Dynamic compensation of MnOOH to mitigate the irregular dissolution of MnO₂ in rechargeable aqueous Zn/MnO₂ batteries

Abstract

As a recognized promising cathode material for rechargeable aqueous Zn batteries, an MnO₂ cathode often suffers from rapid fading of capacity due to irreversible Mn dissolution, which hinders high-performance Zn batteries. Herein, we introduced Ce(SO₄)₂ additives into the electrolyte of Zn/MnO₂ batteries to cope with the irreversible dissolution of MnO₂. During charging, the MnOOH formed by the reaction between Ce⁴⁺ and Mn²⁺ deposited on the cathode with the attraction of H⁺ and was converted subsequently to MnO₂ to achieve dynamic compensation. Meanwhile, MnOOH was generated from the transformation of MnO₂ during discharge, and the reaction between Ce³⁺ and MnOOH was beneficial for the reversibility of Ce⁴⁺, but also competitive with the disproportionation of MnOOH. As a result, Zn/MnO₂ batteries with Ce(SO₄)₂ additives showed high capacity retention of 97.4% at 1.0 A g⁻¹ after 1000 cycles, which far exceeded that of the batteries without Ce(SO₄)₂ (40.5%).

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Jiang Zhou

Professor Jiang Zhou received a bachelor's degree (2011) and doctorate (2015) from Central South University (CSU). He studied in the research team of Professor Hua Zhang at Nanyang Technological University as an exchange doctoral student. He has carried out postdoctoral research in the research team of Professor Ju Li at the Massachusetts Institute of Technology since 2016. He joined CSU as a professor at the end of 2017. His research interests include lithium (sodium)-ion batteries and aqueous zinc-ion batteries. He has published more than 100 articles in international journals, which has attracted more than 18000 citations with an h-index of 71. He has been ranked by Web of Science (Clarivate) as a “highly cited researcher” since 2021.

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1 Introduction

Rechargeable aqueous zinc batteries (RAZBs) have attracted great interest in the energy-storage market due to their prominent strengths: plentiful resource of Zn, high theoretical capacity of the zinc anode, and high safety.^1–3 The cathode plays an important part in battery performance. The cathode materials of RAZBs are based mainly on Mn and V, organic compounds, and Prussian blue analogs. MnO₂ is an extensively applied cathode material due to high theoretical capacity and affordable cost.^4–7 Recently, substantial studies on the MnO₂ cathode have been conducted to raise the electrochemical performance and have made great strides. However, the cycle stabilities remain unsatisfactory, which is caused by the irreversible dissolution of MnO₂.⁸ The reduced utilization rate of the active materials is caused by the dissolution of cathode materials, and the possibility of unfavorable side effects on the electrode interface will be increased. Simultaneously, structural degradation and performance attenuation are induced.⁹

Various strategies have been postulated to solve these issues, including electrolyte additives (e.g., Mn²⁺) and modification of electrode interfaces (inorganic/organic coatings).^10–12 To achieve Mn²⁺ equilibrium in an electrolyte, MnSO₄ was introduced into the ZnSO₄ electrolyte to restrain dissolution of the MnO₂ cathode.¹⁰ Also, “graphene scrolls” can be used as a protector, which inhibits the dissolution of MnO₂ effectively and enhances conductivity.¹¹ For the Zn/MnO₂ battery, the structural transformation of MnO₂ during cycling will likely be responsible for the instability of MnO₂, which promotes the dissolution of MnO₂. Moreover, during discharging/charging, MnO₂ is reduced to Mn²⁺ and Mn³⁺, and the disproportionation reaction of Mn³⁺ (which helps to enhance of Mn²⁺) will promote dissolution further.^12,13 Introduction of a certain amount of Mn(CF₃SO₃)₂ to Zn(CF₃SO₃)₂ will lead to inhibition of the unfavorable dissolution of Mn²⁺, with in situ generation of a uniform porous MnO_x layer. The latter will deposit on the cathode and have a significant role in maintaining the integrity of the MnO₂ cathode.¹⁴ Hence, such in situ generation could be a tactic to alleviate the dissolution of MnO₂. Inspired by the mechanism of MnO₂ dissolution and the concept of in situ generation, we postulated a scheme whereby MnOOH was generated in situ in the electrolyte and deposited on the MnO₂ cathode and “dynamic compensation” occurred.

