Effect of salts on the electrochemical performance of Mg metal‒organic battery

Mg rechargeable batteries are a highly perspective battery system for the future post Li-ion batteries. However, their practical application seems quite distant due to poor Mg2+ insertion into inorganic cathode materials. Organic cathodes offer an opportunity for realization of practical Mg rechargeable batteries due to circumvention of insertion limitations encountered in inorganic hosts. Herein, we investigate the electrochemical performance of polyanthraquinone (PAQ) organic cathode in two Mg electrolytes based on different combinations of the salts, Mg(TFSI)2-2MgCl2 and MgCl2-AlCl3. PAQ in electrolyte containing Mg(TFSI)2-2MgCl2 exhibits good capacity retention and utilization. MgCl2-AlCl3 based electrolyte shows poorer electrochemical performance and a peculiar upshift of the voltage plateau for 300 mV, which is caused by the presence of AlCl3 in the electrolyte. Mg anode morphology after cycling reveals the formation of pits and cracks on the Mg foil anode as well as uneven Mg deposits several μm in size in both electrolytes. The maximum capacity and capacity retention are referenced with polyanthraquinone performance in Li battery and show that at 1C cycling rate 75% of the maximum capacity in Li battery can be obtained in Mg battery. Long-term capacity fade is comparable, demonstrating the potential for practical realization of Mg‒organic battery.