heavy metal toxins are the only real concern here, such as the lead and Nickel and cadmium used on rechargables, other than that there`s no real issue, in fact Zinc/Carbon or even Alkaline batts are (when "rotted" down" actualy beneficial to plant life.
giving Zinc, Manganesse, nitrogen, chlorides, potassium and iron back to the soil.
a plant cannot live without any one of these elements either in trace or Macro nutrient form