An Alternative to Carbon Additives: The Fabrication of Conductive Layers Enabled by Soluble Conducting Polymer Precursors - A Case Study for Organic Batteries

Abstract

Utilizing organic redox-active materials as electrodes is a promising strategy to enable innovative battery designs with low environmental footprint during production, which can be hard to achieve with traditional inorganic materials. Most electrode compositions, organic or inorganic, require binders for adhesion and conducting additives to enable charge transfer through the electrode, in addition to the redox-active material. Depending on the redox-active material, many types and combinations of binders and conducting additives have been considered. We designed a conducting polymer (CP), with a soluble, trimeric unit based on 3,4-ethylenedioxythiophene (E) and 3,4-propylenedioxythiophene (P) as the repeat unit, acting as a combined binder and conducting additive. While CPs as additives have been explored earlier, in the current work, the use of a trimeric precursor enables solution processing together with the organic redox-active material. To evaluate this concept, the CP was blended with a redox polymer (RP), which contained a naphthoquinone (NQ) redox group at different ratios. The highest capacity for the total weight of the CP/RP electrode was 77 mAh/g at 1 C in the case of 30% EPE and 70% naphthoquinone-substituted poly(allylamine) (PNQ), which is 70% of the theoretical capacity given by the RP in the electrode. We further used this electrode in an aqueous battery, with a MnSO₄ cathode. The battery displayed a voltage of 0.95 V, retaining 93% of the initial capacity even after 500 cycles at 1 C. The strategy of using a solution-processable CP precursor opens up for new organic battery designs and facile evaluation of RPs in such.

Keywords: conducting polymers; conductivity additives; organic battery; organic electrode; quinones; redox-active polymer.

Figure 1

Schematic showing the process for creating CP/RP composites. The materials are composed of a CP precursor, EPE, which polymerizes to pEPE and PNQ. EPE and NQ are mixed and dried, resulting in a PNQ@EPE composite; subsequently, an oxidative potential (0.8 V vs SHE) is applied in a 1 M p-toluenesulfonic acid (aq) solution until the residual current is below 0.1 mA, which creates CP/RP composite PNQ@pEPE. Also, the structure of the reference material, pEP(NQ)E, is shown on the left.

Figure 2

In situ conductance for PNQ@pEPE at different CP/RP ratios (10–100 wt % pEPE) in the PNQ@pEPE blend, as evaluated using bipotentiostat measurements on PNQ@pEPE-covered interdigitated array electrodes. A potential bias of 10 mV between the two working electrodes was applied, and the potential was varied cyclically at a scan rate of 10 mV/s.

Figure 3

Specific capacity of materials with different ratios of pEPE (10–90 wt %) and PNQ (90–10 wt %) in the electrode composition and pEP(NQ)E. Their specific capacity is calculated for the total electrode weight and given for different charging currents ranging from 15 (0.2 C) to 750 mA/g (10 C). Also, the capacity that would be expected to result from the pEPE additive is given (diamond) together with the capacity that would result if the entire theoretical specific capacity of PNQ would be accessed (black line).

Figure 4

Electrochemical characteristics of 3:7 PNQ@pEPE; cyclic voltammogram of the redox response (a) at a scan rate of 5 mV/s and potential profile (b) at different charging currents ranging from 15 (0.2 C) to 750 mA/g (10 C).

Figure 5

Electrochemical properties of the polymer-manganese secondary battery. Schematic image of the battery (a) charging and (b) discharging.

Figure 6

Battery characteristics: voltage profile (a), cycling stability during 500 cycles at 1 C (b) for charge (black) and discharge (gray) with corresponding Coulombic efficiency (light gray), and voltage profiles at different rates from 20 (corresponding to 0.3) to 750 mA/g (corresponding to 10 C) for 3:7 PNQ@pEPE (c).