Sony explained to the press what's wrong with all the li-ion batteries that were recalled recently.
The problem was first acknowledged in August when Dell issued a recall for 4.1 million batteries and until now had been explained as metallic particles that got into the battery during the manufacturing progress. Today Sony expanded on this and said the particles, believed to be nickel, likely got into the battery during two stages in production: when a groove was created in the battery case and when the electrolyte was poured into the cell.
But that alone wouldn't be enough to cause the fires that have been reported by laptop owners. For that to happen Sony believes that the particles would have to fall into a small triangular gap in the cell body right at the point where the cathode ends between two layers of spacer material. Then, depending on system configuration, the conditions could be right for a fire to start in the battery.