Over the last year I've had a number of Rolleiflex 6008/6000 series users tell me about a bad experience with one particular ebay seller who is a Lithium battery solution for 6000 series cameras. The seller on ebay is from Quebec, Canada and the batteries are 3D printed blocks fitted with 3 Li cells. I've been sent several of these batteries after they fail. One user told me that they failed shortly after buying but not before they left feedback on ebay. I know that one big problem with Lithium batteries is fire - and that each cell needs to be charged independently to avoid that potential problem. These Li batteries from the Canadian ebay vendor charge all the cells in a sting or series connection - so that should be the first clue that these are not well designed.
Now I'm also hearing from my service technician that these batteries could be causing main circuit board failures. This is an expensive fix. Rolleiflex 6000 series cameras use more than just the two pins needed for charging to monitor battery and load and temperature. The Li batteries from Canada don't use all pins, don't have a thermal resistor the cameras read, and can provide higher current. This is also why those batteries don't work in all cameras. I strongly recommend against buying battery rebuilds from this vendor.
I have seen Lithium battery solutions that work well, for example the ones made by Wiese Fototechnik, and some nice home built ones as well. The 3D printed battery block is an interesting idea, but unfortunately can not recommend.