| Aqueous secondary zinc batteries feature high theoretical capacity(820 mAh g-1 for Zn metal anode),relatively low redox potential（-0.76 V vs.SHE）,high safety,and low cost,which are considered as the competitive candidates for next-generation high-performance electrochemical energy storage devices.However,the practical application of Aqueous secondary zinc batteries is still hindered by the problems of dendrite growth and unfavorable side reactions of hydrogen evolution and corrosion.Therefore,fabrication of Zn anode with high reversibility is of vital importance to promoting the development and commercial application of Aqueous secondary zinc batteries.This dissertation focuses on the design,preparation,electrochemical performances,and relative mechanism of highly reversible Zn anodes.By increasing the gain size of Zn anode to decrease its grain boundary number or mechanical polishing to decrease the surface roughness of Zn anode,the reversibility of Zn anodes are highly improved.The main progresses are summarized as followed.（1）By enlarging the grain size of Zn anode to decrease its grain boundary number in a given surface area,the problems of Zn metal dendrite growth and unfavorable side reactions are effectively depressed.As reported,the grain boundary in Zn metal has relatively high surface energy,which is favorable for the dendrite growth and unfavorable side reactions.In this study,the grain size of Zn anode is effectively controlled and the Zn anode with large grain size is obtained by modulating the annealing temperature and time.The characterizing results show that the grain boundary number decreases with the increasing of grain size.Thereby the problems of dendrite growth and unfavorable side reactions are gradually depressed,which finally promote the reversibility of Zn anode.The Zn||Zn symmetric cell fabricated by Zn foil with the average grain size of around 300μm exhibits a long cycling performance of 570 h under 1 mA cm-2 and 1 mAh cm-2.The Zn||Cu asymmetric cell shows high reversibility over 300 cycles with high Coulomb efficiency of～98%under the test condition of 5 mA cm-2 and 1 mAh cm-2.By coupling with porous carbon positive electrode,the capacitor steady cycles for over 20000 cycles under the current density of 2 A g-1.The fabricated Zn||MnO2 battery also exhibits a high cycling performance for 300 cycles with high capacitance retention of 96%under the current density of 0.5 A g-1,exhibiting the high reversibility of the optimized Zn anode.（2）By mchanical polishing to decrease the surface roughness of Zn anode,the dendrite growth and nfavorable side reactions are effectively depressed.Commercial Zn foil undergoes multiple processes such as winding,calendering,and cutting during the preparation process,which leads to cracks and protrusions on the surface of Zn foil,resulting in increased surface roughness.This is unfavorable for the high reversibility of Zn anode.In this work,machine polishing was used to reduce the roughness of the Zn foil surface,which greatly promote the reversibility of Zn foil. The results show that the reduction of the surface roughness of the Zn metal anode can effectively improve its cycling stability.The Zn||Zn symmetric cell assembled with polished Zn foil（polished with 10000 mesh diamond grinding paste for 10 min）can cycle for 280 h under 1 mA cm-2 and 1 mAh cm-2,which is three times longer than the original Zn foil under the same test conditions.When assembled with a porous carbon cathode to form a Zn ion capacitor,it can maintain～98% Coulomb efficiency during 20000 cycles.The fabricated Zn||MnO2 battery also exhibits a high cycling performance for 300 cycles with high capacitance retention of 99%under the current density of 0.5 A g-1,exhibiting the high reversibility of the optimized Zn anode.The two methods used in this study for elevating the reversibility of Zn anode are all featured with simple fabrication process and low cost,which should be favorable for the practical application of Aqueous secondary zinc batteries. |
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