While Eneloop batteries are the best NiMH AA and AAA batteries that you can get, they have only 1.2 V and some devices need in fact the full 1.5 V of a normal AA battery (e.g. 3.3 V logic that assumes 2 x 1.5 V AA batteries). Li-ion batteries can disguise as 1.5 V AA batteries by using a voltage regulator that step down the Li-ion voltage to 1.5 V. Newer variants have even a USB charging port built in, so that you can charge them without a NiMH charger, but just with any USB power source.

Such Li-ion-based AA batteries typically have two disadvantages:

• The keep the 1.5 V as long as possible and when they are empty, they simply shut down: a device that assumes a normal 1.5 AA battery uses however the voltage drop that a AA 1.5 V battery has when it gets drained to display in advance whether the battery needs to be replaced. This will not work with the Li-ion-based batteries.
• The Li-ion-based batteries can be deep discharged: while they have a protection circuit that switches them off, they recover after some time and the protection circuit enabled the battery again which drains them further and so on.

According to tests (in German), the Keeppower P1450TC (ca. 6.45 EUR with USB C charging port) seems currently to be the only exception that does not have that disadvantage. (However, it starts to drop the voltage somewhat too early so that devices that really need 1.5 V switch off far too early. Would be interesting to see whether future brings even better alternatives).