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Lithium metal dendrite growth in Li/poly(ethylene oxide)–lithium bis(trifluoromethanesulfonyl)imide (PEOxLiTFSI) and in an ionic liquid, N -methyl- N -propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) composite solid polymer electrolyte/Li was examined by direct in situ observation. Lithium dendrite formation was observed after 15 h of polarization at 0.5mAcm−2 and at 60°C on PEO18LiTFSI . The onset time was increased to 35 h at 0.5mAcm−2 by the addition of PP13TFSI into PEO18LiTFSI up to 1.44 mol. The dendrite onset time decreased with increasing current density and deviated from Sand’s law in the current density range of 0.1–0.5mAcm−2 at 60°C . The electrical conductivity of PEO18LiTFSI was enhanced, and the interface resistance between Li and PEO18LiTFSI was suppressed by the addition of PP13TFSI. The lithium ion transport number of the composite electrolyte decreased with increasing x in PEO18LiTFSI–xPP13TFSI . The suppression of the lithium dendrite formation could be explained by the low resistance of the interface layer between lithium and the composite polymer electrolyte.

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