In-situ Transmission Electron Microscopy Study On The Electrochemical Reaction Mechanism Of Potassium Secondary Batteries

Potassium secondary battery is a new type of energy storage and conversion device.It is an extension of lithium secondary battery.It has the advantages of abundant raw materials,low cost,and good cycle performance.It may be used as a low-cost and long-life battery in the future.The number of related studies on potassium secondary batteries is also growing rapidly,but research on its microscopic reaction mechanism is still in its infancy.The microscopic scale study of the morphology,structure,and composition of the potassium secondary battery during the reaction process is helpful for understanding the mechanism of the battery.It can also provide microscopic guidance for improving battery performance.In this paper,several typical nano-electrode materials are selected,includingα-manganese dioxide（α-MnO₂）nanowires,copper oxide（Cu O）nanowires and carbon nanotubes（CNTs）.By using in-situ transmission electron microscopy,it is possible to explore the real-time change of morphology,structure,and composition in potassium secondary batteries（including potassium ion batteries,potassium oxygen batteries,and potassium carbon dioxide batteries）on the nano-level,especially in the potassiation/depotassiation process.The energy storage mechanism in the battery explains the failure mechanism of the material in the energy storage process,and provides a microscopic theoretical basis for finding suitable nano-electrode materials or modifying nano-materials.A potassium ion nano-battery is constructed by usingα-MnO₂ nanowires with channel structure as the positive electrode material.The electrochemical potassiation/depotassiation process ofα-MnO₂ nanowires is observed in-situ by means of applied voltage.The nanowire shows obvious volume expansion during the potassiation process,and theα-MnO₂ converts to K_xMnO₂.The volume of the nanowire can not be restored to the initial state during the depotassiation process,which explains the low efficiency and the faster capacity decay of the first cycle exhibited by the coin-type battery performance.In the environmental transmission electron microscope,α-MnO₂ nanowires are used as the air cathode material to construct a potassium oxygen nano-battery.The electrochemical oxygen reduction reaction（ORR）process ofα-MnO₂ nanowires is observed in-situ by means of applied voltage,α-MnO₂ exhibits volume expansion during ORR process and converts to manganese oxide（MnO）.At the same time,a stable ORR product,potassium superoxide（KO₂）,is produced on the surface of the nanowire.It is found that the ORR process can be promoted by coating the nanowire with nano-glod.These results provide a basis for the realization of potassium-oxygen batteries.In the environmental transmission electron microscopy,Cu O nanowires are used as air cathode materials to construct potassium-oxygen batteries in nano-scale,and the possibility of non-channel structure of conventional nanomaterials as air cathodes is explored.In-situ electron microscopy characterization is used to observe the electrochemical ORR process of Cu O nanowires.The abnormal volume shrinkage phenomenon is observed.Cu O is transformed into porous nano copper,which is explained by the electrochemically driven Kirkendall effect.A potassium-carbon dioxide nano-battery is constructed in ETEM by using CNT as the air cathode material.The electrochemical carbon dioxide reduction process of CNT nanowires is observed in-situ by means of applied voltage.The reaction mechanism of charging and discharging process is clarified.The carbon monoxide（CO）is produced during the discharging process,and the carbon in the CNT is consumed during the charging process to achieve multiple reversible cycles,which provides a microscopic reaction mechanism for the design of potassium-carbon dioxide battery.