Solid electrolyte interphase formation on metallic lithium

Andrzej Lewandowski,Agnieszka Swiderska-Mocek,Lukasz Waliszewski

doi:10.1007/s10008-012-1786-4

Andrzej Lewandowski, Agnieszka Swiderska-Mocek + Show 1 more

Open Access

https://doi.org/10.1007/s10008-012-1786-4

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Abstract

Solutions of three salts (LiBF4, LiNTf2, LiPF6) in N-methyl-2-pyrrolidone (NMP), selected arbitrarily as a reference solvent, were investigated by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy techniques. The lithium surface in contact with LiPF6 in NMP electrolyte was covered with a protective layer (SEI) which morphology comprise small particles (of ca. 0.2 μm in radius). This salt was selected for further studies. The impedance of the Li|(LiPF6 in NMP + additive)|Li system was measured immediately after cell assembly and after galvanostatic charging/discharging. Fifteen different additives (10 wt.%) were used. The efficiency of individual additives was evaluated in terms of the Li|electrolyte system resistance (ΔR) or total cell impedance reduction, both deduced from EIS. Some of the additives were able to form the SEI layer and to reduce resistance/impedance of the Li|electrolyte interphase. In such cases, the lithium surface was covered with relatively uniform conglomerates, or regions separated by cracks, of ca. 1–2 μm in dimension.

Highlights

Both lithium-metal and lithiated graphite anodes applied in lithium or Li-ion batteries react with electrolytes with the formation of a passivation layer, usually called the solid electrolyte interphase (SEI), protecting them against further
Electrolyte was obtained by dissolution of the solid LiPF6 salt in N-methyl-2-pyrrolidone (NMP, Fluka) (1 M solution of LiPF6 salt in NMP)
In the case of all salts, the impedance of the system increased after the electrochemical SEI formation

Summary

Introduction

Lithium foil (0.75 mm thick, Aldrich), lithium hexafluorophosphate (LiPF6, battery grade >99 %, Fluka), vinylene carbonate (VC, >97 %, Aldrich), vinyl ethylene carbonate (VEC, >99 %, Aldrich), ethylene carbonate (EC, anhydrous 99 %, Aldrich), propylene carbonate (PC, anhydrous 99,7 %, Aldrich), dimethyl carbonate (DMC, anhydrous >99 %, Aldrich), diethyl carbonate (DEC, anhydrous >99 %, Aldrich), ethylene sulphite (ES, 98 %, Aldrich), vinyl acetate (Vac, >99 %, Fluka), gamma-butyrolactone (γ-BL, >99 %, Aldrich), triphenyl phosphate (TPhPh, Aldrich), phenyl isocyanate (PhIsCy, >99 %, Fluka), methyl cinnamate (MCin, >99 %, Aldrich), dimethyl sulfoxide (DMSO, Merck), and styrene (>99 %, Aldrich) were used as received. Electrolytes were prepared in a dry argon atmosphere in a glove box. They contained various additives (VC, VEC, EC, PC, DMC, DEC, ES, Vac, γ-BL, TPhPh, PhIsCy, MCin, DMSO, styrene) at 10 wt.%

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