Comparative Study of PEDOT- and PEDOT:PSS Modified δ-MnO₂ Cathodes for Aqueous Zinc Batteries with Enhanced Properties

Materials and electrodes preparation

Electrochemical measurements

Figure 1a presents XRD patterns of as-synthesized δ-MnO₂ powder. Very intense peaks at 12.46° and 25.09° are observed with several coupled and low-intensity peaks which are typically observed for layered-type materials. These diffraction peaks can be indexed to monoclinic potassium-doped birnessite 2H phase with molecular formula K_0.46Mn₂O₄ · 1.55 H₂O matching the ICCD card no. 01–75–8312. This finding is in close agreement with reported chemical formulae of layered-type manganese dioxide, synthesized by the same procedure. ¹⁹

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In the SEM images of the powder (Fig. 1b) nanoflower-like MnO₂ is observed as agglomerated grains with highly porous morphology. A tendency to the formation of separated layers is also noticed. The ratio Mn:O observed from EDX analysis (Table SI) is close to 1:1.8.

SEM images of PEDOT powder demonstrate that highly porous amorphous structures are observed with additional ideal spheres (Fig. 2a). From the EDX analysis ratio of components is close to the theoretical for PEDOT (Table SII).

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In the SEM images of the electrode materials a more dense and smooth surface is observed for PEDOT-containing electrodes, especially for MnO₂/PEDOT:PSS electrode, in comparison with bare MnO₂ electrode (Figs. 2b–2d). From the EDX analysis for all three electrode materials the manganese and oxygen distributions are uniform. The sulphur distribution in MnO₂/PEDOT electrode is also well-random (Fig. S4) and in case of MnO₂/PEDOT:PSS electrode it is uniform but in correlation with manganese presence (Fig. S5). Thus, the addition of ICP dispersion allows us to obtain a novel type of MnO₂-based cathode material, covered with polymer.

Electrochemical properties of all types of prepared electrodes were evaluated by cyclic voltammetry and galvanostatic charge/discharge.

Figure 3 shows the cyclic voltammograms of MnO₂-based cathodes. In general, CV shapes for different cathodes were very similar and they displayed two redox couples which are typical for Zn/δ-MnO₂ system and agree with literature data. ^8,20,21 Two cathodic peaks located at E = 1.37 and E = 1.24 V correspond to reduction of Mn⁴⁺ to Mn³⁺; they are accompanied by intercalation of protons (1.37 V) and of Zn²⁺ ions (1.24 V). The interpretation of the electrode process at the second peak (1.24 V) is more debatable, the peak is commonly assigned to Zn²⁺ intercalation, complicated by chemical formation of byproducts like zinc sulfate hydroxide Zn₄(OH)₆SO₄·xH₂O (ZHS) during the cathodic process at E = 1.24 V. ²² A detailed study of electrochemical reactions was performed with quartz microgravimetric analysis. ²³ The magnitude of apparent molar mass of species transferred is approximately proportional to the molecular mass of H₃O⁺ ions for the first peak (E = 1.4 V) and significantly higher than the molecular mass of Zn²⁺ ions for the second peak (E = 1.26 V). Based on the molecular weights of species taken into account in the redox process, the first peak was attributed to intercalation of H⁺ (as H₃O⁺) and the second peak is related to precipitation of ZHS with Zn²⁺-intercalation which was also assumed in Ref. 24 for a Zn(TFSI)₂-based electrolyte.

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Two not well-resolved anodic peaks at E = 1.56 and E = 1.62 V were observed which are related to Mn³⁺-ion oxidation with corresponding extraction of zinc ions and protons, respectively. Both cathodic and anodic peaks gradually increased with the number of cycles, indicating the progressive electrodeposition of newly formed MnO_x or ε-MnO₂ layers onto electrode surface from Mn²⁺ ions contained in the electrolyte solution.

The discharge capacities from the CV curves are a little higher for MnO₂/PEDOT and MnO₂/PEDOT:PSS electrodes because of the widening of the cathodic peaks, especially the second one, which could be associated with progressive ZHS precipitation on the more porous surface and formation of "diffusion valleys." During charge, a gradual capacity increase is observed due to the presence of Mn²⁺ ions and MnO_x layers deposited on the electrode surface.

