Aqueous Mg-Ion Battery Based on Polyimide Anode and Prussian Blue Cathode

The magnesium-metal battery, which consists of a cathode, a Mg-metal anode, and a nonaqueous electrolyte, is a safer and less expensive alternative to the popular Li-ion battery. However, the performance of Mg batteries is greatly limited by the low electrochemical oxidative stability of nonaqueous electrolytes, the slow Mg diffusion into the cathode, and the irreversibility of Mg striping and plating on the Mg metal anode. Here, we report the first Mg-ion battery using a Mg aqueous electrolyte, nickel hexacyanoferrate cathode, and polyimide anode. The operation depends on Mg intercalation−deintercalation at the cathode and reversible enolization at the anode, accompanied by Mg transport between cathode and anode. The cell exhibits a maximum cell voltage of 1.5 V and a supercapacitor-like high power, and it can be cycled 5000 times. This system points the way to improved Mg-based rechargeable batteries. Magnesium (Mg) metal is an attractive anode material for rechargeable batteries because it has a low reduction potential of −2.37 V (vs NHE), a volumetric capacity that is higher than that of lithium, low cost, and high abundance; is environmentally benign; and never forms dendrites during plating−stripping cycles. Accordingly, the Mg-metal rechargeable battery has long been considered as a safe and inexpensive alternative to the popular Li-ion battery. To date, many efforts have been devoted to improving the performance of Mg-metal batteries using nonaqueous electrolytes. However, the cell performance of the currentMg-metal batteries is still impeded by several factors such as limited electrochemical oxidative stability of nonaqueous electrolytes, slow Mg-ion diffusion into the cathode, and irreversible Mg/Mg conversion at the anode. Most of the reported Mg-metal rechargeable batteries display limited power density, insufficient energy density, and poor cycle life. Indeed, theworking voltage of current Mg-based rechargeable batteries is close to that of aqueous electrolyte-based batteries (Table S1). Moreover, the use of unsafe organic electrolytes runs counter to the inherently safe and environmentally benign characteristics of Mg. These considerations motivated us to shift from current organic electrolyte-based Mg-metal batteries to aqueous Mg-ion batteries (Figure S1a,b). This enables the elimination of the passivation problem and replacing toxic and flammable organic electrolytes (such as acetonitrile or tetrahydrofuran) by aqueous electrolytes. Recently, aqueous Li-ion and Na-ion batteries have been developed as promising alternatives for organic Li-ion and Na-ion batteries for large-scale applications because of their low cost, high safety, and long cycling life. Compared with aqueous Li-ion batteries, the aqueous Mg-ion battery has a lower cost, and as mentioned above, the Mg has high abundance and environment-friendly characteristics. Moreover, being a divalent Mg ion, the electrolyte salts of aqueous Mg-ion battery will be only half that of aqueous Liand Na-ion batteries which have monovalent ions of Li and Na, respectively. Here, we report an original aqueous electrolyte Mg-ion battery system that involves a reversible Mg intercalation−deintercalation in a nickel hexacyanoferrate cathode and a reversible enolization at a polyimide anode, accompanied by Mg transfer between cathode and anode. The electrode reactions do not involve dissolution or deposition of Mg metal electrode, and we demonstrate that the kinetics of both cathode and anode electrodes is not limited by ion diffusion or phase conversion. As a consequence of these features, this cell has the promise to achieve a long cycle life and high power density as super-