The older Power Functions electrical system includes the option to purchase a lithium ion rechargeable battery box. Is something like this available for the newer Powered Up system?
This isn't currently available.
The Powered Up team responded to a question about this in May 2019. The gist is that they were able to say that the removable battery holder in the hub was designed with a future rechargeable battery in mind. This can also be inferred directly from the design:
They cited several reasons that this isn't yet available, including TLG's strong commitment to product quality and safety, particularly in a product like this that would involve a direct interface to mains power for recharging. The challenges of localizing the charging adapter to all markets was also mentioned.
The team didn't provide a release date, but there was a sense that this would be coming at some point.