposed include zirconium disulfide, cobalt(II,III) oxide, tungsten selenide, vanadium pentoxide and vanadate based cathodes. Cobalt based spinels showed inferior kinetics to insertion compared to their behaviour with lithium.^[1] In 2000 the chevrel phase form of Mo₆S₈ was shown to have good suitability as a cathode, 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 and 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]

As of 2014 secondary magnesium battery research has not progressed as far as producing a commercialisable battery, with specific challenges being the electrolytes and cathode materials.^[22]

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