Hello Paul,
Thank you. The ES300h utilize the nickel metal hydride battery and not the trendier lithium batteries. Based on the warranty booklet, it is covered for 8 years or 100K miles but since I live in CA, it is extended to 10 years or 120K miles (dealer said it's a state law). In my opinion, the electric motor batteries are maintenance free. I have spoken with a few veteran Prius owners when i still have my Camry hybrid for service and said that they didn't have any issues with the battery which they owned for 8 years. There is a vent in the left side back seat that sucks in air to keep the battery cool during operation.
Yes, the 2013 ES300h and ES350 have subtle differences in the interior (EV mode and smaller trunk space for hybrid). Outside, there is hybrid badge, blue outline on the Lexus logo and it comes with a rear lip spoiler as standard (according to my dealer). Not sure about the ES350 exhaust if they are dual chrome plated but in the hybrid model, it is hidden.
I hope this helps. :)