Bismuth-Enhanced Electrochemical Stability of Cobalt Hydroxide Used as an Additive in Ni/Cd and Ni/Metal Hydride Batteries

Abstract

Ni-based rechargeable alkaline batteries, including Ni-Cd and the rapidly growing Ni-metal hydride (MH) technology, are widely used in today's power tools and portable applications and are in strong competition with the Li-ion technology. Such a competition is an incentive for improvements of both technologies in terms of autonomy, cycle-life, and reliability. For the Ni-MH technology, such improvements could come from improving either the metal hydride (MH) electrode or the Ni electrode, as reported below.Whatever the Ni/Cd or Ni/MH technology used, Co additives are added to the positive Ni electrode for better electrochemical performances. [1][2][3][4] However, these improvements were found to depend on whether Co is added by coprecipitation, (Ni 1Ϫx Co x (OH) 2 , or simply incorporated within the positive material composite under specific conditions (dispersed vs. coated). Detailed electrochemical studies 5,6 of the Co(OH) 2 /Cd cell have recently thrown some light on the Co(OH) 2 redox chemistry. Upon charging the cell, Co(OH) 2 is oxidized to CoOOH at 0.73 V. However, this oxidation mechanism was found to depend strongly on the reaction kinetics, producing at the end of charge "CoOOH" phases that differ in their chemical/physical properties. For instance, the formation of the electronically conducting nonstoichiometric phase, ␤-Co x 4ϩ Co 3ϩ 1Ϫx OOH 1Ϫx (0.1 < x < 0.2), previously reported by Butel et al., 4 was found to be favored at high charging rates >C/5. In contrast, low charging rates (<C/5) favored Co dissolution together with the formation of the stoichoimetric and poorly conducting Co III OOH phase.Electrochemical reduction of the electronically conducting ␤-Co x 4ϩ Co 3ϩ 1Ϫx OOH 1Ϫx to a poorly conducting CoOOH phase (i.e., reduction of Co ϩ4 to Co ϩ3 ) occurs at 0.9 V vs. Cd/Cd(OH) 2 . Further lowering the cell potential results in the reduction of CoOOH to Co(OH) 2 at around 0.67 V vs. Cd/Cd(OH) 2 . This reduction mechanism was shown to involve a dissolution/crystallization process with the overall result being a nonuniform Co(OH) 2 phase distribution through the positive electrode, and, more specifically, preferential crystallization of Co(OH) 2 around the Ni current collector, thereby jeopardizing the cell performance. Occasionally, under specific chemical conditions 7 (in solution for T > 110ЊC) or electrochemically 8,9 (2 years storage of a Ni alkaline battery at low potential), other authors show evidence for the formation of the spinel phase Co 3 O 4 from CoOOH. The present study shows that well-defined crystals (a few micrometers in size) of the poorly conducting Co 3 O 4 spinel phase can easily be formed under "quasi" normal cell operations. Thus, any chemical/physical means that could decrease or block the formation of Co 3 O 4 and Co(OH) 2 phases should help in improving the performance of the Co-containing nickel positive electrodes.Recently, Larcher et al. 10 succeeded in controlling the kinetics of -MnO 2 r ␣/␥-MnO 2 phase transformation that proceeds through a dissolution/crystalli...

Characterization of Pretreated Co(OH)2-Coated Ni(OH)2 Positive Electrode for Ni-MH Batteries

“…It is well known that Co(OH) 2 dissolves into concentrated alkaline aqueous solution to form Co(OH) 4 2-(1). The Co(OH) 4 2-in the electrolyte solution has absorption peaks at 533, 584 and 629 nm (6). In the UV-Vis spectrum of the electrolyte solution immersed for 7.5 h without discharging, the above absorption peaks were distinctly observed, as shown in Fig.…”

Bi2O3 modified cobalt hydroxide as an electrode for alkaline batteries

“…After 10 days, the product was recovered. The PXRD pattern of the recovered product does not show any trace of Co 3 O 4 formation [14]. The isostructural relationship between manganese dioxide and cobalt hydroxide promotes us to investigate the effect of Bi 2 O 3 , on cobalt hydroxide electrode from fundamental point of view.…”

“…In the first method, when the discharge of MnO 2 [14,19]. Several authors report the formation of Mn-Bi complexes in alkaline medium [19][20][21].…”

“…The PXRD patterns clearly show the structural changes that can take place during the charge discharge process of cobalt hydroxide. Cobalt hydroxide will also undergo oxi- dation in KOH to form highly insulating phase of Co III OOH [17]; Co III OOH further transforms to Co 3 O 4 [14,17]. This is the cause of an irreversible electrochemical behaviour in cobalt hydroxide.…”

“…Wroblowa and Gupta blended small amount of Bi 2 O 3 with manganese oxide during electrode fabrication and succeeded in inducing 2e − exchange reversibility in manganese dioxide electrode [13]. Bi 2 O 3 coated cobalt hydroxide was used as additive to improve the electrochemical performance of nickel hydroxide electrode [14] and NaOH (1 M) to a beaker containing an aqueous solution of sodium carbonate (0.5 M) results in the precipitation of the solid. The precipitation was stopped at pH at 9 and the product was aged in mother liquor at 65 • C for 18 h prior to filtration [15].…”