This is a Sony rechargeable battery BP-2 or BP-2EX designed for Sony Discman CD Players. The battery cell has been replaced with a high-capacity Polymer Li-ion cell, boasting approximately 1200mAh, which is twice the capacity of the original battery. The battery can be recharged by the CD player itself or an compatible external charger. 

This battery works for many CD players (D-Z555 in the picture), including:

SONY: D-66,  DT-66, D-82，D-88，D-90，D-99，D-150，D-250，D-303，D-515，D-555，D-Z555.

DENON: DCP-100，DCP-150.

AIWA: XP6，XP7.

PHILIPS: AZ6808，AZ6819，AZ6829.