Solid electrolyte interphase (SEI) electrode. Part III. Deposition-dissolution process of lithium in thionyl-chloride solution

The deposition-dissolution mechanism of lithium on stainless steel and calcium electrodes in 1 M LiAlCl₄ -thionyl-chloride solution is studied by pulse galvanostatic and ac techniques. The metal -solution interfacial capacitance of the stainless steel electrode is about 30 μF cm^-2 which is higher by an order of magnitude than the capacitance of lithium-coated stainless steel and either pure or lithium-coated calcium. The lower capacitance is attributed to the existence of a solid electrolyte interphase (SEI) on the coated stainless steel or the calcium electrode. Significantly different is observed upon deposition of lithium on stainless steel or calcium. Deposition on stainless steel takes place only after prior formation of a SEI on the electrode (by passage of about 20 mC cm^-2), while deposition on calcium starts immediately after the electrode capacitance has been charged (by about 5 μC cm^-2). Furthermore, deposition of about 3% of a monolayer of lithium on calcium is enough to stabilize its potential at 0.0 V vs. LiRE. On the lithium-coated stainless steel electrode, a linear relationship between the current and over-potential is observed for up to 700 mV. This indicates a Tafel slope > V. During lithium deposition on stainless steel, the SEI resistivity is about 1.5 × 10⁷ Ω-cm and its thickness is about 10 nm. Under open circuit potential, the deposited lithium corrodes at an apparent rate of 100 μA cm^-2. Rapid fluctuations of the electrode potential during the corrosion or dissolution process are accounted for by a break and repair mechanism of metallic contact between lithium deposited within the SEI and the current collector.