Effect of Lanthanide Oxide Additives on the High-Temperature Charge Acceptance Characteristics of Pasted Nickel Electrodes

Abstract

The development of electric and hybrid vehicles is proceeding against the backdrop of environmental problems, and high-output, high-energy-density nickel-metal hydride cells (NiMH cells) are finding application as the cardinal components for these vehicles. Because such uses require many NiMH cells to be connected in series and installed in small spaces inside vehicles, battery performance and service life are affected by how one controls the heat produced during the charging and discharging processes. This makes the cooling system an essential element, but the impossibility of completely preventing increases of battery temperature and evening out the temperature distribution among batteries has imposed the important technical task of raising the charge acceptance of batteries in the high-temperature range of 35-60ЊC. 1 However, the charge acceptance of NiMH cells declines considerably at high temperatures due to the small oxygen overpotential of the positive nickel electrodes. 2 The charging reaction of nickel electrodesis followed by the evolution reaction of oxygenbut at high temperatures the oxygen overpotential of the nickel electrode drops rapidly, and the charge acceptance decreases owing to conflict between the charging and the oxygen evolution reactions. For that reason one can find reports on various methods employed to increase the oxygen overpotential of nickel electrodes and improve charge acceptance. 2-6 These include the addition of cobalt in solid solution to nickel hydroxide, 3,6 which makes the oxidation potential of nickel electrodes decrease; the addition of compounds with group two elements, such as zinc oxide (ZnO), 4 cadmium oxide (CdO), 2 and calcium oxide (CaO), 4 or of rare earth compounds such as yttrium oxide (Y 2 O 3 ), 4 which act to raise the oxygen evolution potential; or the addition of NaOH or LiOH to the KOH electrolyte. 5 Especially effective is the addition of cobalt in solid solution to nickel hydroxide, but there is a limit to the amount added because it entails a drop in discharge potential. Therefore, actual batteries use these methods in combination, but it is still difficult to maintain a high charge acceptance at high temperatures above 50ЊC.Nevertheless, as noted previously, the oxygen evolution potential of nickel electrodes is suppressed by the addition of, for example, metal oxides whose metal ions have sp closed shells, but not much research has been focused on this mechanism or on the systematic relationship with the electron configurations of the metal ions making up those oxides. We therefore investigated oxides, and their combinations with lanthanide elements (Ln 3ϩ :[Xe] 4f n ), which are characterized by the progressive filling of the 4f orbitals from La 3ϩ to Lu 3ϩ and have sp closed shell-like ion structures,such as Zn 2ϩ , Ca 2ϩ , Cd 2ϩ , and Y 3ϩ , to determine their effects on the high-temperature characteristics of pasted nickel electrodes. ExperimentalNickel electrode preparation.-For the nickel hydroxide active material we used high-density spherical ...

Preparation of porous spherical α-Ni(OH)2 and enhancement of high-temperature electrochemical performances through yttrium addition

“…Regarding the roles of rare earth elements, Oshitani et al [27] assumed that the compound might exist in a thin film as hydrous hydroxides on the surface of Ni(OH) 2 active materials. The thin film may act as a barrier for electron transfer in oxygen evolution process because of its poor conductivity.…”

“…However, Ni-MH rechargeable batteries behave badly at high temperatures because of strong oxygen evolution on nickel electrodes. In order to improve high-temperature electrochemical performances of nickel electrodes, many studies have been carried out on the addition of cobalt [22,23], zinc [24], cadmium [25], calcium [26] and lanthanide elements [27][28][29][30] in ␤-Ni(OH) 2 . We have ever found that yttrium element is very effective to improve hightemperature performances of ␤-Ni(OH) 2 [29][30][31], so Y element was also added to ␣-Ni(OH) 2 and the effects were investigated on high-temperature performances of ␣-Ni(OH) 2 .…”

Effect of rare earth oxide additives on the performance of NiMH batteries

“…When adding heavy rare earth oxides (Er, Tm and Yb) provided a charge efficiency of 90%. This happens because the oxygen evolution overpotential of nickel electrodes shifted to the noble potentials [5][6][7] and indicates that heavy rare earth oxides effectively raise high-temperature charge efficiency. On the other hand, when light rare earth oxides (La, Ce and Pr) were added to nickel electrodes, charge efficiency was the same as when there were no additives.…”

“…Because the oxygen overpotential of nickel electrode drops rapidly in the high-temperature range of over 35 • C, the charging reaction and the oxygen evolution reaction occur in conflict with one another, thereby it causes the charge efficiency to decrease. While various ways to improve charge efficiency have been reported by others [1][2][3][4], we have reported that addition of heavy rare earth oxides can achieve substantial improvement [5][6][7]. The use of this technique has attained excellent charge efficiency in EVs and HEVs applications [8,9].…”

Fabrication of bipolar nickel metal hydride batteries with nanometer copper oxide as anodic additive

“…Current researches most concern on the first aspect of increasing the performance of the electrodes. In the past, various methods have been introduced to improve the performance of the anode and cathode electrode, such as mixture of various additive, alkaline or acid etching, ball-milling and micro-encapsulation [8][9][10][11][12][13][14][15]. Feng and Northwood have studied the properties of the MH electrode microencapsulated Cu, and they found that the microencapsulated electrode had higher exchange current density and better high-rate dischargeability compared with the untreated one [16,17].…”