Preparation Of Soft Carbon-based Microcrystalline Carbon And Its Electrochemical Capacitance Characteristic

Supercapacitors are widely used in new energy vehicles,rail transportation,industrial production for its high power density,long cycle life,fast charging and discharging.Compared with secondary batteries,the energy density of supercapacitors is low,which limits further application of supercapacitors.Therefore,improving the energy density of supercapacitors has become the major objective of supercapacitor research,and increasing the specific capacitance and operating voltage are two strategies to achieve this objective.This paper focuses on the carbon electrode materials for supercapacitors.The first step is to obtain the microcrystalline carbon electrode with high specific capacitance and good withstand voltage characteristics by preparing the new type of soft carbon-based microcrystalline carbon.The second step is to further improve the specific capacitance and withstand voltage characteristics of the microcrystalline carbon electrode by changing the use of organic electrolyte.Ultimately,the improvement of the energy density of supercapacitors can be achieved.Using mesocarbon microbeads（MCMBs）as raw materials and KOH as activator,the new type of MCMB-based microcrystalline carbon was prepared by pre-carbonization and KOH activation.The effect of alkali-carbon ratio（KOH usage ratio）on the morphology,structure and capacitive performance of MCMB-based microcrystalline carbon were studied by structural characterization methods such as SEM,HRTEM,XRD with electrochemical performance test such as galvanostatic charge-discharge.The results show that the interlayer spacing of graphite-like crystallites of MCMB-based microcrystalline carbon increases relatively with the increase of alkali-carbon ratio,the specific capacitance and withstand voltage characteristics of the electrode are also improved.The electrode of microcrystalline carbon（NMC4）prepared with an alkali-carbon ratio of 4:1,not only exhibits a discharge specific capacitance of 129.1 F/g for the first time,but also withstand a high voltage of 3.5 V.The NMC4 prepared with the optimal alkali-carbon ratio was used as the electrode material,and the capacitive behavior of NMC4 electrode in DMPBF₄/PC and TEABF₄/PC electrolytes with different cation sizes were studied by electrochemical performance test such as galvanostatic charge-discharge.The results show that the new type of DMPBF₄/PC electrolyte further improves the specific capacitance and withstand voltage characteristics of the NMC4 electrode:compared with TEABF₄/PC,the NMC4 electrode in DMPBF₄/PC not only exhibits a discharge specific capacitance of 145.5 F/g for the first time,but also withstand a high voltage of 3.7 V.The supercapacitor assembled with NMC4 electrode and DMPBF₄/PC electrolyte finally obtain a high energy density of 65.6 Wh/kg.In addition,in this paper,an innovative 3.5 V cut-off voltage charge-discharge method（3.5 V activation）was used to improve the rate capability and cycling performance of MCMB-based microcrystalline carbon electrodes at regular 2.7 V voltage.Among them,the rate capability and cycling performance of the NMC4electrode show a good improvement:in DMPBF₄/PC,after 3.5 V activation,the discharge specific capacitance of NMC4 electrode in rate performance test decreases from 100.3 F/g for the first time to 88.8 F/g for the 80th time,and the capacitance retention reaches 88.5%,while the discharge specific capacitance of NMC4 electrode in cycle performance test decreases from 92.5 F/g for the first time to 85.9 F/g for the500th time,and the capacitance retention reaches 92.9%.