Structural Regulation Of Layered Manganese Oxides For Aqueous Electrochemical Energy Storage

With the excessive consumption of traditional fossil energy and environmental pollution caused by their burning,the demand for alternative renewable energy sources such as wind,solar,and tidal energy is becoming an inevitable trend and,in the spotlight,to meet increasing energy demands.Therefore,our country have put forward the"double carbon"strategy of"carbon peak"and"carbon neutrality".However,low-cost and high-performance energy storage devices are urgently needed to effectively achieve large-scale energy storage of this renewable energy because of its intermittent nature and instability.Among the various energy storage technologies and devices,rechargeable batteries have been extensively developed as the most promising and reliable devices for a diverse range of applications.Therefore,developing aqueous energy storage devices with the advantages of high safety,low cost and environmental friendliness has drawn more attention.Among them,aqueous sodium ion supercapacitors and aqueous sodium ion batteries（ASIBs）are good choices for practical application because of long cycling performance and fast charging/discharging rate.However,the low energy density of these devices limited their application.Hence,it is critical to select appropriate electrode materials to improve energy density.Layered manganese oxides have attracted extensive attention as cathode electrode materials due to the low cost,wide potential window（0–1.2 V）and high theoretical capacity(1370 F g^–1).However,the electrochemical performance is strictly limited by the relatively poor ionic and electronic conductivity and poor structural stability of layered manganese oxides,thus hindering its practical application.Therefore,aiming at improving the practical electrochemical capacity and cycling stability of layered manganese oxides materials for aqueous electrochemical energy storage,this thesis proposes various modification methods and carry out the three aspects of research:（1）In the first work,metal ion doping method is used to suppress J–T distortion and improve structural stability ofδ-Mn O₂.A structure reinforced birnessite with improved electron transfer is developed via Cr doping inδ-Mn O₂ using hydrothermal synthesis.The structural analyses illustrate that Cr³⁺dopedδ-Mn O₂ obtained via a substitutional doping route(by replacing the Mn³⁺ion),could efficiently stabilize the layered structure.The morphologic and structural analysis of the cycled indicates that the Cr³⁺doping inδ-Mn O₂can effectively suppress the phase transition and Mn dissolution during cycling.Combined with first-principles calculations,Cr doping inδ-Mn O₂ can narrow down the band gap and increase the binding energy,strengthening the layered structure.Besides,the decrease of Mn vacancy formation energy for Cr-dopedδ-Mn O₂ samples demonstrate the restrain of Mn dissolution after Cr doping,which is in accordance with the experimental results.Therefore,Cr-dopedδ-Mn O₂ electrode materials with high mass loading exhibit high specific capacitance,good rate performance and excellent cycling performance.（2）In the second work,we construct the P’3-type Na-birnessite with fast reaction kinetics and stable structure by increasing the exposing{010}active facets.We use a hydrothermal sodiation method to prepare a high Na-content P’3-type Na-birnessite with exposing{010}active facets for aqueous energy storage.In hydrothermal sodiation process,the O’3-type Na-birnessite nanosheets transform into P’3-type Na-birnessite nanobelts with the dissolution-redeposition reaction.The exposing{010}active facets of nanobelts increase due to the splitting along the[010]direction of nanosheets,improving the{010}/{001}ratio of nanobelts.Therefore,the highly ordered layered structure is well retained with effectively suppressed Mn dissolution during cycling,resulting in ultralong cycle life of P’3-type Na-birnessite in aqueous electrolyte.With a typical two-electrode device,a 2.2V P’3-type Na-birnessite//Na Ti₂（PO₄）₃ aqueous full cell was fabricated,delivering high energy density and excellent cycling performance.（3）In the third work,construction ofδ-Mn O₂@Mn PO₄·H₂O heterostructure is to enhance charge transport and high Na⁺reactivity by built-in electric field in the heterointerface.We propose a simple one-step hydrothermal method to in situ form theδ-Mn O₂@Mn PO₄·H₂O heterostructure with rich oxygen-vacancies for aqueous energy storage.The formation process ofδ-Mn O₂@Mn PO₄·H₂O heterostructure shows that the two phases can transform into each other during the reaction process,resulting in the construction of heterogeneous interface.The structural and morphologic analysisδ-Mn O₂@Mn PO₄·H₂O heterostructure samples show that the presence of PO₄^3–inhibits the{001}facets ofδ-Mn O₂,resulting in a heterogeneous structure with porous structure and rich oxygen-vacancies.Moreover,the difference of work function between two phases at the heterogeneous interface spontaneously develop a built-in electric field.This enables the free electrons to accumulate near theδ-Mn O₂ surface,inducing extra electrostatic attraction with sodium ions,which surely improves the utilization ratio of cations and drastically accelerates the transport of charge carriers forδ-Mn O₂@Mn PO₄·H₂O heterostructure.Therefore,the electrochemical capacity of theδ-Mn O₂@Mn PO₄·H₂O heterostructure is increased to 746 F g^–1 at a current density of 0.2 A g^–1.Besides,the cyclic stability of the heterostructure is significantly improved due to the protective effect of the structurally stable Mn PO₄·H₂O and the strong covalent P–O tetrahedron structure at the interface.