Energy Storage Mechaniam Investigation And Improvement Strategies For Aqueous Rechargeable Zinc-manganese Batteries

Under the national development strategies of“peak emissions”and“carbon neutrality”background,the society exhibits great demands for renewable and clean energy.Due to the intermittent and uncertainty of clean renewable energy,developing an efficiency grid-scale energy storage system(EES)is the key to make the most advantage of it.Among various advanced energy storage equipment,aqueous rechargeable zinc-manganese(AR-Zn/Mn)batteries appear to be hugely promising for use in grid-scale ESS because of their low price and toxicity,high reliability,and softy.Unfortunately,the application and developments of the AR-Zn/Mn battery still suffer from the poor cycling stability.The poor reversibility,complicated structure and phase evolution of MnO₂cathode have the main responsibility for the inferior stability.Although a lot of strategies have been proposed to improve the stability of cathode,such as composite,doping and ion pre-intercalation of MnO₂,improvement of electrolyte and optimization of charge-discharge program,the improvement of the electrochemical performance is still limited because of the unclear and contradictory energy storage mechanism of AR-Zn/Mn batteries.Based on the above analysis,regarding improving the electrochemical stability of the AR-Zn/Mn batteries as the goal,beginning with the investigation of energy storage mechanism,this article carries out a series of researching work about AR-Zn/Mn batteries.The electrochemical reaction mechanism of Mn²⁺conversion deposition and the deposition-dissolution energy storage mechanism assisted by ZSH are proposed,which well explain a series of electrochemical phenomena in AR-Zn/Mn batteries.Based on the above two reaction mechanism models,AR-Zn/Mn batteries with curable capacity and AR-Zn/Mn batteries without MnO₂ cathodes that can be cycled stably are respectively proposed,which greatly improve the cycle stability of the battery.The main contents are as follows:(1)AR-Zn/Mn batteries using Mn²⁺as the electrolyte additive have recently attracted remarkable attention owing to their largely improved cycling stability.The conventional mechanism holds that the improved stability after introducing the Mn²⁺electrolyte additive is caused by suppressing the dissolution of MnO₂.Herein,the experiment reveal that the Zn/MnO₂ batteries with Mn²⁺additive still exhibit rapid capacity fading when cycling between 0.8 and 1.6 V vs.Zn/Zn²⁺,its improved performance is only observed when charged to a higher slope region(>1.6 V vs.Zn/Zn²⁺),which strongly disprove the viewpoint about the improved stability of AR-Zn/Mn batteries.Inspired by this discovery,the Mn²⁺conversion deposition reaction behaviore is scrutinized and proved for evaluating the performance of the AR-Zn/Mn batteries after using Mn²⁺as the electrolyte additive.During the charging process of 1.5-1.6 V vs.Zn/Zn²⁺,Zn_xMnO(OH)₂ nanosheets was deposited on the electrode surface and then partly converted into MnO₂ nanoparticles with low crystallinity during the high potential area(1.6-1.8 V vs.Zn/Zn²⁺).During the discharge process,the Znx MnO(OH)₂ nanosheets that did not undergo a conversion reaction were dissolved.Obviously,during the electrochemical cycle,Mn²⁺is converted and deposited on the electrode surface through a mesophase Zn_xMnO(OH)₂ nanosheet,form MnO₂ nanoparticles.By adding a certain amount of Mn²⁺into the electrolyte,the batteries can improve the capacity and cycling ability through the converted electrodeposition of Mn²⁺on the electrode surface and form MnO₂ nano-particles.Finally,through a special zinc/carbon nanotube(Zn/CNTs)battery,the proposed Mn²⁺conversion deposition reaction behavior is further verified.