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.

References

[1] M. A. Al-Maghrabi, et al, J. Electrochem. Soc., 160, A1587 (2013).

[2] H. Takezawa, et al, J. Power Sources, 324, 45 (2016).

[3] K. Yasuda, et al., J. Power Sources, 329, 462 (2016).