The stakes keep getting higher for American discount brokerage Robinhood, which today disclosed that it has added hundreds of millions of dollars to its previously disclosed funding round. Including the $280 million that the company had already announced, Robinhood said that it was “pleased to share” that it “raised an additional $320 million in subsequent closings.” Its now $600 million funding round brings its post-money valuation to $8.6 billion. Fortune first reported the news. (A detail, but the new capital is part of the same round as it was raised…