Electrochemical Performance and Mechanism of Bimetallic Organic Framework for Advanced Aqueous Zn Ion Batteries

Widespread interest has been generated by aqueous zinc batteries (AZIBs), which have excellent theoretical capacities (820 mA h g^-1), a low redox potential (-0.76 V vs SHE of Zn metal), and high security. Suitable cathodes for constructing high performance AZIBs are of great signification. Metal-organic frameworks (MOFs) with adjustable structure via metals and organic units show great potential in AZIBs. In this work, ZnMn-Squaric acid (ZnMn-SQ) was synthesized using squaric acid through coprecipitation and served as the cathode for AZIBs. The ZnMn-SQ electrode demonstrated a high capacity of 489.1 mA h g^-1 at 0.2 A g^-1. Meanwhile, ZnMn-SQ can obtain 80.7 mA h g^-1 after 1300 cycles, showing an outstanding long cycle life. More importantly, ex situ characterizations of XRD, XPS, and FT-IR revealed that ZnMn-SQ undergoes a structural transformation from the initial ZnMn-SQ framework to manganese oxide accompanied by Zn-SQ and then reduced to MnOOH, ZnMn₂O₄, and Zn₄SO₄(OH)₆·5H₂O (ZHS) in subsequent cycles. In addition, a modified zinc anode using cubic porous Zn-SQ-3d was used to construct ZnMn-SQ // Zn-SQ-3d@Zn(Zn-SQ-3d-coated Zn) high performance AZIBs, the capacity of which reaches 171.3 mA h g^-1 at 1 A g^-1 after 660 cycles. This work provided chances for constructing high-performance zinc ion batteries using MOF compounds.