Cycling Behavior of Silicon-Containing Graphite Electrodes, Part B: Effect of the Silicon Source

Abstract

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.

The success story of graphite as a lithium-ion anode material – fundamentals, remaining challenges, and recent developments including silicon (oxide) composites

“…In fact, the application of a lower lithiation potential of 5 mV did not result in any signicant improvement despite the increased risk of lithium metal plating, while the complete lithiation also leads to a more severe volume expansion when forming the fully lithiated Li 15 Si 4 phase. 353 Aer an extrapolation of the expected capacity values for higher silicon contents, Schott et al 352 also studied electrodes with a ratio similar to that suggested previously by Yim et al 351 The resulting electrodes containing 10 wt% of silicon were able to compensate for the incomplete utilization of graphite when applying the higher cut-off potential of 50 mV, with silicon contributing with more than 50% to the total specic electrode capacity of ca. 650 mA h g À1 .…”

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“…This deleterious volume change stimulated the development of Si nanomaterials, because Si at the nano scale can withstand swelling without fracture during LiB cycling. This has been demonstrated for a variety of morphologies including Si nanoparticles (SiNP) [ 6 , 7 , 8 , 9 , 10 , 11 ], Si nanowires (SiNW) [ 3 , 12 , 13 , 14 ], Si nanotubes [ 15 , 16 ], and Si porous nanomaterials [ 17 , 18 , 19 ]. However, nanostructuration can be a costly process that needs to be carefully optimized for a dedicated application.…”

“…A critical diameter was also predicted by a mechanical numerical model [ 23 ] at 90 nm for SiNPs and 70 nm for SiNWs. By contrast, several electrochemical studies in LiB investigated the impact of particle size on Si-based anode cycling for small [ 6 , 8 ] and large [ 7 , 8 , 9 ] SiNPs, with the best results at diameters of 20–50 nm. As for SiNWs, some studies have suggested the optimal diameter as being around 30 nm [ 10 , 24 ].…”

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“…As seen in the differential capacity curve, the Si−P1 also shows a small spike in accordance with this phase, as opposed to the amorphous nature of the Si−M1 and Si−P2, but this plateau fades with cycling. The formation of the Li 15 Si 4 phase in some Si particles and not others may arise from differences in the reaction kinetics during lithiation . Si particles can form a Li 15 Si 4 shell as the core slowly continues to lithiate.…”