Preparation And Electrochemical Capacitance Performance Of Layered Double Hydroxides

Supercapacitors as a new energy storage device has many applications in aerospace, transportation, electro-communication, due to its high specific power, high specific capacitance, long cycle life, low cost and other advantages. Layered double hydroxides (LDHs) have a typical hydrotalcite-like lamellar structure, which can provide high specific surface areas. Electrical double layer capacitance and faradaic pseudocapacitance can be simultaneously acquired because of their abundant channels and electrochemically active sites. Thus, LDHs can serve as alternative materials in supercapacitor with simple preparation and low cost. In this thesis, the electrochemical performance and stability of LDHs are improved by the modulation of composition and structure of laminae. Details are as follows:(1) Co-Al LDHs have smaller and thinner sheets synthesized in alcohol media than in aqueous, and the electrochemical performances have increased dramatically with preparing in nonaqueous media. The number of hydroxyl groups in the alcohols used as synthetic media exerts a great influence on the electrochemistry. The increase of the number results in the elevation of electrochemical performances with much smaller sheets. At a current density of 1 A g-1, the Co-Al LDH samples synthesized in ethylene glycol exhibits the specific capacitance of 354 F g-1. The specific capacitance increases gradually from 148 to 299 F g-1 at the current density of 2 A g-1 when the solvent varied from water, methanol, ethylene glycol, to glycerol in sequence. In the meantime, cycle stability of the LDHs has been greatly improved with smaller resistance and better reversibility than prepared in the aqueous solution. In addition, the specific capacitance of LDHs synthesized in glycerol has less loss after 2000 cycles.(2) LDHs with different metal ions were prepared by modulating the composition of divalent and trivalent cations. Insights into the impact of trivalent cations, the substitution of Al(III) with In(III) and Cr(III) can broaden the potential window, and perform a better cycle stability and high current retention rate, while the Co-Al LDHs have a superior capacitance over others. The divalent metal ions are the primary part for generating pseudocapacitance, and have a considerable influence on the electrochemical performances of LDHs. Ni-Al LDHs can achieve a large specific capacitance only at a low current density.(3) CoxNi1-x-Al LDHs with hierarchical structure were hydrothermal synthesized with glycine as a template. The proportion of Co2+ in the divalent metal cations was modulated to obtain the composite structure containing macropore and mesopore. The presence of glycine in the preparation has a great impact on the morphology of LDHs, and markedly improves the value and high current retention rate of specific capacitance. While the percentage of Co2+ to the divalent metal cations is 60%, Co0.6Ni0.4-Al LDHs has the most outstanding performances with the specific capacitance of 1207 F g-1 at the current density of 1 A g-1. (4) A high performance LDHs with stable structure is prepared by doping the rare earthLanthanum into ordinary Ni-Al LDHs in the coprecipitation process. The La doped LDHs have a improved charge/discharge cycle stability with a more stable crystal structure which can promote the stable existence of the generatedα-Ni(OH)2. After 2000 cycles at high current charge/discharge current density, La doped LDHs yet still can rapidly recover the specific capacitance nearly to 1000 F g-1, at 1 A g-1, and maintain a slow decay rate in the next 1800 cycles. The specific capacitance of ordinary Ni-Al LDHs fades to 30% of initial capacitance after cycles at high current density of 2 A g-1.