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

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 Si) 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 Si NMR analysis revealing that they comprise small Si domains that are surrounded by amorphous SiO, the domain size and crystallinity growing with heat treatment. In and ex situ Li and Si solid-state NMR combined with detailed electrochemical analysis reveals that a characteristic metallic Li Si 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 LiSi, 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 SiO matrix are present, a phase resembling LiSi forms, albeit at a higher overpotential. The continuous formation/decomposition of amorphous Li Si phases without the hysteresis and phase change associated with the formation of c-LiSi, along with a partially electrochemically active SiO/lithium silicate buffer layer, are paramount for the good cyclability of a-SiO.