Exploring Reliable Organic Cathode Materials for High-Performance Mg-Ion Batteries
Authors: Pan Baofei , Vaughey John T. , Zhang Zhengcheng , Burrell Anthony K. , Liao Chen 
Published: February 2020

The lack of efficient energy storage devices is still the major bottleneck for the practical exploit of sustainable and renewable energy sources such as solar, wind or tide. The energy and safety barriers make the state-of-the-art Li-ion batteries not desirable to meet the requirements for large and safe energy storage. As the alternative, multivalent metal-base rechargeable batteries such as Mg, Ca, Zn and Al are more promising as larger and safer energy storage devices due to their advantages in safety, cost and capacity.1-3 For instance, in contrast to lithium, magnesium metal is non-toxic, more air resistant and not suffering from the dangerous dendrite formation. The naturally much more abundance makes magnesium more than 20 times cheaper than lithium in cost. In addition, the divalent characteristic of magnesium ion (Mg/Mg2+) enables magnesium to have nearly doubled volumetric capacity than lithium (3832 mA cm-3 vs 2062 mA cm-3). All these merits have brought significant attention on magnesium as the new generation rechargeable energy storage device.

Since the pioneer work by Aurbach group in 2000 with the design of the prototype Mg-Mo6S8 battery using the famous dichloro-complex electrolyte (nBu2Mg + EtAlCl2 in THF),4,5 a lot of efforts have been devoted to the development of high-performance magnesium electrolytes, including the discovery of a few examples with above 3 V (vs Mg/Mg2+) oxidative stabilities.6-10 In contrast to the rapid progress of efficient electrolytes capable of plating/stripping magnesium with wide electrochemical windows, the development of high-performance cathode material is far behind for magnesium storage. Chevral phase Mo6S8 is still the only reliable cathode for Mg-ion batteries to date. However, the relatively low working voltage (< 1.3 V vs Mg) and low theoretical capacity (130 mAh g-1) cannot meet the requirement for high density energy storage. Unfortunately, most of the state-of-the-art metal oxide-based cathode materials were proven unsuccessfully for magnesium storage. In addition to the traditional intercalation type materials, redox-active organic molecule is an alternative solution to provide high voltage high capacity cathodes Mg-ion batteries.11-13 Here, we would like to report our recent progress on the development of a series of redox-active organic molecules as potential high voltage and high capacity cathode materials for efficient magnesium storage. Excellent electrochemical reversibility has been established for the Mg-Organic battery cell by the steady-state cyclic voltammetry measurement. The sustainable cycling performance was further highlighted by the very small amount of capacity loss upon cycling. At a reasonable rate of 0.5C, above 100 mAh g-1 discharge capacity was obtained initially, of which 99% can be retained in at least 100 cycles. The excellent cyclability further highlights the great potential of redox-active organic molecules as high-performance cathode material for Mg-ion batteries.

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