Infinitesimal sulfur fusion yields quasi-metallic bulk silicon for stable and fast energy storage

Abstract

A fast-charging battery that supplies maximum energy is a key element for vehicle electrification. High-capacity silicon anodes offer a viable alternative to carbonaceous materials, but they are vulnerable to fracture due to large volumetric changes during charge-discharge cycles. The low ionic and electronic transport across the silicon particles limits the charging rate of batteries. Here, as a three-in-one solution for the above issues, we show that small amounts of sulfur doping (<1 at%) render quasi-metallic silicon microparticles by substitutional doping and increase lithium ion conductivity through the flexible and robust self-supporting channels as demonstrated by microscopy observation and theoretical calculations. Such unusual doping characters are enabled by the simultaneous bottom-up assembly of dopants and silicon at the seed level in molten salts medium. This sulfur-doped silicon anode shows highly stable battery cycling at a fast-charging rate with a high energy density beyond those of a commercial standard anode.

Fig. 1

Structural evolution and characterization of quasi-metallic silicon. a, b Schematic of each stage for (a) QMS and (b) bare Si. c Scanning transmission electron microscopy (STEM) image. d, e Corresponding energy-dispersive X-ray spectroscopy (EDS) maps for (d) Si and (e) sulfur. f EELS spectra for elemental sulfur, bare Si, and QMS (inset: STEM image of the EELS measurement position). g, h High-magnification STEM images of QMS. Scale bars, 1 μm (c–e); 50 nm (f); 5 Å (g, h)

Fig. 2

Metallicity of quasi-metallic silicon. a, b Single-particle I–V plots (a) and bulk (pellet) conductivity results (b) of bare Si, carbon-coated Si(Si@C), QMS(0.1), QMS(0.3), and QMS(0.7) samples. c, e Calculated band structure of QMS samples with different substitution doping concentrations of (c) 0.39%, S₁Si₂₅₅, and (e) 1.59%, S₁Si₆₃. The charge density distribution at these impurity levels (in the insets) is shown in red and blue lines to intuitively illustrate the contribution of Si to the metallic properties. The isosurface of the density is 0.0005 e/ų. d Hall effect measurement results of QMS(0.7)

Fig. 3

Sulfur-fusion-induced channel formation in quasi-metallic silicon. a HR-TEM image of QMS (inset: corresponding fast fourier transform image). b Enlarged TEM image showing column formation between characteristic Si (111) planes. c Intensity profiles of selected areas in (a). d XRD patterns of Si and a series of QMS between 27.6–29.2°. e Sulfur chain structure under applied pressure depending on different channel sizes, as calculated by DFT. f Li-ion diffusion coefficient versus the state of charge (SOC) during the first cycle. Scale bars, 2 nm (a); 1 nm (b)

Fig. 4

Lithium sulfide formation inside quasi-metallic silicon. a–f TEM images, and corresponding selected area diffraction patterns for (a, d) pristine, (b, e) fully lithiated, and (c, f) fully delithiated states of QMS particle. g, h HR-TEM image and corresponding elemental maps for Si and S (g) and EELS spectrum (h) of QMS after cycles and SEI elimination. Scale bars, 500 nm (a, c, e); 2 1/nm (b, d, f); 5 nm (g)

Fig. 5

Li-ion storage properties of quasi-metallic silicon and Si electrodes. a First charge–discharge profiles of QMS(0.7) and Si electrodes at a C-rate of 0.05 C. b Charge areal capacity retention of both electrodes at 0.5 C with the corresponding Coulombic efficiency. c Specific capacity plots for both electrodes at different C-rates from 0.2 C to 5 C. d Electrode expansion ratio during 300 cycles for both electrodes. e Full-cell cycle retention at 1 C (3.3 mA cm⁻²) along with the coulombic efficiency