Morphology Changes of Amorphous-SiO x Thin-Film Negative Electrodes of Different Oxygen Contents

Abstract

The silicon–oxygen binary system (SiO x ) has attracted attention as a negative electrode with a reasonably high energy density and good cyclability. The oxygen-content dependence of the reversible capacity and cycle performance of SiO x electrodes have been reported.1,2 However, effects of oxygen content on morphology changes in SiO x electrodes upon cycling have not been clarified well in spite of their importance in understanding the capacity fading mechanism, and to improve the cycle performance of SiO x electrode. Therefore, we investigated the cycle performance and morphology changes in SiO x films with various oxygen contents. SiO x thin films were prepared by RF magnetron sputtering on oxygen-free Cu foil, in which the deposited masses of Si were constant. The flow rate of Ar gas was fixed at 20 sccm, while that of O2 gas was varied in the range of 0-0.20 sccm to change the oxygen content in SiO x . The RF power and processing pressure were optimized to obtain amorphous SiO x films without any crystalline phases. After deposition, the SiO x film was transferred to an Ar-filled glovebox without exposure to air, and a coin-type half-cell was assembled. Solutions of 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DMC) (3:7 by vol.) with and without 10 wt.% VC were used as electrolytes. O/Si atomic ratio was determined from energy dispersive X-ray spectroscopy (EDX) analysis. The discharge capacity of lightly O-doped SiO x films (x = 0.21 and 0.48) did not change appreciably compared to that of pure-Si film, meaning that almost all Si atoms in the SiO x film contributed to the Li alloying/dealloying reactions. The cyclability of the lightly O-doped SiO x films was improved little. On the other hand, an irreversible fraction of Si in SiO x at high oxygen contents (x = 1.09 and 1.78) increased owing to the formation of an inactive Li4SiO4 phase.3 In return for the decrease of reversible capacity, the capacity retention of the heavily O-doped SiO x films improved owing to the buffer effect of Li4SiO4 matrix against volume changes. Moreover, the capacity retention of the SiO x films was improved substantially by VC addition, especially for the heavily O-doped SiO x films. The morphology changes with cycling in the SiO x films were affected significantly by the oxygen content and presence of VC. For the pure-Si film, a sponge-like porous structure appeared after cycling owing to repeated crack formation and inhomogeneous volume changes, and resulted in massive electrode swelling. For the heavily O-doped SiO x films, the morphology change was suppressed by the volume buffer of Li4SiO4 matrix. We found that a relatively high oxygen content is required to obtain a stable Li4SiO4 phase because SiO x decomposes to LiO2 and Li-Si alloy in the lightly O-doped SiO x . The morphology change was further suppressed by VC addition because the thin and uniform SEI layer derived from VC inhibited the electrolyte decomposition and allowed for uniform Li alloying/dealloying. The volume buffer of Li4SiO4 and the uniform SEI are important to achieve a long-term cycle life of SiO x negative electrode for lithium-ion batteries.