Herein, we introduced Ce(SO₄)₂ as an additive of the basic electrolyte (2 M ZnSO₄ + 0.1 M MnSO₄) for a Zn/MnO₂ battery, which was noted as a Zn–Ce electrolyte. In the basic electrolyte, the Mn²⁺ generated from the disproportionation with MnOOH in discharging is regarded to dissolve in the electrolyte, which cannot take advantage of the MnO₂ cathode. While the introduced Ce⁴⁺ reacted with Mn²⁺ can make a contribution to the conversion of Mn²⁺ to MnOOH, and the generated MnOOH deposits partly on the cathode due to the attraction of H⁺ formed during charging. The part of MnOOH that transforms to MnO₂ during charging compensates for the active substance to achieve dynamic compensation. Meanwhile, MnOOH is generated from the transformation of MnO₂ during discharging. The reaction between Ce³⁺ and MnOOH is beneficial for the reversibility of Ce⁴⁺, but also competes with the disproportionation of MnOOH. As a result, compared with the basic electrolyte, the Zn/MnO₂ battery with a Zn–Ce electrolyte shows an excellent cycle life and capacity retention (97.4% vs. 40.5% at 1.0 A g⁻¹ after 1000 cycles).

2 Results and discussion

Visualization of the mechanism of dynamic compensation was investigated through a series of demonstrations conducted on MnO₂ cathodes and electrolytes at different charge/discharge states (Fig. 1). The conversion of MnO₂ was found to be related with eqn (1) (Fig. 1a).^10,15 According to the results of ex situ X-ray diffraction (XRD), MnOOH was transformed from MnO₂ during discharging and the opposite reaction occurred during charging. However, during discharging, parts of MnOOH tended to transform into Mn²⁺via disproportionation, and this phenomenon was consistent with increases in levels of Mn-OSO₃²⁻ and Mn in the electrolyte (Fig. 1b and S1†).¹⁶ Also, parts of Mn²⁺ would diffuse into the electrolyte and could not return back to the cathode spontaneously during charging, which is responsible for the capacity fading of MnO₂.

H⁺ + e⁻ + MnO₂ ↔ MnOOH (1)

Fig. 1 Investigation of the dynamic compensation mechanism. Ex situ (a) XRD patterns for the MnO₂ cathodes and (b) Raman spectra for the electrolyte of the Zn/MnO₂ battery with the basic electrolyte at different charge/discharge stages. Ex situ (c) XRD patterns for MnO₂ cathodes and (d) Raman spectra for the electrolyte of the Zn/MnO₂ battery with the Zn–Ce electrolyte at different charge/discharge stages. (e) UV-vis spectroscopy and (f) Raman spectra of the Zn–Ce electrolyte at different charge/discharge stages.

For the Zn–Ce electrolyte, the increasing diffraction peaks of MnOOH on the cathode and decreasing peaks of Mn-OSO₃²⁻ in the electrolyte were found during charging, which was not in accordance with eqn (1) (Fig. 1c and d). Addition of Ce(SO₄)₂ was considered to be the reason for this deviation, which was investigated further by ultraviolet-visible (UV-vis) spectroscopy. As shown in UV-vis and Raman spectra, periodic oscillation could be found for Ce³⁺ and Ce⁴⁺ (Fig. 1e and f).^17,18 Based on these analytical results, the role of Ce⁴⁺ could be proposed as shown in eqn (2). With respect to the reaction, addition of Ce⁴⁺ is related to the redox couple for Mn²⁺. As mentioned above, the MnOOH generated during discharging tends to be converted to Mn²⁺ with the Jahn–Teller effect. During charging, parts of Mn²⁺ cannot return back to MnO₂. Ce⁴⁺ can react with Mn²⁺ to form MnOOH via a redox reaction during charging. Subsequently, parts of MnOOH will deposit on the cathode by H⁺ attraction and transform further to MnO₂ for participation in the electrochemical reaction on the cathode, which helps to increase the mass of active substances and enhance the capacity (Fig. 1c and S2†).

