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. 2025 Feb 28;10(9):9096–9105. doi: 10.1021/acsomega.4c08749

The Role of Catholyte Modulation in Suppressing the Initial Capacity Fade of Zinc Electrolytic Manganese Dioxide Coin Cells

Eugene Engmann 1,*, Pete Barnes 1, Lorenzo Vega-Montoto 1, Abderrahman Atifi 1,*
PMCID: PMC11904440  PMID: 40092764

Abstract

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Despite its potential for zinc–manganese oxide batteries, electrolytic manganese dioxide (EMD) can experience capacity fade due to a deficiency in the Mn2+ supply at the cathode electrolyte interphase (CEI) from side reactions, even in the presence of an electrolyte additive. In this work, electrolyte loading modulation at the cathode electrolyte interface (CEI) was correlated with Zn∥EMD cell capacity retention and cycling performance, as a proposed measure to curb the initial capacity fade observed in EMD. Initial galvanostatic charge/discharge cycling, with varied electrolyte loading, revealed severe capacity fade (from ∼188 to 10 mAh g–1 for the highest loading of 200 μL) within the first 15 cycles. Such a decrease in cell capacity is correlated with the formation of a Mn4+ deposit on the current collector and consequently, the Mn2+ depletion at CEI, as was supported by elemental and Raman analyses. Interestingly, confinement of the electrolyte to the CEI at a lower (≤15 μL) electrolyte loading mitigated Mn4+ side-deposition, maintaining the cell capacity at >80% over the first 15 cycles. Interfacial Mn supply/depletion could be monitored via voltammetric analysis based on changes of the Zn2+ insertion-reduction peak. Additional galvanostatic experiments corroborated the voltammetric interpretation and the proposed degradation pathway in the studied cell conditions. The outcomes of this work provide practical insight into coin-cell design and configuration strategies for developing Zn∥EMD batteries.

1. Introduction

Aqueous ion batteries, which are promising alternatives to lithium-ion batteries, have seen little commercialization due to their inadequate energy density and short lifespan. Because of their safety, economic viability, and green-chemistry advantages, aqueous batteries have garnered considerable research attention as the search for alternatives to lithium-based chemistry intensifies.1−9 Among the various charge carriers employed for an aqueous system, zinc—with a high theoretical capacity (819 mAh g–1) and low electrochemical potential (−0.762 V vs the standard hydrogen electrode [SHE])10−12—stands out. Manganese dioxides (MnO2) are also considered as an attractive cathode material for Zn∥Mn batteries due to their relative abundance, benign nature, low cost, and high theoretical capacity of 308 mAh g–1 and 616 mAh g–1 for 1e– and 2e– transfer, respectively.13−20

Electrolytic manganese dioxide (EMD), a variant of MnO2, has been the preferred choice for Zn2+ storage from a commercial standpoint due to its higher Mn content and purity compared to chemically achieved reagent or naturally occurring MnO2.21 Early investigations and commercialization of battery chemistries involving EMD were widely confined to alkaline-based electrolytes. At present, there is a shift to mild, acidic electrolyte-employing salts such as zinc sulfate (ZnSO4), zinc trifluoromethanesulfonate (Zn[CF3SO3]2), and zinc bis(trifluoromethylsulfonyl)imide (Zn[TFSI]2). These salts prevent the formation of side reactions prone to alkaline electrolytes, which are detrimental to cell capacity.21−24 While EMD has shown promise as an adequate choice for Zn2+ storage in these salt systems, the observed capacity fade experienced during the first 15–20 cycles25−27 has prompted researchers to explore various remediation techniques. Such techniques as doping, using surfactants, and incorporating a metal ion into EMD fabrication have been employed to improve cell capacity and decrease capacity fade.21,28−35 Other attempts by researchers to curb this underlying loss in capacity have resorted to the use of electrolyte additives such as Mn2+14,36−39 and the synthesis of MnO2 with different properties.40−43

