Preparation And Electrochemical Storage Properties Of Nickel-Cobalt Compounds

Recently, 3d transition-metal-oxides, especially nickel and cobalt oxides, were very popular in the electrode materials of lithium-ion battery and supercapacitor owing to high theoretical capacity. Nevertheless, poor electron conductivity of nickel and cobalt oxides would result in their undesirable cycle performance and rate capability and had been a key scientific issue for their real application. The study found that the multi-metal oxides containing two or more metal elements could reduce their band gap, improving the electron conductivity. Furthermore, nickel and cobalt elements had similar physiochemical properties and electrochemical window, Therefore it was feasible to compound multi-metal oxides containing nickel and cobalt. It was a bran-new way to improve the poor cycle performance and rate capability of transition metal oxides. Another factor affecting the electrochemical performance was the size and morphology of electrode materials. The unique nanostructure could shorten the diffusion path of ions in electrolyte into electrode materials, contributing to reduce their migration resistance and enhance ionic conductivity, thereby improving the electrochemical performance of lithium-ion battery and supercapacitor. My research is focus on improving the conductivity of nickel and cobalt oxides including electronic and ionic conductivity through the formation of nickel cobalt-sulfide(NiCo2S4) and nickel-cobalt molybdate(NixCo1-xMoO4) and the design of novel nanostructure. In this thesis, the following were mainly studied:A facile twice-hydrothermal method and a hydrothermal-calcination technology were respectively developed for the growth of NiCo2S4 nanotube array and NixCo1-xMoO4(x=0、0.25、0.50、0.75 and 1.00) nanoflake array on Ni foam with robust adhesion. It could be observed that as-prepared NiCo2S4 nanotube directly grow on Ni foam with an average inner/external-diameter of ~ 50/100 nm, length of ~ 2 μm, and tube wall of 20—30 nm, and the array was an orderly, high conductivity, large specific surface electrode structure. The crystal structure and micromorphology had some connection with the relative content of Ni element in NixCo1-x MoO4 materials. If the x value was 0 or 0.25, the crystal of NixCo1-xMoO4 would be α/β-CoMoO4 structure, but 0.50, 0.75 or 1.00 for α/β-NiMoO4. The morphology of NixCo1-xMoO4 materials was nanoflake directly grown on Ni foam, and an ordered three-dimensional-network array. It’s worth mentioning that the different Ni content would bring change to the size and thickness of nanoflake and porosity among nanoflake, etc.The electrochemical performance of NiCo2S4 nanotube array and NixCo1-xMoO4(x=0, 0.25, 0.50, 0.75 and 1.00)nanoflake array was evaluated as the binder-free materials for lithium-ion batteries. Among them, NiCo2S4 and Ni0.5Co0.5MoO4 array exhibited high special capacity, excellent cyclic stability and desirable rate capability. At 0.2C rate, NiCo2S4 nanotube array delivers the average discharge capacity of 720 mAh g-1 during 50 cycles that slightly surpasses its theoretical capacity of 703 mAh g-1. When the current comes back to 0.2C after rate measurements, NiCo2S4 nanotubes can still stably deliver the discharge capacity of 652 mAh g-1, reaching 92.7% of that at the initial 0.2C. Cyclic Voltammetry(CV) measurements reveal that NiCo2S4 nanotube array contains two sets of electrochemical reaction behavior come from Ni and Co. Impressively, Ni0.50Co0.50MoO4 array exhibited 89% retention of discharge capacity undergoing 50 cycles at 0.2C rate, roughly equaling to 86% of CoMoO4 array and outclassing 37% of NiMoO4. Moreover, 1100 mAh g-1 of average discharge capacity was 250 mAh g-1 high than that of CoMoO4 array. At 4C rate, Ni0.50Co0.50MoO4 array still delivered a discharge special capacity of 828 mAh g-1, reaching 85% of theoretical capacity of CoMoO4 material. Even the current recovered to 0.2C, the discharging capacity reached 81% of that at initial 0.2C. CV curve revealed Ni, Co and Mo elements could participate in the electrochemical reaction with lithium-ion in electrolyte, contributing to the enhanced capacity of Ni0.50Co0.50MoO4 materials.The pseudocapacitance of NixCo1-xMoO4(x = 0, 0.25, 0.50, 0.75 and 1.00) nanoflake array/Ni foam composite was studied using a three-electrode system, and the specific capacitance, cycle performance and rate capability is closely related to the relative Ni content of NixCo1-xMoO4 materials. CV measurement confirmed that the pseudocapacitance of NixCo1-xMoO4 array mainly was from the Faraday reaction of Ni2+ and Co2+ with OH- in electrolyte. Furthermore, the increase of Ni content could contribute to the enhanced capacitance of NixCo1-xMoO4 array. In 4000 cycles, it was a similar linear capacitance degradation of Ni0.50Co0.50MoO4 and Ni0.75Co0.25MoO4 array, and the capacitance retention of respectively 83% and 85% roughly equalled to that of 86% of CoMoO4 array. While respectively 1072 and 1137 F g-1 of the 4000 th cycle was significantly higher than 940 F g-1 of CoMoO4 array, exhibiting superior charge-discharge characteristics and outsanding cycle stability. When the current came back to 2 mA cm-2 after 20 mA cm-2 rate measurement, the special capacitance of Ni0.50Co0.50MoO4 array could still reach 92% of that at the initial 2 mA cm-2, showing desirable rate capability and excellent cyclic stability.