E0200

Site Preference of Cobalt in Complex AB5 Electrodes - A Neutron Powder Diffraction Study Using Isotope Substitution. Tom Vogt, Physics Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA

Expansion and contraction occur when intercalating interstitials such as lithium or hydrogen into materials. This is a process occurring in batteries. Most loss of capacity is related to surface passivation and pulverization of electrodes due to the huge mechanical stresses created by expansion and contraction during electrochemical cycling. The nickel-metal-hydride (Ni/MH) rechargeable battery has emerged as the most promising environmentally acceptable substitution for current Ni/Cd batteries in non-automotive applications for the consumer and industrial market. Rechargeable nickel-cadmium (Ni/Cd) batteries are currently on the edge of an outright ban or a heavy tax due to their toxicity. After Sonyo Electric Company began marketing Ni/MH batteries in 1990 the production quantities in Japan alone reached $200 million in 1994 and grew to $300 million in fiscal year 1995. No other battery has ever experienced such a dramatic increase in such a short period of time. A huge body of work on the interplay between structural substitutions and electrochemical properties has been done for so-called AB5 alloys used as anodes in Ni/MH batteries and an understanding of the microscopic mechanism is emerging. The expected use in automotive applications increases the requirements for the next generation of Ni/MH. Currently the need to substitute nickel by the more expensive cobalt to increase the thermodynamic stability of the hydride phase and its corrosion resistance is at the focus of a world wide research effort. A site preferably occupied by cobalt was identified by neutron diffraction in LaNi3.55 Co0.75Mn0.33Al0.3, an industrial electrode using isotope substituted samples. Theoretical calculations point to a magneto-elastic origin of the beneficiary role of cobalt in these alloys. This could open up new possibilities for cheaper and better electrodes.