The idea behind employing Mn2+ as an electrolyte additive is to suppress both the dissolution of Mn2+ during the disproportionation reaction of Mn3+ and the formation of zinc hydroxide sulfate,14,36−39 thus enabling capacity retention and improving cycling. While this has been the main consensus, a handful of reports in some cases have shown only slight improvement in capacity fade and retention in the presence of a Mn2+ additive. For example, while investigating the structural transformation of layered MnO2 in zinc aqueous batteries, Alfaruqi et al. observed a capacity drop of 40% at a cycle rate of 50 mA g–1 by the tenth cycle in the presence of a 100 mM Mn2+ additive.44 Similarly, while studying the importance of a 200 mM MnSO4 additive at a cycling rate of 30 mA g–1, Yang et al. observed that the addition of Mn2+ does not significantly change the first discharge capacity. Rather, Mn2+ enhances the cyclability of the cell by providing a supply of Mn2+ that is lost to the electrolyte during the discharge cycle as a result of Mn2+ dissolution.22 Poosapati et al. also observed that, at higher cutoff charging potentials, the capacity of EMD decreased by 40% after the tenth cycle.45 These results suggest either the nonutilization of Mn2+ at the CEI or the exhaustion of Mn2+ via a different mechanism. Therefore, the key to solving such intrinsic capacity loss is not only related to the presence of Mn2+ in the electrolyte but also to maintaining an adequate supply of Mn2+ at the CEI and mitigating any side reactions.

Interfacial reactions at the CEI are critical to the structural stability of EMD. The electrochemical conversion of Mn2+ to Mn4+ during charging is crucial to the facilitation of Zn2+ storage.46 In Zn∥Mn batteries, key parameters of anode thickness, cell impedance, and electrolyte loading significantly influence the quality and reversibility of the electrochemical reactions at the CEI.47 Our findings underscore the particular importance of electrolyte loading, which is consistent with previous studies highlighting the importance of these parameters in coin-cell battery performance.48,49 Following a study on electrolyte formulas of aqueous zinc-ion batteries, Xu et al. postulated that a decrease in electrolyte volume during cell fabrication could aid in capacity retention as well as prevent the dissolution of MnO2. However, no investigations were conducted, and this area remains underexplored.50

In this work, interfacial contributions of Mn2+ at the CEI in neatly prepared coin cells are explored via electrolyte-volume loading to address the concern of why initial capacity fade may still occur in a Zn∥EMD coin-cell configuration. The impact of electrolyte loading was rationalized through cyclic voltammetry exploration and validated through galvanostatic cycling. Additionally, while voltammetric analysis was used to probe the reaction of Mn2+ at the CEI, post surface and elemental characterizations were employed to analyze the electrode structure and deposits formed during cycling experiments. Table S1 compares this work with other reported studies addressing Mn2+ at the CEI to explain that electrolyte loading remains underexplored as a means of suppressing the initial capacity fade for Zn||MnO2 batteries. Furthermore, the studied system included dimethyl sulfoxide (DMSO) as a diluent, which is added to the electrolyte to suppress dendrite formation. The use of DMSO and the selected ratio to H2O was based on research conducted by Kao-Ian et al. During their research, 20 vol % DMSO:H2O was identified as optimal for dendrite suppression, which improved electrochemical performance.51 This work furthers the development of the Zn||EMD battery by addressing one of the many pathways for capacity fade.

2. Experimental Section

2.1. Chemicals and Materials

EMD was obtained from Borman Specialty Materials. 1-Methyl-2-pyrrolidinone (NMP), poly(vinylidene fluoride) (PVDF), zinc sulfate monohydrate (≥99.9% trace metals basis), and DMSO (≥99.7%) were purchased from Sigma-Aldrich. Carbon black, acetylene, and manganese sulfate monohydrate were purchased from Thermo Fisher Scientific. Zn foils (0.05 mm thick, 99.95% purity) were purchased from GoodFellow. Ti foil (0.25 mm thick, 99.5% purity), and stainless steel 304 foil (0.025 mm) were purchased from Thermo Scientific. Fisher brand glass fiber G4 (1.2 μm pore size and 280 μm thickness) was purchased from Fisher Scientific. Stainless steel current collectors (1 mm thick) were purchased from the MTI Corporation.

