Neo 1973 Battery
The battery is a one 3.7V (nominal) cell. It has internal protection circuitry, and a capacity of 1200mAh.
On the battery, which has dimensions of approximately 53mm*38mm*7mm is written "FIC, First International Computer, Model: GTC-01/GTA-01, Standard Battery Pack, 1200mAh Li-ion, Rechargeable Battery: 3.7V".
It is apparently compatible with the Sony BLC-5 See this mailing list post These are available very inexpensively from ebay. Though quality will of course vary.