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Lithium electrode surface morphologies during cycling were measured in LiAsF6‐normalethylene carbonate false(ECfalse)/2‐normalmethyltetrahydrofuran false(2normalMeTHFfalse) and LiAsF6‐2normalMeTHF electrolytes. Particulate and needle‐like Li were observed on the Li electrode after cycling. Although the particulate Li could be stripped during discharge, much of the needle‐like Li remained. It appears that the needle‐like Li tends to become “dead‐Li” and is responsible for the loss of cycling of lithium electrodes.

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The lithium cycling efficiencies of the lithium anode in the ethylene carbonate ͑EC͒-based electrolytes were improved by adding vinylene carbonate ͑VC͒ to the electrolyte. We analyzed the surface films of deposited lithium on a nickel substrate in a VC-containing electrolyte with scanning electron microscopy, Fourier transform infrared spectroscopy, two-dimensional nuclear magnetic resonance, gel permeation chromatography, and X-ray photoelectron spectroscopy. The corresponding surface films comprise various polymeric species including poly-͑vinylene carbonate͒ ͓poly-͑VC͔͒, oligomeric VC, and a ring-opened polymer of VC. Furthermore, the surface film of carbon double bonds (C ϭ C-O) and lithium carboxylate ͑RCOOLi͒ as reduction products of VC were formed on deposited lithium. These structures of the surface film on the lithium anode were similar to those on the graphite anode. At elevated temperatures, the VC-containing electrolyte led to the formation of surface films comprising poly-͑VC͒. The VC-derived polymeric surface film, which exhibited gel-like morphology, could prevent the deleterious reaction which occurs between deposited lithium and the electrolyte, resulting in an enhanced lithium cycling efficiency.