2.2. Cathode Fabrication

Ultrafine EMD from Borman was used without modification (Figure S1). The electrode slurry was made based on the ratio 70 wt % EMD, 20 wt % conductive material (carbon black, acetylene), and 10 wt % binder (PVDF) following electrode optimization tests, which are further elaborated on in the accompanying Supporting Information (Figure S2). NMP was the solvent. The ratio of NMP to PVDF was 95:5 wt %. The electrode was prepared by mixing the conductive material and binder-solvent mix in a Thinky ARE-310 mixer at 2000 rpm for 15 min. The Thinky mixer was purchased from Thinky, USA. The active material was then introduced into the binder-active material mix and mixed for 15 min. The slurry consistency was thinned using the binder and solvent mix. The slurry was cast onto Stainless Steel 304 foil with a doctor blade set to 500 μm. Following casting, the foil and electrode material were placed in an oven at 70 °C overnight. Following drying, 12.7 mm-diameter electrodes were punched out as cathode electrodes. The mass loading was 5–8 mg cm–2, with an average aerial loading and thickness of 4.0–6.3 mg/cm2 and 61 μm, respectively.

2.3. Electrolyte

2 M ZnSO4 + 100 mM MnSO4 were employed with an electrolyte diluent (20 vol %), DMSO, to suppress dendrite formation. DMSO was selected based on works reported by Feng et al.52 and Kao-Ian et al.51

2.4. Cell Assembly

Fabricated 2032 coin cells employed Zn foil (50 μm thick, 14.3 mm diameter) as the anode material and EMD or Ti as the cathode material, with Fisher brand G4 glass fiber (1.2 μm pore size, 280 μm thick, 15.9 mm diameter) as the separator. Zn and Ti foils were used as received, and SS304 was used as the current collector. During cell fabrication, the EMD was prewetted with ∼10 μL of electrolyte to enhance electrolyte–electrode contact. Excess electrolyte was wiped from the electrode surface using clean absorbent paper. Wet/flooded and lean electrolyte loading consisted of ≥15 and ≤15 μL, respectively. The fabricated cells were handled under ambient conditions during cell construction and recovery studies. It should be noted that under said cell configuration, the current collector at the cathode is not completely covered by the cathode due to a difference in diameter. Thus, the current collector is exposed to the electrolyte, as seen in Figure S3a.

2.5. Electrochemical Measurements

Cyclic voltammetry (CV) studies were conducted in a three-electrode coin cell to investigate the effects of DMSO on the electrolyte and the effect of electrolyte loading. Potentials were measured versus a Zn-wire reference electrode (Figure S3b). Depending on the experiment performed, MnO2 or Ti was the working electrode, and Zn was the anode or auxiliary electrode. Biologic SP-150 and Espec134 cell chambers, in combination with EC Lab software, were employed for CV tests. To elucidate the effects of DMSO in the electrolyte, cyclic voltammograms were conducted in the potential range of −0.2 to 0.3 V vs Zn/Zn2+ using a Zn∥Ti cell, as seen in Figure S4. The scan rate applied for this study was 1.0 mV/s, and aqueous 2 M ZnSO4, with and without DMSO, was used as the electrolyte. For studies involving DMSO, a mixture of 20:80 vol % DMSO:H2O was employed. Detailed electrolyte analysis and findings on the benefits of DMSO are reported in the Supporting Information.

An Espec134 cell chamber in combination with EC Lab software was employed for galvanostatic cycling tests. Aging cycles were conducted based on the following protocol, and a minimum of three cells were tested for each experiment to confirm reproducibility.

2.6. Electrode Conditioning

Constant current (CC) charge at C/10 (∼30 mA g–1) → rest 1 h → constant voltage (CV) charge at 1.8 V for 1 h or until the current dropped to C/20 (∼15 mA g–1), whichever came first. →CC discharge at C/10 → rest 1 h. Repeat the cycle three times. The capacity of the third discharge cycle from the conditioning tests was employed as the capacity for the cell for the reference performance and aging tests. All tests were run at 25 °C.

2.7. Reference Performance Tests

CC charge at C/10 → rest 1 h → CC discharge at C/10 → rest 1 h. Repeat the cycle twice.

2.8. Aging Cycles for Electrolyte Degradation

CC charge at C/5 (∼60 mA g–1) → rest 15 min → CC discharge at C/5 → rest 15 min. As needed, reference performance tests (RPTs) were taken during aging cycles.

2.9. Analysis and Characterization

Following galvanostatic cycling tests, current collectors, and electrodes were washed with copious amounts of deionized (DI) water and characterized by scanning electron microscopy (SEM), EDS, and X-ray diffraction (XRD). SEM/EDS analysis employed the JEOL JSM-6610LV and EDAX AMETEK Apollo X, controlled by EDAX software, and XRD analysis employed the Rigaku SmartLab (Co Kα radiation, λ = 1.78897 Å).

