Lithium Dendrite Formation at the Interface Between Lithium Metal and PEO Complexed with Various Lithium Salts

A water-stable lithium electrode (WSLE) based on Li_1+x+y(Ti,Ge)_1-xAl_xP_3-ySi_yO₁₂(LTAP) requires a lithium conducting interlayer between lithium metal and LTAP. Polyethylene oxide (PEO) based polymer electrolytes are the best candidate as the interlayer, because they are stable in contact with lithium metal and large-sized sheets can be easily fabricated. The interface resistance between lithium metal and polymer electrolytes in the WSLE was found to exhibit a major part of the electrode resistance [1], which can be suppressed by addition of ionic liquids, nanofillers and oligomer ethers into PEO. Further, the passivation film may play an important role for the lithium dendrite formation [2], especially the anion of lithium salts is expected to participate in forming the surface sublayer adjacent to lithium metal, whose role for the lithium dendrite formation is yet unveiled. 

    In this study, the polymer electrolytes of PEO₁₈LiX with different lithium salts of LiN(FSO₂)₂ (LiFSI), LiN(CF₃SO₂)₂ (LiTFSI), LiN(C₂F₅SO₂)₂ (LiBETI), and LiC(C₂F₅SO₂)₃ (LiTFSM) were prepared, and the effects of lithium salt in PEO on the interface resistance and the lithium dendrite formation have been examined. Electrochemical impedance spectroscopy measurements were performed in the symmetrical lithium pouch cells of Li/PEO₁₈LiX/Li as a function of storage time at 60 ^oC. The passivation film resistances (R_p) for PEO₁₈LiTFSM and PEO₁₈LiTFSI increased from 75 to 135 Ω·cm² and 92 to 204 Ω·cm² after 148 h, respectively. On the other hand, R_p for PEO₁₈LiBETI and PEO₁₈LiFSI decreased from 181 to 143 Ω·cm² and 372 to 278 Ω·cm², respectively after 148 h. The capacitance of the passivation film was in a descending order of PEO₁₈LiTFSM, PEO₁₈LiTFSI, PEO₁₈LiBETI and PEO₁₈LiFSI. Activation energies for the total interfacial resistance were calculated as 84.4 kJ·mol^-1 for PEO₁₈LiFSI, 84.1 kJ·mol^-1 for PEO₁₈LiTFSI, 85.8 kJ·mol^-1 for PEO₁₈LiBETI, and 90.9 kJ·mol^-1 for PEO₁₈LiTFSM. These results imply that FSI^-could take part in the generation of a dense resistive passivation film.

    The short-circuit time (t_s) for the Li/PEO₁₈LiX/Li cell was examined at 0.1 mA·cm^-2 and   60 ^oC, where the thickness of PEO₁₈LiX was ca. 100 μm. Figure 1 shows the potential change with the polarization period for the Li/PEO₁₈LiX/Li cell at 60 ^oC. The longest t_s of 145 h was observed for the Li/PEO₁₈LiFSI/Li cell with over-potential of 55 mV. t_s for the Li/PEO₁₈LiTFSI/Li cell was 63.7 h and the over-potential 62 mV. However, cells of Li/PEO₁₈LiTFSM/Li and Li/PEO₁₈LiBETI/Li were shorted at 3.2 and 8.7 h and the over-potentials were 32 and 40 mV, respectively. The cycling performance measurements were carried out for the Li/PEO₁₈LiTFSI/Li and Li/PEO₁₈LiFSI/Li cells at 0.1 mA·cm^-2 and 60 ^oC, where the current was passed for 5 h in each half cycle. The total charge passed before short-circuit was calculated as 19.8 C·cm^-2 for PEO₁₈LiTFSI and 70.2 C·cm^-2 for PEO₁₈LiFSI. These results suggest that FSI^-is a promising anion for suppressing lithium dendrite formation and enhancing the flexibility of the passivation film. Detailed study of the composition of the passivation film on the lithium metal is under investigation by in-situ FTIR, ex-situ XPS and SEM. We attempt to correlate lithium dendrite formation with anions of novel lithium salts for the development of WSLEs.

References

[1] T. Zhang, N. Imanishi, O. Yamamoto, et al., J. Electrochem. Soc., 155, A965 (2008).

[2] H. Wang, O. Yamamoto, N. Imanishi, et al., J. Electrochem. Soc., 160, A728 (2013).