Challenges of a Rechargeable Magnesium Battery

The energy density of rechargeable batteries has improved more than six times over the past 150 years. Currently commercial versions of lithium-ion boast an impressive 265 Wh/kg with possible improvements to 315 Wh/kg. Due to its high energy density, this technology has started to recapture automotive markets worldwide. However, post-lithium-ion batteries promise energy densities of more than 500 Wh/kg which will be high enough to power aviation. A stringent requirement of a post-lithium-ion battery is the replacement of currently commercial graphite anodes with energy-dense metal anodes. Unfortunately, the safety of metallic lithium is hindered by dendritic growth so smooth plating magnesium metal anodes have been proposed instead. Magnesium also has double the volumetric energy density of lithium and a far cheaper cost due to higher abundance.

A rechargeable magnesium battery is a post-lithium-ion battery which promises the safe use of a high-energy magnesium metal anode. While electrolytes aimed at magnesium electroplating have been reported for 100 years, a great challenge has been the development of noncorrosive, high-voltage electrolytes which are compatible with magnesium metal and are aimed for battery use. The last 30 years have resulted in rapid progress of electrolytes for battery applications. Such modern electrolytes are compatible with magnesium metal and possess high conductivities (>10 mS/cm), high voltage stabilities (>4 V vs. Mg/Mg²⁺), and a noncorrosive character and can be used to develop high-energy cathodes for a rechargeable magnesium battery. The current challenge of the magnesium electrolyte field is the development of simple, transferable synthetic routes which can be reproduced by nonspecialists.

High-voltage, oxide-based insertion cathodes are commercial favorites for lithium-ion batteries. However, due to the double charge density, the magnesium cation (Mg²⁺) tends to bond covalently with the oxygen in the electrode structure which precludes its smooth and reversible intercalation. With the exception of lithium titanate, reversible magnesium intercalation has only been reported for sulfide-based electrodes which offer a low energy density due to low voltage and capacity. However, higher reversibility and energy density have been reported with conversion cathodes such as selenium and iodine. The current challenge for the development of a suitable cathode for a rechargeable magnesium battery with a metal anode is improving the rates of battery charge/discharge as well as increasing the cycle life. The use of modern magnesium electrolytes will accelerate this effort.

The ultimate post-lithium-ion battery will contain a metallic anode. However, since the use of lithium metal anode has been precluded due to the dendritic nature of its electrodeposition, there has been a rush for alternative, non-dendritic metal anodes. Magnesium has been shown to electroplate smoothly, without dendrites, and champions the race toward a post-lithium-ion battery with a safe metallic anode. Progress in magnesium electrolytes has been marked by the successful transition from low-voltage stability electrolytes used for electroforming toward high-voltage, noncorrosive electrolytes used in rechargeable magnesium batteries. Modern magnesium electrolytes have high oxidative stabilities in excess of 4 V vs. Mg/Mg²⁺, are not corrosive, have high conductivities of 5 ̶ 10 mS/cm, and are compatible with metallic magnesium anodes. The current challenge in this field is the ease of synthesis and transferability of electrolytes and their synthetic routes for wide access by the community. The last piece of the rechargeable magnesium puzzle is a cheap, stable, high-energy cathode. While great progress has been in done in recent years, it is still difficult to pair a cathode with a voltage >1.5 V vs. Mg/Mg²⁺ with a magnesium metal anode. In addition, while reported rates of operation are rapidly improving, they are still lower than those of lithium cathode counterparts. So far, high-capacity conversion cathodes such as selenium, sulfur, or iodine outcompete intercalation cathodes which are plagued by low voltage and low capacity. Important progress in the area of noncorrosive high-voltage electrolytes and high-energy cathodes with high cycle life suggests that the future of a rechargeable magnesium battery looks brighter than ever. Research focus on easy synthetic routes for high-voltage electrolytes will ensure that a commercially viable cathode will be soon discovered. Such a breakthrough will undoubtedly improve the quality of life for all mankind.