Structure and Anodic Discharge Behavior of Lithium‐Boron Alloys in the LiCl ‐ KCl Eutectic Melt

Abstract

Anodic discharge curves in the eutectic melt have been obtained for two lithium‐boron alloys (70 and 80 weight per cent Li) between 673° and 873°K (400° and 600°C) and from 2–8 A/cm2. The behavior of Li contained in wicks of porous Ni was also studied for comparison. Calculated Li contents for Li‐B alloy anodes, at the main break in curvature of discharge curves, clustered closely around 0.67 mole fraction Li. We consider this strong evidence for the existence of a compound in this alloy system. Between 748° and 873°K (475° and 600°C) and up to at least 8 A/cm2, there is no diffusional limitation to the anodic extraction of Li from the Li‐B alloys. The shape of discharge curves, the lack of anodic diffusional limitation, and the longer low‐V plateaus of the alloy richer in Li, are consistent with the following picture of the discharge process. The discharging material behaves like a sponge of solid compound containing a Li‐rich, Li‐B liquid alloy, probably a conjugate phase to the compound. Lithium anodizes into the melt at constant V out of this liquid alloy until it is wholly transformed to solid. This process constitutes the main plateau of alloy discharge curves. At this point (the main break in discharge curves) voltage rises to values where tighter‐bound Li can be anodized out of the decomposing compound. The Li‐B alloy electrodes are markedly superior to the Li‐Ni wicks both in gravimetric coulombic capacity and in the ability to retain Li at high temperatures. Useful coulombic capacities of the Li‐B alloys ranged up to 7700 coulombs/g of at about 3V vs. a chlorine reference electrode.