Developing new strategies to improve the performance of organic-based sodium-ion batteries (SIBs) has gained tremendous scientific interest in recent years. The design flexibility and resource renewability of organic electrode materials offer new avenues towards sustainable battery systems for better energy storage.^[1] Towards the end of 20^th century, the research on organic electrode materials for battery applications was much focused on capacity retention. Immobilization driven materials design has alleviated the poor capacity retention issues to a great extent in recent years.^[2] However, organic materials have an inherent drawback of low reduction potential and consequent low discharge potential, which remains as a catastrophic block for its applications at the cathode side.  3,4,9,10-perylenetetracarboxylic (PTCDA), with a low lying LUMO and eight free bay and ortho positions for substitutions, is an ideal system to tailor a molecule with higher reduction potentials.^[3] Polyimides of PTCDA are very important class of engineering plastics. Herein, we demonstrate a simple and efficient approach to tune the redox properties of perylene poly/diimides as high voltage cathodes for organic-based SIBs. Firstly, a perylene based polyimide (PI) was synthesized through polymerization of PTCDA with hydrazine linker to yield a low molecular weight polymer which offers a high reversible capacity of 126 mAhg^-1with a good cyclic stability. An all-organic sodium ion full cell is also fabricated for the first time, with PI as cathode and disodium terephthalate (NaTP) as anode, offering an average cell voltage of 1.4 V.

Redox potentials of a molecule can be tailored by tuning its HOMO and LUMO energy levels.^[4] According to the molecular orbital theory, a lower LUMO level means higher electron affinity and thus a higher reduction potential for the molecule.^[5] Lowering the LUMO levels could be achieved in two ways; (i) substituting electron withdrawing groups to the redox active molecule and (ii) extending the conjugation in a molecule through aromatic rings.^[4-5] Extending the conjugation is a conventional approach and could increase the electrochemical dead weight of the molecule, resulting in reduced theoretical capacity. Here we chose the first route and demonstrate a remarkable tunability in the discharge potential from 2.1 to 2.6 V vs. Na⁺/Na, with a sodium intake of ~1.6 ions per molecule, using various electrophilic substitutions on perylene diimides. Further, acknowledging the importance of a single plateau voltage profile in commercial rechargeable batteries, we attempt to achieve a single redox peak in PDIs by tuning certain parameters. With the understood inherent advantage of greater coordination energy, we presumed that the dianion species could shift the equilibrium just by having a naked state energy (energy before coordination) similar to that of the radical anion.^[6] The naked state energies of a perylene diimide derivative can be tuned by changing the dihedral angle in the ring. A single plateau discharge profile is obtained for tetra bromo substituted perylene diimide with dihedral angles of θ₁ & θ₂=38^o. Detailed structural analysis and electrochemical studies on substituted PDIs unveil the correlation between molecular structure and voltage profile. The results are promising and offer new avenues to tailor the redox properties of organic electrodes, a step closer towards the realization of greener and sustainable electrochemical storage devices.

 

References

[1]        Z. Song, H. Zhou, Energy & Environmental Science 2013, 6, 2280-2301.

[2]        Y. Liang, Z. Tao, J. Chen, Advanced Energy Materials 2012, 2, 742-769.

[3]        C. Huang, S. Barlow, S. R. Marder, The Journal of Organic Chemistry 2011, 76, 2386-2407.

[4]        G. S. Vadehra, R. P. Maloney, M. A. Garcia-Garibay, B. Dunn, Chemistry of Materials 2014, 26, 7151-7157.

[5]        Z. Song, H. Zhan, Y. Zhou, Angewandte Chemie International Edition 2010, 49, 8444-8448.

[6]        S. Seifert, D. Schmidt, F. Wurthner, Chemical Science 2015, 6, 1663-1667.