The invention relates to the manufacture of manganese dioxide by a chemical process. The resulting manganese dioxide product takes the form of particles characterized by filament-like protrusions jutting out from its surface. The manganese dioxide particles having such surface features can be manufactured by reacting manganese sulfate with sodium peroxodisulfate in an aqueous solution. The process can be controlled to yield high density manganese dioxide. The manganese dioxide formed in the process can be deposited directly onto the surface of electrolytic manganese dioxide (EMD). The manganese dioxide product is particularly suitable for use as a cathode active material in electrochemical cells.