With increasing scan rate (from 0.2 mV·s⁻¹ to 0.5 mV·s⁻¹) for all three electrodes the pair of cathodic and anodic peaks associated with zinc ion insertion/extraction are significantly diminished (Figs. 4a, S6). It is clearer for the cathodic curve, where the second reduction peak completely disappeared at high scan rates. So, this allows us to suppose that at high scan rates insertion and extraction of protons become the dominant redox processes.

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From the double logarithmic plot for MnO₂/PEDOT:PSS electrode (Fig. 4b), the slopes of the lgI_p , lgv-dependencies are 0.64 and 0.71. Based on general descriptions, both diffusion-controlled and pseudocapacitive currents may flow. The experimental data will obey an empirical equation, in which the dependency of peak currents on the scan rate is a power function:

$$\begin{eqnarray}&&{I}_{{\rm{p}}}=a\cdot {\nu }^{b},\end{eqnarray} \tag{ 1 }$$

with I_p—peak current, ν—scan rate, b—the exponent. This equation can be rewritten in logarithmic form

$$\begin{eqnarray}&&\left(\mathrm{log}\,{I}_{p}\right)=\,\mathrm{log}\,a+b\cdot \,\mathrm{log}\,\nu \end{eqnarray} \tag{ 2 }$$

When the b value is close to 0.5, it is suggested that ion diffusion limits the current of the electrochemical process, b value close to 1.0 correspond to (pseudo)capacitive response. Thus, insertion and extraction of protons is controlled dominantly by diffusion of H⁺-ions and it may influence the pH value near the electrode surface with contributions from a pseudocapacitive response. Because of formation of the excess of OH⁻-anions, the formation of zinc hydroxyl sulfate takes place during discharge process. This mechanism is in agreement with reported ones intensively discussed. ^25,26

Electrochemical performance of the MnO₂-based cathodes is presented on Fig. 5. At low current densities increase of specific capacity can be explained by MnO_x (ε-MnO₂) layer formation on the electrode surface. In case of PEDOT:PSS-coated electrode capacity values (over 300 mAh g⁻¹ at low current densities) are approaching the theoretical value until current density 1.0 A·g⁻¹ and are much higher at the last cycles at 0.1 A·g⁻¹ due to additional formation of electroactive layer MnO_x formed on the electrode and in spaces at E = 1.7 V and higher where a visible dark precipitate was observed. At high current density (5.0 A·g⁻¹) all three electrode materials delivered zero capacity.

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During long-term cycling capacity increases in initial cycles at 0.1 A·g⁻¹, and in following 20–30 cycles at 0.3 A·g⁻¹ a capacity increase of 15% for MnO₂, 25% for MnO₂/PEDOT and 20% for MnO₂/PEDOT:PSS electrodes is observed (Fig. 5b). Furthermore for pristine δ-MnO₂ cathode capacity decrease by 10% while for modified electrodes no capacity fading is observed after stabilization (capacity retention from maximum value is 90% for MnO₂, 99.5% for MnO₂/PEDOT and 99% for MnO₂/PEDOT:PSS). Capacity values of PEDOT:PSS-coated electrode materials are twice higher than for MnO₂-based electrode which can be associated with increased electronic conductivity of composite material and, as a consequence, fast ion transport between electroactive grains. These values are also on par with the other state-of-the-art manganese-based cathodes for aqueous zinc-ion batteries (see Table SIII).

Charge/discharge profiles of all types of electrodes (Fig. 5d) show one long charge plateau close to 1.55 V and two discharge plateaus at 1.4 and 1.25 V. It is clearly seen on the discharge curves that the plateau at E = 1.4 V is a bit longer than the second one, which allows to suggest that proton intercalation is dominant energy storage process instead of Zn²⁺-ion intercalation. On the discharge curves (Figs. 5c, 5d) at E = 1.26–1.28 V an increase of the potential is detected. It corresponds to zinc ion intercalation or/and chemical precipitation of a Zn²⁺-containing insoluble byproduct zinc sulfate hydroxide. In the presence of the ICP the slope of the curve at the end is smoother than for pristine MnO₂-based cathode (Fig. 5d) which is associated with the shape of CV curves of MnO₂/PEDOT and MnO₂/PEDOT:PSS electrodes during discharge process.

