Is a lithium battery a substitute for a radium battery?

Answer 1

Absolutely not.

While there are radioisotope batteries, radium is too expensive and dangerous for it to be used in them! Most radioisotope batteries use either plutonium-238 or tritium as the radiation source. It can be quite dangerous to handle these batteries. Radioisotope batteries can generate usable electricity for one or two halflives of the isotope and can thus function for years to centuries (or even longer) before going dead. Radioisotope batteries are almost always custom designed for the application and sometimes cannot be replaced as the device using them (e.g. deep space probe, equipment for operation in inaccessible remote sites) will not be operating at a location where any repairs or replacements would be practical.

Lithium batteries are chemical batteries, that generate electricity by electrochemical "corrosion" of plates inside the battery. Primary lithium batteries can go dead in weeks to months. Secondary lithium batteries may not go dead for years, but need to be recharged. Most lithium batteries are designed to be user replaceable.