JOURNAL OF APPLIED ELECTROCHEMISTRY 25 (1995) 833-840 Kinetic aspects of self-discharge of nickel-hydrogen batteries and methods for its prevention A. VISINTIN*, A. ANANI*, S. SRINIVASAN, A. J. APPLEBY Center for Electrochemical Systems and Hydrogen Research, Texas Engineering Experiment Station, Texas A&M University, College Station, Texas 77843-3402 USA H. S. LIM Hughes Aircraft Company, Electron Dynamics Division, Torrance, CA 90505, USA Received 9 December 1992; revised 19 October 1994 Results of microcalorimetric experiments, in relation to self-discharge in Ni/H 2 batteries are reviewed; the mechanism of self-discharge, as well as possible methods for its inhibition, are discussed. These studies indicate that: (i) the self-discharge is due to a direct reaction of hydrogen with the charged active material (nickel oxide); (ii) the presence of metallic nickel sinter particles does not affect the reaction rate; (iii) the reaction rate depends linearly on hydrogen pressure indicating that the reaction is first order with respect to hydrogen; (iv) the reaction rate is higher under starved-electrolyte rather than flooded electrolyte conditions, indicating that the rate is affected by a diffusion process of dissolved hydrogen; and (v) the microcalorimetric heat evolution rate correlates with that of a decrease in electrode capacity due to the self-discharge reaction. The effects of additives to the active material of the nickel electrode were tested as an approach to reduction of the intrinsic rate of self-discharge. An alternate method for minimizing this rate is by storing the hydrogen as a hydride and thereby lowering the cell operating pressure. Some alloys were thus examined for their hydriding/dehydriding characteristics. 1. Introduction The nickel-hydrogen (Ni/H2) battery is currently the most reliable secondary battery for space and other high power applications. An extremely long cycle life (up to 40000 cycles at 80% depth of discharge) has been demonstrated [1]; its specific energy is approximately 40Whkg -1 [2]. A Ni/H2 battery can be overcharged for an extended period of time and overdischarged for a short duration without serious damage [3]. It also has the desirable feature of a built-in state-of-charge indicator for the cell pres- sure. The disadvantages of the Ni/H2 battery are its relatively high self-discharge rate and low volumetric energy density, which thus makes it unsuitable for some applications. An understanding of the mecha- nism of the self-discharge reaction will be of value in attempts to minimize its rate. It is also necessary to investigate methods of increasing the volumetric energy density. The reaction of hydrogen with the charged active material (nickel oxide) of the nickel electrode controls the rate of self-discharge in the Ni/H2 celt. At the typical operating pressures (30-50 atm), a Ni/H2 cell will lose 50% of its capacity in approximately ten days at 20 °C [4]. According to some publications, the self-discharge rate of Ni/H2 cells is a first order chemical reaction with respect to hydrogen gas pressure [5, 6]. Anotfier publication suggests that the self-discharge depends not only on hydrogen gas pressure but also on the amount of undercharged nickel oxyhydroxide [7]. Although the self-discharge rate can be reduced by lowering the cell operation pressure, this approach causes an increase in cell volume, and hence reduces the volu- metric energy density of the cell. This option is thus not viable for many applications. The self-discharge reaction mechanism was previously investigated in our laboratory by microcalorimetry [8-10]. One approach to reduce the self-discharge rate, which is due to the intrinsic reaction between hydrogen and nickel oxyhydroxide (NiOOH), is by using additives to the electrode materials. Another approach is to lower the hydrogen pressure in the battery by using a hydrogen absorbing alloy. This approach was previously explored by using LaNi s as the hydrogen storage alloy in a Ni/H2 cell [5, 11]. The alloy, LaNis, has attractive absorption/ desorption isotherm characteristics [12], but it presents some problems due to its long term chemical instability [11, 13], especially in a Ni/H 2 cell environ- ment, and its mechanical instability during cycling [12]. Over 1000 charge/discharge cycles have been * PresentAddress:INIFTA-UNLP,C.C. 16, Sucursal4, (1900)La Plata, Argentina. PresentAddress:Motorola,Energy ProductsOperations,8000WestSunriseBlvd.,Plantation, FL 33322,USA. 0021-891X © 1995Chapman&Hall 833