Controllable Preparation Of Layered Potassium Manganese Oxides And Investigation Of Working Mechanism For Potassium/Sodium Ion Storage

Potassium/sodium-ion batteries have a similar“rocking chair”charge storage mechanism to lithium-ion batteries.Owing to the elemental abundance in the earth’s crust and lower prices of potassium and sodium elements,potassium/sodium-ion batteries become possible alternatives to lithium-ion batteries for a large-scale energy storage application.Cathode materials are one of the key to improving the electrochemical performance of these batteries.Layered transition metal oxides have become one of the most concerned cathode materials due to the simple preparation method,high theoretical capacity and controllable composition.However,because of the much larger ionic radius of potassium/sodium ions,the diffusion kinetics of most layered transition metal oxide is sluggish.In addition,the cathode materials always undergo large anisotropic volume-change and irreversible phase transitions during repeated insertion/extraction of ions,which ultimately result in structural collapse and a decrease in reversible capacity.Therefore,suppressing or eliminating the irreversible phase transition during the insertion/extraction of potassium/sodium ions is crucial to the improvement of the electrochemical performance of the materials.Among the various transition metal oxides,potassium manganese oxides have the advantages of high theoretical capacity,low cost,environmental friendliness,and rich chemical valence.Based on this,this thesis will start with layered manganese-based transition metal oxides to explore the effect of element substitution on materials and the mechanism of structure evolution during the potassium/sodium ion insertion/extraction processes.The main research results are as follows:Controllable preparation of Cu substituted P3-phase K_0.45MnO₂（KMO）and investigation of working mechanism for potassium storage:the strategy of substituting partial Mn with Cu element improves the structural stability,charge storage kinetics,and air stability of P3-phase K_0.45MnO₂.By optimizing the composition,K_0.45Mn_0.8Cu_0.2O₂（KMCO）with a good cycle performance and a higher rate performance is obtained.The material still possesses a capacity retention of 71%after 100 cycles at a current density of 0.2 C,and delivers a specific discharge capacity close to 60 m Ah g^-1 at a current density of 2 C.The spherical aberration corrected transmission electron microscope is used to deeply study the crystal structure of KMCO at the atomic scale.Geometric phase analysis demonstrates that less stress is observed inside the KMCO compared to the pure KMO.In situ X-ray diffraction technology is used to study the structural evolution of KMCO during the insertion/extraction of potassium ions,and the results show that the reversible phase transition of P3（?）O3 occurred during the charge/discharge process.Fourier infrared spectroscopy proves that KMCO material exhibits good air stability.Controllable preparation of K⁺supported layered oxide K_0.45Mn_0.8Cu_0.2O₂（KMCO）and investigation of working mechanism for sodium storage:P3-phase KMCO with K⁺riveted in interlayer is used in the sodium-ion battery system by electrochemical ion exchange method.The existence of heterogeneous ions sustaining or pinning in alkali-metal layer will greatly stabilize the structure during charging and discharging,restrain the gliding effect of transition-metal layers,and facilitate the insertion/extraction of more alkali-metal ions thereby providing higher specific capacity and cycling life.This cathode achieves the specific discharge capacity close to 111 m Ah g^-1 at 0.2 C between 1.5 and 4.0 V,and exhibits a good cycle performance and rate performance.The capacity retention after 150 cycles is 86%at 0.2 C.XRD analysis after cycling indicates that some potassium ions always exist in layer during the charge/discharge process and play a key role in stabilizing the structure.