Excellent Performance of Mesoporous Carbon with High Crystallinity for Lithium Air Battery Cathode
Authors: Nakajima Yuuka , Matsuda Shoichi , Kameda Takashi , Yamaguchi Shoji , Takada Junji , Morishita Takahiro 
Published: December 2024

1.Introduction
Lithium-air batteries (Li-airs) have four times higher energy density by weight than current lithium-ion batteries because Li-air avail oxygen from outside of batteries. Therefore, Li-airs are expected as next generation batteries 1). Furthermore, specific capacity of cathode depends on specific pore volume of carbon materials in the cathode 2).
On the other hand, a lifespan of Li-air is lower than conventional lithium-ion batteries because carbon materials in cathode is decomposed by lithium peroxide generated in discharge process.
Therefore, carbon material with high oxidation resistance have been investigated for the cathode 3).
In this study, mesoporous carbon materials “MPC” which have high specific pore volume and oxidation resistance were synthesized to be applied to cathode in Li-air. Furthermore, we investigated the relationship between an oxidation resistance of MPC and a cycle performance of the cathode in Li-air.
2.Experimental
Mesoporous carbons were prepared by using the MgO template method 4). Phenol resin as carbon source was mixed with MgO. The mixture was heat-treated at 900 °C in N2 atmosphere to obtain carbon-MgO composite. The composite was rinsed by 1 mol L-1 H2SO4aq to remove the MgO from the carbon-MgO composite. Obtained MPC resulting from cleaning process (MPC-1) were heat treated at 1600, 1800, 2100, and 2400 °C (MPC-2, 3, 4, and 5). BET specific surface area (BET SSA), mesopore diameter, and pore volume of samples were calculated by BET and BJH methods with nitrogen adsorption measurements.
Electrochemical performance of cathode in Li-air was measured under following conditions. Charge-discharge test was performed in a pure oxygen atmosphere at a current density of 0.4 mA cm-2 between 2.0 and 4.8 V until the specific capacity of cathode decreases to 3.2 mAh cm-2.
3.Results and Discussion
Characteristics of MPC-1, 2, 3, 4, 5 and Ketjen Black (reference sample) were represented in Table 1. BET SSA and total pore volume of MPC gradually decreased with heat treatment temperature up to 2100 °C, and it drastically decreased at 2400 °C. BET SSA and total pore volume of Ketjen Black decrease significantly due to a shrinkage of micropores and the other pores on the carbon by heat-treatment above 1600 °C. It is considered that MPC heat treated above 1600 °C have high BET SSA and total pore volume because 3D-structure of MPC derived from MgO template method mitigated the shrinkage of pores by heat-treatment. MPC-1 exhibit specific capacity comparable to Ketjen Black at 1st discharge process.
MPC-1 has poor cycle performance despite having a large first discharge capacity. On the other hand, MPC-2 – 4 performed lower specific capacity and better cycle performance with heat temperature. From the results, there are obvious correlation among specific capacity, cycle performance, surface area and pore volume. MPC-5 exhibits the lowest specific capacity and cycle performance among MPC. It would come from decreasing of BET SSA and total pore volume by heat treatment at 2400 °C. Figure 1 shows the XRD patterns of MPC and Ketjen Black. A peak attributed to (002) of graphite was observed around 26° on the patterns of MPC-3 and 4. This result indicates that crystallinity of MPC was increased by heat-treatment above 1800 °C. Contrastly, there are no peak observed on the pattern of MPC-5. The result shows crystal structure of MPC is collapsed when heat treated at 2400 °C. From the results, it is turned out that crystallinity increasing of MPC would prevent degradation of cathode in Li-air during charge-discharge cycles.
In this study, MPC-3 was the material with both high specific surface area and high crystallinity.
Reference
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48, 10, 2690-2707 (2010).

Figure 1