2.10. Raman Spectroscopy Measurements

Raman microspectroscopic mapping measurements were conducted by using a Senterra II Raman spectrometer (Bruker Optics) integrated with a BX-53 M microscope (Olympus). The apparatus employed a 532 nm laser-excitation source focused onto the specimen through an Olympus objective lenses. The objective lens employed—featuring a 50× magnification and a numerical aperture of 0.75—resulted in a laser spot size of 5 μm. The scattered light collected by the objective lens was dispersed using a 400 grooves/mm grating and captured by a thermoelectrically cooled charge-coupled device detector (Bruker Optics) at −60 °C. Detailed Raman analysis can be found in the accompanying Supporting Information.

3. Results and Discussion

3.1. Voltammetric Analysis

The structural and electrochemical stabilities of the EMD cathode are dependent on electrolyte/CEI redox reactions. Voltammetric analysis was employed to investigate and evaluate the reaction mechanisms involving Zn2+ storage in EMD. The two-step reduction/oxidation process within layered MnO2 follows the reactions (eqs 1–4):53

3.1. 1
3.1. 2
3.1. 3
3.1. 4

As expected, cyclic voltammetric traces shown in Figure 1a illustrate a single peak (Red2) during the first discharge (i.e., reduction cycle) from 1.8 to 1 V, corresponding to the formation of Mn2+ at 1.24 V vs Zn/Zn2+. The corresponding oxidation peak (Ox2) at 1.56 V vs Zn/Zn2+ is observed during the reverse scan (1–1.8 V).

Figure 1.

Figure 1

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(a) First, second, and fifth cycles of voltammetric traces. (b) Discharge capacity trend observed with the Zn∥EMD cells with an electrolyte loading of 40 μL.

Voltammetric traces for the second cycle reveal two peaks during the forward scan at 1.37 V (Red1) and 1.25 V (Red2) vs Zn/Zn2+. Upon voltage reversal, the reverse scan displays a prominent peak at 1.55 V vs Zn/Zn2+ and a second obscure peak at 1.6 V vs Zn/Zn2+. Furthermore, the appearance of a peak shoulder during the fifth cycle (1.63–1.58 V vs Zn/Zn2+) has been reported as an extraction of Zn2+ from the MnO2 and the conversion of the cathode to the +4 oxidation state.44,54 The presence of a single peak pair during the first cycle has been attributed to the activation of the MnO2.44 Previous studies have also attributed Red2 and Ox2 to zinc insertion and extraction within MnO2, while Red1 and Ox1 have been attributed to the insertion and extraction of protons, respectively.42,54−57 An increase in capacity (area under CV traces) from Cycles 1 to 5 is suggestive of increasing active sites for both proton and Zn2+ shuttling and the activation process taking place in the cathode. The intercalation of a proton along with Zn2+ has been extensively reported.58−60 It should be noted that the peak current density for Red2 decreases from Cycles 1 to 5.

To better understand the basis behind capacity fade under voltammetric conditions, coin cells with electrolyte loadings of 40 μL were subjected to galvanostatic cycling. The electrolyte loading of 40 μL was chosen from the literature and has been reported with adequate cycling.61 RPTs were conducted following cell conditioning and were repeated after every 10 cycles. Galvanostatic discharge studies in Figure 1b reveal an initial capacity of 185 mAh g–1, followed by a steady loss of both specific capacity and normalized capacity, accruing a 40% loss in capacity by the tenth cycle, with the coulombic efficiency generally greater than 95%.

