| Anode materials are typically considered as an important component to influence the electrochemical energy storage performance of batteries.Hard carbons are promising anode materials due to their advantages such as high specific surface area,abundant pore structure and low production cost.Several modification strategies,including electrolyte engineering,surface chemical functional groups and microstructure design,have been proposed to address certain key problems such as low initial coulombic efficiency（ICE）and poor cycling performance.However,a fundamental understanding and systematic investigation on origin of the above-mentioned issues are still needed.Our article mainly focuse s on elucidating the relationship of structure-electrochemical performance-potassium storage mechanism of hard carbon,and figuring out the influence of morphologies,structures and pore size distributions of hard carbon materials on potassium storage.Usin g in-situ characterization techniques（in-situ Raman spectroscopy and alternating-current impedance）,we have explored the structural evolution and kinetics of carbon anodes upon K+intercalation/de-intercalation,and clarified the key factors that influen cing potassium ion storage.Our results reveal the internal relationships among the structure-properties-mechanism of hard carbon,and provide a thorough understanding of the effects of morphology and structure on hard carbon’s potassium storage performance.（1）The relationship between the microstructure of hard carbon materials and the potassium storage performance is systematically studied.The porous hard carbon anode materials with spherical structure（HC-A）,micro/mesoporous yolk-shell structure（HC-B）and mesoporous sheet structure（HC-C）are prepared by spray pyrolysis combined with salt template assisted method.In-situ Raman and in-situ AC impedance techniques are used to systematically study the potassium storage mechanism,kinetics and electrochemical performance of potassium storage.Our results show that:1)Potassium ions are absorbed on the defect-/edge-sites and/or trapping in micropores at high potential region（denoted as capacitive contribution）,and intercalated into the graphite layers at low potential（diffusion-controlled charge contribution and/or intercalation contribution）;2)Micropores are beneficial to provide more potassium storage sites and enhance the specific capacity.Compared with micropores,mesoporous structure could enhance the ion diffusion kinetics,and the rate performance of hard carbon anode is significantly improved;3)Our results indicate that the porous hard carbon（HC-B）with rich micro/mesoporous structure exhibit s relatively high ICE（44.4%）,and could maintain a reversible specific capacity of 197.7m Ah g-1 after 200 cycles at 0.1A g-1 current density,showing excellent electrochemical potassium storage characteristics,which is due to the high specific surface area,abundant porous structure and large interlayer spacing.In addition,the potassium ion hybrid capacitor construct with HC-B as the negative electrode exhibits a perfect electrochemical performance(74.5 Wh kg-1,184.4 W kg-1).（2）The relationship between the graphitization degree of hard carbon materials and the performance of potassium storage is systematically studied.The hard carbon materials with different graphitization degrees（ID/IG:2.34（HC-600）>2.22（HC-1000）>1.84（HC-1400）are synthesized.The effects of carbonization temperature on the structural characteristics of hard carbon（graphitization degree,layer spacing）are elucidated systematically.The effects of graphitization degree difference on the electrochemical reaction characteristics,reaction kinetics and electrochemical properties are investigated by using a variety of spectroscopic characterizations and galvanostatic intermittent titration technique（GITT）.Our results show that:1）At low potential region,hard carbon materials with high graphitization degree exhibit more specific capacities due to intercalation contribution;2)At high potential region,the low graphitization hard carbon materials show higher specific capacities originated from adsorption/desorption mechanism which could promote the rapid diffusion of ions and maintain excellent structural stability.HC-600 hard carbon materials with low graphitization degree show good cycling stability and rate performance.At the current density of 0.1 A g-1,the high reversible specific capacity of 170.7 m Ah g-1 is obtained after 280 cycles with a high specific capacity retention（82.6%）,which is much higher than those with high graphitization degree.Moreover,HC-600 exhibits a relatively high reversible specific capacity of 164.5 m Ah g-1 at a high current density of 5 A g-1. |
