Polymeric Schiff bases as low-voltage redox centers for sodium-ion batteries

The redox entity comprising two Schiff base groups attached to a phenyl ring (-N=CH-Ar-HC=N-) is reported to be active for sodium-ion storage (Ar=aromatic group). Electroactive polymeric Schiff bases were produced by reaction between non-conjugated aliphatic or conjugated aromatic diamine block with terephthalaldehyde unit. Crystalline polymeric Schiff bases are able to electrochemically store more than one sodium atom per azomethine group at potentials between 0 and 1.5 V versus Na(+)/Na. The redox potential can be tuned through conjugation of the polymeric chain and by electron injection from donor substituents in the aromatic rings. Reversible capacities of up to 350 mA h g(-1) are achieved when the carbon mixture is optimized with Ketjen Black. Interestingly, the "reverse" configuration (-CH=N-Ar-N=HC-) is not electrochemically active, though isoelectronic.