Hidden among the architectural landmarks of General Motors’ sprawling Warren Tech Center outside Detroit is a new cornerstone of the automaker’s $900 million bet on its electric future. The nondescript 500,000-square-foot pair of off-white boxes, which house GM’s new Battery Cell Development Center, might not look like much. But locked inside is the key to GM’s plan to slash the cost of its EVs by nearly 10%. At a time when some car companies are pulling back on EVs, GM’s new Battery Cell