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 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 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 chloride, palladium chloride, copper iodide, copper thiocyanate, manganese dioxide and air. 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.
Secondary cells
Overview
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 energy density per unit mass under half that of lithium, but a volumetric energy density around 50% higher. In comparison to metallic lithium anodes, magnesium anodes do not exhibit dendrite formation, 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 materials for magnesium ions.
Research
Anodes and electrolytes
A key drawback to using a metallic magnesium anode is the tendency to form a passivating layer when recharging, blocking further charging ; 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.Early attempts to develop magnesium batteries explored the use of 'magnesium insertion electrodes', based on reversible insertion of magnesium metal into metal alloy anode. 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.
Examples of insertion anode types researched include Sn, Mg2Sn.
Grignard based ethereal electrolytes have been shown not to passivate; Magnesium organoborates also showed electroplating without passivation. The compound Mg2 was used in the first demonstrated rechargeable magnesium battery, its usefulness was limited by electrochemical oxidation. Other electrolytes researched include borohydrides, phenolates, alkoxides, amido based complexes, carborane salts, fluorinated alkoxyborates, a Mg solid state electrolyte, and gel polymers containing Mg2 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. The primary advantage of this electrolyte is a significantly larger positive limit of the voltage window 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 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 – though dimers are often formed. The movement of the magnesium ion into cathode host lattices is also problematically slow.
In 2018 a chloride free electrolyte together with a quinone based polymer cathode demonstrated promising performance, with up to per kg energy density, up to 3.4 kW/kg power density, and up to 87% retention at 2,500 cycles. The absence of chloride in the electrolyte was claimed to improve ion kinetics, and so reduce the amount of electrolyte used, increasing performance density figures.
A promising approach could be the combination of a Mg anode with a sulfur/carbon cathode. Therefore, a non-nucleophilic electrolyte is necessary which does not convert the sulfur into sulfide just by its reducing properties. Such electrolytes have been developed on the basis of chlorine containing and chlorine-free complex salts. The electrolyte in is a Mg salt containing Mg cation and two boron-hexafluoroisoproplylate groups as anions. This system is easy to synthesize, it shows an ionic conductivity similar to that of Li ion cells, its electrochemical stability window is up to 4.5 V, it is stable in air and versatile towards different solvents.
Cathode materials
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 oxide, tungsten diselenide, 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, as well as a low voltage, c. 1.2V, and low energy density. 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 disulfide cathode was reported in 2015.Manganese dioxide based cathodes have shown good properties, but deteriorated on cycling. Modified manganese-based spinels are an active topic of research for magnesium-ion insertion cathodes.
In 2014 a rechargeable magnesium battery was reported utilising an ion exchanged, olivine type MgFeSiO4 cathode with a bisimide/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. Quinone based cathodes also formed the cathode a high energy density magnesium battery reported by researchers in 2019.
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.