Poly(acrylamide-co-acrylic acid) Gel Electrolytes for Ni-Zn Secondary Batteries

trolyte was prepared using poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)). The electrochemical performances of the Ni-Zn cell with the gel polymer electrolyte and the morphology of the zinc electrode during charge-discharge cycles were investigated. The anode (Zn electrode) was prepared by pasting a mixture consisting of the pristine or modified ZnO powder (82 wt %), Ketjen Black (KB; 10 wt %), Ca(OH)2 (1 wt %), Bi2O3 (3 wt %), and polytetrafluoroethylene (PTFE; 4 wt %) on a porous Ni foam. The addition of Ca(OH)2 reduces the shape change of the zinc electrode and improves the cycle stability of the Ni-Zn batteries, 12 and Bi2O3 suppresses the hydrogen evolution. 13 The electrode was dried at 70 o C and pressed (700 kgf·cm �2 ) to a thickness of ~0.3 mm. The Ni electrode consists of Ni(OH)2 powder (86 wt %), Ni powder (3 wt %), Co(OH)2 (3 wt %), polyethylene (PE; 7 wt %), and PTFE (1 wt %). It has been reported that the addition of cobalt to the active material improves the reversibility of the electrochemical reaction. 2 The standard electrolyte is an aqueous solution consisting of 3.8 M KOH, 2.5 M NaOH, and 1.2 M LiOH because this composition has a maximum ionic conductivity and suppresses the shape change of zinc electrode. 14,15 The gel polymer electrolyte was prepared by the following steps. P(AAm-co-AAc) was added to alkaline solution, and the mixture was stirred until they form a homogeneous solution. Then, the solution was poured onto an airtight glass vessel to avoid water evaporation and absorption of carbon dioxide. The conductivities of gel polymer electrolyte and normal alkaline electrolyte were measured by IM6 impedance analyzer (Zahner elcktrik). The morphologies of ZnO electrode and the microstructure of ZnO powders were investigated by field emission scanning electron microscopy (FE-SEM, Hitachi S-4300, Japan) with energy dispersive spectroscopy (EDS, Horiba EX-200). The electrochemical performances of the various compounds were evaluated using a WBCS 3000 instrument (WonA Tech., Korea). The three-step charge regime 2,5 from Samsung advanced institute of technology was used and the discharge rates were 0.1 C (until the 3rd cycle) and 0.3 C (after the 3rd cycle). The gel polymer electrolyte is a viscous liquid mixture. As increase in the concentration of P(AAm-co-AAc) the viscosity of the electrolyte increases. The conductivity of the gel polymer electrolytes as a function of the concentration of P(AAm-co-AAc) was measured at room temperature. The ionic conductivity of gel polymer electrolyte slightly decreases with an increase in the concentration of P(AAm-coAAc). Considering the conductivity and the viscosity of the gel polymer electrolyte, the concentration of P(AAm-co