Ferrocene-functionalized polyheteroacenes for the use as cathode active material in rechargeable batteries

Abstract

Herein we report the synthesis and characterization of new conjugated polymers bearing redox-active pendant groups for applications as cathode active materials in secondary batteries. The polymers comprise a ferrocene moiety immobilized at a poly(cyclopenta[2,1-b:3,4-b']dithiophene) (pCPDT, P1) or a poly(dithieno[3,2-b:2',3'-d]pyrrole) (pDTP, P2) backbone via an ester or an amide linker. Electrochemical and oxidative chemical polymerizations were performed in order to investigate the redox behaviour of the obtained polymers P1 and P2 and to synthesize materials on gram-scale for battery tests, respectively. During galvanostatic cycling in a typical battery environment, both polymers showed high reversible capacities of 90% and 87% of their theoretical capacity and excellent capacity retentions of 84% and 97% over 50 cycles.

Scheme 1. Syntheses of CPDT–Fc M1, DTP–Fc M2, and corresponding polymers pCPDT–Fc P1 and pDTP–Fc P2.

Fig. 1. Crystal structure of CPDT–Fc M1: (a) molecular structure, (b) unit cell, and (c) intermolecular contacts: π–π interactions (red dotted lines) and H–π herringbone interactions (cyan dotted lines). Displacement ellipsoids are drawn in (a) and (b) at the 50% probability level. The ferrocene units in (c) are shown in light grey for clarity.

Fig. 2. Electropolymerization of M1 (a) at a scan rate of 200 mV s⁻¹ and M2 (c) at a scan rate of 100 mV s⁻¹ in 0.1 M TBAPF₆ in CH₂Cl₂ (monomer concentration 10⁻³ M). Cyclic voltammograms of pCPDT–Fc P1 (b) and pDTP–Fc P2 (d) in 0.1 M TBAPF₆ in CH₂Cl₂ applying different scan rates.

Fig. 3. UV-vis-NIR spectra obtained from spectroelectrochemical measurements on polymer P1 (a) and P2 (b). Applied voltages are referenced vs. Ag/AgCl. Arrows show the spectral changes upon increasing the applied voltage. Artefacts are marked with * in the spectra.

Fig. 4. Cyclic voltammograms of (a) pCPDT–Fc P1 and (b) pDTP–Fc P2 in 1 M LiPF₆/EC : DMC (1 : 1 by weight) at a scan rate of 0.1 mV s⁻¹ and with a stepwise increase of the upper vertex potential.

Fig. 5. Galvanostatic cycling performance of pCPDT–Fc P1 (a and c) and pDTP–Fc P2 (b and c) in 1 M LiPF₆/EC : DMC (1 : 1 by wt.), at a current rate of 0.1C. In (c) the open symbols denote the charge capacities and the closed symbols denote the discharge capacities relative to the theoretical capacities.