I'm a PhD candidate in chemistry and love this kind of stuff, so at the risk of sounding like a massive pedant, an impromptu chemistry lesson...
Well, to seriously answer the question, I forget the scientific term but some molecules are very high on the positive potential, while others are very high on the negative. And I forget which each of these is which, but hydrogen and fluoride are the furthest apart on this measurement. So if you mix those two together (which is extremely difficult) they make the strongest acid that can even eat glass. Hydroflouric. Now the potential of a battery depends on the two metals involved and the strength of the acid (or alkali). Lead, copper and sulphuric acid make a pretty good, easy to make and therefore affordable battery.
Lemons and limes just aren't strong enough to make such a good battery. However, if you're squeezing them for juice, you'd better wear gloves. I made a key-lime pie without PPE and burned the cr@p out of my hands *once*. Learnt my lesson.....
You're sort of on the right track, but have a few terms confused here:
I forget the scientific term but some molecules are very high on the positive potential, while others are very high on the negative.
What you're describing here is
electronegativity, which is the propensity of an atom to attract an electron. Fluorine (not fluor
ide, which already has an "extra" electron) is indeed the most electronegative element, but hydrogen and fluorine are not on opposite ends of this scale (fluorine has an electronegativity of ~4, hydrogen is 2.2 (values differ slightly depending on who you ask)). The least electronegative element (or most "electropositive" as we'd say) that we have reliable data for is cesium, the one that is the most diagonally opposed to fluorine in the periodic table. The redox reactions involving fluorine/fluoride and dihydrogen/H+ are also not on opposite ends of the scale, e.g. the alkali metals (especially lithium) are much more "negative" than dihydrogen/H+.
So if you mix those two together (which is extremely difficult)...
You're right that combining hydrogen gas and fluorine gas would be a terrible and super difficult way to make HF. Rather, the industrial route involves the decomposition of calcium fluoride with sulfuric acid, which is much easier (both those things are readily available bulk chemicals) (don't do this though, see below!!).
...they make the strongest acid that can even eat glass. Hydroflouric.
In chemistry the term "strong acid" means something specific. A "strong" (Brønsted) acid like hydrochloric acid will completely separate into H+ and X- (e.g. Cl-, or whatever the anion is) in water, whereas a "weak" acid will only partially separate (acetic acid, CH3COOH, the stuff in vinegar, is a quintessential weak acid - in water, there is some H+, CH3COO-, and leftover CH3COOH). Note that this has nothing at all to do with the "corrosiveness" of the acid. Hydrofluoric acid is actually a
weak acid by this metric -- the reason it "eats glass" is due not to its acidity, but a
different property of fluoride (F-). Glass is mostly silica (silicon dioxide, SiO2), and the silicon-fluorine bond is much stronger than the silicon-oxygen bond, so HF tears apart the Si-O bonds (i.e. destroys the glass) to make silicon fluorides and water. HF is also tremendously dangerous for a different reason: it readily permeates your skin and leaches the calcium from your bones, and other tissues. It is not something we take lightly.
But yes, the lead/sulfuric combination does indeed make a convenient and inexpensive high capacity battery, so it remains used to this day despite the hazards. Most hazards are manageable with appropriate controls!