Structure Design And Electrochemistry Properties Of Mn-Based Layered Cathode Materials For Potassium-Ion Batteries

Electrochemical energy storage has become an advanced technology to achieve carbon peak before 2030 and carbon neutrality before 2060.However,for commercial Lithium-ion batteries（LIBs）,the limited resources and surge prices make them unfavorable for large-scale energy storage.Potassium sources are abundant and cost-effective,based on the same working mechanisms with LIBs,potassium-ion batteries（PIBs）are ideal alternative for large-scale energy storage applications.As the crucial role of PIBs,the cost and properties of cathode materials directly determine the overall cost and the electrochemical performance of batteries.Therefore,the in-depth research on electrode materials can effectively enhance the core competitiveness of PIBs in the field of energy storage.Specially,manganese-based layered cathodes,as reversible K⁺intercalation hosts,have been regarded as one of the most promising cathode materials owing to their low cost,sustainability,and high theoretical capacity.However,their development is still limited by low rate capacity and poor cycling performance.To solve these issues,through microstructure modulation,composition optimization,and surface modification strategies,a variety of manganese-based cathode materials with excellent electrochemical performance were designed and prepared,and further establishing the well-defined relationship between structure properties and electrochemical behaviors,ultimately demonstrating their application prospects in the field of large-scale energy storage by full cells and thick electrodes.The main research contents are summarized as follows:1.The local lattice distortion induced by the Jahn-Teller effect of Mn³⁺usually results in limited capacity and unsatisfactory cycling lives.To solve these issues,a new low-cost P3-type K_0.45Mn_0.9Al_0.1O₂ material is designed via riveting electrochemical inactive Al³⁺in the octahedral Mn³⁺sites.It is experimentally proved to play a key role in suppressing misfit dislocations at[MnO₂]layers,and building the regular and enlarged K layers to facilitate fast K⁺transportation.While the stronger Al-O bonding solidifies K-O-Al/Mn-O-K interaction to improve its structural and thermal stability,thus effectively suppressing the P3′′phase formation at high voltage and relieving exothermic phase transition at elevated temperature（350 ℃）.Furthermore,It helps to form a stable and uniform CEI layer synergistically which can suppress electrolyte erosion and prevent the manganese dissolution.Thanks to these inherent merits,K_0.45Mn_0.9Al_0.1O₂ delivers a high specific capacity of 152 m Ah g^-1 and excellent cycling performance with the capacity retention of 67%over 1000 cycles.Impressively,the full battery achieves a high energy density(291 Wh kg^-1,based on the cathode mass)over the state-of-the-art layered cathodes for KIBs,and considerable cycling stability at 45 ℃,thus providing a cost-effective,stable and safe alternative beyond LIBs for large-scale energy storage applications.2.To further combat the significant challenges against poor cycling stability caused by structural degeneration and particle microcracks of layered oxides.Herein,compositionally complex（high-entropy MgAlCrCoTi）substitution is introduced to design P3-type K_0.45Mn_0.75Mg_0.05Al_0.05Cr_0.05Co_0.05Ti_0.05O₂（HE-KMO）.It plays bifacial effects in the charge compensation and the structural stabilization by Cr redox and stronger TM-O bonding,supporting reversible K⁺（de）intercalation.Moreover,the multicomponent coordination accommodates local variations around TM ions,thus suppressing the adverse high-voltage O3-P3’’phase transition.Synergistically,the reduced local strain concentration inside particles conduces to restrain the microcrack generation which can validate its mechanical stability.The as-designed HE-KMO delivers enhanced cyclic stability and the capacity retention increases by 14%.This strategy was also successfully applied to other materials,demonstrating its universality.In addition,the first inorganic potassium-based cathode thick electrode was prepared and achieved the highest areal capacity of 4.0 m Ah cm^-2,which provides a feasible reference for designing the electrode materials with reversible K⁺storage as well as improving the energy density of K-ion batteries.3.The service life of K_xTMO₂ is dramatically limited by cathode-electrolyte side reactions and thus interfacial instability issues.To solve these issues,an amorphous and K⁺-conducting Fe PO₄（a-FP）skin is successfully built on the K_0.5Ni_0.1Mn_0.9O₂ surface,covering the particles conformally.Its elasticity for strain relaxation guarantees its integrity for sealing function,and could withstand mechanical strain which significantly suppresses the mechanical cracking and transition metal dissolution,thereby providing long-term protection for the cathode surface.Accordingly,a uniform and robust KF-rich CEI is in situ formed,which guarantees fast and continuous K⁺migration during repeated cycles,while a well-defined relationship between the surface engineering of K_xTMO₂and their electrochemical behaviors has been established.Specifically,KNMO@a-FP exhibits faster K-storage of 66 m Ah g^-1 at 500 m A g^-1,and realizes the longest lifespan of 2500 cycles among the currently reported K_xTMO₂ cathodes.The full battery achieves a high energy densities(252 Wh kg^-1,based on the cathode mass).The progress made on the interfacial issues of K_xTMO₂ gains an insight into further development of PIBs cathodes with high performance and service life.In this thesis,several effective solutions are proposed to solve the issues of manganese-based layered materials.The as-designed materials improve the capacity,rate and cycle properties,and the modification mechanisms of these cathodes have been revealed through the established relationship between the structure and the electrochemical properties.And the energy storage devices established in this thesis show great possibility in applications of large-scale energy storage.The research results provide new ideas for the design strategy and research direction of manganese-based cathode materials.