Redflow of australia actually makes zinc bromide flow batteries now. It has not been a financial success to put it mildly... Much as I like the idea of flow batteries I think their window of opportunity may be closing.
Closed batteries have no inherent advantage on large scale stationary projects, only problems. The only gain they bring is that you can ride on the scale of mobile applications.
Redflow I believe costs as much as a lithium battery but claims to degrade less and be able to be 100% discharged. In practice they have proven hard to manufacture reliably or competively. They seemed better when they stayed theoretical. Share price is down about 95%...
1. lack of efficiency - only 80% roundtrip, much less than Lithium.
2. they go completely offline for periodic maintenance. This is done automatically by Redflow, but the loss of availability is problematic in some circumstances.