Effect of Cu As an Additive in the MnO2 Cathode in Rechargeable Zn-MnO2 Alkaline Batteries
Authors: Lin Yi Min , Lee Changhee , Sakamoto Ryo , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

Aqueous rechargeable Zn-MnO2 batteries are drawing much attention as a solution for grid-scale energy storage because of their characteristics of high safety, low cost, and ease of manufacturing [1]. However, in alkaline aqueous solutions, due to the formation of electrochemically inactive compounds such as hausmannite (Mn3O4) during the charging/discharging process [2], developing highly rechargeable Zn-MnO2 alkaline batteries remains a challenge.
In 2017, Yadav et al. reported that they successfully achieved the full theoretical 2-electron capacity of MnO2 for >6000 cycles by adding bismuth oxide (Bi2O3) and copper to the MnO2 cathode to form Bi-intercalated (Bi-δ-MnO2) intercalated with Cu2+ ions [3]. Some researchers are paying attention to the effect of Bi2O3 as an additive. However, the study on the role of Cu is still incomplete.

Objective

To unravel the effect of Cu as an additive, we used MnO2, MnO2 + Cu, and MnO2 + Bi2O3 + Cu as active materials to observe the electrochemical behaviors in the alkaline electrolyte without the existence of [Zn(OH)4]2−. Moreover, we investigated the product of each cycled cathode.
In this study, we have mainly focused on the effects of adding Cu to the MnO2 cathode without Bi2O3.

Experimental

The cathode mixture is made by mixing the active materials, acetylene black (AB) and polytetrafluoroethylene (PTFE) by hands at the weight ratio of 80:14:6. Electrolytic manganese dioxide (EMD), EMD + the powder of metallic Cu (6:2 wt%), and EMD + Bi2O3 + Cu (6:1:2 wt%) were mixed and used as the active materials. A 2-electrolyte Zn-MnO2 cell was used for electrochemical measurements. The cathode mixture, Zn plate, and Hg/HgO electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. Solutions of 6 mol dm−3 (M) KOH and 6 M KOH saturated with ZnO are used as the cathode electrolyte and the anode electrolyte, respectively. FAAM-PK-75 was used as the separator between the two electrolytes. The galvanostatic test was performed at 0.2C (1C = 616 mA g−1) for 20 cycles in a voltage range of −1.0 – 0.45 V (vs. Hg/HgO).

Results and discussion

The galvanostatic data of the EMD + Cu and EMD cathodes is shown in Fig. 1. The plateau at around −250 mV, which indicates the oxidation of MnⅡ during the charging process, was observed at the fifth cycle. In contrast, such plateau cannot be observed in the case of the EMD cathode. However, the capacity retention at the seventh cycle for the EMD + Cu cathode is lower than that for the EMD cathode (Fig. 2). We suggest that Cu not only induces the second electron reaction of Mn but also the formation of electrochemically inactive compounds. Furthermore, other Cu compounds are expected to be formed rather than metallic Cu because of the poor reversibility of the EMD + Cu cathode after discharge.
From the XPS result for the EMD + Cu discharged cathode, the Cu2+ satellite peak was hardly observed, which shows no existence of the possibly formed compound CuMn2O4. Based on the results, we conclude that the electrochemically inactive compounds formed in the cycled EMD + Cu cathode are Mn3O4 and Cu2O instead of any Cu-Mn compounds. Since Cu induces the second electron reaction of Mn, the species of MnⅡ, such as Mn(OH)2 and [Mn(OH)4]2− increase, which finally allows MnⅢ species to encounter MnⅡ species easier and form Mn3O4.

Conclusion

We found that the MnO2 + Cu cathode shows poorer cyclability even compared to the MnO2 cathode. We assume that this is because while Cu induces the second electron reaction of MnO2, it does not prevent the formation of Mn3O4, which deteriorates the performance of the Zn-MnO2 cells. Moreover, no Cu-Mn compounds were found in the XPS result.

References

[1] A. Zhou, R. Chi, Y. Shi, X. Zhao, X. Li, Z. Kou, Z. Zhang, X. Zhang, G. Nie, Mater. Today Chem., 27, 101294 (2023).
[2] B. J. Hertzberg, A. Huang, A. Hsieh, M. Chamoun, G. Davies, J. K. Seo, Z. Zhong, M. Croft, C. Erdonmez, Y. Shirley Meng, D. Steingart, Chem. Mater., 28, 4536–4545 (2016).
[3] G. G. Yadav, J. W. Gallaway, D. E. Turney, M. Nyce, J. Huang, X. Wei & S. Banerjee, Nat. Commun.,
8, 14424 (2017).

Figure 1