The researchers discovered that, while there are two forms of lithium filaments, the way they grow depends on the applied current. Clustered, ‘mossy’ deposits, which form at low rates, turn out to grow from their roots and can be relatively easy to control. The sparser and rapidly advancing ‘dendritic’ projections grow only at their tips. The dendritic deposit, the researchers said, are harder to deal with and are responsible for most of the problems.
“Those small fibres can go right through the separator and cause explosions or fires,” said Peng Bai, a senior post doctorate student.
The root-growing mossy growth, the team found, can be blocked by adding a separator layer made of anodic aluminium oxide, which is a nanoporous ceramic material.
“It’s a big discovery, because it answers the question of why you sometimes have better cycling performance when you use ceramic separators,” Bai explained.
The tip-growing dendritic growth, however, cannot be easily blocked, but supposedly should not occur in practical batteries. The normal working currents of practical batteries are much lower than the characteristic current associated with the tip-growing deposits, so these deposits should not form unless significant degradation of the electrolyte has occurred.
The team concluded that these growths can be controlled at lower current levels while demonstrating the upper limits on battery performance to prevent the truly damaging dendritic filaments.
