This is Turnigy Soft Silicone Lipo Battery Protector(1000-1300mAh).

The Turnigy Lipo Protector is made entirely from soft compliant silicone, its purpose is to help stop your Lithium Polymer battery from moving around in your model during use. Whilst also adding a layer of abrasion protection and vibration absorption.

This sleeve is fit for most 1000 – 1300mah 3S packs, it is suitable for batteries with dimensions up to 74 x 36 x 21mm.

Note: The sizing is approximate as the silicone material is stretchy.