Samsung Electronics announced on Friday that it   would recall   2.5 million Galaxy Note 7 smartphones after finding a flaw in the battery cell that resulted in fires. It is the latest problem for the lithium-ion battery, the power source at the heart of most modern devices. Q. What is a lithium-ion battery? A.   It is a   powerful, lightweight battery   that includes lithium-ion particles. Lithium-ion particles in the batteries move back and forth between a negative and positive electrode as they are charged and discharged. The advantage for companies in Silicon Valley and Detroit is that the batteries don’t take up much room and can quickly recharge repeatedly without wearing out. Q.  What are they used in? A.   Chances are you encounter these on a daily basis. They are in smartphones, laptops,   electric cars , airplanes and even e-cigarettes. Of course, the ones in airplanes and cars are much larger than those used in phones...