Elsevier

Journal of Power Sources

Volume 321, 30 July 2016, Pages 135-142
Journal of Power Sources

Operando identification of the point of [Mn2]O4 spinel formation during γ-MnO2 discharge within batteries

https://doi.org/10.1016/j.jpowsour.2016.05.002Get rights and content

Highlights

  • Discharge of γ-MnO2 was tracked within sealed batteries as a function of position.

  • Spinel formation immediately followed phase transition to α-MnOOH.

  • Well-formed α-MnOOH occurred after insertion of 0.79 H+ per Mn atom.

  • Insertion of 0.79 H+ correlated to 104% of the 2 × 1 ramsdellite tunnel capacity.

Abstract

The rechargeability of γ-MnO2 cathodes in alkaline batteries is limited by the formation of the [Mn2]O4 spinels ZnMn2O4 (hetaerolite) and Mn3O4 (hausmannite). However, the time and formation mechanisms of these spinels are not well understood. Here we directly observe γ-MnO2 discharge at a range of reaction extents distributed across a thick porous electrode. Coupled with a battery model, this reveals that spinel formation occurs at a precise and predictable point in the reaction, regardless of reaction rate. Observation is accomplished by energy dispersive X-ray diffraction (EDXRD) using photons of high energy and high flux, which penetrate the cell and provide diffraction data as a function of location and time. After insertion of 0.79 protons per γ-MnO2 the α-MnOOH phase forms rapidly. α-MnOOH is the precursor to spinel, which closely follows. ZnMn2O4 and Mn3O4 form at the same discharge depth, by the same mechanism. The results show the final discharge product, Mn3O4 or Mn(OH)2, is not an intrinsic property of γ-MnO2. While several studies have identified Mn(OH)2 as the final γ-MnO2 discharge product, we observe direct conversion to Mn3O4 with no Mn(OH)2.

Introduction

The oxide γ-MnO2, which is a staple of global electrochemical energy storage, shows a high complexity in its structure and discharge mechanism. The mechanism in alkaline electrolyte is often treated in shorthand as a first electron reaction followed by a second electron reaction [1], [2].MnO2 + H2O + e → MnOOH + OHMnOOH + H2O + e → Mn(OH)2 + OH

This understanding is true only in limited circumstances. While discharge always begins via the proton insertion in Reaction (1), upon reaction beyond a poorly defined extent various other manganese oxides are observed, including the spinels Mn3O4 (hausmannite) and ZnMn2O4 (hetaerolite), the latter of which is produced in batteries with zinc anodes. These spinels have low electrochemical activity, and will not recharge to MnO2. Despite their importance in keeping γ-MnO2 batteries from being widely used rechargeably, the reaction mechanisms leading to these spinels are largely unknown, as are their precise points of formation during discharge.

Details from the literature show a strong influence of experimental factors, for example the mass percent of γ-MnO2, which must be diluted with a conductive material like carbon. Various dilutions show a range of discharge results, and the studies most cited are difficult to compare directly. In one, constant current discharge at 87.5% mass loading led first to Mn3O4 followed by Mn(OH)2, while in a second, potential scanning at 60% loading led to Mn(OH)2, which was recharged through γ-Mn2O3, β-MnOOH, and γ-MnOOH, and gave Mn3O4 only on the second cycle [3], [4]. In the ensuing years there has been no resolution of this mechanism. A later in situ study at 80% loading showed simultaneous formation of Mn3O4 and Mn(OH)2 at 0.8 extent of Reaction (1) [5]. Another study at 3% loading showed Mn(OH)2 with no Mn3O4, suggesting that Mn3O4 in earlier reports was a consequence of sample preparation [6]. Addition of a zinc anode has complicated interpretations further. It has been reported that ZnMn2O4 was observed in situ during the first electron reaction, and that ZnMn2O4 and Mn3O4 form at different stages, governed by the cell potential [6], [7]. These variations can also be attributed to data interpretation, as the manganese oxides are difficult to distinguish experimentally. Also in play is the nature of porous electrodes, which almost always display nonuniform reaction rates, especially at high mass loading or high rate. Reaction extents calculated assuming a bulk average will differ from the true values and thus call mechanisms into question.

