The ML2032 is a Lithium Manganese Dioxide cell with a nominal voltage of 3.0V and a capacity of approximately 65mAh. These cells require a constant charging voltage of 3.1V.
The ICR2032 60mAh Lithium cobalt oxide cell with a nominal voltage of 3.7V.
IFR2032 45mAh Lithium iron phosphate with a nominal voltage of 3.2V
Also commonly available is the LIR2032 coin cell, with LIR standing for Lithium Ion Rechargeable. As with normal Lithium Ion cells theyhave a nominal voltage of 3.6V and a capacity of around 40mAh and a fully charged voltage of 2.4 volt.