If you search for "1.5V rechargeable", there are lots of lithium ion batteries with electronic voltage regulators now. Some designs will step down to ~1V when

If you search for "1.5V rechargeable", there are lots of lithium ion batteries with electronic voltage regulators now. Some designs will step down to ~1V when nearly depleted, so that low battery indicators still work. I'm not sure if an NiMH sleeve was ever practical given the space constraints, but it's kind of moot now that better technology exists.