Hi,
Whether a device can handle 6.5 V or not depends on the technology and the foundry.
Means you have to check with the design files provided by the foundry to see the breakdown voltage.
a 0.35u technology with 7.5nm oxide thickness can hold 5v supply according the technolgy file. however,when used it in 6.5v,it works too. maybe the difference is the lifetime.