Lithium air secondary batteries (LABs) have the highest theoretical energy density among secondary batteries reported so far. However, major problems are poor cycle characteristics and large discharge/charge overpotential. To improve these properties, various kinds of solid-phase catalysts loaded into air electrodes have been reported [1]. One of the significant problems is that the solid-phase catalysts become inactivated gradually because discharge product Li2O2 accumulats on the surface of the air electrode as a result of imperfect decomposition during the charging process after a large number of cycles [2]. Since a solution-phase catalyst dissolved in the electrolyte solution could work stably during the cycles, it would overcome these issues associated with the solid-phase catalyst. Recently, as solution-phase catalysts, transition metal complexes such as manganese phthalocyanine (MnPc) [3], cobalt phthalocyanine (CoPc) [4] have been reported [3-5]. It is necessary to investigate the electrocatalytic activities for various kinds of solution-phase catalysts because the catalytic mechanism is unclear. We focused on manganese-containing salen-type complexes (MnSl) as a new solution-phase catalyst (Fig. 1). The electrochemical properties would be improved by involving oxygen adsorbed on the central Mn-ion site with the reaction over the air electrode [6]. Here, we report discharge/charge properties in LABs using MnSl as solution-phase catalyst.
Experimental
MnSl [(R,R)-(−)-N,N-Bis(3,5–di–tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)chloride] and MnPc [manganese(II) phthalocyanine] as a reference catalyst were purchased from Sigma-Aldrich Co. LLC. A 2.0 wt% of both catalysts vacuum-dried at 90 °C for 12 h was added to an electrolyte solution of 1.0 mol/l LiTFSA / TEGDME (tetraglyme). Ketjenblack EC-600JD (KB, 80 wt%) and polyvinylidene difluoride (PVdF, 20 wt%) coated on carbon paper (Toray, TGP-H-120) was used as the air electrode. A Li metal sheet was used as the negative electrode. The experimental LAB cell was assembled using a commercial cell (ECC-Air, EL-Cell GmbH). Electrochemical experiments were conducted under galvanostatic condition at 200 mA/g in dry air atmosphere. Current density and discharge/charge capacity were normalized by the mass of the reaction layer (KB + PVdF) in the air electrode.
Results and discussion
Figure 2 shows the first discharge/charge curves of LAB cells with MnSl, with MnPc catalyst, and whithout the solution-phase catalysts. The cells with MnSl catalyst and MnPc show higher average discharge voltages of 2.64 and 2.55 V and larger discharge capacities of 4903 and 3309 mAh/g, respectively, compared to the cell without the catalysts. In addition, the cell with MnSl catalyst shows lower average charge voltage of 3.93 V and larger charge capacity of 6390 mAh/g than the other cells. These results suggest that the solution-phase catalyst could have electrocatalytic activities. Moreover, the MnSl catalyst exhibits rather higher activity than the conventional catalyst, MnPc. However, with the MnSl catalyst, the first charge capacity was larger than the discharge capacity by about 2000 mAh/g. This behavior suggests that some side reactions might co-occur with the charge (oxygen evolution) reaction in the high-voltage region above 4.0 V.
Figure 3 shows SEM images of the air electrodes as prepared and after the first discharge without and with MnSl catalyst. Compared to the electrode surface as prepared shown in Fig. 3(a), the fine sub-micrometer discharge products with similar structures were deposited in a part of the electrode surface both without and with MnSl catalyst as shown in Fig. 3(b) and (c). In particular, there were spherical micrometer structures of the discharge products only with MnSl as shown in Fig. 3(c). MnSl catalyst three-dimensionally catalyzed deposition and decomposition of Li2O2 over the air electrode, which led to significant increase in the capacity.
According to the above results, the MnSl catalyst exhibited higher activities than MnPc. One of the reasons might be that the oxygen adsorption on the central metal Mn containing N-Mn-N bonds is stronger in MnSl than in MnPc. At the IMLB meeting, we will also discuss the cycle characteristics of LABs with the MnSl catalyst.
References
[1]A. Debart et al., J. Power Sources, 174, 1177 (2007).
[2] Y. Shao et al., J. Adv. Funct. Mater., 23, 987 (2013).
[3]S. Heo, Master's thesis “The study on the non aqueous electrolyte for lithium air battery.” (2011). (URL: http://hdl.handle.net/10076/12935, accessed September 1, 2015).
[4]S. Matsuda et al., Abstracts of the 55th Battery Symposium in Japan，1B25，p.130 (2014)．
[5]D. Sun et al., J. Am. Chem. Soc, 136, 8941-8946 (2014).
[6]N. S. Venkataramanan et al., Coord. Chem. Rev, 249, 1249 (2005).