In this study we target the pathway to and point of spinel formation under the most relevant experimental conditions: those found within commercial γ-MnO2 batteries. Knowledge of the precise molecular pathway leading to spinel formation may be used to engineer a method to block or reverse this pathway. For practical use, γ-MnO2 electrodes operate at high mass loading and significant discharge rate. The technique of using photons of high energy and high flux for energy dispersive X-ray diffraction (EDXRD) has been used to track material evolution within electrochemical systems that more resemble actual devices than model systems, which may exclude factors that have a significant impact [8], [9], [10], [11]. Thus we use commercial alkaline batteries as the test bed. These have qualities not found in many model systems: ∼97% active material loading, thick electrodes, and relatively starved electrolyte. We have previously demonstrated that high energy EDXRD has the sensitivity required to spatially distinguish ZnMn2O4 and Mn3O4, which have a d-spacing difference of only 0.04 A, within the sealed steel casing of a battery, using a moderate data collection time of 20 s per location [12]. To account for the distribution of current within the thick electrodes, we use a proven macrohomogeneous battery model to calculate local discharge rates based on the bulk discharge rate. The practical interest in this mechanism is due to the emerging need for inexpensive and safe electricity storage on the scale of the power grid [13]. Because γ-MnO2 is inexpensive, non-toxic, and non-flammable, it is uniquely suited for massive-scale stationary batteries, fulfilling requirements challenging for other battery chemistries [14].

Section snippets

Experimental

Because the photons used were highly penetrating it was possible to use LR6 commercial alkaline batteries as a test bed, and these were Duracell MN1500 AA with a rated capacity of 2.85 Ah at 0.8 V. The reported specific and volumetric energy densities were 143 Wh kg−1 and 428 Wh L−1. In operando battery discharge was performed using an eight channel MTI battery cycler.

EDXRD experiments were conducted at Brookhaven National Laboratory (BNL) at beamline X17B1 of the National Synchrotron Light

Battery discharge rates

Batteries were discharged at 400, 143, 100, and 50 mA, as shown in Fig. 2. Upon reaching 0.02 V the discharge continued at constant potential until the experiment was halted. The gradual drop in cell potential was chiefly due to the γ-MnO2 cathode material, whose potential drops from its value of E0 = 0.26 V vs. Hg|HgO (1.64 V vs. Zn) as it is discharged. The batteries were continually mapped by EDXRD to track the crystal structure of the active material as a function of position inside the

Discussion

Direct observation showed that [Mn2]O4 spinels formed immediately following appearance of the α-MnOOH (400) reflection. This transition to the α-MnOOH structure occurred in a tight range of x = 0.78–0.81 at all locations in all batteries, regardless of local reaction rate. We thus define a single value of state of charge at the phase transition of xt = 0.79, the most common value. The γ-MnO2 was calculated to have a fraction of 0.39 pyrolusite layers in the crystal lattice, and state of charge

Conclusions

In a discharging γ-MnO2 electrode, formation of ZnMn2O4 and Mn3O4 follow the same mechanism, which occurs immediately following the appearance of α-MnOOH as a distinct phase. Time of formation of the α-MnOOH phase is identified by the splitting off of its (400) reflection from the hexagonal (100)h reflection of ε-MnO2. After this split, the ε-MnO2 (100)h reflection smoothly transitions to the α-MnOOH (210) peak upon further discharge. If the electrode is recharged, the ZnMn2O4 and Mn3O4

Acknowledgements

The authors would like to thank Hui Zhong for helpful assistance at the beamline. This work was supported by the Laboratory Directed Research and Development Program of Brookhaven National Laboratory (LDRD-BNL) Under Contract No. DE-AC02-98CH10866 with the U.S. Department of Energy. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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