Relationship Between Discharge Profile and Pore Structure for Lithium-Air Secondary Battery

Abstract

Introduction High-capacity next-generation secondary batteries have been desired to spread plug-in hybrid vehicles and electric vehicles. Lithium-air secondary batteries have attracted much attention as the large storage secondary batteries due to their high theoretical capacity (5200 Wh kg-1). However, the reversible capacity is limited because the discharge products block the oxygen diffusion path in air electrode. The influence of the pore structure of carbon materials on the deterioration of electrochemical performance is not clear, although the carbon materials are widely studied as air electrode for lithium-air secondary batteries. In this study, we have prepared the carbon electrodes using carbon materials (CNovel MH (MH) and ketjen black (KB)) with different pore volume, analyzed the discharge products and change of pore structure by discharging. Experimental The carbon materials (MH and KB) were mixed with PVdF (carbon : PVdF = 93 : 7 wt. %). After coating on the carbon paper as a current collector (the loading of carbon material 1 mg, electrode area 2.0 cm2), it was dried overnight at 80 oC in a vacuum to obtain carbon electrodes. The CR2032 coin type cell was assembled in an Ar-filled glovebox. It consisted of the prepared carbon electrode as a cathode, lithium foil as an anode, 1 mol dm-3 LiCF3SO3/TEGDME as an electrolyte, and Whatman glass filter GF/A as a separator. The carbon electrodes were analyzed by using FE-SEM, EDX, XRD, and the nitrogen adsorption method. For the electrochemical measurements, the discharge-charge tests were carried out (current value: 100 mA, voltage range: 2.0-4.5 V, measured temperature: 25 oC, and atmosphere: dry air). Results and Discussion From the discharge-charge tests, the MH electrode showed higher discharge capacity than the KB electrode. Fig. 1 shows SEM images of MH and KB electrodes before and after 6 h discharge. The MH electrode has much larger volume of micropore than the KB electrode, as shown in the pore size distribution. The spherical Li2O2 particles is deposited on the surface of the KB electrode after 6 h discharge, while they are not observed on the MH electrode. The micropore structures prevent growth of Li2O2 particles for MH. The mesopore structure of the MH electrode does not change by 6 h discharge, while mesopore volume of the KB electrode clearly decreases by 6 h discharge. From these results, it was found that the deposited spherical Li2O2 particles changed not only micropore structure but also mesopore structure of carbon electrode to affect the discharge profile. References 1) Y. Gao et al., Int. J. Hydrog. Energy, 37, 12725 (2012). 2) T. Cetinkaya et al., J. Power Sources, 267, 140 (2014). Acknowledgments This research was partially supported by ALCA-SPRING, JST. Figure 1