Mn²⁺ + Ce⁴⁺ + 2H₂O ↔ MnOOH + Ce³⁺ + 3H⁺ (2)

Furthermore, the Zn–Ce electrolyte was centrifugated and analyzed (Fig. 2). The peaks located at 882.6/886.6 eV and 883.0 eV in the high-resolution XPS Ce 3d spectrum for the Zn–Ce electrolyte represented Ce⁴⁺ and Ce³⁺, respectively. The peaks located at 642.5/653.9 eV and 643.1/655.0 eV in the high-resolution XPS Mn 2p spectrum for the sediment represent Mn³⁺ and Mn²⁺, respectively (Fig. 2a).^19–23 Furthermore, the lattice-oxygen Mn–O–Mn and surface-adsorbed oxygen Mn–O–H also confirmed the generation of MnOOH.²⁴ The XRD pattern showed that the sediment was mainly MnOOH (space group: Pbnm, JCPDS card number: 01-089-2354) (Fig. 2b). Moreover, the obvious co-existing signals of Ce⁴⁺ and Ce³⁺ demonstrated the reversibility of the Ce⁴⁺/Ce³⁺ conversion reaction (Fig. 2c). Combined with the oscillation of Ce³⁺/Ce⁴⁺ and Mn²⁺/MnOOH, we speculated that eqn (2) was reversible. That is, the decline of Ce³⁺ upon discharge was related to negative eqn (2), consistent with the increase in Ce⁴⁺ and Mn²⁺ (Fig. 1d and f). Moreover, the increase in Ce⁴⁺ and decrease in Ce³⁺ were shown by UV-vis spectroscopy, which was related to the reaction between Ce³⁺ and MnOOH during discharge (Fig. 1e). During discharge, the reaction between Ce³⁺ and MnOOH would be competitive, with the disproportionation of MnOOH, which would be beneficial for the Ce⁴⁺/Ce³⁺ reversible conversion reaction, and the opposite process would be found simultaneously. Herein, Hess's law can be introduced to explain this phenomenon (see ESI Discussion†). The change in Mn²⁺ and MnOOH with eqn (1) is the driving force of eqn (2). Based on this interpretation, the assumption that positive eqn (2) is influenced by the peak content of Mn²⁺ in the electrolyte at the termination of discharge is reasonable. The Mn content stayed about the same after one cycle because the balance of Ce⁴⁺/Ce³⁺ is obtained and negative eqn (2) competes with the disproportionation of MnOOH (Fig. S1†). As a result, stronger peaks of MnOOH can be found after 10 cycles, which is caused by the addition of Ce⁴⁺ (Fig. S3†). At the beginning of the cycle, positive eqn (2) is stronger due to the amount of Ce⁴⁺, whereas the balance is built later (Fig. 2c). Moreover, the variation in Mn at the cathode was studied by X-ray fluorescence (XRF) spectrometry (Fig. S4†). The mass of Mn was ∼45% after 50 cycles compared with the first cycle in the basic electrolyte, whereas it was 84% in the Zn–Ce electrolyte, showing that the dynamic compensation mechanism could inhibit the dissolution of Mn effectively. In summary, Mn²⁺ generated from the disproportionation of MnOOH during discharge was reconverted to MnOOH with the addition of Ce⁴⁺ during charging. MnOOH would deposit on the cathode by H⁺ attraction and be converted to MnO₂ for capacity compensation during charging. Meanwhile, during discharge, Ce³⁺ would react with MnOOH and be converted to Ce⁴⁺ to achieve dynamic reversible Ce⁴⁺/Ce³⁺ conversion (Fig. 2d).

Fig. 2 Characterization of the results in the Zn–Ce electrolyte. (a) High-resolution X-ray photoelectron spectroscopy (XPS) of Ce 3d, Mn 2p, and O 1s of the Zn–Ce electrolyte. (b) XRD pattern of the sediment in the Zn–Ce electrolyte. (c) UV-vis spectroscopy of the Zn–Ce electrolyte in different cycles. (d) Dynamic compensation mechanism for the Zn/MnO₂ battery with the Zn–Ce electrolyte (schematic).

