XPS study of duplex stainless steel as a possible current collector in a Li-ion battery

Abstract

The surface chemistry and corrosion property of a duplex LDX 2101 steel that had been cycled in a Li-ion battery with a 1M LiPF6 in EC/DMC 1:1 electrolyte was studied. The results are compared to those of steel stored for the same length of time. Cyclic voltammetry was used to sweep the steel between 0V and 5V and the different reduction products were identified with SEM, XRD and XPS. A conversion reaction occurred during the cathodic sweep between 2.0 and 1.5V where chromium and iron oxides were reduced forming Li2O and metal. At 0.5V vs. Li+/Li a Solid Electrolyte Interface (SEI) was irreversibly formed predominantly during the first cycle. During the oxidation sweep the typical stainless steel passive layer of chromium and iron oxides/hydroxides formed at 2.5V vs. Li/Li+. Li2O also decomposed at this potential. Simultaneously metal fluorides are formed. The XPS revealed a thicker SEI containing organic and inorganic species on the cycled electrode than on the stored. The stored sample showed chemical formation of CrF3 on the surface. Depth profiling of the cycled electrode by Ar+ etching showed a thick layer of CrF3 and a thin layer of FeF3. We conclude that the level of corrosion of this duplex steel is acceptable in the 3–4.5V vs. Li+/Li region. However, in a Li-ion battery it is too reactive at low potentials to be considered as a replacement for copper as an anode current collector. We also observe that the PF6− anion from the electrolyte salt plays an important role in the formation of metal fluorides which is a fact generally neglected in the discussion of conversion reactions of metal-oxide anodes for Li-ion batteries. For stainless steel to be considered as current collectors for Li-ion batteries optimisation of alloy compositions need to be made to reduce corrosion occurring during cycling in organic solvents.