Under such meticulous cell preparation, it was expected that the capacity of the cell would be stable during the first few cycles; however, capacity fade was present in all of the cells. Moreover, RPTs display a decreasing trend in cell capacity, even at slower cycling rates, suggesting that the active sites on the cathode material responsible for zinc storage were depleting.62

In published reports, the lower initial capacity of EMD has been attributed to the mixture of phases (ε-MnO2 (53%), ramsdellite (34%), and γ-MnO2 (13%)), which can hinder access to active material due to defects in the crystalline structure.14 A continual decrease in cell capacity has been attributed to low crystallinity and transformation of EMD from a layered-type structure to a spinel-type (ZnMn2O4) structure during cycling.32,39,46,63 While the CV traces in Figure 1a align well with the reported literature and are typical of EMD and MnO2 analogues, the steady decrease in cell capacity during cycling remains unanswered from an electrolyte-loading perspective. It is reasonable to suggest that the deposition of MnO2 during charging is not tied to only the surface of the cathode (CEI) but can take place at the current collector.14,22,39,46,63−65 Thus, if electrolyte loading does not affect capacity recovery, then no notable improvement in capacity retention during higher or lower electrolyte-loading tests should be observed. Interestingly, during post-electrolysis studies, a dark-brown deposit was observed on the current collector at the cathode, as seen in Figure S5. This deposit could be key to understanding capacity loss at the CEI.

Depending on the cell design, excessive electrolyte loading can provide a pathway for Mn2+ to deposit as Mn4+ on exposed conductive surfaces, such as current collectors. The loss of Mn2+ from the CEI can lead to loss of capacity due to a potential decrease in Mn2+ population in the electrolyte necessary for curbing Mn3+ disproportionation at the CEI. Furthermore, the deposition of Mn4+ from an electrolyte to a conductive surface has been reported.22 Thus, this postulate is further explored through electrolyte loading.

3.2. Galvanostatic Cycling of Varied Electrolyte Loadings

There are severe consequences in dipping below a minimum supply of Mn2+ at the CEI during the charging (oxidation) of Zn∥MnO2 batteries; these can be controlled via the amount of electrolyte loading. During charging, Mn2+ is oxidized via a two-step process to MnO2, and any loss of Mn2+ from the CEI during redeposition of MnO2 on the cathode can result in a substantial decrease of cell capacity during cycling.66

Figures 2a and S6a–e show controlled electrolyte loading experiments employing wet to lean conditions, 200 μL, 100 μL, 40 μL, 15 μL, and 10 μL. When electrolyte loadings exceed 15 μL, substantial loss of capacity is observed. Electrolyte volume loadings of ≤15 μL display better cycling with sustained capacities of approximately 80% over 15 cycles. Furthermore, for electrolyte loadings ≥15 μL, a similar deposit on the current collectors is observed (see Figure S6f). Visualized in Figure 2b is the correlation between the electrolyte loading and the mass of a deposit (Figure S6f) formed at the current collector following post-analysis. EDS analysis (Figure S6g) confirms the presence of Mn along with oxygen in the deposit, which is attributed to a Mn oxide complex. The presence of this Mn deposit suggests the loss of Mn from either the active material or electrolyte or both. This deposit is later studied in detail employing the use of Raman spectroscopy.

Figure 2.

Figure 2

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(a) Discharge capacity of electrode at different electrolyte loadings of 200, 100, 40, 15, and 10 μL. (b) Average weight of deposited layer on the SS304 electrode. Cycle 5 is the reference RPT from which the normalized capacity values were calculated.

Elsewhere, the deposition of a brown deposit on a bare nickel-mesh current collector during the first charge cycle in 2 M ZnSO4 has been reported and characterized as birnessite-type MnO2 with a similar elemental composition.22

The role of the electrolyte is to shuttle charge carriers between electrodes, help maintain CEI stability, and act as a supplementary source for Mn2+ at the CEI.67 Therefore, it is expected that confining the electrolyte volume to the CEI while avoiding electrolyte contact at the current collector can help improve the initial capacity retention and cycling. Figure 3 shows three possible scenarios for MnO2 conversion and deposition during charging for electrolyte loading in which the electrolyte is in contact with the current collector. In Scenario 1, Mn2+ species (Inline graphic) present at the active material are oxidized to Inline graphic In Scenario 2, Mn2+ from the electrolyte (Inline graphic) is deposited as an active layered oxide (Inline graphic), capable of Zn2+ intercalation on the cathode. Furthermore, in Scenario 3, during which the electrolyte is in contact with the current collector (Figures 3 and S7), Inline graphic is deposited on the cathode and the current collector. This phenomenon depletes the concentration of Mn2+ in the electrolyte needed to curb the disproportionation of Mn3+ during discharge and the enhancement of Deposition Pathway 2 (deposition on the cathode for Zn2+ insertion). However, when the electrolyte is confined to the vicinity of the CEI, deposition occurs only on the cathode, and Deposition Pathway 3 is eliminated. Hence, there is less depletion of Mn2+ concentration from the electrolyte. While the capacity will eventually decrease in both scenarios, the capacity of the cell configuration in which the electrolyte is kept within the vicinity of the electrode would be solely due to structural transformation and not due to the loss of active material deposited on the current collector.

