It cannot work on it's own as there is nothing at stake, allowing an attacker with large holdings in the future to rewrite the chain at no cost.

However, it can work in a hybrid model, where the sole purpose of POS is to solve the 51% hashpower attack problem. For more info, see Interleaved Mining,
https://bitcointalk.org/index.php?topic=1317240.0

The primary benefit of this approach is to not punish miners who are not hostile. It allows them to retain their investment in SHA256 hashing hardware.