Unraveling the vertical expansion and hysteresis in SiOx/graphite composite electrodes via in-situ dilatometry and cracking origins via X-ray diffraction and scanning electron microscopy

Silicon-containing electrodes offer high capacity but suffer from pronounced volume changes and voltage hysteresis during lithiation and delithiation, which limits energy efficiency and accelerates degradation. In this work, we investigate the role of SiO x in SiO x /graphite composite electrodes with constant active material loading but varied SiO x fractions. Electrodes were cycled at C/20 with in situ dilatometry monitoring thickness evolution, complemented by differential voltage and incremental capacity analysis. We define the hysteresis-induced energy loss E loss as the integrated area of the voltage hysteresis loop. With increasing SiO x content, electrode expansion and E loss both increased mainly linearly, likely dissipated as heat and irreversible structural change. In-situ dilatometry further revealed a lithiation-dependent and heterogeneous expansion dynamic. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis on a representative 25 wt% SiO x electrode elucidated the structural origins at different lithiation states. XRD confirmed reversible graphite staging and a persistent high-angle shift of the (002) peak of graphite, suggesting compression possibly associated with SiO x expansion. SEM revealed crack initiation in the overlapped working potential of SiO x and graphite due to the bi-directional diffusion, propagating along Si-containing particles, which eventually caused a fragmentation into isolated islands. These findings establish clear correlations between SiO x fraction, expansion, and hysteresis-induced energy loss, providing mechanistic insights and practical design guidelines for improving the mechanical stability and the performance of SiO x /graphite composite electrodes. • In-situ dilatometry reveals that SiO x content has an almost linear effect on the thickness change of composite electrodes. • Voltage hysteresis-induced energy loss depends linearly on the SiO x content. • Bidirectional Li⁺ diffusion at overlapping lithiation potentials initiates cracks at SiO x –graphite interfaces. • Highly lithiated Si-containing particles exhibit a homogenous chessboard-like cracking on the surface.