The scanning electron microscope (SEM) images, corresponding mapping images, and energy dispersive spectroscopy (EDS) of MnO₂ cathodes upon discharging to 0.8 V and charging to 1.8 V revealed that the structures of MnO₂ in the two types of electrolytes were distinct (Fig. S5–S7†). The concentration of Zn, O, and S suggested that Zn₄SO₄(OH)₆·4H₂O appeared on the MnO₂ cathode in the basic electrolyte (Fig. S6†).²⁵ During discharge, the increasing OH⁻ in the electrolyte was the result of positive eqn (1), and reacted with Zn²⁺ and SO₄²⁻ on the cathode to form Zn₄SO₄(OH)₆·4H₂O, which corresponded to the cathode in the basic electrolyte (Fig. S5a†). In contrast, the cathode surface in the Zn–Ce electrolyte was mainly MnOOH instead of Zn₄SO₄(OH)₆·4H₂O because OH⁻ was likely to react with the extra H⁺ generated in eqn (2) rather than Zn²⁺ and SO₄²⁻ (Fig. S7 and S8†).

Cyclic voltammetry (CV) at scan rates from 0.1 to 1 mV s⁻¹ was performed to investigate the electrochemistry of Zn/MnO₂ batteries with the Zn–Ce electrolyte (Fig. 3a). As reported previously, the scan (v) and peak current (i) can be used to analyze the contribution of diffusive-controlled and capacitive-controlled effects based on eqn (3) and (4).^26,27

i = av^b (3)

log(i) = b log(v) + log(a) (4)

where a and b are adjustable parameters, 0.5 ≤ b ≤ 1 denotes a greater diffusive-controlled contribution in the electrochemical process if b is close to 0.5, and a greater capacitive-controlled contribution in the electrochemical process if b approaches 1.

i = k₁v + k₂v^0.5 (5)

where i is the peak current, v is the scan rate, k₂v^0.5 represents the diffusive-controlled contribution, and k₁v represents the capacitive-controlled contribution.

Fig. 3 Electrochemical performance of the Zn/MnO₂ battery. (a) CV curves at various scan rates from 0.1 to 1.0 mV s⁻¹, (b) relationship between log(i) and log(v) plot at corresponding redox peaks, and (c) capacitive contribution ratio of the Zn/MnO₂ battery with the Zn–Ce electrolyte. (d) CV curves of the Zn/MnO₂ battery with the Zn–Ce electrolyte and basic electrolyte at 0.2 mV s⁻¹. (e and f) Charge/discharge curves and (g) rate performance of Zn/MnO₂ batteries with the basic electrolyte and Zn–Ce electrolyte at different current densities. (h) EIS curves of Zn/MnO₂ batteries with the Zn–Ce electrolyte at different temperatures. (i) Activation energy (E_a) values of Zn/MnO₂ batteries with the basic electrolyte and Zn–Ce electrolyte.

The two peak-current values were utilized as the i values in eqn (3) and (4), and the linear relationship between log(i) and log(v) is shown in Fig. 3b. The corresponding b₁ and b₂ values of peaks were 0.63 and 0.75, respectively.²⁸ Furthermore, the capacitive contribution ratio could be calculated using eqn (5). The result showed that the electrochemical behavior of the Zn/MnO₂ battery with the Zn–Ce electrolyte was dominated by a diffusive-controlled contribution (Fig. 3c).²⁹

According to the CV curves at 0.2 mV s⁻¹ (Fig. 3d), one pair of redox peak corresponding to the MnO₂/MnOOH conversion reaction could be found in the Zn/MnO₂ battery with two types of electrolytes, which was consistent with the charge/discharge plateaus in galvanostatic charge/discharge curves (GCDs).^30–32 Meanwhile, the higher current density of Zn/MnO₂ batteries with Ce(SO₄)₂ additives in comparison with that without Ce(SO₄)₂ additives indicated a higher specific capacity, which was ascribed to the dynamic compensation mechanism (Fig. 3e–g).³³ Therefore, the Zn/MnO₂ battery with the Zn–Ce electrolyte exhibited a reversible capacity of 190.4 mA h g⁻¹ at 0.5 A g⁻¹ and maintained 105.2 mA h g⁻¹ at 3 A g⁻¹, and the capacity returned to 202.4 mA h g⁻¹ when the current density returned to 0.5 A g⁻¹ after 40 cycles, thereby demonstrating outstanding cycling stability (Fig. 3f). The charge-transfer resistance of the Zn/MnO₂ battery with the Zn–Ce electrolyte at 30 °C was 217 Ω, which was far lower than that of the Zn/MnO₂ battery with the basic electrolyte (608.6 Ω), indicating faster charge-transfer kinetics (Table S1†). Compared with the higher activation energy of the Zn/MnO₂ battery with the basic electrolyte (39.9 kJ mol⁻¹), that of the Zn/MnO₂ battery with Zn–Ce electrolyte was lower (33.7 kJ mol⁻¹), thereby indicating better reaction kinetics and resulting in a satisfactory rate performance (Fig. 3h–i and S9†). The lower R_ct of the Zn/MnO₂ battery with the Zn–Ce electrolyte at different cycles was attributed to the accelerated kinetics process shown above. Furthermore, accelerated kinetics at the anode was also shown with activation energy using electrochemical impedance spectroscopy (EIS) at different temperatures (Fig. S10 and Table S2†).³⁰

