Organic electrode materials have the typical advantages
of flexibility,
low cost, abundant resources, and recyclability. However, it is challenging
to simultaneously optimize the specific capacity, rate capability,
and cycling stability. Radicals are inevitable intermediates that
critically determine the redox activity and stability during the electrochemical
reaction of organic electrodes. Herein, we select a series of aromatic
imides, including pyromellitic diimide (PMDI), 1,4,5,8-naphthalenediimide
(NDI), and 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which
contain different extending π-conjugated aromatic rings, to
study the relationship between their electrochemical performance and
the size of the aromatic ring. The results show that regulating the
aromatic ring size of imide molecules could finely tune the energies
of the lowest unoccupied molecular orbital (LUMO), thus optimizing
the redox potential. The rate performance of PMDI, NDI, and PTCDI
increases with the aromatic ring size, which is consistent with the
decrease in the LUMO–HOMO gap of these imide molecules. DFT
calculations and experiments reveal that the redox of imide radicals
dominates the charge/discharge processes. Also, extending the aromatic
rings could more effectively disperse the spin electron density and
improve the stability of imide radicals, contributing to the enhanced
cycling stability of these imide electrodes. Hence, aromatic ring
size regulation is a simple and novel approach to simultaneously enhance
the capacity, rate capability, and cycling stability of organic electrodes
for high-performance lithium-ion batteries.