Figure 3.

Figure 3

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Proposed mechanism resulting in a loss of cell capacity. Inline graphic represents the Mn2+ structure in the active material. Inline graphic represents the Mn2+ present in the electrolyte, and Inline graphic represents the MnO2 deposited on cathode and current collector.

It has been widely reported that interfacial reactions at the CEI involve the dissolution of the cathode into the electrolyte via a disproportionation reaction. MnO2 in the form of Mn4+ is reduced to Mn3+ during the discharge phase. Mn3+ disproportionates to form Mn2+ and Mn4+, and Mn2+ dissolves into the electrolyte whereas Mn4+ remains at the cathode due to its insolubility. This lattice distortion of the outer Mn atoms at the surface of the cathode into the electrolyte creates a chain reaction that leads to structural instability and collapse.47,68−70 A detailed review of the reaction at the CEI is given by Tang et al. in a recently published report.47 The presence of these species has been deconvoluted by reports employing X-ray photoelectron spectroscopy (XPS). Huang et al. employed XPS to differentiate the various species (+2, + 3, and +4) of Mn during cycling of an ε-MnO2 cathode. They observed that Mn4+ is the dominant state in ε-MnO2. Furthermore, following H+ and Zn2+ insertion at full discharge, the present species at the cathode were indexed to Mn2+ and Mn3+.71 Jin et al. observed through XPS that at initial Zn2+ intercalation, the cathode consists of Mn4+ and Mn3+. At the end of the discharge, the dominant species is Mn3+.72 Zhong et al. also deconvoluted the speciation of Mn2+ and Mn3+ during Zn2+ and H+ insertion into the cathode by observing an appearance of the Zn 2p peak and an increase in the proportion of low valence peak at 1.0 V.73 These reports suggest that the structural stability of the CEI is dependent on the suppression of the disproportionation reaction via Mn3+. Thus, one pathway to preserving cell life is to confine the electrolyte with the Mn2+ additive solely to the CEI.

Correspondingly, such modulation of the Mn2+ supply at the interface is examined via further cyclic voltammetric analysis of two cells, each with different electrolyte loadings. For these cyclic voltammetric studies, the cells require two separators. Hence, an electrolyte volume of 15 μL will cause cell dry-out due to absorption of electrolyte by the separators. Thus, in this special case, an electrolyte of 40 μL is used for wet and 120 μL is used for flooded, to account for the second separator needed to prevent contact between the anode, cathode, and reference electrode (Figure S1).

Figures 4a and S8a,b show the CVs for the second, fifth, and tenth cycles of two electrolyte loadings of 120 and 40 μL. It is observed that, for a lower electrolyte loading, the overall charge density of the cell is generally higher. More interestingly, the peak charge density at Red2 for 40 μL is significantly greater than 120 μL for Cycles 2, 5, and 10; this is illustrated in Figure 4b. A similar observation is observed for Red1, as displayed in Figure 4c. Further illustrated in Figure 4d, the ratio of Red2:Red1 increases and decreases for lean and flooded electrolyte loadings, respectively. This indicates that Mn2+, when confined to the CEI, aids in capacity retention. However, due to the deposition of MnO2 at the current collector at higher electrolyte loadings, capacity diminishes over time unless there is an excessive source of Mn2+ in the electrolyte. This is validated by increasing the concentration of Mn2+ to 500 mM in a cell with an electrolyte loading of 120 μL. Figure S9 shows that with an increase of supply in Mn2+ in the electrolyte, capacity retention can be maintained. However, an increase in the concentration of Mn2+ could result in an undesirable increment of electrochemically irreversible and inactive deposition of ZnMn2O4, consequently resulting in capacity loss.74 These observations suggest that CVs could aid in offering valuable information during cell diagnostics based on the loss of Mn2+ at the CEI.

Figure 4.

