ORCID Profile
0000-0002-2403-7794
Current Organisation
National Institute for Materials Science
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Publisher: Royal Society of Chemistry (RSC)
Date: 2019
DOI: 10.1039/C9CP00425D
Abstract: The hopping/exchange-dominated Li ion transport is attributed to liquid electrolytes with solvent-bridged, chain-like Li ion coordination and aggregated ion pairs.
Publisher: The Electrochemical Society
Date: 07-07-2022
DOI: 10.1149/MA2022-01158MTGABS
Abstract: Rechargeable lithium-air battery (LAB) is considered to be one of the most promising energy storage devices because of its large theoretical energy density. Despite significant efforts by many research groups, LAB is still far from practical use and many problems must be overcome [1]. The first problem is associated with the use of air as it contains many components other than oxygen including N 2 , water, and CO 2 , which interfere battery reactions. Thus, most of the fundamental studies are not on LAB but on Lithium-oxygen battery (LOB), which uses pure oxygen as an active material. Even LOB has many serious problems, including low cyclability caused by (1) high charging overpotential, which induces degradation of positive electrode (carbon) and electrolyte solution, and (2) degradation/dendrite formation of negative Li metal electrode. To improve the cyclability, it is essential to clarify the mechanism of degradation of electrodes and electrolyte. Here we employed mass spectroscopy to follow products generation during charging so that we can clarify the degradation mechanisms of electrodes and electrolyte. Porous carbon sheet (KJCNT from KJ Specialty Paper), lithium metal (Honjo Metal), 1 M LiTFSI TEGDME (both from Sigma-Aldrich) solution, PE separator (W-Scope), and stainless steel mesh (Hohsen) were used as positive electrode, negative electrode, electrolyte, separator, and gas diffusion layer, respectively. Isotope exchanged TEGDME, i.e., 13 CH 3 -TEGDME and CD 3 -TEGDME, which were synthesized in our laboratory, and 18 O 2 (Taiyo Nippon Sanso) were used for mass spectroscopy for precise products assignments. Details of set-up for mass analysis have been reported elsewhere [2]. We carried out online QMS (Quadrupole Mass Spectrometer) measurement for mass numbers of decomposition products between 12-90 during the potential sweep from OCP to 4.8 V (0.05 mV/s). Figure 1 shows the current and mass signals of m/z = 18 (H 2 O), 32 (O 2 ), and 44 (CO 2 ) as a function of potential. It is clear that current increased immediately as potential was scanned from OCP (~2.8 V ), reached a maximum at 3.3 V, decreased gradually, reached a minimum at 4.0 V, increased again to reach a maximum at 4.4 V, decreased to reach a minimum at 4.6 V and increased again. The potential dependence of O 2 signal is very similar to that of current as the main battery reaction is Li 2 O 2 → 2Li + + O 2 + 2e - . Two clear peaks were observed at around 3.3 V and 4.4V, showing the presence of two kinds of Li 2 O 2 [3]. The H 2 O signal started to increase just after the 1st current/O 2 peaks where the current efficiency for O 2 generation current efficiency started to decrease, suggesting the H 2 O formation is related to the decrease of the current efficiency. The CO 2 signal started to be significant at 4.1 V where current and the O 2 signals started to increase again and reached a maximum at ca. 4.5 V, which is more positive than those of current and O 2 signal but equal to H 2 O peak. Although current increased again significantly at potentials more positive than 4.65 V, no mass signals of O 2 , H 2 O and CO 2 increased. Instead signals such as m/z = 15, 29, 31, 45, and 60, which are related to organic molecules derived from TEGDME, became significant. Figure 2 shows the online QMS results at 4.65 V. We can see that various mass signals such as 15, 29, etc. were observed at this potential. By careful analysis of the results with the assistance of isotopes 18 O 2 , 13 CH 3 -TEGDME, and CD 3 -TEGDME, presence of CH 3 OH, HCHO and other organic molecules with high vapor pressure originated from TEGDME were confirmed. Additionally, we detected molecules with low vapor pressure such as CH 3 O(CH 2 ) 2 OH, CH 3 OCH 2 COCH 3 , etc., which were derived from TEGDME by GC/MS analysis of s les collected every 1hr. Based on the products detected by online QMS and GC/MS, possible degradation mechanism of electrolyte can be deduced. I will discuss the degradation mechanism of electrolyte based on the above results. [1] Liu T., et al. Current challenges and routes forward for nonaqueous lithium–air batteries. Chemical reviews, 2020, 120(14): 6558-6625. [2] Ue, M., et al. Material balance in the O 2 electrode of Li–O 2 cells with a porous carbon electrode and TEGDME-based electrolytes, RSC Advances, 2020, 10(70): 42971 - 42982. [3] Nishioka K, et al. Isotopic depth profiling of discharge products identifies reactive interfaces in an aprotic Li–O2 battery with a redox mediator. Journal of the American Chemical Society, 2021, 143(19): 7394-7401. Figure 1
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