Morphological Changes of Silicon Nanoparticles and the Influence of Cutoff Potentials in Silicon-Graphite Electrodes

Abstract

Silicon-graphite electrodes usually exhibit improved cycling stability when limiting the capacity exchanged by the silicon particles per cycle. Yet, the influence of the upper and the lower cutoff potential was repeatedly shown to differ significantly. In the present study, we address this discrepancy by investigating two distinct degradation phenomena occurring in silicon-graphite electrodes, namely (i) the roughening of the silicon particles upon repeated (de-)lithiation which leads to increased irreversible capacity losses, and (ii) the decay in the reversible capacity which mainly originates from increased electronic interparticle resistances between the silicon particles. First, we investigate the cycling stability and polarization of the silicon-graphite electrodes in dependence on different cutoff potentials using pseudo full-cells with capacitively oversized LiFePO 4 cathodes. Further, we characterize postmortem the morphological changes of the silicon nanoparticles by means of scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) as a function of the cycle number. To evaluate the degradation of the entire electrode coating, we finally complement our investigation by impedance spectroscopy (EIS) with a gold-wire micro-reference electrode and post-mortem analyses of the electrode structure and coating thickness by cross-sectional SEM. Silicon is among the most promising anode materials for future lithium-ion batteries.1,2 For example, a prismatic hard case cell comprising a silicon-carbon anode with 1000 mAh gand an NMC811 cathode would offer a specific energy of up to ∼280 Wh kgcell . 3 In contrast to state-of-the-art graphite electrodes, where lithium is inserted into the interlayers between the graphene sheets, silicon reacts with lithium and forms Li x Si alloys.4-6 Because the (de-)alloying reaction allows a higher lithium uptake per silicon atom (3579 mAh g −1Si , Li 15 Si 4 ) compared to the intercalation of lithium into the graphite host structure (372 mAh g −1 C , LiC 6 ), silicon offers an about ∼10 times larger theoretical specific capacity. However, while the intercalation chemistry reveals excellent cycling stability with only minor irreversible changes of the graphite's morphology (ca. +10%), 8 the (de-)alloying reaction causes significant morphological and chemical changes to the silicon particles, including (i) a large volume expansion of up to +280% and (ii) repeated breakage and formation of Si-Si bonds, which leads to severe mechanical stress and particle fracturing. [9][10][11][12] Upon continued cycling, these morphological changes cause a rapid capacity decay of silicon-based electrodes, which is largely driven by the electrical isolation of the fractured silicon particles.13-17 Nanometer-sized structures, including nanoparticles and nanowires, were shown to mitigate the mechanical stress which results from volumetric changes during the (de-)alloying reaction.12,18-20 However, there exists a trade-off, because the reduction of the particle size als...

Integration of Graphite and Silicon Anodes for the Commercialization of High‐Energy Lithium‐Ion Batteries

“…The repeated cycling after a large volume change seriously damages the material, and proceeds until the morphology reaches the steady state. For example, the Si nanoparticles transform into nanoporous structure with a very large surface area as a result of repeated cycling, which coincides with the large volume expansion of about 700 % (Figure ) . This morphological change leads to rapid electrode swelling at the initial cycling, with rearrangement of the electrode components.…”

“… Changes in the morphology of Si particles observed by scanning transmission electron microscopy (STEM) during cycling . Copyright: The Electrochemical Society, 2018.…”

The critical role of carbon in marrying silicon and graphite anodes for high‐energy lithium‐ion batteries

“…The repeated volume changes of Si during cycling process can seriously change the morphology of Si/G composite. It has been reported that the Si nanoparticles turn into nanoporous structure with very large surface area as a result of repeated cycling, which leads to electrode swelling with rearrangement of the electrode components, and thus disrupts the electrode . The huge increase in the surface area due to the change into the nanoporous structure provides abundant sites for the electrolyte decomposition.…”

Differentiating the Degradation Phenomena in Silicon-Graphite Electrodes for Lithium-Ion Batteries

“…48 In contrast to the silicon particle degradation mechanism, the electrode degradation mechanism is highly dependent on the electrode composition, with an observed earlier (in terms of cycle number) and more severe decay of the reversible capacity with increasing silicon ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address.…”