Quantitative investigation of the cycling behavior and SEI formation of tin through time-resolved microgravimetry

Abstract

Insufficient understanding on the mechanism of electrode failure restricts the development of sensitive electrode materials such as tin-based alloys. Here-in, a non-destructive and time-resolved in-situ characterization method is proposed for the investigation of the electrode failure. The cycling behavior of tin electrode and the evolution of the Solid-Electrolyte Interface (SEI) were investigated via combining quartz crystal micro-gravimetry (QCM) with cyclic voltammetry (CV). Key findings were further confirmed by ex-situ SEM, TEM and XPS. Remarkably, opposite to the general behavior, SEI thickness grows linearly with the electrode's thickness. This is ascribed to the continuous cracking and oxidization of Sn. However, below a critical thickness (Xo), the SEI growth by cracking (Xsn > Xo) transitions to diffusion-controlled growth (Xsn ≤ Xo). Surprisingly, after serious fracture of Sn, the QCM mass spectrometry reveals, the mass of Li2O also cycled reversibly along with Li insertion. Despite the general belief that the SEI is formed before the lithiation, the inorganic SEI mainly composed of Li2O and Li2CO3 unexpectedly forms in the later cycles during the lithiation. This inorganic part of the SEI plays the most important role of stabilizing the SEI. Preventing its formation, by using an unsuitable voltage window, results in a completely porous non-self-limiting SEI along with a porous electrode.