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Article
Effect of Water and HF on the Distribution of Discharge Products at Li-O Battery Cathode 2
Kentaro Tomita, Hidenori Noguchi, and Kohei Uosaki ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00584 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018
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ACS Applied Energy Materials
Effect of Water and HF on the Distribution of Discharge Products at Li-O2 Battery Cathode Kentaro Tomita, Hidenori Noguchi, Kohei Uosaki* Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN) and Center for Green Research on Energy and Environmental Materials (Greater GREEN), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan.
ABSTRACT
Li-air battery has attracted much attention due to its very high theoretical energy density but its actual performance is still very low. Most important reasons are the slow kinetics and low reversibility of electrodeposition/dissolution of Li-O2 species at the cathode. Thus, much effort has been devoted to understand the mechanisms of these processes but low reproducibility makes the full understanding of the mechanism difficult. Here we demonstrate that low reproducibility is caused by impurities in solution by showing how HF and H2O, major impurities, affect the potential dependent product distribution during discharge at Li-O2 cathode. HF causes significant mass increase as a result of the deposition of fluorine-containing species and H2O converts Li2O2 to proton containing side products such as H2O2, LiHO2 and LiOH and induces the solvent, DMSO, decomposition. These results demonstrate the importance of the impurity control in the operation of Li-air battery.
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KEYWORDS: Li-air battery, oxygen reduction reaction, non-aqueous electrochemistry, potential dependent product distribution, electrochemical quartz micro balance (EQCM), rotating ringdisk electrode (RRDE), in situ surface enhanced Raman scattering (SERS)
INTRODUCTION Li-air battery (LAB)1 has been considered as one of the most promising energy storage devices for electric vehicles because of its high theoretical energy density, which is more than 10 times that of the current lithium ion batteries and is comparable to those of fossil fuels2, and, therefore, attracts much research interests3. There are, however, many issues to be solved before it becomes a practical devise. Practical energy density is far below the theoretical value as Li2O2, the discharge product at a cathode, is an insulator and blocks further reaction. Insulating nature of Li2O2 causes very high overpotential for the oxidation reaction of Li2O2, i.e., charging reaction, leading to the decomposition of electrolyte and low charge-discharge cycles. To solve these problems, numerous efforts to understand the cathode reaction mechanism have been made4 and it has been suggested that the discharge reaction consists of multiple steps with the formation of LiO2 as a soluble and unstable intermediate as follows5-6, Li+ + O2 + e- → LiO2
(1)
O2 + e- → O2-
(1)’
Li+ + O2- ↔ LiO2
(2)
2LiO2 → Li2O2 + O2
(3)
We carried out electrochemical quartz crystal microbalance (EQCM) study to clarify the reaction mechanism of the cathode reaction at a gold electrode in a DMSO solution containing
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ACS Applied Energy Materials
0.1 M LiPF6 and found that the deposition of LiO2 by Reaction (1) proceeds in relatively positive potential as the mass per mol-electron (mpe) was 37 g/mol-e, which is in good agreement with the theoretical value of 39 g/mol-e for Reaction (1), but O2- is formed in the solution by Reaction (1)’, the mpe of which is 0 g/mol-e, in relatively negative potential regions and most of the formed O2- were diffused away before being associated with Li+ by the Reaction (2) as mpe was 7 g/mol-e7. However, following EQCM studies reported different conclusions8-11. These discrepancies in EQCM studies may be due to the fact that these reactions are greatly influenced by the components of the electrolytes, which are not necessary well controlled. Recent reports demonstrated that the H2O impurity in electrolyte solutions changes the morphology of the Li2O2 deposition from film to particles, resulting in the increase of the practical capacity12-15. Other researchers reported that H2O works as a proton source to convert the discharge product to LiOH in polar solvents such as dimethyl sulfoxide (DMSO) and acetonitrile16-17. Another impurity to be considered is HF. It is known that residual HF is contained in LiPF6, one of the most common electrolytes, because anhydrous HF is used as a solvent in the preparation of LiPF6. The decomposition reaction of LiPF6 with H2O also results in the formation of HF. The concentrations of H2O and HF in commercially available LiPF6 solutions are as high as 15 ppm, i.e., 1.1 mM, and 50 ppm, i.e., 3.2 mM, respectively18. They are comparable to saturated O2 concentration in DMSO that is 2.1 mM19. Commercially available DMSO of anhydrous grade also contains as high as 50 ppm, i.e., 3 mM, of H2O20. In this paper, we demonstrate how HF and H2O, major impurities, affect the amount, composition and morphology of discharge products in DMSO-based electrolyte solutions, leading to the low reproducibility of electrochemical processes. Reaction mechanisms were discussed based on the amounts of the deposits, the amount of dissolved products, the
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composition of the products near the electrode surface and surface morphology and composition of the cathode after discharge investigated by EQCM21, rotating ring-disk electrode (RRDE)5-6, in situ surface enhanced Raman scattering (SERS)
22-23
, and scanning electron microscope
(SEM), respectively. It is found that HF increased the amounts of deposits, the mass of which is dependent on the HF concentration. In H2O rich (~1000 ppm) solution, byproducts such as H2O2 and LiHO2 were detected. H2O also induces decomposition of DMSO at the gold electrode surface in relatively negative potential regions. These results suggest that a small amount of impurities controls the cathode reactions. Discrepancies among reported results of ORR in LiPF6 containing electrolyte solutions are caused by the HF-induced deposition and H2O induced side reactions such as formation of H2O2 and LiHO2 and the decomposition of DMSO. EXPERIMENTAL SECTION Materials. LiPF6 and DMSO were purchased from Sigma-Aldrich. DMSO was dried with molecular sieves 4A (Wako Pure Chemical Industries, Ltd.) prior to use. The electrolyte solutions, DMSO solutions containing 0.1 M LiPF6, were prepared and stored in an argon-filled globe box. As-received LiPF6 was used for solution 1 and 2. LiPF6 for solution 3 and 4 was treated in vacuo with a liquid N2 trap at room temperature for 4 h initially and at 398 K for 20 h. Solution 3 was stored with molecular sieves 4A (ca. 10 wt%) to remove HF. The electrolyte solutions were filtrated with a 0.2 µm nylon filter prior to use. Electrochemical Measurements. EQCM and in situ SERS measurements were carried out with a gastight three-electrode spectroelectrochemical cell made by a PTFE with an optical quartz window. RRDE measurement was carried out with a Au ring (φI.D. = 5.5 mm, φO.D. = 8.2 mm) – Au disk (φ = 5.3 mm) electrodes unit in a three-electrode-beaker-type cell with a luggin capillary. All measurements were performed in a super dry room (water contents:
