Bismuth Contribution to the Improvement of the Positive Electrode Performances in Ni/Cd and Ni/MH Batteries
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Bismuth Contribution to the Improvement of the Positive Electrode Performances in Ni/Cd and Ni/MH Batteries V. Pralong, A. Delahaye-Vidal, B. Beaudoin and J-M. Tarascon Laboratoire de Réactivité et de Chimie des Solides, Université de Picardie Jules Verne & CNRS, 33, rue St Leu 80039 Amiens, France ABSTRACT In this study we investigated the evolution of nickel hydroxide, which acts at the positive electrode of the Ni/Cd, Ni/MH and Ni/H2 alkaline batteries. We found that the addition of bismuth oxide in the course of the active material preparation prevents the dissolution-re-crystallization processes of the nickel hydroxide that are harmful to the electrode efficiency. From XRD and SEM studies, it is shown that treatment of the bismuth doped-nickel hydroxide by hydrogen in 5 N KOH electrolyte prevents metallic nickel formation. Moreover, it appears to stabilize the α-type nickel hydroxide structure, preventing its transformation into the β-Ni(OH)2 phase. Finally, an implementation of these findings towards the most efficient use of nickel positive electrodes is shown. INTRODUCTION Ni-based rechargeable alkaline batteries, Nickel-Cadmium and Nickel-Metal Hydride cells are widely used in portable and power tool applications despite a strong competition with Li-ion technology. Actually, this alkaline battery technology offers numerous advantages including low cost, high power capabilities and a long cycle life. Such a competition is an incentive for the improvement of both technologies in terms of autonomy, cycle life and reliability. For the Ni-MH technology such improvements could either be obtained on the Metal Hydride (MH) electrode or the Ni electrode. The redox processes occurring at the Nickel Oxyhydroxide Electrode (NOE) mainly involve the Ni3+-Ni2+ redox couple. The crystal chemistry complexity of this system was initially reported by Bode et al. [1](Figure 1). These authors established the occurrence of four phases in the oxydo-reduction processes of the NOE. In the discharged state of the battery, the nickel hydroxide Ni(OH)2 usually adopts the β structural form. Occasionally, another structural form denoted as α can be obtained. Both forms crystallize in the hexagonal system with the brucite-type structure. Each Ni(OH)2 layer consists of a hexagonal planar arrangement of octahedrally coordinated Ni2+ ions. The main difference between the two α and β-type structures resides in the stacking of the layers along the c axis. In the charged state of the batteries, namely the NiOOH nickel oxy-hydroxide, two structural forms (β and γ ) are known to coexist in relative amounts depending upon the degree of charge and the amount of additive used to improve the performance of the electrode (usually Co based) 2-7. Thus two redox processes may occur, a β/β transformation (reversible potential at about 1.25 V vs. Cd/Cd(OH)2) and occasionally a γ/α transformation (reversible potential at about 1.17 V vs. Cd/Cd(OH)2). The fact remains that most of the phase transformations occurring in the NOE during a charge-discharge proc
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