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).