Figure 4

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Cyclic voltammetric validation of reaction leading to capacity fade at CEI. The scan rate was 0.1 mV/s. (a) The second cycle comparison between cells loaded with 40 and 120 μL, respectively. (b) Peak charge density of Red2 for both electrolyte loadings. (c) Peak charge density of Red1 for both electrolyte loadings. (d) Peak charge density ratios of Red2:Red1 for both electrolyte loadings.

3.3. Raman Spectroscopy Analysis of the Deposit at the Current Collector

Raman analysis can be employed to determine the state of oxidation and concentration increase or decrease of a species in a compound. Due to its sensitivity to the basic structure of MnO2, Raman analysis has been used to investigate different phases MnO2.75−79 In this section, a systematic approach is used to track the loss of active material from the CEI and the type of MnO2 phase of the deposit at the current collector. Figure 5a shows a comparison between a pristine MnO2 electrode and the electrode after electrolysis, employing the peak at 1350 cm–1 as the internal reference for normalization. A strong peak centered at 665 cm–1 suggests the stretching mode of the MnO6 octahedra in β-MnO2. A similar peak was reported by Julien, Massot, and Poinsignon while investigating the lattice vibrations of various manganese oxides.76 Furthermore, a decrease in the intensity of the peak at 665 cm–1 for the electrode after electrolysis is observed. Ye and Spencer reported that, as the concentration of a species increases, the Raman intensity increases, and vice versa.80 Thus, a decrease in the peak at 665 cm–1 for the electrode after electrolysis suggests a decrease in the MnO2 concentration of the active material. Further analysis of the brown deposit at the current collector could suggest a possible pathway for the loss of MnO4 at the CEI.

Figure 5.

Figure 5

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(a) Comparison of Raman spectra between a pristine MnO2 electrode (blue trace) and the electrode after electrolysis (orange trace), showing a decrease in intensity for the used electrode. (b) Photographic image of the deposit on the current collector employing a Senterra II Raman spectrometer (Bruker Optics) integrated with a BX-53 M microscope (Olympus). (c) Depth profile of the brown deposit showing an uneven deposition. (d) Depth profile of the pristine electrode. (e) Raman spectra for the dark-brown deposit on the current collector, suggesting a MnO2 deposit.

Figure 5b is a photographic image of the deposit at the SS304 current collector, and Figure 5c shows an inhomogeneous deposition based on the surface morphology of the deposit produced using a Senterra II Raman spectrometer (Bruker Optics) integrated with a BX-53 M microscope (Olympus). Compared to the pristine electrode, the height of the deposit is five times greater, as observed in Figure 5c (2400 μm) and Figure 5d (500 μm). This height increase could be a contributing factor to capacity loss by separating active material from the CEI. Raman analysis of the dark-brown deposit in Figure 5e revealed a strong peak at 665 cm–1, as well as other prominent peaks at 670, 491, and 404 cm–1. The peaks at 670 and 491 cm–1 have been reported as the stretching mode of MnO6 octahedra in γ-MnO2 with pyrolusite intergrowth. The peak at 404 cm–1 has been reported as α-Mn2O3.75,76,79 These results show that the deposit at the current collector is mostly composed of various phases of Mn4+, with traces of Mn3+ from the oxidation of Mn2+ during the charging cycle. The presence of the peak at 665 cm–1 for both the pristine electrode and the deposit strongly confirms the deposit as mostly Mn4+. Thus, the confirmation of Mn4+ at the current collector supports the idea that electrolyte loading plays an important role in maintaining the cell capacity.

