Polyimides: promising energy-storage materials.

Based on this redox mechanism, we developed a series of polyamides with different structures and investigated their electrochemical activity as cathode materials for rechargeable lithium batteries. The results show that the polymer framework endows these samples with excellent cycling stability. Additionally, their capacity and rate capability are also satisfactory. The five polyimides were obtained through a simple polycondensation between commonly used dianhydrides and diamines. Coin cells consisting of the cathode (polyimide: conductive carbon: PTFE= 6:3:1), lithium anode and electrolyte (1 M LiTFSI/DOL+DME) were used for cycling tests, with the cutoff voltage of 1.5~3.5 V. FTIR characterization confirms the obtaining of our target products. In the charge/discharge measurements, all the samples show efficiency close to 100%. The Table below summarizes the results of five typical products. The theoretical capacity is calculated based on a four-electron mechanism shown above. The strong resemblance in the average discharge voltage and voltage profile between the sample PI-1 and PI-2, PI-3 and PI-4 seems to imply that the average discharging voltage are mainly determined by the functional group adjacent to the carbonyl group rather than that attached to the nitrogen atom. Comparing the theoretical capacity and experimental value, it can be generalized that almost half of the active site is available to be attached by Li, which possibly suggests that two electrons have been transferred during the charge/discharge process. Considering the importance of cyclabilty and rate performance, polyimide containing NTCDA(1,4,5,8-naphthalenetetracarboxylic dianhydride) unit seems to be a promising energy-storage material. Authors would express their sincere thanks to the Nature Science Foundation of China (No. 20873094) for the financial support.