Unraveling The Effects Of Cation Distribution In Spinel Cobaltites M_xCo_3-xO₄ On Pseudocapacitive Charge Storage Mechanism

Environmental pollution and energy crisis caused by the wide application of fossil energy have become two major problems in today’s world.The development of new generation devices based on electrochemical energy storage is of great significance for alleviating environmental pollution and solving energy crisis.The intrinsic properties of electrode materials determine the performance,cost and reliability of new energy devices to a large extent.It is of great scientific and practical significance to design high-performance electrode materials via adjusting compositions and“electronic/phase/micro”structures of the electrode material that can promote charge transfer and ion diffusion.Compared to rechargeable batteries,electrical double-layer capacitors can provide higher power density and longer cycle life.However,its low energy density greatly limits its application in many practical scenarios.The pseudocapacitive materials that depends on“reversible surface or near-surface Faraday reaction”to store charge transcend the capacity limitations of electrical double-layer capacitors and the mass transfer limitations of batteries,thus enabling the construction of“high energy-high power density”syupercapacitors.Among various pseudocapacitive materials,spinel oxide Co₃O₄ has attracted much attention in recent years due to its low cost,rich availability,good capacity and many other advantages.However,the electrochemical performance of Co₃O₄ electrode is still much lower than expected due to its poor electrical conductivity and slow electrochemical kinetics.The introduction of environmentally friendly and inexpensive metal elements to form bimetallic oxides(M_xCo_3-xO₄)is considered to be an ideal way to achieve better electrochemical properties,such as high specific capacitance and good redox capacity.However,the geometrical distribution of host and substitutional cations in spinels and their influence on M_xCo_3-xO₄’s pseudocapacitive charge storage mechanism are still vague at atomic level.Based on the above background,the distribution of substituted cations in the octahedral and tetrahedral coordination of spinel M _xCO_3-xO₄ and the effect of redox capacity on the pseudocapacitance behavior of spinel M _xCO_3-xO₄ have been studied at atomic scale.Through theoretical calculation,it is further revealed that the electrochemical activity of octahedral sites comes from the covalent competition between tetrahedral sites and octahedral sites.The main results of this work are as follows:（1）The critically important role of geometrical distribution and intrinsic properties of host and substitutional cations in tetrahedral/octahedral sites in determining spinel cobaltites’pseudocapacitive properties have been identified via XAFS,in-situ Raman,in-situ EIS and ex-situ XPS.The results reveal that i)Zn incorporation leading to cation redistribution,with Zn mainly occupy tetrahedral sites and Co move to octahedral sites.and ii)The substituted Mn mainly occupy the octahedral units,which are preferred link toget her upon cycling,leading to irreversible surface reconstruction to form layeredδ-Mn O₂ that change the reaction mechanism.The electrochemical analysis reveal that substitution of Co with Mn and Zn leads to×4.4 and×2.9 increments in specific capacitance,as well as×71 and×1.7 increments in ion diffusion coefficients.Covalency competition between tetrahedral and octahedral sites has been identified as the intrinsic reason for determining spinel cobaltites’activity via theoretical calculations.The M_O-O interactions present much weaker metal-oxygen covalency upon cycling,indicating easier breakage of M _O-O bonds with Zn and Mn substitution and thus leading to the“exposure”of“octahedrally-coordinated cations”that are active,which are two prerequisites for the spinel’s activity and enhanced charge storage.The surface energy comparison also confirms that the cations in octahedral units are more likely to be exposed due to covalency competition upon cycling,which are served as active sites for pseudoc apacitive reactions.（2）On the basis of the first work,we further regulat e the active site of spinel Mn_xCO_3-xO₄ by simple acid treatment.XAFS data show s that the content of octahedral coordination Co and Mn in Mn_xCO_3-xO₄ do not change significantly after acid treatment,while some tetrahedral coordination cations Co and Mn lost their coordination and dissolve at the interface,resulting in tetrahedral vac ancy.At this time,octahedral coordination cations on Mn_xCO_3-xO₄ surface recombine to form favorable edges of sharing octahedral hydroxyl oxides Co OOH and Mn OOH.The dissolution of tetrahedral coordination cations promots the reconstruction of octahedral coordination Co and Mn on the surface of Mn_xCO_3-xO₄,forming hydroxyl oxides Mn_xCo_1-xOOH with octahedral coordination different from the volume phase structure,thus promoting the exposure of more surface active sites and becoming the real active sites.Elect rochemical analysis shows that a pair of significant redox peaks appeare s in the cyclic voltammetry curve after acid treatment,and the mass ratio of capacitance is nearly doubled.The significantly enhanced electrochemical performance can be attributed to the enrichment of surface octahedral coordination cations on the one hand,and the existence of tetrahedral vacancies can provide a larger interface for carrier embedding,activate more reaction sites and expand faster transport paths on the other hand.