Investigation On Modification Strategies And Optimization Mechanisms Of Potassium-Ion Layered Oxide Cathodes

The development of long life,high safety,cost-effective,and globally distributed energy storage systems with advanced electrode materials is crucial for new energy power generation,particularly under the dual carbon goal.Ascribing to the natural abundance of potassium,low standard electrode potential of potassium（-2.93 V vs.the standard hydrogen electrode）,smaller size of the solvated potassium ions and similar working principles to lithium-ion batteries,potassium-ion batteries come with big advantages for large scale energy storage.Cathode is a crucial component for batteries,which ultimately determines the cost,cycle life,energy density,and power density.Among the well-developed cathode materials,potassium layered oxides is an attractive option as it features high energy dense,structural modifiability,and commercial viability of large scale preparation.However,their potential remains untapped,due to the bottlenecks including sluggish kinetic,complex phase transitions,poor air stability and K-deficiency.To solve these issues,in this thesis,we designed and prepared several potassium layered oxides from different perspectives and dimensions,including molecular design,interlayer modification,and phase modulation.Moreover,we explore the effects of chemical composition and crystal structure on their energy storage mechanism and main indicators of batteries in terms of energy density,power density,coulombic efficiency,and cycling life.The potassium full cells employing the designed cathodes with outstanding performance further demonstrate their potential for practical applications.The main conclusions are summarized as follows:Firstly,a binary Ni/Mn based K_0.44Ni_0.22Mn_0.78O₂ cathode was designed,aiming to mitigate the lattice distortion caused by the Jahn-Teller effect of Mn³⁺and suppress the structural phase transition induced by interlayer sliding in unary Kx MnO₂ cathode.Notably,the material is a Co-free one to avoid cobalt panic originated from its rising prices.The cathode delivers a reversible specific capacity of 126 m Ah g^-1 in the voltage range of 1.5-4.0 V as well as the improved rate performance.In addition,the Ni²⁺substituted material undergoes single solid-solution process during K⁺intercalation/deintercalation,leading to capacity retention of 67%after 500 cycles,which is higher than that of K_0.4MnO₂（Decaying to almost 0）.Moreover,a cathode electrolyte interphase layer（CEI）induced by the carbonate-solvent-based electrolyte degradation was observed on the surface of the electrode,which was found to grow rapidly in the initial cycles followed by a much slower speed after several cycles.And the components of the film were revealed as potassium alkyl carbonates（RCO₂K）,potassium alkoxides（KOR）,K₂CO₃ and KF,beneficial for understanding the interfacial and mechanical degradation for potassium layered cathodes.Secondly,selectively doping at transition metal sites utilizing elements with ideal functions and lower prices than nickel achieved the simultaneous cost reduction and performance enhancement.Specifically,the soft-mode buckling property provided by high-spin FeO₆ at the end of charge could lower the K diffusion barrier,while the stronger orbital hybridization between Ti-O is helpful in suppressing MO₆ movements by sharing oxygen atoms with other transition metals,thus guaranteeing a stable framework.As a result,the designed K_0.4Fe_0.1Mn_0.8Ti_0.1O₂ deliveres a specific capacity of 71 m Ah g^-1 at a high current density of 1000 m A g^-1.In addition,it presents a unique zero-strain characteristic accompanied by a single solid-solution process（unit cell volume change of 0.5%）,and thus the electrode remains 74%of the initial capacity after 500 cycles,with no cracks observed.Owing to the novel reaction mechanism and multifunctional framework,its Na⁺-storage performance surpassing most reported sodium layered cathodes was also displayed:a high reversible specific capacity of 160m Ah g^-1 between 1.8 and 4.0 V,along with a capacity retention of 82%after 300 cycles.Thirdly,we propose a universal strategy to make breakthroughs in kinetics property and air stability of potassium layered oxide cathodes.Introducing H₂O molecules into the K-layer induces a phase transition towards monoclinic symmetry,layer spacing expansion,the inhibited K⁺/vacancy ordering as well as the enhanced intercalation pseudocapacitance.Owing to these merits,the hydrated K_0.4Fe_0.1Mn_0.8Ti_0.1O₂·0.16H₂O enables fast and durable K⁺storage at 2 A g^-1,with a capacity retention of 90%after 1000 cycles.The organic potassium-ion full cell derived from the cathode exhibits an ultrahigh power density up to 2640 W kg^-1,which is outstanding compared to prior works.Moreover,we unveiled the underlying degradation mechanism of the typical layered oxides in air and concluded that the hydration strategy assists in air stability through effectively restraining K⁺/H⁺exchange reaction and the leaching of Mn.As a result,K_0.4Fe_0.1Mn_0.8Ti_0.1O₂·0.16H₂O can remain stable in lab air ambient for over 60 days without crystal structure and chemical composition variation.The characteristic also boosts high feasibility in aqueous electrolyte,contributing to specific capacities of 150 m Ah g^-1 at 100 m A g^-1 and 105m Ah g^-1 at 1 A g^-1,rendering a new option for high-safety aqueous potassium-ion batteries.Lastly,the application of potassium layered oxides has been expanded from liquid to solid-state system.The development of solid-state batteries has gained momentum worldwide,yet the progress of K-ion solid-state batteries has always been hindered by the lack of high-conductivity solid-state electrolytes.A series of antimony-based layered K-ion conductors were explored based on the inducing effect induced by the non-metallic element antimony.Furthermore,efficient ion channels were otained through regulating the mobile ion concentration,crystal phase and microstructural arrangement.Among them,the Ni/Sb-based cathodes possessing redox-active Ni²⁺ions show the highest bulk conductivity(1.5×10^-4S cm^-1)with the potassium-ion concentration as 0.68.Contrary to typical K-deficient layered cathodes,it could offer an initial coulombic efficiency nearly 100%,promising for both liquid and solid-state systems.The Mg/Sb-based solid-state electrolyte exhibits a stable upper voltage limit exceeding 6 V.Moreover,the P2/P3 composite oxide with K-ion content as 0.62combines high bulk conductivity and low grain boundary resistance due to the face-to-face crystalline growth.As a result,high-speed and long-term transportation of potassium ions throughout the quasi-solid-state battery based on them is achieved,which exhibits an energy density of 174 Wh kg^-1（calculated by cathode mass）and retains 80%of its capacity after 200 cycles.The antimony-based layered potassium-ion conductors emerge as a promising approach to high-safety potassium-ion storage.In summary,in this thesis,we have developed a series of advanced potassium layered cathodes,taking advantage of their molecule modifiability.Additionally,advanced characterization techniques and theoretical analysis were jointly combined to investigate the properties and mechanism of their electrochemical processes.Through modifying the intrinsic crystal structure,chemical composition,and local environment of the materials,the issues that confine their practical application,including air instability,K⁺/vacancy ordering,and poor stability of the bulk phase were successfully tackled.We hope that the systematic developments promoted in this thesis could provide valuable guidance for high-performance potassium layered cathode materials and push potassium-ion batteries towards practical application as well as the associated large-scale energy storage systems.