Supercapacitors And Related Materials Research

In recent years,supercapacitors have attracted global attentions due to its fast charge/discharge,high efficiency,long cycle life,wide temperature range and high reliability.However,its energy density is much lower than that of secondary batteries, e.g.lithium-ion batteries.According to the energy equation of supercapacitor E=1/2 CU²,two effective approaches can be used to improve the energy density of supercapacitors:One is to increase the specific capacitance of the electrode material (C);the other is to promote the output voltage(U).The present work focuses on improving the capacitance of carbon by the following method:1).Optimize the pore structure of highly ordered mesoporous carbons(OMCs) in both the pore length and pore size,2).Develop a novel non-porous electrode material:high surface-area graphite(HSG),it showed a high volumetric specific capacitance as well as low cost compared to the conventional activated carbon(AC);3).Load an organic polyradical material onto AC to obtain a composite electrode with higher capacitance.Finally,we also developed a novel nonaqueous hybrid capacitor of high work voltage by introducing a 5 V lithium intercalation compound LiNi_xMn_2-xO₄ as the positive electrode and AC as the negative electrode.1.Synthesis of OMCs with different pore lengths by hard-template method and its capacitance propertiesOMC has attracted much attention due to its highly ordered pore structure and narrow pore size distribution in mesopore range.We synthesized two OMCs with same pore size(4 nm) but different pore length by using highly ordered mesoporous silica as the hard temple and sucrose as the carbon precursor through a successive process of solution impregnation,pyrolysis,and template elimination.The pore length of LOMC derived from the conventional SBA15 template was over 2μm,while SOMC derived from a novel short-axis-oriented-growth SBA template showed a much shorter pore length of 200～300 nm.Shown by the cyclic voltammetry,a maximum specific capacitance of 14μF/cm² was obtained for SOMC in 6 M KOH solution compared with a capacitance of 10μF/cm² for LOMC.The electrochemical impedance analysis indicated that LOMC accounted for a larger ion diffusion resistance than SOMC.Moreover,the capacitance of LOMC also decayed faster than that of SOMC in the case of low electrolyte concentration.The electrochemical results suggested that SOMC can provide more facility for electrolyte accessibility and rapid ion diffusion thus it exhibited higher surface efficiency and better rate capability than LOMC.2.Synthesis of OMCs with different pore sizes by self-assembly and its electrochemical propertiesDi-OMC with a pore diameter of 3.1 nm and tri-OMC with a pore diameter of 6.7 nm were synthesized by organic-organic di-constituent self-assembly and organic-inorganic-organic tri-constituent co-assembly,respectively.As the electrode material for supercapacitor,tri-OMC showed a large specific capacitance of 117 F/g in organic electrolyte as well as a good retention of capacitive characteristic even at a high scan rate of 200 mV/s;while di-OMC showed nearly no capacitance in nonaqueous electrolyte since its surface area was difficult for the accessibility of organic electrolyte due to its small pore size and high proportion of micro-porosity. In aqueous electrolyte whose ion size was quite smaller than that of organic electrolyte,the surface area of di-OMC can be easily accessed by electrolyte thus it showed a high surface efficiency,e.g.a capacitance of 117 F/g was obtained.As for tri-OMC,it showed a specific capacitance as large as 210 F/g in aqueous electrolyte, and its rate capability was much better than that of di-OMC due to its large pore size. The results indicated that larger pore size was helpful to get better rate capability; while for specific capacitance,different electrolyte desired different pore size,a highest surface efficiency can be reached when the pore size was most appropriated for the electrolyte ion.In addition,we also investigated the possibility of these two carbons as the anode material for lithium-ion battery.Tri-OMC exhibited a reversible capacity of 1048 rnAh/g,nearly three times of that of di-OMC,and the cycle performance of tri-OMC was also superior to that of conventional hard carbon materials.3.Organic polyradical-carbon composite as the electrode material of supercapacitorA quite stable organic nitroxide polyradical material(PTMA, Poly-2,2,6,6-tetramethylpiperidinyloxy methacrylate) was synthesized by a successive process of esterification,polymerization and oxidation。It showed a specific capacity of 82 mAh/g in 1 M LiClO₄/PC electrolyte.When discharged at a high current rate of 50C,it can still exhibit a flat voltage plateau,implying its much higher power potential.However,PTMA was an insulator,which limited its application in supercapacitor.We creatively loaded PTMA onto the high surface activated carbon to prepare a capacitor-battery composite(PTMA-AC) through a dispersing-depositing procedure.The capacity of this composite electrode was 30%larger than that of pure AC electrode.A hybrid capacitor fabricated by a PTMA-AC composite as the positive electrode and AC as the negative electrode exhibited good rate capability as well as long cycle life.AC in the composite served as both active material and electronic conducting support during charge/discharge,by this means,the advantages of AC and PTMA can be well combined.This present work provided an effective approach for the application of other nitroxide polyradical materials in supercapacitor.4.HSG obtained by high-energy ball milling and its application in EDLCA new kind of electrode material called high surface-area graphite(HSG) was developed for EDLC through a high energy ball milling method using natural graphite as the starting material.The BET surface of HSG can be increased from 7 m²/g to 580 m²/g by high energy ball milling.Long time milling led to a much high specific capacitance over 200 F/g but at the same time with deteriorated rate capability due to the decreased conductivity caused by the strong destroy of graphite crystal.HSG sample obtained at a medium milling time can be employed as a good electrode material for EDLC in both aqueous and nonaqueous electrolytes.The large specific capacitance of HSG was demonstrated to derive of not only the high specific surface area but also the high density of lattice defects and surface functional groups produced during high energy ball milling.Attribute to the large quantity of graphite microcrystallines,HSG possessed of a better conductivity than AC thus it can exhibit good power ability.The cycle performance showed that HSG can well maintain its capacitance during 5000 cycles charge/discharge processes.Moreover,the low porosity of HSG promised a high electrode density thus led to a much higher volumetric energy density than AC.The results demonstrated that HSG is hopeful to be commercialized especially take its low cost,simple preparation and good electrochemical properties into consideration. 5.A novel nonaqueous hybrid capacitor using high voltage lithium intercalation compound as the positive electrode and activated carbon as the negative electrodeA high voltage lithium intercalation compound LiNi_0.5Mn_1.5O₄ was first introduced as a positive electrode for a nonaqueous hybrid capacitor combined with an AC negative electrode in 1.0 M LiPF₆ EC/DMC electrolyte.A Ni completely doped compound LiNi_0.5Mn_1.5O₄ was synthesized by a sol-gel process,it showed a single flat discharge plateau at 4.7 vs.Li⁺/Li with a large capacity of 123 mAh/g.The novel hybrid capacitor fabricated by LiNi_0.5Mn_1.5O₄ positive and AC negative with a weight ratio of positive:negative=1:3 showed a capacity of 26 mAh/g in comparison with 17 mAh/g of an EDLC;and the average work voltage of the hybrid capacitor was also promoted to 2.1 V compared with 1.4 V of an EDLC.Both the increase of average voltage and specific capacity resulted in a much high total energy density of 55 Wh/kg for the hybrid capacitor,approximately twice that of an EDLC. The rate performance showed that the hybrid capacitor can maintain 80%of its initial capacity at a discharge current of 10C.After 1000 cycles charge/discharge,the capacity loss was also lower than 20%,showing a desirable cycling ability. Moreover,the charge/discharge process of this novel hybrid capacitor was associated with the transfer of Li-ion between the two electrode,which overcame the drawback of electrolyte depletion during charge process in conventional EDLC and other hybrid systems such as Li₄Ti₅O₁₂/AC.