Preparation And Capacitance Properties Of Ca-Mn-O Electrode Materials

Due to its high power density and long cycle life,supercapacitors,a new type of electrochemical energy storage device between secondary battery and common physical capacitor,have attracted extensive attention worldwide.Active electrode materials play a pivotal role in the electrochemical performance of supercapacitors.In this paper,Ca₃Mn₂O₇,CaMn₂O₄ and ABO₃ perovskite oxide CaMnO₃ have been studied comparatively as active materials for electrochemical capacitor from the aspects of the influence of different Ca:Mn ratio and the substitution of Mn with Co and Fe.Given the potential essential role of flexible supercapacitors in the future portable digital gadgets,the way of producing flexible asymmetric supercapacitor are explored.The method of preparing conductive flexible electrode substrate from nonconductive cotton cloth has been preliminary developed.Based on this cost-effective technique,an asymmetric all-solid supercapacitor are constructed and its energy storage property are characterized.The main contents of the paper are as follows:（1）CaMnO₃ powder are prepared by citrate method.The effects of different cooling rates on the phase,morphology and capacitive performance of CaMnO₃ are studied.The results show that CaMnO₃ prepared by furnace cooling is single phase composed and larger in size than CaMnO₃ prepared by water cooling which has tiny amount of CaMnO_2.5.galvanostatic charge-discharge（GCD）tests show that the specific capacitance of water cooled CaMnO₃ is 503 F g^-1,higher than 380 F g^-11 of furnace cooled CaMnO₃ at a current density of 1 A g^-1.The specific capacitance retention ratio after 1500 cycles of charge and discharge is only 30.44%and 38%for water cooled and furnace cooled CaMnO₃,respectively,indicating a pending problem to be addressed.（2）Both Ca₃Mn₂O₇ and CaMn₂O₄ powder are prepared by citrate method.GCD tests show that the specific capacitance of Ca₃Mn₂O₇ at a current density of 1 A g^-1is 119 F g^-1,while the specific capacity of CaMn₂O₄ can reach 441 Fg^-1 under the same conditions.The specific capacitance retention of Ca₃Mn₂O₇ and CaMn₂O₄ after 1500 cycles of charge and discharge at a current density of 1A g^-1are 51%and 32%,respectively.The specific capacitance dwindles as:water cooled CaMnO₃>CaMn₂O₄>furnace cooled CaMnO₃>Ca₃Mn₂O₇.（3）Both CaCoO_2.5.5 and CaFeO_2.5.5 powder are prepared by citrate method.FESEM showed the CaCoO_2.5.5 is sheet shape and uniformly distributed.CaFeO_2.5.5 is spherical and agglomerated.The electrochemical results show that the specific capacitance of CaCoO_2.5reaches 440 F g^-1,higher than 220 F g^-1 of CaFeO_2.5.5 at a current density of 1 A g^-1.After GCD tests at a current density of 1A g^-1 for 1500 times,the specific capacitance retains70.9%for CaCoO_2.5.5 and 15.0%for CaFeO_2.5.The specific capacitance descends in the order of:water cooled CaMnO₃>CaCoO_2.5>furnace cooled CaMnO₃>CaFeO_2.5.（4）CaMnO₃ are supported on a flexible conductive cloth to prepare an asymmetric flexible supercapacitor.The electrochemical tests show an excellent result with energy density of the flexible supercapacitor to be 17.9 Whkg^-1 at a power density of 750 Wh kg^-1.