(2)Through the research in the last chapter,it is easy to know that as the cycle progresses,continuous and uncontrolled deposition and dissolution of manganese oxide can cause the cathode and electrolyte to change drastically.Once a critical point is reached,the capacity of the battery will rapidly fade.Consequently,Therefore,this work proposes a capacity self-healing AR-Zn/Mn batteries(Zn/cCNTs-MnO₂)using carboxyl-modified carbon nanotubes(cCNTs)as the cathode substrate,ZnSO₄+MnSO₄ mixed aqueous solution as the electrolyte,and Zn foil as the anode.Based on the controllable electrodeposition reaction of MnO₂,the specific capacity of Zn/cCNTs-MnO₂ batteries can be achieved or recovered by operating several cycles under a low current density(0.1m A cm^-2).Then,the batteries can stably perform under a high current density(1 m A cm^-2).By repeating the above steps,a capacity self-healing usage scheme was established,which can significantly improve the cycling performance of Zn/cCNTs-MnO₂ batteries.(3)Through careful study of the electrochemical process of AR-Zn/Mn batteries,it is easy to find that some electrochemical phenomena that cannot be explained by the traditional reversible ion intercalation and deintercalation mechanism of energy storage,which overshadows the further development of AR-Zn/Mn batteries.shadow.Herein,by using Zn₄SO₄(OH)6·4H₂O(ZSH)as the cathode material,using ZnSO₄+MnSO₄ as the electrolyte,we propose a ZSH-assisted deposition-dissolution model to depict AR-Zn/Mn batteries.During the charging process,the ZSH on the surface of the electrode can drive the Mn²⁺in the electrolyte to undergo an electrodeposition reaction,thereby form Znx MnO(OH)₂ nanosheets on the surface of the electrode.In the subsequent discharge process,the Znx MnO(OH)₂ nanosheets also undergo proton solid-phase reaction,but its high activity and the reformation of ZSH that can change the pH value of the electrode surface,causing the Znx MnO(OH)₂ nanosheets to undergo 2-electron transfer and dissolution.In AR-Zn/Mn batteries using MnO₂ as the cathode material,the MnO₂cathode undergoes a proton solid-phase reaction during the first discharge process,which causes a large amount of ZSH to be deposited on the electrode surface and dissolves Mn²⁺into the electrolyte.Therefore,in the subsequent charge and discharge process,the MnO₂cathode can undergo a ZSH-assisted Znx MnO(OH)₂ nanosheet deposition-dissolution reaction.Obviously,the proposed ZSH-assisted deposition-dissolution reaction model demonstrates that the reversibility of AR-Zn/Mn batteries results from the deposition of Mn²⁺derived by the discharge by-product of ZSH.(4)Similar to MnO₂ electrode,the ZnO,MgO,and CaO electrodes can also induce the deposition reaction of ZSH in the ZnSO₄+MnSO₄ electrolyte.Based on this,according to the proposed ZSH-assisted deposition-dissolution mechanism,a kind of no MnO₂ cathode AR-Zn/Mn batteries are proposed,including Zn/ZnO,Zn/MgO,and Zn/CaO.All of the proposed batteries exhibit the same reversible energy storage behaviors with AR-Zn/Mn batteries in electrolyte with Mn²⁺additive.Then,in order to improve the cycling stability of no MnO₂ AR-Zn/Mn batteries,a PO₄^3-electrolyte additive is successfully applicated,which can form a stable Zn₃(PO₄)₂·3H₂O phase on the electrode surface.It can maintain the stability of electrode structure,and further improve the cycling performance.Specifically,under the current density of 1 A g^-1,the Zn/ZnO batteries can stably operate 200 cycles with no obvious capacity decay.In summary,this paper creatively proposed Mn²⁺conversion deposition behaviors and ZSH-assisted deposition-dissolution energy storage mechanism.At the same time,based on the proposed reaction model,two novel AR-Zn/Mn batteries are proposed,the special composition,and superior cycling stability broaden the possibilities of the AR-Zn/Mn batteries.Obviously,this work is a creative innovation of AR-Zn/Mn battery theory and technology,which will greatly promote its further development.