3.4. Galvanostatic Cycling with Modulated Electrolyte Loadings

To corroborate the voltammetric interpretation as well as observation during Raman characterization, cells were loaded with 100 μL and cycled at C/5 (∼60 mA g–1) until substantial capacity loss. The 100 μL load was chosen because it is sufficient to cause side reactions at the cathode due to contact with the current collectors. Recovery tests included cell disassembly, replacement of current collectors and separators, and addition of fresh electrolyte while maintaining the same cathode and anode, as seen in Figure 6a. Upon cycling with a fresh electrolyte, the normalized capacity did not improve after the initial capacity loss. RPTs at lower cycling rates revealed little improvement in the capacity. A second recovery test was performed in which the EMD cathode was replaced and the Zn anode was retained. After cathode substitution, an increase in the normalized capacity to 80% was observed. This corroborates that at higher electrolyte loading, the access to active material due to deposition on the current collectors is lost. To verify this access, newly fabricated cells were cycled at C/10 (∼30 mA g–1) with an electrolyte loading of 15 μL. These cells exhibited good capacity retention over 20 cycles, as seen in Figure 6b. A similar study, confining the Mn2+ species to the vicinity of the CEI, showed that the use of organic ligands improved the capacity by 2-fold.66 These results confirmed that modulating the electrolyte loading to restrict Mn2+ to the vicinity of the CEI is an alternate pathway to improving the initial-capacity retention. This is essential for EMD coin cells with lower electrolyte additive concentration, where longer cycling (>100 cycles) is not required. Overall, an electrolyte loading of ≤15 μL seems to be optimum for preventing the loss of Mn2+ at the CEI under our studied cell conditions. This is particularly the case where the stainless steel current collector is partially exposed.

Figure 6.

Figure 6

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(a) Systematic investigation correlating capacity fade to electrolyte volume employing recovery tests. (b) Cycling of cell at C/10 with optimized electrolyte volume showing a better capacity retention after 20 cycles.

4. Conclusion

Zn∥EMD aqueous coin-cell batteries are susceptible to early capacity fade during initial cycling. This trend is directly related to the dissolution of Mn2+ in aqueous batteries and is well-documented in various cell configurations but remains underexplored in coin cells. A gap in understanding the impact of electrolyte loading on Mn2+ dissolution is crucial to the advancement of research in Zn∥EMD aqueous batteries. Literature frequently provides instructions that prove difficult to repeat in coin-cell fabrication to meet specific electrochemical benchmarks for Zn∥EMD cells. Initial capacity fade is prevalent, even in the presence of an electrolyte additive, when electrolyte loading extends beyond the surface of MnO2 to exposed current collectors.

Examination of various electrolyte loadings revealed that the capacity of the cell can be maintained when the electrolyte is restricted to the surface of the active material and prevented from contacting conductive surfaces outside the cathode surface area. Modulation of the catholyte to ≤15 μL maintained 85% of the cell capacity over the first 15 cycles, whereas higher loading led to severe decay in cell capacity, in conjunction with a side MnO2 deposition at the current collector. From a mechanistic point of view, electrolyte contact with such conductive surfaces as a current collector triggers side reactions, which strip the electrolyte of Mn2+ during charging. This side reaction leads to an irreversible dark-brown deposit on the current collector, identified through Raman analysis as Mn4+. Loss of Mn2+ from the electrolyte results in the continuous dissolution of the active material, which depletes active sites for Zn insertion during discharge. Together with galvanostatic cycling, voltammetric analysis provided insight into the capacity fade due to the interfacial dynamics of Mn2+ supply and depletion at the CEI. Without catholyte modulation, capacity fade could ultimately be restored by cathode replacement.

Thus, care should be taken to prevent contact of the electrolyte with conductive surfaces, such as current collectors, during cell fabrication. Such an approach would prevent the loss of the Mn2+ additive from the electrolyte. For scenarios in which electrolyte contact on conductive surfaces cannot be avoided, an increase in Mn2+ additive in the electrolyte could help curb capacity loss. While this work sheds light on a specific degradation pathway in Zn∥EMD coin cells, much research is still needed in this area to enhance and promote a depth of understanding and development of the Zn battery.

Acknowledgments

The authors wish to express their sincere thanks to Yu Lu, Jana Howard, and Sidharth Sukumaran Nair, from the Center for Advanced Energy Studies (CAES), for offering their support and expertise during XRD, SEM, and EDS analysis.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c08749.

  • Experimental procedures on electrode characterization, SEM, EDS, and XRD analyses, as well as cell optimization procedures; contains charge and discharge plots for tested cells and cyclic voltammetry from electrochemical studies (PDF)

Author Contributions

E.E., A.A., and P.B. contributed equally to the writing and conceptualization of this work. E.E. conducted all experiments, including electrochemical and characterization acquired. L.V.-M. contributed to the writing of this paper and conducted Raman analysis.

This work was supported by the Department of Energy under the DOE-ID/BEA INL contract number DE-AC07-05ID14517.

The authors declare no competing financial interest.

Supplementary Material

ao4c08749_si_001.pdf (1MB, pdf)

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