The Study Of Storage Mechanism Of Multicalent Cations In ?-MnO₂

As an extensively applied material for energy storage,manganese dioxide has been utilized in various fileds of battery,such as primary alkaline manganese battery,rechargeable lithium ion battery,sodion battery and various of novel batteries.In the family of manganese dioxide,?-MnO₂ has been detailedly researched due to its favourable crystal form and great electrochemical performance.However,so far most researchers are focusing on the improvement of electrochemical performance,and there are few researcher focues on the energy storage mechanism between?-MnO₂ and different cations by both calculation and experiment.The comprehensive research method composed by the first principle calculation and experimental verification are used to explore the storage behaviour of five kinds of cations(Na⁺,Ca²⁺,K⁺,Ba²⁺and Zn²⁺)in?-MnO₂,and the reaction mechanism between Zn²⁺and?-MnO₂ is been systematically studied.The First-principle calculation method is used to optimize the lattice parameter.Because the storage mechanism of Li⁺and Mg²⁺in?-MnO₂ has been widely studied,our research focues on alkalis Na⁺and K⁺,which are in the same main group with Li⁺,and the alkaline earth Ca²⁺and Ba²⁺,which are in the same main group of Mg²⁺.It has been revealed in our research that these four cations are thermodynamicly able to insert into?-MnO₂,but they have different storage behavior with Li⁺and Mg²⁺.Because the insertion process is limited by the radius of cations,the insertion concentration of Li⁺is able to reach LiMnO₂,while the all four cations are not able to reach A_0.25MnO₂?A is the cations?.The electrochemical performance of supercapacitors assembled by the four cations electrolyte are measured.The agreement between experimental results and calculated results verifies the storage mechanism of multicvalent ions,which means under certain conditions,the univalent cations are able to be replaced by muiltivalent cations to obtain better electrochemical performance.Based on the conclusion above,combining with experiment test,we devided the calculation procedure into insertion path and conversion path to further explore the controversial reaction mechanism in Zn²⁺/?-MnO₂ reaction.When the whole insertion process of Zn²⁺in?-MnO₂ was simulated,it is revealed that the largest concentration of Zn²⁺are not able to reach Zn_0.25MnO₂,accompanying by a insertion voltage of 1.08 V.The formation energy of conversion reaction is calculated.Comparing with the insertion path,we find that the insertion reaction and conversion reaction are complementary.The insertion process of Zn²⁺is the requirement for the conversion reaction of inner?-MnO₂,as well as the conversion reaction to generate ZnMn₂O₄ phase is thermodynamically easier to proceed.This conversion reaction voltage is 1.54 V,corresponding to the first platform in the charge-discharge curve.The XRD and TEM phase analysis proves that the two main reactions in the zinc ion battery is the insertion-conversion reactions of both proton and zinc ion.As a result,the hybrid raction mechanim in the zinc ion battery is revealed.