Airbus was worried about late additional compliance issue that have been introduced by EASA (European Aviation Safety Agency). Airbus plans to mature the advanced technology further nonetheless, but initial flight tests will be performed with lithium-ion batteries. The Airbus said that its too late to make some implement the change for early part of flight test program.
The Airbus A350 itself later will be certified with Nickel-Cadmium batteries. Also batteries made from lithium-ion is considered to be used by Airbus A320 NEO, unfortunately they drop the plan and use nickel-cadmium batteries.