Synthesis And Modification Of δ-MnO₂ Cathode Materials For Aqueous Zinc-ion Batteries

As a new energy storage technology with high energy density,high safety,low cost and ecological friendliness,aqueous rechargeable zinc-ion batteries（ZIBs）have been widely studied in recent years.Among the cathode materials studied,manganese oxide has a broad application prospect because of its high working voltage and large capacity.However,MnO₂material has poor cyclic stability and low conductivity,which limit its development in ZIBs field.Therefore,MnO₂ cathode materials with high specific capacity and good cycling stability were prepared by interlamellar filling of alkali metal ions,composite with carbon materials and in-situ synthesis of carbon-containing manganese oxides.The main research contents of this thesis are as follows:（1）To reveal the effect of interlayer cations on the electrochemical performance and reaction mechanism ofδ-MnO₂ electrodes,the X-δ-MnO₂ with different interlayer spaces（K-δ-MnO₂:0.64 nm;Na-δ-MnO₂:0.58 nm;Li-δ-MnO₂:0.55 nm）were prepared by one-step method at room temperature as the cathode materials of ZIBs.The results show that the cyclic stability,multiplier performance and reversibility ofδ-MnO₂ increase with the increase of the interlayer spaces（K-δ-MnO₂>Na-δ-MnO₂>Li-δ-MnO₂）.K-δ-MnO₂ exhibits the best electrochemical properties and cycling stability,with the capacity of 270.5 and 95.1 m A h g^-1at 0.1 and 3 A g^-1,respectively.After 1000 cycles at the current density of 2 A g^-1,the capacity remains above 50%.In addition,the storage mechanism ofδ-MnO₂ was revealed in detail by XRD and SEM,namely,a two-step embedding process of H⁺and Zn²⁺accompanied by reversible deposition/dissolution of Zn₄SO₄（OH）₆·4H₂O.Finally,it is confirmed that K-δ-MnO₂ has the better conductivity,stronger adsorption of H⁺and Zn²⁺and lower diffusion energy barrier of Zn²⁺by density functional theory（DFT）.（2）On the basis of the previous chapter,a K⁺filled three-dimensional network structure MnO₂（K-δ-MnO₂/CNT）grown from CNT skeleton was prepared by ultrasonic and redox reaction at room temperature.The introduction of CNT inhibits the accumulation and aggregation of MnO₂ nanosheets and forms a good line-plane microscopic conductive connection,which is beneficial to promote ion/electron transfer.K-δ-MnO₂/CNT showed significantly improved electrochemical performance,with a capacity of 291.7 m A h g^-1 at the current density of 0.3 A g^-1.High rate performance(173.8 m A h g^-1 at 2 A g^-1)and impressive stability（71%retention rate for 1000 cycles）.Compared with K-δ-MnO₂,K-δ-MnO₂/CNT has the better reaction kinetics and faster diffusion capacity of Zn²⁺.（3）MnO₂-carbon composite materials（GLC-MnO₂）were synthesized by low temperature redox method.The materials have the potential of large-scale and low-cost industrial application.The effects of different amount of KMnO₄ on the structure of carbon-containing GLC-MnO₂ and the storage performance of Zn²⁺were investigated through a variety of physical properties and electrochemical tests.Among them,GLC-MnO₂-2 electrode has high reversible capacity close to theoretical value(277.2 m A h g^-1 after 100 cycles at 0.3A g^-1),coulomb efficiency of about 100%,excellent rate performance and ultra-long cycle stability(the capacity of GLC-MnO₂-2 electrode is 117.9 m A h g^-1 after 1500 cycles at the current density of 2 A g^-1,with a capacity retention of 62%).Subsequently,a flexible quasi-solid zinc-ion battery（GLC-MnO₂-PVA）was assembled with GLC-MnO₂-2 electrode as positive electrode,PVA hydrogel as electrolyte and zinc sheet as negative electrode.The flexible quasi-solid zinc-ion battery demonstrates superior safety and durability in extreme conditions.