Some preliminary experiments with copper cathodes look promising. While the voltage may ultimately be lower, it seems like a very strong battery can result from the formation of the copper silicates. I don't know if the liquid electrolyte is actually necessary, but I used it ( Paint the aluminum, allow to cure/dry, drip fresh aluminum silicate on the dried cake, and then lay a copper sheet on top, charge). So perhaps a simple copper/silicate/aluminum "solid state" cell is possible. Seems to exhibit a wonderful 1.7-1.5v chemistry - with no seperator, just the Al Silicate and Cu silicate, it's capable of a rapid charge at higher voltages, and a good 15mA discharge for nearly an hour. This might actually be the next stage in developing this battery... lower voltage, but intense durability.