Samsung SDI supplies Rivian with 2170-type cylindrical cells (the same form factor as Tesla uses in the Model 3/Model Y and Lucid in the Air).
Update: Yangmo Ku, Senior Professional Communication Team at Samsung SDI contacted InsideEVs, explaining that the rumors about talks with Rivian are not true:
"Samsung SDI has not specifically discussed battery JV with Rivian."
The Elec reports that the talks over forming a potential joint venture in the U.S. were dropped. Nonetheless, Samsung SDI remains Rivian's supplier.
First of all, a slower than anticipated ramp up of Rivian production had "caused trouble between Rivian and Samsung SDI’s talks, sources said."
On top of that comes Rivian terms, which, according to the sources, includes a transfer of the battery making technology, rights to inspect the interior of factories, and no definite contract on the battery volume. That, if true, sounds pretty one-sided:
"Rivian had requested a transfer of the battery making technology and also rights to inspect the interior of factories, they said, which was unacceptable to Samsung SDI.
The US firm also didn’t guarantee to buy certain amounts of the battery produced, they said."
Meanwhile, Samsung SDI announced a battery joint venture with Stellantis, with an initial annual production capacity of 23 GWh (40 GWh in the future). Production will start in 2025, but the location is not yet announced.
In December, Rivian announced a plant to build a second EV plant in Georgia - east of Atlanta, in Morgan and Walton Counties. It was said that there might be a co-located battery cell production at the site, but it's hard to imagine without a battery partner.