Studies On The Controlling Synthesis And Electrochemical Performance Of Hydrated Cupric And Nickelous Oxysalts

As one of the most important electrical energy storage devices, rechargeable lithium-ion batteries (LIBs) have been widely used in mobile phones, laptops, electric vehicles, etc. Researchers have explored various electrode materials for LIBs in order to improve their high-energy and high-power performances for the high-level demands of energy storage. Transition metal oxysalt (e.g., transition metal oxalate and carbonate) can be regarded as one of the most promising anode materials because of its higher theoretical lithium-storage capacity than LIB commercial graphite. Generally, crystal water in oxysalts (e.g. cupric and nickelous oxysalts) is unavoidable when obtained from an aqueous reaction system, that is, therein crystal water can hardly be removed via a thermal dehydration route. Also, it is easy to understand that, resembling moisture, crystal water in active sunstance may exert an adverse effect on the electrochemical properties of LIB anodes. Furthermore, transition metal oxalates and/or carbonates practically have a reversible lithium-storage capacity higher than the theoretical ones, however, the involved lithium-storage mechanism and the origin of an extra-capacity contribution are still ambiguous nowadays. Therefore, the controlling syntheses and electrochemical performance of cupric and nickelous oxysalt dehydrate are chosen as main topics in this thesis, focusing on the influences of crystal water and/or interfacial lithium-storage behavior on the electrochemical properties of nanostructured LIB anodes. This dissertation mainly deals with two aspects, shown as below.(1) As discussed previously, dehydrated cupric oxalate cannot be obtained from hydrated cupric oxalate by a. simple dehydration method. Hydrated and dehydrated cupric-cobaltous oxalates were prepared by co-precipitation and dehydration method. Then, the hydrothermal treatment of these binary oxalates with freshly prepared graphene oxide (GO) are adopted to combine the hydrated or dehydrated oxalate with GO. Their electrochemical performances were studied as anode materials to research the effect of unavoidable crystal water. As a LIB anode, Cu1/3Co2/3C2O4·xH2O possessed a reversible capacity of 565.0 mAh g"1 at 1000 mA g-1 over 200 discharge-charge cycles, higher than that of the dehydrated counterpart, indicating a positive effect of crystal water. After combined with GO, the reversible capacity of Cu1/3Co2/3C2O4·xH2O goes up, possessing a reversible capacity of 388.9 mAh g-1 at an ultra-high current density of 1000 mA g-1 over 1000 discharge-charge cycles, indicating a jointly positive effect of crystal water and GO on the high-rate electrochemical performance of cupric-cobaltous oxalate.(2) In this chapter, at 180℃ the dispersion of hexamethylene amine (urotropine) and nickel acetate tetrahydrate in triethylene glycol has been used for the unique nanofabrication of roselike hollow architectures of dehydrated carbonate with a formula of Ni2(OH)2-x(CH3COO)xCO3·2H2O. The controlling synthesis process or the template effect of fogdrip in solvent triethylene glycol has also been investigated to understand the formation of Ni2(OH)2-x(CH3COO)xCO3·2H2O roselike hollow nanostructures. As LIB anodes, the hydrated carbonate roselike hollow nanostructures deliver an initial discharge capacity of 1776.9 mAh g-1 at a small current density of 100 mA g-1. Then, the specific discharge capacity goes down to the minimum value of 265.4 mAh g-1 in the 64th discharge-charge cycle. And then, this capacity starts to go up and reaches the maximum value of 611.3mAh g-1 in the 171th cycle. These indicate that the subsequent upward trend of discharge capacity could be due to the gradually emerged interfacial storage capability. However, this phenomenon could not appear when Ni2(OH)2-x(CH3COO)xCO3·2H2O roselike hollow nanostructures have been continuously cycled at a high current density of 1000 mA g"1.Considering a close relationship between the interfacial storage capability (i.e., an extra lithium-storage capacity beyond the theoretical value) and the specific surface area, roselike NiO porous hollow nanostructures have been further obtained using the high-temperature decomposition of Ni2(OH)2-x(CH3COO)xCO3·2H2O nanostructures, giving a high specific surface area of 115.7 m g-1 (pore size ～23.2 nm). As LIB anodes at 200 mA g-1, roselike NiO porous hollow nanostructures exhibit an initial discharge capacity of 1584.7 mAh g-1 (coulombic efficiency,78.2%). During hundreds of the continuous discharge-charge cycles, the porous NiO anode can be reversibly converted into Ni/Li20 nanocomposites and can unexpectedly deliver an ultra-high discharge capacity of 2124.8 mAh g-1. On the principle of differential capacity, we measure the whole discharge capacity in each cycle and quantitatively calculate the percentage of interfacial storage contribution therein. This indicates that the above results of porous NiO anode may be referred to as the gradual battery-to-pseudocapacitor transformation of lithium storage, also as the gradually enhanced pseudocapacitive behavior. Comparing the interfacial storage behavior of Ni2(OH)2-x(CH3COO)xCO3·2H2O roselike hollow nanostructures with that of derived porous NiO nanostructures, probably there is a proportional relationship between interfacial storage capacity and specific surface area.