S&P Dow Jones Indices announced on Friday were among eight additions to the index as part of the September 2022 quarterly rebalance.
Sayona has a market capitalisation of A$2.3 billion, while Capricorn is valued at $1.3 billion.
Suspended lithium developer AVZ Minerals will be removed from the ASX 200 and ASX 300.
The stock has been suspended since May due to a permitting spat with Zijin Mining over the world-class Manono lithium project in the Democratic Republic of Congo.
Fellow company on the long-term suspended list, troubled Mali gold miner Firefinch, will also be removed from the ASX 300.
Another two Africa-focused miners, newcomer Sierra Rutile and gold producer Resolute Mining, have lost their spots in the ASX 300.
Joining the ASX 300 is new nickel miner Mincor Resources, magnetite miner Grange Resources, uranium developers Boss Energy and Deep Yellow, rare earths developer Arafura Resources, battery minerals stocks Argosy Minerals, Neometals and 5E Advanced Materials and mining services company Maas Group.
The changes take effect before the market opens on September 19.
