Unraveling the Reaction Mechanisms of SiO Anodes for Li-Ion Batteries by Combining <i>in Situ</i> ⁷Li and <i>ex Situ</i> ⁷Li/²⁹Si Solid-State NMR Spectroscopy

Abstract

Silicon monoxide is a promising alternative anode material due to its much higher capacity than graphite, and improved cyclability over other Si anodes. An in-depth analysis of the lithium silicide (Li _xSi) phases that form during lithiation/delithiation of SiO is presented here and the results are compared with pure-Si anodes. A series of anode materials is first prepared by heating amorphous silicon monoxide (a-SiO) at different temperatures, X-ray diffraction and ²⁹Si NMR analysis revealing that they comprise small Si domains that are surrounded by amorphous SiO₂, the domain size and crystallinity growing with heat treatment. In and ex situ ⁷Li and ²⁹Si solid-state NMR combined with detailed electrochemical analysis reveals that a characteristic metallic Li _xSi phase is formed on lithiating a-SiO with a relatively high Li concentration of x = 3.4-3.5, which is formed/decomposed through a continuous structural evolution involving amorphous phases differing in their degree of Si-Si connectivity. This structural evolution differs from that of pure-Si electrodes where the end member, crystalline Li₁₅Si₄, is formed/decomposed through a two-phase reaction. The reaction pathway of SiO depends, however, on the size of the ordered Si domains within the pristine material. When crystalline domains of >3 nm within a SiO₂ matrix are present, a phase resembling Li₁₅Si₄ forms, albeit at a higher overpotential. The continuous formation/decomposition of amorphous Li _xSi phases without the hysteresis and phase change associated with the formation of c-Li₁₅Si₄, along with a partially electrochemically active SiO₂/lithium silicate buffer layer, are paramount for the good cyclability of a-SiO.