Wonder if those neg ends have something that was put on them? Maybe to help prevent accidental discharge? Its now a little late, but take a dremel or some fine grit sandpaper and clean the neg ends. Wipe the ends off with motor spray or denat alcohol.

When solder won't stick, there is usually something on that surface that is preventing the flux and solder from wetting that surface. Thats why you clean and abrade or rough up the ends of the cells and wipe them with a cleaner that doesn't leave any residue.