Effect of Interfacial Cations of MnO2 Nanosheet-Based Catalystson Air-Electrode Reaction for Rechargeable Li-Air Batteries
Authors: Mikami Kento , Tachikawa Yusuke , Ishikawa Taro , Fujinami Taichi , Saito Morihiro 
Published: February 2020

In recent years, development of new-generation batteries with high energy densities exceeding that of Li-ion batteries (LIBs) has been intensively studied. One of the candidates is a non-aqueous Li-air (O2) battery (LAB) and the energy density is more than 5 times larger than those of LIBs. Also, the cost is relatively low because of no containing of precious metals in the positive electrode (PE). However, for the practical use, the LAB still have some problems, e.g., Li metal dendrite growth at the negative electrode, electrochemical durability for electrolytes especially against O2
- radical at the PE, large overpotential to decompose the Li2O2 deposition as a discharge product, because it leads to deteriorate the air-electrode and electrolyte, and then lowers the energy efficiency of LABs. Therefore, many researchers have researched on air-electrode catalysts, e.g., noble metal nanoparticles (Pt, Au) and oxides (Co3O4, RuO2) loaded on nano-carbon materials. However, such kinds of solid electro-catalysts have not exhibited sufficient performance yet because the Li2O2 product generates and covers on the catalysts. To prevent this problem, we have synthesized a new composite catalyst composed of MnO2 nanosheets (Mn-NS) and Ketjen black (KB), i.e. a KB-comp. Mn-NS catalyst, and investigated the electro-catalytic activity against Li2O2 deposition/ decomposition reaction using a LAB cell, together with the durability [1]. In this study, we focused on the effect of interfacial cations for the MnO2 nanosheets on the ORR/OER reactions by comparing the performance of Li+, Na+ and K+-form KB-comp. MnNS catalysts.
Mn-NS colloid solution was synthesized by a one-step solution method [1, 2], and then a KB ethanol dispersion was added to form a mixed solution. Afterward, 0.10 M LiCl, NaCl and KCl aqueous solutions were gradually dropped into the mixture to form the KB-comp. MnNS catalysts with each cation (Li+, Na+ and K+). Characterization of the obtained catalysts was carried out by a XRD, TG-DTA, BET measurement and TEM observation. The electro-catalytic activities were evaluated by using a LAB cell using the catalysts, Li metal NE and 0.20 M LiN(SO2CF3)2/diglyme (G2) electrolyte, and tested in constant current (CC) mode at 0.20 mA cm-2 at 30oC. Cyclic voltammetry (CV) and AC impedance measurements were also conducted to elcidate the reaction mechanism for ORR and OER at the PE.
From XRD analysis, crystal phases of the synthesized KB-comp. Mn-NS catalysts clearly correspond to the birnessite-type MnO2 layer (JCPDS No. 80-1098) with different distance between the nanosheets, implying the existence of each cation. Fig. 1 shows charge/discharge curves and cycleability of the capacities for the LAB cells using the KB-comp. Mn-NS catalysts, respectively. As a result, the Li+ form clearly exhibited the lowest overpotential during both discharge and charge processes and the best cycle performance among them. This indicates the Li+ ions around the MnO2 nanosheets enabled to promote a Li2O2 generation near them, and keep good contact between the catalytic sites and Li2O2 product. From the CV measurement (Fig. 2), the order of magnitude for both ORR and OER currents was Li+ > Na+ > K+. This was equal to the trend of cell performance. The AC impedance suggested that the Li-form KB-comp. Mn-NS catalyst enhanced the Li2O2 decomposition reaction during charge process as compared with the other cation-form ones. The enhancement mechanism in more detail will be reported in the meeting.
This work was supported by JST “A Tenure-track Program” and JSPS “KAKENHI” (25870899), Japan.
[1] M. Saito et al., Electrochim. Acta, 252, 192 (2017).
[2] K. Kai et al., J. Am. Chem. Soc., 130, 15938 (2008).

Figure 1