Preparation And Modification Of Layered Oxide Cathode Materials For Sodium/Potassium-Ion Batteries

To date,lithium-ion batteries(LIBs)have been successfully used in electric vehicles,storage systems(ESSs)and flexible electronic device,due to high energy density and long cycle performance.However,the huge consumption of lithium resources,sharp rise in price and uneven distribution seriously limits the sustainable development of LIBs.Based on similar principles to LIBs,rich resources and low cost,sodium/potassium-ion batteries(SIBs/PIBs)are expected to supplement/replace LIBs.Compared with anode materials,cathode materials usually exhibit low specific capacity,poor rate capability and structural instability,which significantly affects the development of SIBs and PIBs.There,cathode materials with long cycle life,rapid reaction kinetics and low cost paly a decisive role in the realization of high-performance SIBs and PIBs.In this thesis,the cathode materials of Na Cr O₂ for SIBs and K_0.45Co_1/12Mg_1/12Mn_5/6O₂(KCMMO)for PIBs are synthesized by sol-gel method,respectively.The physical and electrochemical properties,Na⁺/K⁺(de)intercalation kinetics and structural evolution of the as-synthesized cathode materials are systematically investigated.Meanwhile,Na Cr O₂@C flexible free-standing cathode is designed and constructed by the electrospinning technology with Na Cr O₂ as electrochemical active materials,which can directly act as SIBs cathode without coating,binder and conductive agent,to meet the needs for long cycle life,high energy density and good flexibility.The research contenets,main results and conclusions are as follows:(1)O3-layered Na Cr O₂cathode material for SIBs is synthesized by a freeze-drying assisted sol-gel method.Due to the high structural reversibility,rapid Na⁺diffusion and high electronic conductivity,the Na Cr O₂ cathode material exhibits outstanding cycle stability and rate performance.At 5C,the capacity retentions are?50%after 3000 cycles at room temperature,and 57.6%after 2000 cycles at 60?,respectively.In addition,the average discharge specific capacity at 50C reaches 82.1 m Ah g^-1.The three-phase coexistence in Na Cr O₂ during charge-discharge process is confirmed for the first time,which is supposed to delay the O3 phase disappearance so as to improve the structural reversibility.The density functional theory(DFT)calculation verifies that the Na Cr O₂ has lower Na⁺diffuiosn activation barrier and higher electronic conductivity.This study is heful to accelerate the practical process of cathode materials for SIBs,and provides a new perspective for analyzing the structural evolution of Na Cr O₂.(2)A Co³⁺-Mg²⁺co-substituted KCMMO is designed as the cathode materials of PIBs for the first time.The capacity retention of KCMMO is 54.8%after 500 cycles at 1000 m A g^-1,which is higher than 41.6%of the undoped K_0.45Mn O₂(KMO).The improved cycle stability of KCMMO is mainly resulted from the highly reversible single-phase structure evolution during cycling,while KMO experiences an incomplete reversible structural change form P'3 phase to P3 phase.Meanwhile,thanks to the rapid K⁺diffusion,KCMMO also shows better rate performance.Electrochemically active Co³⁺contributes additional capacity,while electrochemically inactive Mg²⁺can stabilize the crystal structure.This work broadens the research idea for improving the electrochemical properties of cathode materials for PIBs and helps to deepen the understanding of K⁺storage mechanism in layered Mn based oxides.(3)Na Cr O₂@C flexible free-standing cathode is designed and constructed by the electrospinning technology with Na Cr O₂ as electrochemical active material for the first time.In the Na Cr O₂@C flexible free-standing cathode,Na Cr O₂ particles are basically adhered or wrapped in cross-linked carbon fibers,conductive to Na⁺diffusion and electron transport.Various nitrogen derived from PAN precursor can provide more electrons to the?-conjugated system of carbon,which also significantly improves the electronic conductivity.At 0.5C,the initial capactity of the Na Cr O₂@C flexible free-standing cathode is 110.0 m Ah g^-1and it remains at 107.4 m Ah g^-1 after 50 cycles,indicating good cycle stability.The Na Cr O₂@C flexible free-standing cathode also displays good mechanical flexibility.This study lays experimental and theoretical foundation for the preparation of oxide flexible free-standing cathodes with excellent electrochemical and mechanical properties.