High capacity battery like lithium air battery (LAB) is demanded for electric car and clean energy battery. Lithium is known as an ideal anode material because of its high theoretical capacity (3860 mA h g-1) and the most negative reduction potential (-3.04 V vs. SHE). However, during charging process, lithium dendrite, which leads to short circuit and/or explosion, is produced. In order to avoid dendritic lithium deposition, the way using solid electrolyte interphase (SEI), which is formed by decomposition of electrolytes and/or solvents on the anode surface during charging/discharging processes, is expected. Relatively flat and flexible SEI with higher lithium ion conductivity and less electron conductivity leads to better battery performance [1].
Electrochemical quartz crystal microbalance (EQCM) technique provides us not only ng leveled mass change (Δm) at the electrode surface from frequency change but also electrode surface area change (ΔA) and/or density/viscosity change (Δρ/Δη) at the electrode/electrolyte solution interface from resonance resistance change (ΔR) [2-5]. In addition, we can get mass per electron (MPE) value from this technique and then, this technique is quite useful to study the electrochemical lithium deposition/dissolution processes.
In this study, we focused on the "pre-SEI" formed by potential pre-cycling before lithium deposition on the Cu EQCM electrode. As compared with the case without pre-SEI, we found that much more ideal MPE values in the case with pre-SEI were obtained in the three electrolyte solutions [6].
Experimental
Using three kinds of electrolyte solutions containing 1 M lithium salts, such as 1 M LiPF6 in tetraglyme (G4), 1 M LiTFSI in G4, and 1 M LiFSI in G4, we measured cyclic voltammograms (CVs) of Cu EQCM electrode and we got simultaneously Δm andΔR. In the case without pre-SEI, the potential was negatively scanned from open circuit potential (OCP) to -0.1 V (vs. Li/Li+), where lithium deposition is already started, and then cycled between -0.1 V and +0.35 V. In the case with pre-SEI, the potential was negatively scanned from OCP to 0 V, where lithium deposition is not started yet, cycled between 0 V and +0.35 V for two cycles, and then, cycled between -0.1 V and +0.35 V. All the electrochemical measurements were carried out in the glove box, in which argon gas was filled. The SEI components were evaluated by IR and XPS.
Results and discussion
When the electrode potential was negatively scanned from OCP, small cathodic peaks due to the electrochemical reductive decomposition of unavoidable water, unavoidable dissolved oxygen, and electrolytes were observed around +2.0 V, +1.5 V, and +0.5 V, respectively, in all the solutions, with increases of Δm and ΔR. During pre-SEI formation, Δm and ΔR also increased although no conspicuous current peaks were observed. From IR and XPS, components of the SEI prepared from the three electrolyte solutions were almost same as each other. On the basis of the MPE values during lithium deposition and dissolution, it is concluded that the formation of the pre-SEI layers, which have the appropriate thickness and lithium ion conductivity with a relatively flat surface, is effective in regulating the lithium deposition/dissolution processes.
References
[1] X.-B. Cheng, R. Zhang, C.-Z. Zhao, F. Wei, J.-G. Zhang, Adv. Sci. 3 (2016) 1500213.
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[3] D. Aurbach, M. Moshkovich, Y. Cohen, A. Schechter, Langmuir 15 (1999) 2947.
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[5] N. Serizawa, S. Seki, K. Takei, H. Miyashiro, K. Yoshida, K. Ueno, N. Tachikawa, K. Dokko, Y. Katayama, M. Watanabe, T. Miura, J. Electrochem. Soc. 160 (2013) A1529.
[6] K. S. Smaran, S. Shibata, A. Omachi, A. Ohama, E. Tomizawa, T. Kondo, J. Phys. Chem. Lett. 8 (2017) 5203.