Photoelectron Spectroscopic Evidence for Overlapping Redox Reactions for SnO2 Electrodes in Lithium-Ion Batteries

In-house and synchrotron-based photoelectron spectroscopy (XPS and HAXPES) evidence is presented for an overlap between the conversion and alloying reaction during the cycling of SnO 2 electrodes in lithium-ion batteries (LIBs). This overlap resulted in an incomplete initial reduction of the SnO 2 as well as the inability to regenerate the reduced SnO 2 on the subsequent oxidative scan. The XPS and HAXPES results clearly show that the SnO 2 conversion reaction overlaps with the formation of the lithium tin alloy and that the conversion reaction gives rise to the formation of a passivating Sn layer on the SnO 2 particles. The latter layer renders the conversion reaction incomplete and enables lithium tin alloy to form on the surface of the particles still containing a core of SnO 2 . The results also show that the reoxidation of the lithium tin alloy is incomplete when the formation of tin oxide starts. It is proposed that the rates of the electrochemical reactions and hence the capacity of SnO 2 -based electrodes are limited by the lithium mass transport rate through the formed layers of the reduction and oxidations products. In addition, it is shown that a solid electrolyte interphase (SEI) layer is continuously formed at potentials lower than about 1.2 V Li + /Li during the first scan and that a part of the SEI dissolves on the subsequent oxidative scan. While the SEI was found to contain both organic and inorganic species, the former were mainly located at the SEI surface while the inorganic species were found deeper within the SEI. The results also indicate that the SEI dissolution process predominantly involves the organic SEI components.