I will be completing an unpaid, 2 month internship in Nanjing, which visa should I apply for?

Hi,
I've recently returned from a four month study visit to China under an F visa, and I will be returning in mid-April to complete and unpaid internship in Nanjing. I will be finished around mid-June, probably around the 22nd.

What kind of visa should I apply for and what documentation will I need from my host organisation? (Keeping in mind I won't be getting paid)