Studies On The Multi-level Construction Of δ-MnO₂ Cathode Materials And Their Electrochemical Zinc Storage Performances

Aqueous zinc-ion battery as a promising eco-friendly energy storage device can well meet the needs of large-scale energy storage systems due to the high volumetric energy density,low cost,and high safety of zinc.δMnO₂is regarded as an ideal cathode material,because its layered structure is suitable for diffusive transport of Zn²⁺,high voltage platform,and low cost.However,δ-MnO₂ materials still suffer from poor structural stability,few ion-diffusion paths,and sluggish reaction kinetics.Therefore,it is urgent to carry out systematic multi-level structure construction ofδ-MnO₂ materials,which can stabilize the crystal structure ofδ-MnO₂ and enhance the electrochemical reaction kinetics of the batteries.This method has become an inevitable choice to improve the electrochemical zinc-storage performances of aqueous zinc-ion batteries.This dissertation focuses on four scientific problems ofδ-MnO₂ materials:the enhancement of the microstructure stability of the material,the construction of the ion/electron diffusion path,the improvement of the charge transfer efficiency,and the enhancement of the electrochemical kinetics.Through the strategies of ions pre-intercalation,defect engineering,conductive materials coating,and light-assistance,a few of high-performance zinc storage systems based onδ-MnO₂ materials were designed and constructed.The main research contents and result are given as follows:1.Design and construction of Na⁺/H₂O co-intercalatedδ-MnO₂nanosheets and their zinc storage performances:This chapter aims at solving the problem of poor crystal structure stability ofδ-MnO₂ cathodes.Layered Na⁺/H₂O co intercalatedδMnO₂（NMOH）nanosheets with abundant active sites were prepared by the alkaline solution selective etching method.The pre-intercalated Na⁺/H₂O can not only enlarge the crystal interlayer spacing for the construction of abundant ion diffusion channels but also stabilize the layered structure to suppress the dissolution of Mnand irreversible phase transformation.Therefore,the NMOH nanosheets obtained excellent zinc-storage properties and outstanding cycling stability:a high capacity of 389.8 m A h g^-1 at 200 m A g^-1,and an admirable cyclability with a high capacity of 201.6 m A h g^-1 at a current of 500 m A g^-1 after 400 cycles.Additionally,the electrochemical behavior of Zn²⁺in NMOH nanosheets is deeply explored,and a displacement/intercalation electrochemical mechanism is firstly verified in the Mnbased cathodes.During the first electrochemical cycle,part of Zn²⁺would occupy the original site of Na⁺,and continue to stabilize the layered structure for the manufacture of ion diffusion channels;meanwhile,a fraction of Na⁺will be extracted from the NMOH interlayer.In the subsequent cycles,the electrochemical reaction in NMOH cathodes is dominated by the H⁺/Zn²⁺intercalation/extraction processes,accompanied by a reversible adsorption/desorption process of Na⁺on the cathode surface and the reversible deposition/dissolution reaction of Zn₄SO₄（OH）₆·0.5H₂O.2.Design and construction of oxygen vacancy-rich K-birnessite@C yolk-shell nanospheres and their zinc storage performances:Although the co-intercalation of Na⁺/crystal water can effectively enhance the structural stability ofδ-MnO₂ nanosheets,it is still challenging to improve their reaction kinetics and long-cycle stability.Herein,a two-step diffusion-driven strategy supplemented by alkaline solution etching is designed to fabricate the yolk-shell structured K_0.48Mn₂O₄·0.49H₂O@carbon（KMOH@C）nanospheres with rich oxygen vacancies,in which the KMOH nanosheets are the core,and hollow mesoporous carbon（HMC）is the shell.During the preparation of KMOH@C nanospheres,the diffusion-driven strategy was regulated by the surface charge and pore structure of HMC nanospheres.The etching effect of alkaline solution cooperates with the confinement effect of HMC nanospheres,which not only introduces K⁺and a large number of oxygen vacancies into the KMOH crystals but also significantly increases the specific surface area and the number of active sites of KMOH nanosheets.Therefore,a massive 3D multi-level ion/electron diffusion channels were fabricated for KMOH nanosheets to improve its zinc-storage kinetics.The yolk-shell structure constructs plentiful voids,which is beneficial for buffering the cathode volume fluctuations caused by ion insertion/extraction,enabling the batteries possess extraordinary cycle stability.Besides,the HMC nanospheres can provide three-dimensional conductive networks for the active material,which reduces the charge transfer resistance at the solid-liquid interface between the cathode and the electrolyte.The above unique structural design brings superior electrochemical kinetics and high-capacity to the KMOH@C nanospheres,which exhibits an excellent rate capability of 122.2 m A h g^–1 at 10.0 A g^-1.More importantly,KMOH@C cathodes exhibit a brilliant long-term cyclability with a high capacity of 129.6 m A h g^–1 after 6000 cycles at 3.0 A g^-1.The displacement/intercalation mechanism of zinc storage in KMOH@C nanospheres is also confirmed.3.Design and construction of novelδ-MnO₂-based light-assisted aqueous zinc-ion battery and their zinc storage performances:This chapter aims at solving the problem of poor reaction kinetics ofδ-MnO₂cathode materials.A novel light-assisted manganese-based aqueous zinc-ion battery was successfully constructed based on the photoelectric effect of manganese oxides.Ultrafine Na⁺/H₂O co-intercalated Na_0.55Mn₂O₄·x H₂O（U-NMOH）nanosheets were synthesized by alkaline solution etching followed by ultrasonication and used for the photocathodes of photo-assisted AZIB（hv-AZIB）.The U-NMOH nanosheets exhibit outstanding photoresponsivity.Upon illumination,the photogenerated carriers generated in the U-NMOH nanosheets can efficiently regulate the surface charge distribution of the positive electrode,and effectively reduce the charge transfer resistance and ion diffusion energy barrier.Therefore,the diffusion behaviors of H⁺/Zn²⁺in the cathode are highly promoted,further reducing the polarization potential of the battery upon cycling.As a result,the reaction kinetics of the cathodes is extremely enhanced,which in turn improves the electrochemical performances of the battery.Under illumination,the specific capacity of the hv-AZIB increases from 218.8 to 300.4 m A h g^-1（37.3%performance increase,light intensity:～0.5 sun）.