Improvement of Discharge Characteristic of Primary Al-Air Battery Anode By Sodium Thiocyanate Electrolyte Additive

Abstract

1. Introduction To increase the energy density of battery system for electric vehicles (EV), Metal (Me)-Air batteries are attractive candidate because of highly theoretical capacity of metal anodes. Aluminum anode is the highest capacity in aqueous Me-Air primary battery anode. In order to realize Al-Air battery, capacity loss by self-discharge and passivation in alkaline electrolyte is urgent matter to be solved. According to past researches, self-discharge was inhibited by using Al-alloying (Sn, etc…)1) or electrolyte additives included metal oxide anion2) (ex. NaSnO3, etc…). However, usually these methods have some problems, ex. high cost, obstructions by additive’s metal deposits. In this study, we report the effect of sodium thiocyanate (NaSCN) additive to the electrolyte, which is found to maintain the Al surface active and to reduce self-discharge to a certain degree. As a result, we improved discharge characteristic of Al anode by the additive. 2.Experiment A piece of Al plate 99% in purity (Nilaco, Japan) with thickness of 1 mm and size of 2.5 × 2.5 cm was used as obtained. 1 mol/l NaOH aq (Kanto Chem.) was used as the electrolyte, w/ or w/o the addition of 0.01 mol/l NaSCN (Aldrich). Self-discharge rate of Al was measured by immersing Al plate into each electrolyte in closed vessel and detecting H2 flow rate by commercial H2mass flow meter connected with it. And complementary detected as the integration of current density through Al dissolution. Electrochemical characterization was performed by 3-electrode beaker cell with Al working electrode, Ni counter electrode (#100 mesh, Nilaco), and Hg/HgO reference electrode (Metrohm Autolab). GD-MS was used to detect residuals composition after dissolved Al plate in each electrolyte. 3. Results and Discussion Fig. 1 shows self-discharge rate of Al plate in each electrolyte. To detect the self-discharge rate of Al plate, the sample was immersed into each electrolyte in the closed vessel, and the evolved hydrogen gas was measured as gas flow rate by connected H2 mass flow meter. Self-discharge rate thus detected for pure 1 mol/L NaOH was calculated to be 60 mA/cm2, and the addition of 0.01 NaSCN was found to decrease the rate down to 40 mA/cm2. Fig. 2 shows discharge curves of Al plate in each electrolyte. As a result, we performed electrochemical measurement, where discharge capacity was found to increase from 925 mAh/g to 1316 mAh/g, by the addition of NaSCN. Moreover, the occurrence point of noise shifted to higher capacity, and discharge potential shifted lower, i.e. overpotential was decreased by means of interface resistance. As a cause of these performance improvements, we found the residual of Al, after self-discharge measurement, was pulverized naturally for the electrolyte with addition of NaSCN. These pictures are shown in Fig. 3. According to the GD-MS measurement, the residual is mostly composed of iron was in each residual, which must be derived from the impurity of Al plate. Furthermore, much sulfur component was detected in the residual dissolved in 1 mol/l NaOH added 0.01 mol/l NaSCN. Thus, sulfur element inhibits aggregation of residual, possibly by adsorbing on iron surface, which is the origin of resistive on Al surface. Other analysis results and details will be shown at the conference. 1) J. Hunter, The anodic behaviour of aluminium alloys in alkaline electrolytes, PhD thesis, University of Oxford, 1989. 2) D.D. MacDonald, C. English, J. Appl. Electrochem. 20 (1990) 405. Figure 1