A gradient structured SEI enabling record-high areal capacity anode for high-rate Mg metal batteries

Abstract

Magnesium metal batteries (MMBs) are being considered as promising candidate for next generation batteries, benefiting from the high specific capacity, low redox potential and abundant earth reserve of Mg anode. However, the Mg anode is prone to be passivated from the side reactions between Mg metal and conventional electrolyte, leading to high polarization and poor kinetics. To solve this problem, we propose a facile drip coating method to construct a polymer-alloy hybrid layer on Mg surface, with a polymerized tetrahydrofuran (PTHF) network crosslinked with Mg-Cl and Sn-Cl complexes and a metallic layer with Sn and Mg-Sn domains from top to bottom. This gradient structure is a mixed electron–ion conductor, and the upper PTHF network with rich Mg-Cl moieties allows fast Mg-ion transport and its flux homogenization, while the lower Sn-based magnesophilic domains provide abundant Mg deposition sites with low nucleation and migration barriers, guaranteeing the uniform Mg plating and stripping. This protection layer enables the ultrahigh exchange current density and low interfacial resistance compared with the natural passivation layer of Mg metal. Thus, the modified Mg anode possess the much lower overpotential (300 mV) and ultralong cycling stability (over 3200 h) at 3 mA cm−2 and 3 mAh cm−2, and can even withstand the record-high current density (6 mA cm−2) and area capacity (6 mAh cm−2). When the modified Mg couples with CuS conversion cathode, the corresponding full cells still realize the high reversible capacity over 200 mAh-g-1 and stable cycling at the high current density (500 mA g−1) without any electrolyte addition. This method effectively promotes the possibility of practical application of high-rate and durable MMBs in the future.