Me over here just raw-dogging a Ryobi 18v pack directly into my thinkpad. It's flawless.
You can also get an "official" adapter for the various battery brands such as this: https://www.milwaukeetool.com/Products/Batteries-and-Charger... (45w over USB C) or https://www.dewalt.com/product/dcb094k/usb-charging-kit (100w over USB C)
Plug adapter is https://www.ebay.com/itm/253555244977
And I just hacked up an old cheap hat charger to become an output connector.
But Ryobi batteries are cheaper than Lenovo batteries, and I can use them for more other stuff so it's easier to justify owning a whole pile of them.
I miss the days of the docking-station-shaped "battery slice" which added even more to the bottom of the machine. But evidently they may still make those and I'm just not seeing them in the wild? Anyway, the Ryobi solution is so simple and efficient, it's not worth shopping for alternatives.