Tesla Inc CEO Elon Musk said improvements to be unveiled at the electric-vehicle maker's "Battery Day" event would not reach "serious high-volume production" until 2022, knocking its shares down.

Analysts were expecting Musk to unveil at the event plans for Tesla to produce its own battery cell as it seeks to cement its lead over General Motors, Volkswagen and others.

But in a post on Twitter ahead of the Tuesday event, Musk said: "This affects long-term production, especially Semi, Cybertruck & Roadster, but what we announce will not reach serious high-volume production until 2022."

Tesla expects significant shortages in 2022 and beyond, Musk cautioned, adding it intended to increase cell purchases from Panasonic, South Korea's LG Chem, China's CATL, and possibly other partners.

LG Chem and CATL shares rose after the comments.

But Tesla shares fell more than five percent to 425.50 U.S. dollars in extended trade as the tweets cast doubt on whether Tesla would be able to produce its own batteries any time soon – a concern echoed by experts and industry officials.

"Battery firms believe it is not easy to mass produce batteries. It took them a lot of time ... how can Tesla do it overnight?" a South Korean battery industry source told Reuters.

Tesla is working to produce new, bigger battery cells at its Fremont facility, two people familiar with the matter have told Reuters. The new cells will have a diameter of 42 mm, versus the 21 mm ones made at its joint battery factory with Panasonic that are used in Model 3 sedans, the people said.

Measures of cost

The cost of battery cells and packs is coming down rapidly. Measured in dollars per kilowatt-hour (kWh), experts have pegged 100 U.S. dollars per kWh at the cell level as the point where electric cars will begin to approach the cost of vehicles powered by combustion engines and fossil fuel.

Today's battery packs cost around 10,000-12,000 U.S. dollars, depending on their capacity. Tesla is aiming to reduce the cost of future packs to less than 6,000 U.S. dollars, which would put the cell cost at well under 100 U.S. dollars per kWh.

(With input from Reuters)