Design And Electrochemical Performance Of Manganese-based Oxide Cathode Materials For Aqueous Zinc Ion Batteries

In recent years,aqueous zinc-ion batteries（AZIBs）have become a secondary energy battery with great development potential for their advantages of non-toxicity,high safety,abundant zinc resources and ecological compatibility.At present,the choice of cathode materials is the key to develop high-performance AZIBs.Manganese dioxide（MnO₂）is considered one of the most promising cathode materials due to its advantages of high theoretical capacity,high working voltage,low cost and abundant crystal structures.However,the low conductivity,easy collapse of the structure and the strong electrostatic interaction with Zn²⁺of MnO₂ limits its further applications in AZIBs.To solve these problems,this paper takes hydrothermal method as the core preparation method,adopts oxygen defects design,cationic doping and interface regulation strategies,aiming at developing MnO₂ cathode materials with high rate performance and long cycle stability.The main research contents and results of this work are as follows:（1）The precursor ofα-MnO₂ nanorods were synthesized by a one-step hydrothermal method.Using NaBH₄ as the reducing agent,oxygen-deficientα-MnO_2-x nanorods were prepared by room temperature reduction method.The existence of oxygen defects was demonstrated by XRD,XPS,HRTEM and Raman characterization techniques.The introduction of oxygen defects enhanced the conductivity,activated more zinc storage sites,and improved the zinc storage kinetics ofα-MnO_2-x materials.Therefore,α-MnO_2-x cathode exhibited good Zn²⁺storage ability including a high discharge specific capacity of 241.8 mAh g^-1 at 0.1 A g^-1 and a notable cycling performance with capacity retention of 90.9%after 300 cycles at 0.5 A g^-1.（2）The Zn-dopedδ-MnO₂（ZMO）nanoflowers cathode was synthesized by the simple hydrothermal method.The successful doping of zinc ions was proved by XRD,HRTEM,EDS,and XPS characterization techniques.The TG curve showed the introduction of crystal water in the sample.Zn doping improved the stability of the material and induced lattice defects ofδ-MnO₂,thus increasing the layer spacing of the material.In addition,the presence of crystal water could weaken the strong electrostatic interaction between the host material and Zn²⁺,thus accelerating the ion migration.Therefore,the ZMO cathode showed a specific capacity of 285.2 mAh g^-1 at 0.1 A g^-1,and the capacity retention rate reached 91.0%after 700 cycles at 1 A g^-1.Even with2000 cycles at a current density of 2 A g^-1,it still retained the specific capacity of 122.9mAh g^-1,showing superior cycle stability.In addition,we assembled zinc ion hybrid capacitors using ZMO as cathode and activated carbon as anode,and studied their electrochemical performance.The results showed that the zinc ion hybrid capacitor exhibited superior rate performance and cycle stability.It still showed a specific capacity of 82.6 F g^-1 at a current density of 10 A g^-1.The capacity retention rate remained 84.1%after 2000 cycles at 1 A g^-1.（3）Different crystalline MnO₂（δ-MnO₂,α/δ-MnO₂ andα-MnO₂）with KMnO₄and MnSO₄ as raw materials were synthesized by controlling hydrothermal temperatures and times,and used as cathode materials for AZIBs.Compared with pure phase MnO₂（δ-MnO₂ andα-MnO₂）,mixed-phaseα/δ-MnO₂ electrode exhibited superior rate performance and cycling stability.This is mainly attributed to the structural defects caused by lattice mismatch at the mixed-phase interface,which provided more zinc storage sites.At the same time,kinetic analysis was used to verify that the pseudocapacitance contribution in the mixed-phaseα/δ-MnO₂ electrode was larger,which contributed to faster reaction kinetics.Therefore,theα/δ-MnO₂ cathode showed a high specific capacity of 306.7 mAh g^-1 at 0.1 A g^-1,and retained a capacity of 221.3 mAh g^-1 after 600 cycles at a current density of 1 A g^-1,showing superior cyclic stability.