Preparation And Performance Of Mangance Oxide Cathode Materials For Aqueous Zinc Ion Batteries

Manganese oxide with a diversity of crystal structures is one of the most potential cathode materials for AZIBs due to high theoretical specific capacity(308 mAhg^-1)and low price.However,there are still some key issues that hinder the further development of AZIBs:（1）Poor electrical conductivity as well as sluggish Zn²⁺kinetics;（2）The Jahn-Teller effect,Zn²⁺is repeatedly embedded in the host during cycling,causing structural degradation and poor cycle ability;（3）The dissolution of Mn²⁺,leading to the mass loss of active materials;（4）The storage mechanism is complex,and its energy storage process and products are still in debt.In order to solve the above problems,this paper focuses on manganese oxide and modifies it by carbon nanofiber composite,ion doping,and construction of heterojunction to improve the conductivity and Zn ion kinetics,inhibit the Jahn-Teller effect,aiming to designing and promoting high-performance manganese oxide cathode materials for aqueous zinc-ion batteries,the main contents are as follows:（1）Waste acrylic fibers utilized as raw materials via electrospinning and heat treatment,MnO and C composite nanofibers（MnOC）were synthesized.The one-dimensional nanostructure provides channels for ion/electron transmission,makes contribution to the kinetics of Zn²⁺.The MnOC electrode show a specific capacity of274.5 mAhg^-1at a current density of 3 A g^-1.After 500 cycles,the capacity is 220.1 mAhg^-1.MnOC were tested in different electrolytes and different voltage.The energy storage behavior process during cycling was monitored using a suit of insightful tools of in-situ X-ray diffraction,ex-situ X-ray photoelectron spectroscopy and Raman spectra.Meanwhile,the phase diagram of the Zn-Mn-O system in the chemical potential space was calculated with density functional theory,which further explained the observed energy storage process.The capacity contribution of MnOC electrode in Zn SO₄+Mn SO₄electrolyte can be attributed to the unique energy storage process:1)MnO-based active material evolved into Zn Mn₂O₄for the repeatedly Zn²⁺insertion/extraction,and 2)Zn²⁺insertion/extraction into the MnO_x originated from the oxidation of Mn²⁺in the electrolyte.（2）Compared with MnO materials,layered manganese dioxide is more conducive to the intercalation and desorption of Zn²⁺.However,the stability and conductivity are greatly poor.Nanoflower-likeδ-MnO₂ doped with Group VIII metals（Fe,Co,Ni）were prepared by hydrothermal method.After doping,due to unique d electron specification of Group VIII metal,the periodic potential field generated by MnO₂ was destroyed,improving the ionic conductivity,inhabiting the Jahn-Teller effect as well.When assembled as AZIBs for testing,Fe-dopedδ-MnO₂（FMO）performances a capacity of 338.3 mAhg^-1 at a current density of 1 A g^-1,and the capacity retention is 86.3%after cycling 200^th.The charge storage mechanism of the FMO electrode was explored by in-situ XRD and ex-situ TEM,and the migration energy barrier of Zn²⁺was calculated by DFT.The results show that FMO electrode undergoes a highly reversible phase transition（δ-MnO₂→Zn_xMnO₂→Zn Mn₂O₄/Mn₃O₄→Zn_xMnO₂→δ-MnO₂）.The migration energy barrier of Zn²⁺was reduced on account of the Group VIII metal doping.（3）The key reason for the capacity attenuation of manganese oxides is the Jahn-Teller effect of Mn³⁺during cycling,which leads to structural distortion and capacity reduction.In order to suppress the Jahn-Teller distortion,the method of electrochemical deposition and constant current test was used to build a heterojunction of Mn₃O₄ and Li Mn₂O₄ on carbon cloth（MO-LMO-CC）.The carbon cloth in electrode improves the conductivity of the material,meanwhile the heterojunction destroys the long-range ordered structure of Mn₃O₄and slows down distortion,providing a stable structure and prolongs cycle life.The MO-LMO-CC electrode can stably cycle for 3000 cycles at a current density of 1 A g^-1,showing a capacity of 157.6 mAhg^-1 and the open circuit voltage is 1.37 V,which can be well applied in low-power alarm clocks and diode light bulbs.In-situ XRD and ex-situ SEM were exploded to the energy storage mechanism of MO-LMO-CC electrode.The heterostructure can still be maintained after cycling,doing domination to the increasement of structural stability and cycle stability.