Structure of lithium-boron alloys and their performance as potential anodes for high power molten salt batteries

Anodic discharge curves in the LiCl-KCl eutectic melt have been obtained for two lithium-boron alloys (70 and 80 weight % Li) between 673 and 873 K (400 and 600 C) and from 2-8 A/sq cm. 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. This is considered strong evidence for the existence of a compound Li2B in this alloy system. Between 748 and 873 K (475 and 600 C) and up to at least 8 A/sq cm, there is no diffusional limitations 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 Li2B compound containing a Li-rich, Li-B liquid alloy, probably a conjugate phase to the Li2B compound. Lithium anodizes into the melt at constant V out of this liquid alloy until it is wholly transformed to solid Li2B. 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 7,700 coulombs per g of (Li-B) at about 3V versus a chlorine reference electrode.