Magnesium batteries are batteries that utilize magnesium cations as the active charge transporting agent in solution and as the elemental 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 possible replacement or improvement over lithium-ion–based battery chemistries in certain applications. A significant advantage of magnesium cells is their use of a solid magnesium anode, allowing a higher energy density cell design than that made with lithium, which in many instances requires an intercalated lithium anode. Insertion type anodes ('magnesium ion') have also been researched, primarily as heavy main group metal thin films or as Zintl phases, for instance Mg2Sn.
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). For example, a water activated silver chloride/magnesium reserve battery became commercially available by 1943.
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.
A magnesium–air fuel cell has theoretical operating voltages of 3.1 V 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.
Magnesium is under research as a possible replacement or improvement on lithium-ion battery in certain applications: 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 do not exhibit dendrite formation at low current densities, which may allow magnesium metal to be used without an intercalation compound at the anode;[note 3] the ability to use a magnesium anode without an intercalation layer raises the theoretical maximum relative volumetric energy density to around 5 times that of a lithium ion cell. Additionally, modeling and cell analysis have indicated that magnesium based batteries may have a cost advantage over lithium due to the abundance of magnesium on earth and the relative scarcity of lithium deposits.
Potential use of a Mg based battery had been recognised as early as the 1990s based on a V2O5, TiS2, or Ti2S4 cathode materials and magnesium metal anodes. However observation of instabilities in the discharge state and uncertainties on the role of water in the electrolyte limited progress was reported.  The first successful rechargeable cell was reported in 2000, based on Chevrel-type Mo6S8 cathode with a magnesium organohaloaluminate / THF based electrolyte. 
As of 2018 secondary magnesium battery research had not produced a commercialisable battery, with specific challenges being the electrolytes and cathode materials. As of 2015 the barriers to producing a commercially useful magnesium battery were the lack of demonstrated practical electrolytes and high energy density cathode cathode materials for magnesium ions.
Anodes and electrolytes
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); 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. [[Victor
Early attempts to develop magnesium batteries explored the use of 'magnesium insertion electrodes', based on reversible insertion of magnesium metal into metal alloy anode (such as Bismuth/Antinomy or Tin). These 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.
Grignard|Grignard]] based ethereal electrolytes have been shown not to passivate; 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 (i.e. a low anodic limit of the voltage window). Other electrolytes researched include borohydrides, phenolates, alkoxides, amido based complexes (e.g. based on hexamethyldisilazane), carborane salts, fluorinated alkoxyborates, a Mg(BH4)(NH2) solid state electrolyte, and gel polymers containing Mg(AlCl2EtBu)2 in tetraglyme/PVDF.
The current wave of interest in magnesium-metal batteries started in 2000, when an Israeli group reported reversible magnesium plating from mixed solutions of magnesium chloride and aluminium chloride in ethers, such as THF. Template:Https://www.nature.com/articles/35037553 The primary advantage of this electrolyte is a significantly larger positive limit of the voltage window (and, thus, a higher battery voltage) than of the previously reported Mg plating electrolytes. Since then, several other Mg salts, less corrosive than chloride, have been reported.
One 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. 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. Mg2Cl3+ ). The movement of the magnesium ion into cathode host lattices is also (as of 2014) problematically slow.
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. 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. A hybrid magnesium cell using a mixed magnesium/sodium electrolyte with sodium insertion into a nanocrystalline iron(II) disulfide cathode was reported in 2015.
Manganese dioxide based cathodes have shown good properties, but deteriorated on cycling. Modified manganese-based spinels ("post spinels") are an active topic of research (2014) for magnesium-ion insertion cathodes.
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. MgMnSiO4 has also been investigated as a potential Mg2+ insertion cathode.
Cathodic materials other than non-inorganic metal oxide/sulfide types have also been investigated : in 2015 a cathode based on a polymer incorporating anthraquinone was reported; and other organic, and organo-polymer cathode materials capable of undergoing redox reactions have also been investigated, such as poly-2,2'-dithiodianiline. In 2016 a porous carbon/iodine combination cathode was reported as a potential alternative to Mg2+ insertion cathodes - the chemistry was reported as being potentially suitable for a rechargeable flow battery.
In Oct 2016, Honda and Saitec (Saitama Industrial Technology Center) claimed to have a commercialisable Mg battery, based on a xerogel cathode of vanadium pentoxide/sulfur. A commercialisation date of 2018 was also claimed.
- Li: Standard electrode potential −3.04; cationic charge +1; Faraday constant 96485.33289 C/mol; Energy per mole 293315.411986 J/mol; Atomic mass 6.94 g/mol; Energy density (mass) 42264.4685858 J/g; density 0.534 g/cm3; energy density (volumetric) 22569.2262248 J/cm3
- Mg: Standard electrode potential −2.372; cationic charge +2; Faraday constant 96485.33289 C/mol; Energy per mole 457726.41923 J/mol; Atomic mass 24.305 g/mol; Energy density (mass) 18832.6031364 J/g; density 1.738 g/cm3; energy density (volumetric) 32731.0642511 J/cm3
- The requirement to intercalate the 'metallic' lithium greatly reduces the energy density of a lithium-ion battery compared to a metallic lithium battery i.e. 372 mAh/g vs. 3862 mAh/g (or 837 mAh/cm3 vs. 2061 mAh/cm3) for lithium/graphite (as LiC6) vs. Li metal.
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