Proper dash to use for negative charge
[…] one professor I know who insists that it should be typeset with an en dash.

I disagree! A standard minus sign should be

on the same height as the horizontal line of a plus sign;
as long as an equal sign.

The en dash is

lower than the horizontal bar of the plus sign;
slightly longer than the equal sign.

Unicode has dedicated characters for minus and superscript minus

standard minus: U+2212: −
superscript minus: U+207B: Cl⁻


See also:
The situation, and how to realize the correct typography using $\mathrm\LaTeX$ has been described in the German article Feinheiten bei wissenschaftlichen Publikationen – Mikrotypographie-Regeln, Teil II by Marion Neubauer in Die TeXnische Kommödie, 1997, 1, 25-44 (PDF).