The Metallocene Battery: Ultrafast Electron Transfer Self Exchange Rate Accompanied by a Harmonic Height Breathing

Abstract

The first all-metallocene rechargeable battery consisting of poly-cobaltocenium/- and poly-ferrocene/reduced graphene oxide composites as anode and cathode was prepared. The intrinsically fast ET self-exchange rate of metallocenes was successfully combined with an efficient ion-percolation achieved by molecular self-assembly. The resulting battery materials show ideal Nernstian behavior, is thickness scalable up to >1.2 C cm^-2 , and exhibit high coulombic efficiency at ultrafast rates (200 A g^-1 ). Using aqueous LiClO₄ , the charge is carried exclusively by the anion. The ClO₄ ^- intercalation is accompanied by a reciprocal height change of the active layers. Principally, volume changes in organic battery materials during charging/discharging are not desirable and represent a major safety issue. However, here, the individual height changes-due to ion breathing-are reciprocal and thus prohibiting any internal pressure build-up in the closed-cell, leading to excellent cycling stability.

Keywords: cobaltocene; ferrocene; organic batteries; organometallic electrodes; reduced graphene oxide.

Scheme 1

Schematic illustration of ion breathing. Frustrated and fulfilled 18‐electron rule in the charged and discharged Fe/Co metallocene battery. a) discharging from the charged state; b) charging from the discharged state; X⁻: moving anion.

Figure 1

Mass–height anion breathing. a) Electrochemical redox switching of (PCo@rGO)@CC (red, left) and (PFc@rGO)@CC (orange, right) during AFM image acquisition. Weight ratio Polymer/GO=5; electrolyte: 0.1 M LiClO₄ ⁻/H₂O; excitation: square wave potential (Anode: −0.3 V → −1.3 V → −0.3 V → …; cathode: + 0.8 V → 0 V → + 0.8 V → …) (13 steps with 30 s step time); current response (chronoamperometric): anodic and cathodic spikes. b) Corresponding height change in AFM images: on a 0.07 cm² glassy carbon (GC) as CC; total AFM acquisition time: 400 s, 13 potential steps during AFM acquisition correlate with 13 stripes (up/down) (see Table 2); c) ec‐QCM frequency response during one CV cycle (PCo@rGO: 0 → −1.3 → 0; PFc@rGO: 0 → + 0.8 → 0) in 0.1 M aqueous LiClO₄ at v=50 mV s⁻¹.

Figure 2

Battery performance of the PCo@rGO–PFc@rGO test cell. a) CVs of PCo@rGO (left, red) and PFc@rGO (right, orange) coated on GC substrates in 0.1 M LiClO₄/aq. at v=10 mV s⁻¹; polymer/rGO=9.5/1. b) galvanostatic charging‐discharging curves of the test‐cell at 1 mA cm⁻² (corresponding to 20 A g⁻¹); c) Cycling performance of test cell at different rates (0.25 mA cm⁻²–10 mA cm⁻² (corresponding to 5, 10, 20, 50, 75, 100, 150 and 200 A g⁻¹); d) Percentage of observed capacity (0.25 mA cm⁻²=100 %) vs. mA cm⁻² rates; e) Discharge capacity of test cell over 500 galvanostatic charge‐discharge cycles at 0.5 mA cm⁻².