Under contract law, the firm is obligated to fulfill the terms of its agreement with the EV manufacturer, which likely included delivering a safe, high-performing electrolyte solution. However, the engineering firm does not have a contract with the consumer, nonetheless it may be liable under negligence if they have failed in their duty of care to potential users of the batteries.

Contract law doesn't apply since the actual harm occurred to consumers after completion of contract, and negligence is negated by the innovative nature of the electrolyte solution, which implies acceptance of a certain level of risk by the EV manufacturer.

Breach of contract is likely applicable here due to the duty of care the engineering firm owes to the end-users and the EV manufacturer, especially since the defect poses a direct risk to consumer safety and public health.

The engineering firm can only be held accountable under negligence law if the EV manufacturer proves that the electrolyte solution did not meet the specific technical parameters outlined in their agreement, irrespective of any safety issues that arose.