| The rapidly developing electric equipments require energy storage devices with highcapacity and energy densities. Metal-air fuel batteries can convert the chemical energy inmetals into electrical power directly with relatively high capacity density and energyefficiency. Compared with traditional hydrogen fuel cells, metal-air batteries have lowerprices and maintenance costs. Furthermore, the preparation, storage and transport of metalare more convenient than hydrogen. Aluminum (Al) is an ideal candidate because of tistrivalence and atomic weight of26.98. The theoretical capacity density of Al is up to2.98Ah g-1-Al. Al also exhibits a high energy density of8.195Wh g-1-Al. Besides, Al hasattracted extended attention due to its abundance, low price, non-toxicity, andenvironmental friendliness. In this paper, an all-solid-state polymer Al-air battery with Alanode and polymer alkaline gel electrolyte is proposed to show an excellent capacityperformance. Research on the anodic corrosion, the electrolyte conductivity and thestructural optimization of the battery has also been carried out during this work. Finally, aflexible thin-film Al-air battery and an all-solid-state ultracapacitor are presented to extendthe application field of this work.A typical Al-air battery is consisted of an Al anode, an electrolyte and a porous aircathode. The fluidity of popular aqueous electrolytes may lead to the penetrations andleakages through the capillaries in the porous air cathode. In this paper, cross-linkedpoly-acrylic acid (PAA) based alkaline gel electrolyte is prepared to solve this problem.Acrylic acid (AA) monomers can be polymerized into PAA chains and then cross-linked tobe3-dimensional matrix, in which the aqueous electrolyte can be stored to form a gel. Theconductivity, which is one of the most important characteristics of the electrolyte, firstlyrises and then decreases as the KOH fraction increases. The final polymer gel electrolyte iscomposed of36wt%KOH,6wt%AA,1wt%cross-linker and0.6wt%polymerizationinitiator. The conductivity of the gel electrolyte is460mS cm-1. In addition to the leakage,the anodic corrosion is another problem of Al-air batteries. The corrosion rate reaches thepeak with the KOH fraction of30wt%. It is indicated that PAA is a good corrosion inhibitor for Al.A molecular dynamic method is used to find the mechanism behind the electrolyteconductivity. All atom models of aqueous alkaline solutions with different KOH fractionsare built to calculate the dynamic and structural performances. According to theNernst-Einstein relationship, the conductivity is proportional to the product of the iondensity and the diffusion coefficient. A higher KOH fraction can raise the ion density butreduce the ion diffusion coefficient. As a result, the peak conductivity is obtained with theKOH fraction of36wt%, which is consistent with the experimental result. In moreconcentrated solutions, the coordination number between ion pairs is enhanced and restrictsthe mobility of ions.An all-solid-state Al-air battery has been fabricated with polymer alkaline gelelectrolyte. An Al mesh with a Ni current collector is used as the anode. The cathodic pasteconsisting of conductive carbon, catalyst, polymer binder and organic solvent is casted ontoa Ni foam. The porous air cathode is obtained after evaporation and cold press. The Almesh anode, the polymer alkaline gel electrolyte membrane and the porous air cathode arelaminated sequentially to form an all-solid-state Al-air battery, which presents an excellentperformance during discharging measurements. As the discharging current density risesfrom0.3to43.3mA cm-2, the peak and average voltage drop together, while the powerdensity is promoted up to34.0mW cm-2. The maximum capacity and energy densities canreach1166mAh g-1-Al and1230mWh g-1-Al, respectively. Besides, a separable Al-airbattery is proposed in this work to inhibit the Al corrosion during rest in order to maintainthe available capacity of the battery.Base on the all-solid-state polymer Al-air battery, we present a flexible thin-film Al-airbattery using an Al foil anode and an ultra-thin carbon air cathode. A paper based gelelectrolyte is prepared by the combination of polymer gel electrolyte and double rice papers.The total thickness of the whole battery is only0.32mm, and the area density is0.1g cm-2.The maximum power density of the battery can reach3.7mW cm-2with the current densityof3.9mA cm-2. The peak area capacity and energy density are2.33mAh cm-2and2.48mWh cm-2, respectively, when the battery is discharging at3.1mA cm-2. The capacity and energy densities corresponding to anodic Al are932mAh g-1-Al and992mWh g-1-Al,respectively.What’s more, the gel electrolyte can also be applied to fabricate solid gelultracapacitors. The ultracapacitor prototype is composed of a gel electrolyte membraneand two porous carbon electrodes. During the cyclic voltammetry testing, the voltagewindow is set as1.0V, and an overload of20%is permitted. The promoted voltagescanning rate can enhance the current density, but the capacitance is limited. Thecapacitance density of0.343F cm-2is obtained with the scanning rate of0.01V s-1. In thecontinuous charging and discharging tests, the capacitance is restricted by large currentdensity, while the coulomb efficiency is promoted to approximate to100%over5mA cm-2.The possible of the combination of all-solid-state batteries and solid gel ultracapacitors isconfirmed in this work.As mentioned above, Al has been employed as the anode material in metal-air batteriesto achieve high capacity and energy densities. A PAA based alkaline gel electrolyte is usedto fabricate the all-solid-state polymer Al-air battery to avoid leakage and corrosion. Themechanism of the electrolyte conductivity is investigated using a molecular dynamicmethod. Finally, the results of this work have been extended to flexible thin-film batteriesand solid gel ultracapacitors. |
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