Magnesium battery

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 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). 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; 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. 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