Effect of O2 Supply on Discharge/Charge Performanceof Li-Air Secondary Batteries
Authors: Ozawa Fumisato , Nakamura Sota , Sano Mitsuki , Azuma Shota , Nomura Akihiro , Saito Morihiro 
Published: December 2024

Li-air batteries (LAB) have been attracting much attention because the theoretical energy density are five times as high as that of Li-ion batteries. However, they suffer from a large charging overvoltage and a poor cycleability due to difficulty of oxidative decomposition of Li2O2 as the discharge product during charging. In general, Li2O2 has both parts of high and low crystallinity, and it is desirable to actively increase the proportion of low crystallinity to reduce charging overvoltage.[1] In this study, we focused on oxygen supply such as the amount of dissolved oxygen and oxygen diffusivity in the electrolyte, and we investigated their effects on the discharge/charge performance of LABs. Especially, in order to improve the amount of oxygen supplied to the air electrode, we prepared LiNO3 electrolyte solutions using mixtures of N-mehtylpyrrolidone (NMP), which has a high dielectric constant and low viscosity, and 1H,1H,5H-Octafluoropentyl 1,1,2,2-Tetrafluoroethyl Ether (OTE) which has a high oxygen solubility as solvent. We fabricated LAB test cells using these electrolyte solutions and conducted discharge and charge tests to evaluate the effect of oxygen supply on the charge overvoltage and cycle life.
Lithium nitrate (LiNO3) was dissolved in Tetraglyme (G4), NMP, and NMP:OTE (2:1 volume ratio) solvents at the concentration of 1.0 M. The ionic conductivity and dissolved oxygen of each electrolyte solution were measured using pH/ion meter and dissolved oxygen meter, respectively. Ketjen Black(KB)-PVdF/NMP slurry was coated on carbon paper, and it dried and fixed to form a positive electrode (KB weight: 1 mg cm-2).[2] LAB cells were fabricated by assembling a positive electrode, separator, and Li metal negative electrode and immersing them with the electrolyte solutions. The discharge and charge tests were conducted at 30℃ under an oxygen flow, and were measured in constant current mode with a current density of 0.20 mA cm-2 and a limiting capacity of 0.50 mAh cm-2 in the voltage range of 2.0 to 4.5 V.

Figure 1 shows the discharge/charge curves for the LAB cells using each LiNO3 electrolyte solution. When using LiNO3/G4, the overvoltage was high during both discharge and charge processes, and it became difficult to reach the limit capacity after 20 cycles (Fig. 1a). Whereas, LiNO3/NMP exhibited a lower overvoltage than the LiNO3/G4 one, and the capacity was maintained even after 20 cycles (Fig. 1b). This is suggested that the degree of dissociation for LiNO3 salt and the diffusivity of Li+ in the electrolyte solution are improved by using NMP solvent. In addition, the diffusivity of oxygen in the electrolyte solution also accelerated by reducing the viscosity. Furthermore, when using LiNO3/NMP:OTE, the overvoltage much more decreased during the charge and discharge cycling, and the capacity was maintained even after 25 cycles (Fig. 1c). This indicates that the mixing OTE with NMP also increased the amount of dissolved oxygen in the electrolyte solution. These results suggest that improvement of the oxygen diffusivity and amount of dissolved oxygen leads to produce a low-crystalline Li2O2 on the air electrode surface, and as a result the decomposition overvoltage was reduced. The other results including the physical properties of each electrolyte solutions and the morphology of Li2O2 generated on the air electrode surface will be reported at the meeting to elucidate the effect of oxygen supply in more detail.
References
[1] A. Dutta, K. Ito, A. Nomura, Y. Kubo, Adv. Sci., 7, 2001660 (2020).
[2] Y. Hayashi, M. Saito et al., Electrochemistry, 89, 557 (2021).
Acknowledgement
This study was supported by JST Project ALCA-SPRING (JPMLAL1301) and NIMS Joint Research Hub Program, Japan

Figure 1