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Silicon (Si) is a promising additive for enhancing the specific charge of graphite negative electrodes in Li-ion batteries. However, Si alloying with lithium leads to an extreme volume expansion and in turn to rapid performance decline. Here we present how controlling the lithiation depth affects the performance of graphite/Si electrodes when different lithiation cutoff potentials are applied. The relationship between Si particle size and cutoff potential was investigated to clarify the interdependence of these two parameters and their impact on the performance of Sicontaining graphite electrodes. For Si with a particle size of 30−50 nm, Li 15 Si 4 is only formed for the potential cutoff of 5 mV vs Li + /Li, whereas using a higher cutoff of 50 mV has no impact on the performance. For larger Si nanoparticles, 70−130 nm in size, Li 15 Si 4 is already formed at 50 mV. However, in these larger particles only 70% of the Si initially participates in the lithiation, independent of the cutoff potential (5 or 50 mV), and the performance fades rapidly. For the highest tested cutoff potential of 120 mV, the contribution of larger Si particles to the specific charge of the electrodes was negligible, but for the smaller particles a stable and still significant Si specific charge was obtained.

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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.

Silicon (Si) is a promising candidate to enhance the specific charge of graphite electrode, but there is no consensus in the literature on its cycling mechanism. Our aim in this study was to understand Si electrochemical behavior in commercially viable graphite/Si electrodes. From the comparison of three types of commercial Si particles with a producer-declared particle sizes of 30–50 nm, 70–130, and 100 nm, respectively, we identified the presence of micrometric Si agglomerates and the Si micro- and mesoporosity as the main physical properties affecting the cycling performance. Moreover, ex situ SEM, XRD, and Raman investigations allowed us to understand the lithiation/delithiation mechanism for each type of Si particles. For nanoscale Si particles, the entire Si particle is utilized, resulting in high specific charge, and the stress induced by the formation of Li15Si4 alloy upon deep lithiation is well managed within the Si mesoporosity. This leads to reversible cycling behavior and, thus, to good cycling stability. On the other hand, micrometric Si aggregates undergo a two-phase lithiation mechanism with early Li15Si4 formation in the particle shell. This leads to stress-induced core disconnection during the first lithiation, and shell pulverization during the following delithiation, resulting in overall low specific charge and rapid performance fading.