For detailed investigation of structural changes in the electrode material, XRD patterns of MnO₂-based cathodes were obtained after 100 charge/discharge cycles in charged and discharged states (Fig. 6). In discharged state for bare MnO₂-based electrode, the main diffraction peaks of δ-MnO₂ (ICCD card no. 01–75–8312, gray symbols) stay and additional strong peaks at 18.55°, 33.67°, 36.59° and 65.39° (noted as black symbols) are observed. These peaks are associated with Zn-doped manganese dioxide ZnMn₃O₇·2 H₂O (ICCD card no. 00–047–1825). It should be noted that several peaks (12.35°, 25.14°, 32.75°, 35.12 °) are wider due to the presence of highly amorphous phase Zn₄(OH)₆SO₄·1.55 H₂O (ZHS) matching the ICCD card no. 00–044–0674. For electrodes with ICP diffraction peaks of Zn-doped manganese oxide are smaller, so we can conclude that PEDOT or PEDOT:PSS prevents the active irreversible zinc ion insertion in the electrode material lattice.

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In the charged state (Fig. 6b) peaks related to ZHS phase are not noticed, so we can conclude that additional surface layer of ZHS dissolves during charge process in agreement with the data presented in Ref. 23. The main crystal phase is Zn-doped hydrated oxide ZnMn₃O₇ while the pristine MnO₂ phase almost disappeared. So, irreversible insertion of Zn²⁺ ions in crystal lattice is also confirmed during charge process.

A comparative electrochemical study of δ-MnO₂-based electrodes modified by PEDOT additive or PEDOT:PSS coating was performed. It was shown that addition of PEDOT and especially PEDOT:PSS leads to an increase of specific capacities: the values delivered were 243 and 219 mAh·g⁻¹ at 1.0 A·g⁻¹ while only 119 mA·g⁻¹ was obtained for bare δ-MnO₂ electrode. At low current densities capacity values of MnO₂/PEDOT:PSS electrodes are comparable with the theoretical values for MnO₂ cathodes for AZIBs. The cycling stability of both electrodes modified by ICPs during 100 cycles was close to 99%. The significant increase of specific capacity during cycling is linked with additional formation of ε-MnO₂ layer on the electrode surface.

Enhancing the electrochemical properties of MnO₂ cathodes with conducting polymers can be explained by the increase of both electronic and ionic conductivity of electrode material due to more conductive media between electroactive grains. In addition, a polymer layer on the electrode surface (in case of PEDOT:PSS coating) supports the mechanical integrity and prevents the manganese dissolution during charge/discharge processes. It follows from the experimental data, that addition of the ICP to electrode slurry in form of dispersion of PEDOT with polystyrene sulfonate anion (PEDOT:PSS) was better than of PEDOT only. This can be explained by assuming a more homogeneous and tight thin layer coating of active grains by PEDOT:PSS, facilitating surface electronic and ionic conductivity, which enhances the rate capability and cycling performance. Moreover, PEDOT:PSS as tight thin layer coating supports more reliable electrical contacts between inorganic active species and carbon species, reducing the interfacial Ohmic resistance.

Investigation of the redox processes shows that the introduction/extraction of protons is the dominant process at high current densities (or short charge/discharge times) based on diffusion-controlled currents. This leads to a local increase in pH and the precipitation of zinc hydroxyl sulfate (ZHS) on the electrode surface, inhibiting the zinc ion insertion reaction.

The financial support from Russian Foundation for Basic Research (grant № 21–53–53012) is gratefully acknowledged. Materials characterization was performed at the Center for X-ray Diffraction Studies and the Interdisciplinary Center for Nanotechnology of Research Park of St. Petersburg State University.