Negative ions are called anions. A typical anion is Cl - . A polar solvent is required. The most common solvent for an electrolyte solution is water. Current can be carried by ionized gases as in fluorescent bulbs.
Some unusual solids can conduct cations through a lattice of anions.
Ice can conduct protons and silver chloride can conduct silver ions at elevated temperatures. Molten salts conduct by movement of both anions and cations.