Graphite Particle-Size Induced Morphological and Performance Changes of Graphite-Silicon Electrodes

Silicon is a long-standing candidate for replacing graphite as the active material in negative electrodes for Li-ion batteries, due to its significantly higher specific capacity. However, Si suffers from rapid capacity fading, as a result of the large volume expansion upon lithiation. As an alternative to pure Si electrodes, Si could be used, instead, as a capacity-enhancing additive in graphite electrodes. Such graphite-Si blended electrodes exhibit lower irreversible-charge losses during the formation of the passivation layer and maintain a better electronic contact than pure Si electrodes. While previous works have mostly focused on the Si properties and Si content, this study investigates how the choice of graphite matrix can alter the electrode properties. By varying the type of graphite and the Si content (5 or 20 wt%), different electrode morphologies were obtained and their capacity retention upon long-term cycling was studied. Despite unfavorable electrode morphologies, such as large void spaces and poor active-material distribution, certain types of graphites with large particle sizes were found to be competitive with graphite-Si blends, containing smaller graphite particles. In an attempt to mitigate excess void-space and inhomogeneous material distribution, two approaches were examined: densification (calendering) and blending in a fraction of smaller graphite particles. While the former approach led in general to poorer capacity retention, the latter yielded an improved Coulombic efficiency without compromising the cycling performance. (C) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited.