The Department of Transportation has banned certain kinds of lithium batteries from checked airline luggage, citing worries over short circuiting and fires. Lithium batteries installed inside of electronic devices will be permitted as carryon, but loose batteries, unless sealed inside a plastic bag, will no longer be permitted on board.
A two battery per passenger limit will also be in effect. Lithium ion batteries under 8 grams, the sort that power most cell phones and laptops, will not run afoul of the new rule, which applies to the non-rechargeable variety. The FAA's main concern is that the fire control systems in cargo holds can't suppress sparks from these types of lithium batteries.