Rechargeable Ni-Zn microbatteries employing MCM-41 separator

Abstract

Although MCM-41 inorganic membrane could act as separator in primary cells such as zinc-air and zinc-manganese dioxide, its efficacy in a rechargeable cell such as nickel-zinc (Ni-Zn) has never been determined. The impracticality of MCM-41 incorporation in Ni-Zn battery system is fortified by the fact that silica is etched away by potassium hydroxide (KOH); besides, MCM-41 transforms into MCM-50 in a concentrated KOH. In order to ascertain the hypothesis, MCM-41 inorganic membrane was employed as a separator in the Ni-Zn cells and microbatteries. Its hexagonally arranged pore channels are expected to act as an electrolyte reservoir and ionic diffusion pathways for the electrochemical reactions. It is due to the hydrophilic nature of its pore walls and the high surface area it possesses. A multilayer MCM-41 thin film was synthesized onto the nickel hydroxide electrode by drop coating of the parent solution consisting of cethyltrimethylammonium bromide (CTAB), hydrochloric acid (HCl), distilled water (H2O), ethanol (C2H5OH) and tetraethyl orthosilicate (TEOS) with a molar ratio formulation of 0.05 CTAB, 1.0 TEOS, 0.5 HCl, 25 C2H5OH and 75 H2O. Zinc and nickel hydroxide thin films were electrodeposited onto copper current collectors to form the anode and the cathode, respectively and a zinc oxide slurry was drop coated onto the electrodeposited zinc, forming a complete anode. The structural formation of MCM-41, zinc and nickel hydroxide was confirmed by X-ray diffraction (XRD) measurements. The surface morphologies of the zinc and nickel hydroxide essentially consisted of nanoflakes and hierarchically structured aggregated nanoparticles, respectively. The structural integrity study of MCM-41 separator in the Ni-Zn cells showed that the MCM-41 partially transformed into MCM-50 phase at the early stage of charge-discharge process i.e. the 5th cycle, almost completely at the 25th cycle, while completely at the 80th cycle. The viability of MCM-41 as separator for alkaline secondary cells was confirmed through the structural properties of the resulting separator materials and the discharge capacity profiles. The reversibility of zinc electrodes in various KOH-MCM-41 surrounds was demonstrated in the cyclic voltammetry measurements which lead to a conclusion that 70:30 KOH-to-MCM-41 weight ratio should result in a lower solubility of zinc discharge products in the electrolyte. Very thin, circular shaped rechargeable Ni-Zn microbatteries were fabricated employing a side-by-side-electrode design with an electrode separation distance of ca. 800 µm. The microbatteries sustained > 130 cycles of cycling with a high depth of discharge. The microbatteries were 200 µm thick, measured 6.41 cm2 in area and weighed 1.14 g (excluding the cap and the substrate). The microbattery discharged at a rate of 0.1 mA possessed an energy density of 3.82 Wh l-1 and power levels of 0.014–0.023 mW cm-2 (i.e. a current density of 15.6 µA cm-2). Whereas the microbattery discharged at a rate of 0.2 mA possessed an energy density of 2.65 Wh l-1 and power levels of 0.023–0.040 mW cm-2 (i.e. a current density of 31.2 µA cm-2). Nevertheless, the present microbattery performance was not optimized since it was noted that cuprous oxide and cupric oxide layers were really forming during the charge-discharge process, i.e. based on the XRD results.