Electrochemical Properties of RuO2 Catalyst for Air Electrode of Lithium Air Battery

Introduction

Lithium air batteries exhibit higher theoretical energy density than lithium ion batteries and are expected to be used in the next generation of secondary batteries.  However, there are many problems, such as a decrease in discharge capacities after some cycles and a large difference between discharge and charge voltages, ΔV.  A large/small ΔV means low/high round-trip (discharge-charge) energy efficiency, respectively. High-performance secondary batteries show small ΔV, i.e., high round-trip energy efficiency. Since the first report by K. M. Abraham et al. [1], various kinds of oxygen reduction/evolution catalysts[1-4] and electrolyte[1,5] materials have been intensively investigated for air batteries to improve their electrochemical properties such as the cycleability and the round-trip energy efficiency of the air batteries. 

The purpose of this research is to improve the round-trip energy efficiency by using highly-active catalysts for air electrodes.   We are focusing on RuO₂ as the catalyst and here report the performance of air batteries incorporating this oxide catalyst.

Experimental

Precursor powder of RuO₂, Ru(OH)_n, was prepared neutralizing 0.1 mol/l RuCl₃ aq with 0.1 mol/l NaOH aq.  RuO₂ was obtained by heat-treating the hydroxide powder at 110, 200, and 500^◦C.  An air electrode was prepared by rolling a mixture of RuO₂, KetjenBlack EC600JD (KB), and PTFE (10:54:36 in weight ratio) into a sheet about 0.5-mm thick.  The lithium air battery consisted of the air electrode, a lithium metal shee,t and 1.0 mol/l lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/aprotic organic solvent as positive electrode, negative electrode, and electrolyte solution, respectively. Solvents used were propylene carbonate (PC), tetraethylene glycol dimethyl ether (TEGDME), and dimethyl sulfoxide (DMSO).  The preparation of the battery is described in detail in our previous paper [4].  Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm² in a dry air atmosphere with a dew point of less than -50^◦C at room temperature.  The discharge/charge capacities were normalized by the weight of the air electrodes.

Results and discussion

Figure 1 shows XRD patterns of RuO₂ heat-treated at 110, 200 and 500^◦C.  All the peaks correspond to the PDF data for RuO₂ (#01-070-2662).  The peaks become sharper as the heat-treatment temperature increases.  This indicates that the particle size of RuO₂ became larger.  It is notable that RuO₂ is crystallized even at a low temperature of 110^◦C.  These RuO₂particles would be very fine and suitable as catalyst for the air electrode.

Figure 2 shows the first discharge/charge curves of the air batteries incorporating the RuO₂ catalyst heat-treated at 110, 200, and 500^◦C.  Compared with KB only, the air batteries with RuO₂ catalyst showed larger charge capacities.  The charge capacities became larger with RuO₂ prepared at lower heat-treatment temperature.  As for the cell voltage, the RuO₂ catalyst reduced the charge overvoltage in particular, even though there were no changes in the discharge overvoltages. The charge overvoltages became smaller for RuO₂ with the lower heat-treatment temperature.  The tendencies observed in the capacities and voltages with the heat-treatment temperature were almost the same.   These results indicate that the fine-powder RuO₂catalyst prepared at the lower temperature had great effects on not the discharge but on the charge capacity/overvoltage.

Figure 3 shows the first discharge/charge curves of the air batteries incorporating the RuO₂catalyst in the electrolyte solution of 1.0 mol/l LiTFSI/PC, TEGDME, and DMSO.  The air batteries with the TEGDME solution showed smaller discharge and charge capacities and larger discharge and charge overvoltages compared with the PC solution.  On the other hand, the air batteries with the DMSO solution showed the largest discharge and charge capacities and the smallest discharge and charge overvoltages.  In particular, the air batteries with the DMSO solution showed rather low average charge voltage of about 3.1 V.  Such low charge voltage greatly improves the round-trip energy efficiency.  This superior battery performance would be due to some properties of the DMSO solution such as its stability under the operation condition of the air battery.

             In conclusion, the fine RuO₂powder prepared at the lower temperature reduced the charge overvoltage.  Moreover, the use of the DMSO-containing solution led to the great improvement in the round-trip efficiency due to the decrease in charge overvoltage.  

References

[1] K. M. Abraham et al., J. Electrochem. Soc., 143 (1996) 1.

[2] T. Ogasawara et al., J. Am. Chem. Soc., 128 (2006) 1390.

[3] A. K. Thapa et al., Electrochem. Solid-State Lett., 13 (2010) A165.

[4] H. Minowa et al., Electrochemistry, 78 (2010) 353.

[5] N.-S. Choi et al., J. Power Sources, 225 (2013) 95.