Fig. 4 shows a comparison of the electrochemical performance of Zn/MnO₂ batteries. The R_ct changes of Zn/MnO₂ batteries with the basic electrolyte and Zn–Ce electrolyte were studied (Fig. 4a, S11 and S12†). With an increase in shelf time, Zn/MnO₂ batteries with the Zn–Ce electrolyte exhibited a much slower R_ct growth compared with Zn/MnO₂ batteries with the basic electrolyte, denoting better stability for storage. Moreover, this phenomenon was confirmed by the impendence–time curves of the Zn/MnO₂ battery with the basic electrolyte and Zn–Ce electrolyte at 1000 Hz (Fig. S13†). Meanwhile, the Zn/MnO₂ battery with the Zn–Ce electrolyte delivered greater capacity retention after 200 cycles at different temperatures, indicating superior wide-temperature stability (Fig. 4b and S14†). Furthermore, benefiting from the dynamic compensation of MnOOH, the Zn/MnO₂ battery with the Zn–Ce electrolyte achieved reversible capacity retention of 97.4% and a capacity of 130.1 mA h g⁻¹ at 1 A g⁻¹ after 1000 cycles (vs. 40.5% and 75.8 mA h g⁻¹ for the Zn/MnO₂ battery with the basic electrolyte), and identical results were obtained at a current density of 0.5 A g⁻¹ (Fig. 4c and d). It also had greater capacity retention compared with the partial Zn/MnO₂ battery based on different optimization methods (Fig. 4e and Table S3†).^34–40

Fig. 4 Comparison of the electrochemical performance of Zn/MnO₂ batteries. (a) Changes in R_ct values of Zn/MnO₂ batteries with the basic electrolyte and Zn–Ce electrolyte during 30 h at room temperature. (b) Capacity retention of Zn/MnO₂ batteries with the basic electrolyte and Zn–Ce electrolyte after 200 cycles at 0, 15, 35, and 55 °C. (c and d) Long cycling performance of Zn/MnO₂ batteries with the basic electrolyte and Zn–Ce electrolyte at 0.5 and 1.0 A g⁻¹. (e) Comparison of electrochemical performance of the Zn/MnO₂ battery based on the Zn–Ce electrolyte with other Zn/MnO₂ batteries reported previously.^34–40

3 Conclusions

The Zn–Ce electrolyte was prepared to enhance the capacity retention of Zn/MnO₂ batteries. The dynamic compensation mechanism was proposed to mitigate the disproportionation of MnOOH. Mn²⁺ formed from the disproportionation of MnOOH during discharging diffused into the electrolyte. Introduction of Ce⁴⁺ led it to react with part of Mn²⁺ to form MnOOH, which deposited partly on the cathode via the attraction of H⁺, and subsequently transformed to MnO₂ during charging to dynamically compensate the active substance. Meanwhile, during discharge, Ce³⁺ reacted with MnOOH to reversibly convert to Ce⁴⁺, which inhibited the disproportionation of MnOOH. Benefiting from this strategy, Zn/MnO₂ batteries with Ce(SO₄)₂ additives showed a capacity retention of 97.4% at 1.0 A g⁻¹ after 1000 cycles, which far exceeded that of the Zn/MnO₂ battery without Ce(SO₄)₂ additives.

Author contributions

J. Z. and Y. T. conceived and supervised the research. G. L. and P. R. carried out the experiments and analysed the experimental data. B. L., S. L., and X. H. helped with the analysis of electrochemical data. All authors commented on the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52172263, 51932011), Natural Science Foundation of Hunan Province (2022JJ30051), Hunan Natural Science Fund for Distinguished Young Scholar (2021JJ10064), and Program of Youth Talent Support for Hunan Province (2020RC3011).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta02202a

‡ These authors contributed equally to the work.

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