Donny Nordlicht from Cadillac communications told AutoblogGreen that while the Volt and CT6 batteries are both 18.4-kWh, the shape is completely different. In the Volt/ELR, the battery is T-shaped (see it here). The CT6 has four seats, with a tunnel running between the two in the rear, as you can see here, but the battery in the CT6 PHEV is "a cube-shaped pack, which is between rear seats and the trunk," Nordlicht said. "There is no pass through."
GM has not yet released any technical schematics about this pack, but Nordlicht said that, "The CT6's advanced mixed-material platform was designed to accommodate the PHEV system by design so that it minimally intrudes on the cabin space." It also means that the CT6 can be ordered as an optional PHEV, while the Volt and ELR were purpose-built plug-ins.
GM is also distinguishing between the EREV and PHEV powertrains in its vehicles from this point forward. "We are not discarding the EREV language," Nordlicht said. "The CT6 utilizes a two-motor system mated to a 2.0T 4-cylinder engine, which is an all-new system to Cadillac." We assume that the PHEV packs will use li-ion cells from LG Chem, just like the EREVs do, but Nordlicht did not answer our question on that point. As for other details about the CT6 PHEV – like production, full dimension, and pricing – we will just have to wait until closer to when the vehicle launches for those.