In Situ TEM Observation of Lithium Electrodeposition/Stripping Process in Ionic Liquid
Authors: Chen Chih-Yao , Tsuda Tetsuya , Oshima Yoshifumi , Kuwabata Susumu 
Published: February 2020

Lithium metal is a potentially ideal anode material for rechargeable Li-based batteries owing to the high theoretical capacity (3860 mAh g–1
vs. 372 mAh g–1 for conventional graphite anode) and the lowest redox potential (–3.04 V vs. SHE).1 Unfortunately, it is difficult to electrodeposit a smooth Li metal layer. Such non-smooth Li electrodeposit leading to lower striping rate and dendrite-induced short-circuits have hampered the practical application of Li metal anode to the Li battery. In order to shed a little light on obtaining the smooth Li layer, Li electrodeposition has been studied by a range of in situ techniques including optical microscopy,2 SEM,3 and TEM.4Among them, TEM is a powerful tool to visually understand the nanoscale morphology evolution of Li deposit as well as solid electrolyte interphase (SEI). Most previous studies use a miniature liquid cell with two silicon nitride windows to prevent vaporization of the electrolyte and involve complicated microfabrication processes. Consequently, it is very challenging for ordinary electrochemists. In this work, Li electrodeposition/stripping in a non-volatile ionic liquid (IL) was examined by a novel user-friendly TEM holder that we designed for in situ observation of the electrode reaction in a Li-based battery.
The Li-based battery designed on the TEM holder was assembled in an Ar-filled glovebox. A commercial LiCoO2 positive electrode sheet (capacity: 3.0 mAh cm–2) and a nickel grid mesh negative electrode were used for constructing the battery. Equimolar mixture of lithium bis(trifluoromethanesulfonyl)amide (Li[(CF3SO2)2N]) and tetraglyme (G4), which is often called solvate IL, was used as an electrolyte. A suitable amount of the electrolyte-maintained polypropylene separator was hold between the electrodes. All the electrochemical measurements were conducted in the TEM chamber using a potentio/galvanostat.
Prior to the in situ TEM observation, we verified that the electrochemical behavior of the specialized battery with different negative electrodes. If a sufficient amount of electrolyte was maintained in the separator, the battery showed a good electrochemical behavior without exception, but it was pretty tough to see the electrode reaction because the electron beam could not penetrate the thick IL layer. In contrast, as is often the case with an insufficient amount, the battery did not work although very clear TEM images were obtained. We found that the condition that the IL does not exude from the separator after setting the battery on the holder but is maintained enables us to gain clear in situ TEM images. Typical examples are depicted in Figure 1. After applying a bias of –4.2 V (vs. LiCoO2), interestingly the deposited Li had two different morphologies. One is a fiber-like structure, and another is a mossy one. Both structures have already been reported by other microscopic techniques. However the difference in the TEM holder creates a big difference in the image quality. By our approach that do not need silicon nitride windows, we can confirm that the deposits are fully covered with a thin SEI layer (10–20 nm) without difficulty but not the case with conventional methods. Additionally, electron diffraction analysis revealed that the deposit obtained is a single crystalline Li covered with polycrystalline LiF derived from decomposition of the [(CF3SO2)2N]–. Li stripping process was also observed at 0.0 V (vs. LiCoO2). Complete dissolution of the deposits was difficult. This can be explained by the fact that some deposits dissolved from the root, resulting the tip part electrically isolated (known as dead lithium).1In summary, our designed TEM holder for in situ observation is easy access to various useful information on the electrode reaction in the Li-based battery, and it will help deepen understanding of the electrode behavior, e.g., Li metal anode.

Acknowledgement

Part of this research was supported by the Grant-in-Aid for Scientific Research, Grant Numbers 15H03591, 15K13287, and 15H2202 from the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the ALCA-SPRING program, Japan Science and Technology Agency. We express our thanks to Prof. Masayoshi Watanabe (Yokohama National University) and his colleagues. G4-Li[TFSA] solvate IL was provided by them.

References

1. W. Xu, J. L. Wang, F. Ding, X. L. Chen, E. Nasybulin, Y. H. Zhang, and J. G. Zhang, Energy Environ. Sci., 7, 513 (2014).
2. H. Sano, H. Sakaebe, and H. Matsumoto, J. Power Sources, 196, 6663 (2011).
3. M. Dolle, L. Sannier, B. Beaudoin, M. Trentin, and J. M. Tarascon, Electrochem. Solid-State Lett., 5, A286 (2002).
4. R. L. Sacci, N. J. Dudney, K. L. More, L. R. Parent, I. Arslan, N. D. Browning, and R. R. Unocic, Chem. Commun., 50, 2104 (2014).

Figure 1