This type of battery is now offered on products from WIDEX, OTICON, STARKEY and RESOUND.
I found out a little about them and how they differ to lithium-ion batteries (in the PHONAK AUDEO B- RECHARGEABLE).
The batteries are designed to last more than a year based on daily usage. A replacement pair of rechargeable batteries will cost around £59. A couple of guides are attached on this product: