Study On Mn-based Mixed Metal Oxide For Electrochemical Energy Storage

As two important electrochemical energy storage devices,lithium ion batteries?LIBs?and supercapacitors?SCs?are promising technological choices for large-scale applications in virtue of their high efficiency of energy storage,high energy/power density,and long life span.Generally,both LIBs and SCs are composed of three essential components: cathodes,anodes,and electrolytes.The performances of these electrochemical devices show strong dependence on the nature of electrode materials.Therefore,further breakthroughs in electrode materials are crucial to develop next-generation high-efficiency electrochemical energy storage devices.Among all available materials,Mn-based oxides have attracted considerable attention due to their environmentally benign nature and rich redox reactions involving different ions.However,application of Mn-based oxides is still largely hampered by their intrinsic low charge/ionic conductivity and pronounced volume expansion and contraction during charging/discharging processes,resulting in the pulverization of electrode film.In recent years,Mn-based mixed metal oxides,especially those in the form of nanostructures,are emerging as promising electrode materials for energy conversion and storage owing to their complex chemical composition and enhanced electrical conductivity.In this project,we focus on the rational synthesis of Mn-based mixed metal oxides,as well as their applications in LIBs and SCs.The main research findings are listed below:Tubular nanostructures?NTS?of Mn-based?Co-Mn,Ni-Mn,Cu-Mn,Zn-Mn?mixed metal oxides have been synthesized via a facile hydrothermal and subsequent annealing method.The tubular nanostructures are constructed by randomly oriented ultrathin nanoflakes,which are interconnected with each other to form a hierarchical framework.The resultant tubular nanostructures are advantageous as electrodes for electrochemical energy storage.It is shown that the Co-Mn-NTS sample exhibits outstanding electrochemical performance as electrode materials for LIBs and SCs.Double-shelled Mn-based mixed metal oxides hollow spheres have been developed through a facile hydrothermal approach and subsequent thermal treatment.The distinct micro-/nanostructures endow Co-Mn mixed oxide high capacitance,outstanding rate and cycling performances as electrode for SCs.In addition,asymmetric supercapacitors?ASCs?have been fabricated by assembling Co-Mn mixed oxide cathode and CMK-3 anode in two-electrode simulation cells.The ASCs display a high energy density of 37.1 Wh kg^-1 at a power density of 417.7 W kg^-1 with a stable potential window of 1.6 V without obvious polarization curves.The CNF@Co-Mn mixed metal oxide core-shelled nanofibers have been produced by using CNFs as hard templates,while Co-Mn oxide shells show a complex hierarchical structure constructed by randomly oriented ultrathin nanoflakes interconnected with each other.Benefiting from the robust support of CNFs,the CNF@Co-Mn oxide core-shelled nanofibers show outstanding electrochemical stability with high capacitance retention of 96.9% after 10000 cycles as electrode for SCs.Meanwhile,the transport of electrons is also facilitated along the conductive CNF cores.As a result,the CNF@Co-Mn mixed metal oxide core-shelled nanofibers can still deliver high reversible capacitance at high current rate.Hierarchical porous carbons?HPCs?have been synthesized through a simple thermal treatment of a mixture of sulfonated pitch precursor and KOH.When evaluated as an electrode material for electrical double layer capacitors?EDLCs?,these HPCs exhibit exceptional capacitive behavior with high specific capacitance(157 F g^-1 at 100 A g^-1)and excellent stability?98.4% retention after 10000 cycles?.The sample also presents outstanding electrochemical performance in 1 M Li2SO4 and 1 M TEA BF4/PC electrolytes.