The Electrochemical Behavior of Polyimide Binders in Li and Na Cells

The electrochemistry of aromatic and aliphatic polyimide binders was characterized in composite coatings for Li-ion and Na-ion battery negative electrodes. Aromatic polyimide was found to have a large first lithiation capacity of 1943 mAh/g and a reversible capacity of 874 mAh/g in lithium cells. The large first lithiation capacity is suggestive of its full reduction to carbon. Subsequent cycles of aromatic-PI are also similar to that of hydrogen containing carbons. Aromatic-PI is also active in Na cells, but with less capacity and less hysteresis during cycling, which is also consistent with the behavior of hydrogen containing carbon in Na cells. Therefore, we suspect that after the first lithiation or sodiation, all of the aromatic-PI becomes carbonized. These conductive reaction products lead to excellent cycling in alloy cells. In contrast, aliphatic-PI is inert and leads to poor cycling when used in alloy cells. These results may have large implications for the use of conductive polymer binders, which may just be carbonizing during their first lithiation.