What chemistry are lithium-ion AA rechargeable batteries using?
Lithium-ion.
The internal battery is not connected to the external battery terminals. The battery operates at its own voltage (i.e. lithium-ion) and uses a DC-DC converter to deliver 1.5V to the external battery terminals.
That's the interesting thing going on.   Well, not quite.
We hope that internal electronics also provides battery charge/discharge protection for the lithium cell. But don't count on it.  The 3 vendors you have linked so far are all &quot;cheap Cheese&quot; and deliberately evade the laboratory testing required for a product to enter the US market, e.g. into the bricks-and-mortar retail channel, or true domestic mail-order e.g. from sears.com, Grainger, etc.  They are using either direct mail from Asia, or drop-shipping through Amazon's warehouse, where Amazon claims to be &quot;only a platform&quot;, leaving no one at all responsible for product safety, and somehow, this is &quot;OK&quot;.
That's the interesting thing going on!