Magnesium-ion batteries (MIBs) have shown prospects in the field of large-scale energy storage due to their superiority of high capacity and abundant reserves. However, the development of MIBs is restricted due to magnesium metal anodes that are prone to form passivation layer in organic electrolytes, and the absence of athode materials that reversibly store magnesium ions. 

Compared with traditional inorganic electrode materials, organic electrode materials exhibit excellent competence for magnesium ions storage due to the mild redox reaction between their functional groups and ions. 

If the working mechanism of the dual ion battery can be combined with organic electrode materials, the high reaction potential and fast anion diffusion kinetics of the anion intercalated graphite cathode will improve the working voltage and electrochemical performance of MIBs. 

A research team led by Prof. TANG Yongbing from the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences developed a magnesium-based dual-ion battery (Mg-DIB) based on insoluble organic anode materials. 

The researchers fabricated the Mg-DIB by using 3, 4, 9, 10-perylenetetracarboxylic diimide (PTCDI) as organic anode, expanded graphite (EG) with high potential and fast anion diffusion kinetics as cathode, and ionic liquid as electrolyte. 

The Mg-DIB delivered extraordinary rate performance and cycle performance, with 85% capacity retention at high current density of 20C (1C=100 mA g^-1) and 95.7% capacity retention after 500 cycles at 5C. 

Through the combination of the in-situ fourier transform infrared (FTIR) spectroscopy, theoretical calculation analysis and other characterization methods, it was confirmed that there were the threefold coordination mechanism and hydrogen bond formation during Mg-ion insertion into PTCDI, which contributed to good insolubility in organic electrolytes and good structural stability.

Furthermore, the insolubility played an important role in improving the cycling stability of Mg-DIB.

"By reasonably designing and engineering the functional group structure of small-molecule organic materials magnesium ion battery structure system, a wide range of insoluble organic materials can be available. Simultaneously, taking into account the advantages of dual ion battery system, the research of Mg-DIB will open up a new way to design high-performance MIBs and other energy storage devices," said Prof. TANG. 

The study was published online in Energy Storage Materials on April 28.