Study On Improvement Of Specific Capacity And Rate Performance Of AC/Li₄Ti₅O₁₂ Hybrid Supercapacitor

AC/Li₄Ti₅O₁₂hybrid supercapacitor demonstrates higher specific capacity and energydensity than conventional AC/AC double layer supercapacitor, and higher power densitythan lithium-ion battery. It can more perfectly meet the need of load on the energy densityand power density of power system. But the specific capacity and rate performance ofhybrid supercapacitor are still limited by AC and Li₄Ti₅O₁₂, respectively. So in the thesis,we study systematically on improvement of specific capacity and rate performance ofAC/Li₄Ti₅O₁₂hybrid supercapacitor from the two aspects of positive electrode and negativeelectrode.Composites of LiFePO₄and activated carbon （LiFePO₄-AC） are prepared by ballmilling. And the effects of LiFePO₄content in the composite positive electrodes on theperformance of LiFePO₄-AC/Li₄Ti₅O₁₂hybrid lithium-ion battery capacitors areinvestigated systemically. Scanning electronic microscopy （SEM） shows reduced particlesize for both AC and LiFePO₄by ball milling, and LiFePO₄particles are uniformlydistributed within the AC matrix and have intimate contact with AC particles.LiFePO₄-AC/Li₄Ti₅O₁₂hybrid lithium-ion battery capacitors have energy storagecharacteristics of hybrid supercapacitor and lithium-ion battery. At0.5A·g^-1charge-discharge current, LFP30-AC70/Li₄Ti₅O₁₂, containing30%LiFePO₄, shows aspecific capacity enhancement of42%comparing to AC/Li₄Ti₅O₁₂, and has the samespecific power density with AC/Li₄Ti₅O₁₂. After500charge-discharge cycles at1.0A·g^-1,the specific capacity of LFP80-AC20/Li₄Ti₅O₁₂and LFP30-AC70/Li₄Ti₅O₁₂is53%and33%higher than that of AC/Li₄Ti₅O₁₂, respectively. This result clearly demonstrated thatthe addition of LiFePO₄to AC can effectively enhance the specific capacity ofAC/Li₄Ti₅O₁₂hybrid supercapacitor with the advantages of high power density and longcycle life.Li₄Ti₅O₁₂-Ketjen Black (Li₄Ti₅O₁₂-KB) and Li₄Ti₅O₁₂-Ketjen Black-Multi-walledCarbon Nanotubes (Li₄Ti₅O₁₂-KB-MWCNTs) are prepared by a solution method with theintroduction of KB and MWCNTs. Transmission electron microscopy （TEM） shows thatthe Li₄Ti₅O₁₂particles in two composites have a diameter of ca.40-60nm. At thecharge-discharge rate of0.1C,20C and30C, the discharge capacity of Li₄Ti₅O₁₂-KB and Li₄Ti₅O₁₂-KB-MWCNTs is156.9,109.5,92.9mAh·g-1and157.4,133.0,105.0mAh·g-1,respectively. The existence of asymmetric behavior between charge and discharge ofLi₄Ti₅O₁₂is identified by galvanostatic charge-discharge test. After100cycles at5C and10C, the discharge capacity retention of Li₄Ti₅O₁₂-KB is94.3%and92.9%, respectively.For Li₄Ti₅O₁₂-KB-MWCNTs, the corresponding values are96.3%and92.7%. Comparedto commercial Li₄Ti₅O₁₂, the excellent high rate performance of two composites can beattributed to the improvement of electronic conductivity due to the uniform distribution ofLi₄Ti₅O₁₂particles within the matrix of carbon and the reduction of Li₄Ti₅O₁₂particle size.The higher rate capability of Li₄Ti₅O₁₂-KB-MWCNTs than Li₄Ti₅O₁₂-KB can be ascribedto its higher electronic conductivity than Li₄Ti₅O₁₂-KB due to the formation ofthree-dimensional conductive network via the addition of MWCNTs. SoLi₄Ti₅O₁₂-KB-MWCNTs is most suitable as negative electrode of AC/Li₄Ti₅O₁₂hybridsupercapacitor.