Plug-in hybrid electric vehicles (PHEVs) accounted for a smaller portion of total plug-in vehicle battery capacity due to their lower sales volumes and because they require smaller battery packs than EVs, since they have gasoline-powered engines to extend total vehicle range.
PHEV sales decreased 32% from 2018 to 2019, about the same as the decrease in PHEV battery capacity.
Calendar year 2018 was the first full year of Tesla Model 3 sales, which accounted for the large increase in total battery capacity between 2017 and 2018.
Source: Argonne National Laboratory, Assessment of Light-Duty Plug-In Electric Vehicles in the United States, 2010 – 2019, June 2020.