IBM 5170  -  Battery

Pretty much, any 6 volt non-rechargeable battery with the required plug, is going to do the job.
Fitting a rechargeable battery is pointless, because the 5170 motherboard does not contain battery charging circuitry.

If you are changing the battery because of a 161 error, then note that after battery replacement, the 161 will remain until you complete the SETUP procedure.

Original battery

Below is one of the IBM supplied batteries; there were others.

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It is 6 volts, Lithium (not the same as Lithium-Ion), and IBM part number 8286121
IBM don't sell the battery anymore, but there are many battery suppliers on the Internet who can supply a replacement.
Many of those list their replacement under part number 8286121

The pictured battery has a resistor on top. The replacement battery does not need to have a resistor.

Four 1.5 volt AA cells

Using the battery holder pictured below, four standard 1.5 volt AA cells can be used.

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A search of eBay using battery holder aa should find this battery holder.

3.6 volt Lithium

Some people use a 3.6 volt Lithium battery.

With a 3.6 volt battery fitted, some 5170 motherboards may keep the CMOS settings but lose time. (technical info)

A particular type of 3.6 volt battery is pictured below.

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A search of eBay using battery tadiran 3.6 should find this battery.