Silicon Nanoparticles with a Polymer-Derived Carbon Shell for Improved Lithium-Ion Batteries: Investigation into Volume Expansion, Gas Evolution, and Particle Fracture

Silicon (Si) and composites thereof, preferably with carbon (C), show favorable lithium (Li) storage properties at low potential, and thus hold promise for application as anode active materials in the energy storage area. However, the high theoretical specific capacity of Si afforded by the alloying reaction with Li involves many challenges. In this article, we report the preparation of small-size Si particles with a turbostratic carbon shell from a polymer precoated powder material. Galvanostatic charge/discharge experiments conducted on electrodes with practical loadings resulted in much improved capacity retention and kinetics for the Si/C composite particles compared to physical mixtures of pristine Si particles and carbon black, emphasizing the positive effect that the core–shell-type morphology has on the cycling performance. Using in situ differential electrochemical mass spectrometry, pressure, and acoustic emission measurements, we gain insights into the gassing behavior, the bulk volume expansion, and the mechanical degradation of the Si/C composite-containing electrodes. Taken together, our research data demonstrate that some of the problems of high-content Si anodes can be mitigated by carbon coating. Nonetheless, continuous electrolyte decomposition, particle fracture, and electrode restructuring due to the large volume changes during battery operation (here, ∼170% in the voltage range of 600–30 mV vs Li+/Li) remain as serious hurdles toward practical implementation.

where λ l is the wavelength of the excitation laser (here, 532 nm) and D and G represent the integrated intensities (areas) of the D and G bands, respectively. The Raman spectra were fitted (green curve), and the contributions of the D (blue curve) and G bands (dark yellow) were determined individually.   Figure S4. Analysis of the rate performance of half-cells with FEC-based electrolyte using pristine Si particles (black) or Si/C composite particles (orange). Note that specific delithiation capacities at rates of 1C (gray), 3C (white), and C/10 (pink) are shown. Figure S5. Specific lithiation capacities of pristine Si particles (black) and Si/C composite particles after heating at 700 (orange), 800 (red), and 900 °C (dark red). The areal loading was 1.2-1.3 mgSi cm -2 .

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For in situ pressure measurements, ("zero-strain") LTO was used as the counter electrode. 2 Prelithiation of LTO was done at a rate of C/10, with a CV step at 1.0 V vs. Li + /Li until the current dropped to C/100. The specific lithiation capacity achieved was 170.5 mAh gLTO -1 , which is in good agreement with the theoretical one. The cell using prelithiated LTO cathode and Si/C composite-containing anode was cycled at C/10 in the potential range between 1.6 and 0.9 V. Figure S6. In situ pressure measurement conducted on a cell with LP57 electrolyte using prelithiated LTO and Si/C composite particles as cathode and anode, respectively. The top panel shows the pressure evolution (black), the irreversible contributions due to gassing (red), and the reversible contributions (green) due to volume expansion/contraction of Si during cycling. The cycles used for determination of the volume changes are denoted by dashed gray lines. The bottom panel depicts the cell potential (light blue), the potential of the LTO electrode (dark blue), and the Si electrode potential (orange) at C/10 rate. Figure S7. Specific lithiation (black) and delithiation (gray) capacities and the corresponding Coulombic efficiencies (green) of the cell in Figure S6 used for determination of the volume changes.
S-8 Figure S8. Cross-sectional SEM images of Si/C composite-containing electrodes before (a) and after 20 cycles (b). The thickness of both the Cu current collector and the active electrode layer is indicated. The cycled electrode was washed with 1 mL DMC to remove electrolyte residues prior to imaging. Figure S9. Cross-sectional SEM images of Si/C composite-containing electrodes before (a) and after 20 cycles (b) and EDX maps of O, C, Cu, and Si and O, C, F, Cu, Si, and P for the area denoted by the black and white box, respectively. The cycled electrode was washed with 1 mL DMC to remove electrolyte residues prior to imaging and mapping analysis. Figure S10. DEMS measurement of a half-cell with LP57 electrolyte using the Si/C composite particles. Note that only the 2 nd and 3 rd cycles at C/10 are shown. The cell potential (light blue) is correlated to the evolution rates of H2 (yellow), C2H4 (green), and CO2 (red). Maxima in the H2 and C2H4 evolution rates are denoted by vertical lines.
S-10 Figure S11. Chronoamperometric measurements on Super C65 carbon/Li cells with LP57 electrolyte analyzed via AE (a) and DEMS (b). The cell potential (light blue) after OCV period was controlled as follows: 500 mV, 400 mV, 300 mV, 200 mV, and 100 mV each for 1 min. The upper potential was set to 1.5 V for 4 min, followed by an OCV period of 5 min between the individual steps. The current response is shown in violet.
(a) Cumulated unfiltered (dark green) and filtered hits (red). Each detected event is denoted by a solid square. (b) Evolution characteristics of H2 (yellow) and C2H4 (green). Figure S12. Cyclic voltammetry and the corresponding AE data of a Super C65 carbon/Li cell using LP57 electrolyte. The potential range was set between 3.0 V and 10 mV (light blue) and the sweep rate was 0.58 V h -1 . The current response (violet) as well as the cumulated filtered hits (red) and AE hit rate (gray) are shown. Figure S13. Temperature profile of the heating process to produce Si/C composite particles. The heating rate was 5 °C min -1 with dwell time of 1 h at 400 °C and 700, 800, or 900 °C. The final material was naturally cooled to room temperature.