A Small-Molecule Organic Cathode with Two Electron-Withdrawing Cyano Bonds for Li-Ion Batteries.

Abstract

Perylene-3,4,9,10-tetracarboxylic dianhydride [PTCDA, theoretical specific capacity (C_T) = 130 mAh g^-1] is an efficient n-type organic cathode in Li-ion batteries with the median potential of ≈2.5 V (vs. Li). In order to improve its working potential, a new insoluble small-molecule organic cathode named 1,7-dicyano-perylene-tetracarboxylic dianhydride (PTCDA-2CN) is designed and synthesized with two electron-withdrawing groups cyano bonds (-CN). As expected, PTCDA-2CN can exhibit the high working potential ≈2.8 V (vs. Li) with its decreased lowest unoccupied molecular orbital energy, which is ≈0.3 V higher than PTCDA. And the introduction of two -CN groups can maintain the basis C_T value of 121 mAh g^-1 for PTCDA-2CN. Consequently, PTCDA-2CN exhibits the good cathode performances in Li-ion half/full cells. In half cells, PTCDA-2CN can show the discharge capacities of 117-107 mAh g^-1 with the stable cycling retention of 91% during 100 cycles. Meanwhile, using the reduced state (LiC₆) of graphite (C) as the anode, the LiC₆//PTCDA-2CN full cells can also exhibit good performances of 116-98 at 50 mA g^-1 with the high median voltage of 2.7 V. PTCDA-2CN is one of the highest potentials ever reported for the n-type organic cathodes in Li-ion batteries.