Carbon-Coated, Diatomite-Derived Nanosilicon as a High Rate Capable Li-ion Battery Anode

Abstract

Silicon is produced in a variety of ways as an ultra-high capacity lithium-ion battery (LIB) anode material. The traditional carbothermic reduction process required is expensive and energy-intensive; in this work, we use an efficient magnesiothermic reduction to convert the silica-based frustules within diatomaceous earth (diatomite, DE) to nanosilicon (nanoSi) for use as LIB anodes. Polyacrylic acid (PAA) was used as a binder for the DE-based nanoSi anodes for the first time, being attributed for the high silicon utilization under high current densities (up to 4C). The resulting nanoSi exhibited a high BET specific surface area of 162.6 cm² g^-1, compared to a value of 7.3 cm² g^-1 for the original DE. DE contains SiO₂ architectures that make ideal bio-derived templates for nanoscaled silicon. The DE-based nanoSi anodes exhibit good cyclability, with a specific discharge capacity of 1102.1 mAh g^-1 after 50 cycles at a C-rate of C/5 (0.7 A g_Si^-1) and high areal loading (2 mg cm^-2). This work also demonstrates the fist rate capability testing for a DE-based Si anode; C-rates of C/30 - 4C were tested. At 4C (14.3 A g_Si^-1), the anode maintained a specific capacity of 654.3 mAh g^-1 - nearly 2x higher than graphite's theoretical value (372 mAh g^-1).

Figure 1. Schematic illustration of the process of obtaining C-coated, DE-derived, frustule-like nanoSi structures for use as Li-ion anode active material.

Lauro Zavala is credited for the contribution of this artwork.

Figure 2

SEM characterization of 2 distinct types of DE frustule fragments with unique geometries (a,b), lower-magnification SEM of the powder made up of DE (c), the corresponding geometries of nanoSi structures derived from DE frustules (d,e), and lower magnification SEM of the powder made up of DE-derived nanoSi (f).

Figure 3

HRTEM analysis of bare DE-derived nanoSi, including Si crystals of various orientations and the indexed selected area electron diffraction pattern as an inset (a), select nanoSi particles showing the d-spacing of crystalline Si (b), a select larger Si particle with well-distinguished Si core and amorphous surface layer with FFT inset (c) and a similar larger particle analyzed by dark-field EDX mapping showing the Si core and oxide surface layer (d).

Figure 4

Low to high magnification TEM characterization of a hexagonal honeycomb-shaped frustule-like nanoSi structure showing the SiNPs (a–c), XRD spectra of DE, nanoSi, and C-coated nanoSi (d), the EDX spectrum and elemental composition of DE before and after HCl leaching (e), and BET N₂ adsorption isotherms of DE before and after purification, and nanoSi (f).

Figure 5

Electrochemical characterization of DE-derived nanoSi-based electrodes, including charge-discharge cycling performance for 50 cycles at C/5 based on Si (a), C-rate testing for 75 cycles at C-rates from C/30 – 4C (b), voltage profiling of the charge-discharge data at C/5 for cycles 1, 25 and 50 (c), voltage profiling of various C-rates (d), CV for cycles 1–10 (e) and the ESR values for cycles 1–10 based on EIS analysis (f).

Figure 6

EIS analysis of the DE-derived nanoSi electrodes assembled in a Li-ion half cell, including the EEC based on modeled EIS data (a), standard Nyquist plots for 10 cycles including fitted data (b), enlarged semi-circle/high-frequency region of the Nyquist plots (c), SEI resistance, internal impedance and charge transfer resistance data for 10 cycles (d), and Bode plots for 10 cycles including fitted data (e).