The offtake memorandum of understanding considers the purchase of 20,000-30,000 tonnes per annum of purified spherical graphite from Renascor's proposed battery anode material plant.
The conglomerate is the world's largest anode manufacturer outside of China, with production capacity of 44,000tpa and a further 69,000tpa in construction.
Together with its lithium interests, wants to become the top player in the global electric vehicle battery materials market by 2030, from raw materials to finished battery inputs.
With POSCO in the picture, Renascor has visibility to some 60,000tpa of offtake, twice its proposed stage one production, and it says it is still seeing plenty of interest from tier one companies in the battery supply chain.
Feasibility studies are already underway for an expansion.
Managing director David Christensen said getting POSCO involved was a transformational step change for Renascor.
The pair will now work together to finalise price, term, product quality and other offtake parameters, and product validation testing.
A final investment decision on Siviour is expected in early 2022.
The 3.8Mt Siviour is said to host the world's second largest proven graphite reserves, and the largest outside Africa.
The development will involve a mine and concentrator on the Eyre Peninsula, and an "eco-friendly" PSG facility at Port Adelaide that avoids the use of hydrofluoric acid.
Stage one was estimated to cost $118 million, while the BAM plant would add $90 million.
Start-up is planned for late 2023.
The company completed a A$15 million placement at 8c in April, which it said allowed to be fully funded until the construction phase of Siviour.
Renascor shares jumped up 10% to 11c, valuing it at $188 million. The stock has traded between 1-18.5c over the past year.