Preparation Of Cathode Material For Aqueous Zinc Ion Batteries And Its Electrochemical Performance

Aqueous zinc ion battery（AZIB）has attracted extensive attention in recent years due to its environmental benignity,operational safety,and high energy and power density.Manganese based materials,with considerable voltage and high theoretical capacity,have been reported as one of the most potential cathodes for AZIBs.Among them,the layer type MnO₂ possesses a large interlayer spacing,which allows for a fast diffusion pathway for Zn²⁺ions with alleviated phase transformation.The main problems of MnO₂ remain in the inferior electronic conductivity,sluggish diffusion kinetics and structural degradation during chare/discharge processes,resulting in fast capacity decay,which greatly limits the application in AZIBs.This thesis is addressed to improve the electrochemical performance of MnO₂.The main contents are as follows:（1）K⁺incorporated intoδ-MnO₂（K-δ-MnO₂）are prepared by a facile redox method.The pre-intercalated K⁺acting as structure stabilizing“pillars”allows to tune lattice space and promotes diffusion kinetics in tunnel structures.Furthermore,the presence of cations introduces shallow donor levels to improve the electronic conductivity of hosts and activate more active sites.Besides,the co-intercalating water screens the intercalation between the intercalated ion and cathode,leading to faster intercalation processes.The as-prepared K-δ-MnO₂ cathode exhibits relative high capacity(274 m Ahg^-1at 0.3Ag^-1)and impressive cycling stability(81%retention after1300 cycles at 3Ag^-1).The energy storage mechanism is clarified as H⁺/Zn²⁺coinsertion/extraction via CV and ex-situ XRD characterization.（2）A rapid mass production molten salts method is used to synthesize 2Dδ-MnO₂nanosheets pre-intercalated with alkali ions（K⁺,Na⁺）.The roles of the pre-intercalated ions are investigated by applyingδ-MnO₂with different kinds and varying amounts of alkali ions as cathodes for AZIBs.K⁺with larger radius than Na⁺is found to facilitate the rapid and reversible（de-）insertion of Zn ions by expanding Zn²⁺migration channels and reducing the electrostatic repulsion between Zn²⁺and host materials.The Zn-K-δ-MnO₂ system delivers superior capacity retention of 72%over 300 cycles at 3 Ag^-1 as compared to 52%with Zn-Na-δ-MnO₂ system.（3）Both methodologies are simple and easy upscaling from the laboratory to industry,especially the molten salts one,which is a rapid mass production method.On the other hand,the K-δ-MnO₂ obtained by molten salts method exhibits inferior cycle performance as compared to the one prepared by the facile redox method,although it possesses similar structures but higher contents of K⁺.This could be attributed to the lower contents of crystal water,which acts as pillars to stabilize the layered structures,promotes the Zn²⁺insertion and therefore plays a key factor in improving the electrochemical performance of AZIB systems.This work reveals the elevated electrochemical performance through the elaborate structure design,providing insights for high-performance cathode materials for AZIBs.