Study On The Preparation And Electrochemical Properties Of Anode Materials For Hybrid Capacitor With High Specific Energy Density

With increasing request for intelligent grid and green energy,supercapacitor has been regarded as one key direction for energy storage and power technologies because of its high charge/discharge rate,high safety and high environmental adoptability.However,the relatively low energy density greatly limits its wide application.Improving energy density without compromising power density and safety has been considered as one of the most challenges for supercapacitors.In this work,a series of studies have been performed on various electrode materials such as lithium titanate（LTO）,activated carbon and biomass-based hard carbon to develop energy storage devices with both high energy density and high safety.In Chapters 3 and 4,kilogram’s grade scale production of spherical LTO with different morphologies,porous structures and crystallinities were prepared by the combined spray drying and solid phase synthesis technologies.Their physicochemical properties were characterized by SEM,XRD,XPS and some other analytical techniques.The electrochemical performances of the obtained samples were evaluated in pouch-and prismatic-type hybrid capacitors.The effects of key parameters on the electrochemical performance of the spherical LTO composite material were discussed.In chapter 5,the physicochemical properties of activated carbon electrode during the manufactory procedure of commercial electric double layer capacitor（EDLC）were fully characterized.Meanwhile,the impacts of leakage on the electrochemical performance of EDLC have also been investigated and the solution has been proposed.In chapter 6,a series of spherical hard carbon materials derived from starch were successfully prepared using the solvothermal method.The microstructure and surface chemical properties were deeply investigated.Furthermore,the electrochemical energy storage characteristics in sodium-ion battery and lithium ion capacitor have also been evaluated.Consequently,the following research results could be drawn from the above studies:（1）A simple and scalable synthetic route combing spray drying and solid state reaction was developed to fabricate a special kind of LTOs that were assembled with nanoparticles as the primary unit and micro size particle as secondary unit.The obtained spherical LTO/C composite materials would demonstrate a high specific capacity(175m Ah g^-1),high rate（20 C）,high tap density(0.89 g cm^-3)and low residual alkalinity（0.2 wt.%）when the fabrication parameters are optimized as follows:with nanoscale anatase Ti O₂as the titanium source,the molar ratio of Li₂CO₃ to Li OH·H₂O at 1.5:1 in the hybrid Lithium source,the Li/Ti ratio at 1.04,and at 780 ^oC for 10h and at 735 ^oC for 10h as the pre-and post-calcination conditions,respectively.Furthermore.the hybrid capacitor based on this material could achieve high energy density(up to 51.65Wh kg^-1),high power density(reaching 2.47 k W kg^-1),and excellent capacity retention（～92%@10000 cycle）.That is,the LTO/C anode is promising to help hybrid capacitor meet the requirement for high specific energy.In addition,it is demonstrated that both of the Li/Ti ratio and calcination condition has a significant impact on the LTO/C composite material,such as in its specific surface area,particular size,surface morphology,residual alkalinity,and electrochemical properties.（2）The hybrid capacitors based on LTO composite materials were investigated systematically in the modes of pouch and prismatic cells.The results show that,the electrochemical performance of a device is not only related to the structure of LTO itself,the applied positive material,and the mass ratio of positive and negative electrodes,and the manufacture technology are also important if not more.When these hybrid capacitors worked between 1.5 V and 2.8 V,the limited voltages for their positive and negative electrodes were decided to be 4.38 and 1.41 V（vs.Li/Li⁺）,respectively.The preparation technics for CEP-21KSN-type activated carbon and SBR wetting could satisfy the requirements of hybrid capacitors in their electrodes.The capacitance,specific energy density,and specific power density the as-prepared prismatic AC/LTO hybrid capacitor reached 30000 F,21.49 Wh kg^-1 and,2.15 k W kg^-1,respectively,when the technological parameters were optimized as follows:with the content of conductive agent in the negative electrode at 7 wt.%,the mass ratio of positive and negative electrodes at 2.23～2.82,and the mass of the EMC+PC+DMC electrolyte at 540～550 g,and the stepped way as the formation method.Moreover,it could also meet the safety requirements during the experiment tests such as short circuit,needling,combustion,and extrusion,and thus may make a potential power source for emerging public transit application.Additionally,judging from the test results using pouch-type cells,CNTs may be not suitable for the optimization of negative electrodes in AC/LTO hybrid capacitor.The reason is that,the Ti⁴⁺species would break the electrolyte’s C-C and C-O bonds by its catalytic effect,and cause the trace of water residual in the electrode（or induced during the device assembly）to decompose during the cycling process,which finally leads to the gas generation in the device and its life reduction.（3）The pore-structure evolution of activated carbon during the manufacturing process of its electrode and the leakage of electrolyte were investigated using traditional electric double layer capacitors.It was found that,when the material was made into electrode,the contents of its meso and macro pores were both greatly reduced.In particular,its specific surface area（SSA）was reduced by 43%,and electrodes prepared using various carbons all showed type-I N₂ adsorption/desorption curve.Additionally,the filling effect of binder and dispersant can reduce the effect of mesoporous and macroporous structure on the electrochemical performance of the electric double layer capacitors.The SSA retention of the electrode was only 40%after the performance test.In addition,the floating test showed extremely adverse influence on the positive electrode,where the SSA retention was only 14.3%at the positive electrode’s edge.Thus,to further improve the capacitor’s performance,the contents of binder and dispersant as well as the storage temperature and voltage for the electrodes should be strictly controlled.（4）Starch-based spherical hard carbon materials（SHCs）with high initial coulombic efficiency（ICE）,high specific capacity,and mass-production capacity of 100 kg have been developed by a solvothermal method for sodium-ion battery and lithium-ion capacitor.Using wheat starch as carbon precursor,the structure of starch can be well stabilized at its original morphology by this solvothermal method.Moreover,the effect of the carbonization temperature on the microstructure was studied.The results show that,when using glycerol as the dispersion medium,the initial coulombic efficiency of the sample,prepared by solvothermal stabilization at 230 ^oC and carbonization at 1500^oC,can reach 90.5%,and its reversible sodium-storage capacity can reach 156.9 m Ah g^-1 at 4 C.Moreover,the full sodium-ion batteries based on SHCs can exhibit good rate performance and cycling stability,where the energy density may reach189 Wh kg^-1.However,even that the SHCs deliver a low ICE 76.3%and a relatively low capacity of223.15 m Ah g^-1 in lithium-ion battery,they can output an energy density of 14.7 Wh kg^-1 and a power density of 870 W kg^-1 in lithium ion capacitors,which can claim its potential for commercial application by some extent.