Scalable Ionothermal Synthesis of Porous Silicon for High‑Performance Lithium‑Ion Battery Anodes

Abstract

Silicon offers an extraordinary theoretical capacity of 4200 mAh g⁻¹, making it a prime candidate for next‑generation anodes in lithium‑ion batteries (LIBs). Here we present a facile, high‑yield, and scalable ionothermal route that converts commercial magnesium silicide (Mg₂Si) into nanoporous silicon (pSi) by reacting it with an acidic ionic liquid at 100 °C under ambient pressure. The resulting pSi is highly crystalline, exhibits a BET surface area of 450 m² g⁻¹, and contains uniform micropores of 1.27 nm. When coated with a nitrogen‑doped carbon layer (pSi@NC) and assembled into LIB anodes, the composites show an impressive initial Coulombic efficiency of 72.9 % and deliver 1000 mAh g⁻¹ at 1 A g⁻¹ after 100 cycles. Importantly, the synthesis requires no high‑temperature furnaces or pressurized vessels, making it amenable to large‑scale production.

Introduction

Global reliance on fossil fuels has spurred demand for sustainable, high‑energy‑density storage devices. Lithium‑ion batteries remain the most promising technology, yet the commercial viability of silicon anodes is hampered by two major challenges: (1) a volumetric expansion of up to 300 % during lithiation that induces particle pulverization, and (2) low electronic conductivity (~1.6 × 10^‑3 S m⁻¹). Numerous strategies—nanostructuring, porous composites, and carbon coatings—have been pursued to mitigate these issues, but many synthesis routes are complex, low‑yield, or unsuitable for scale‑up. Here, we introduce a streamlined, ambient‑pressure ionothermal method that yields high‑quality pSi in a single step and demonstrates superior electrochemical performance when paired with a nitrogen‑doped carbon matrix.

Methods

Materials

1‑Butyl‑3‑methylimidazolium chloride ([Bmim]Cl) and aluminum chloride (AlCl₃) were sourced from Shanghai Cheng Jie Chemical Co. Ltd. Commercial Mg₂Si and 1–5 µm silicon powder were purchased from Alfa Aesar. Battery‑grade electrolyte components (EC, DEC, FEC, LiPF₆) were obtained from Shenzhen Kejingstar Technology Ltd. All reagents were used as received.

Synthesis of Porous Silicon Nanoparticles (pSi)

In a glovebox under argon, 1.5 g [Bmim]Cl and 4.5 g AlCl₃ (1:4 molar ratio) were mixed in a Schlenk tube. 500 mg Mg₂Si was added, and the mixture was stirred at 100 °C for 10 h. After cooling, the precipitate was collected and washed sequentially with 1 M HCl, deionized water, and ethanol. The dried product (≈150 mg, 82 % yield) was ready for characterization.

Synthesis of Nitrogen‑Doped Carbon Coated on Porous Silicon (pSi@NC)

0.1 g pSi was dispersed in 250 mL deionized water containing 5 mg sodium dodecylbenzenesulfonate (SDBS) and sonicated for 30 min. The suspension was stirred for 1 h at room temperature, then 200 µL pyrrole monomer, 0.34 g ammonium persulfate, and 1.25 mL 1 M HCl were added. The mixture was kept in an ice bath for 24 h, yielding black pSi@PPy powder. This was filtered, washed, dried, and pyrolyzed at 700 °C (5 °C min⁻¹) under Ar for 3 h to produce pSi@NC. Thermogravimetric analysis determined a carbon content of ~20 wt %.

Electrochemical Measurements

Half‑cells (CR2032) were assembled in an Ar‑filled glovebox. The working electrode comprised 70 wt % pSi@NC, 20 wt % Super P, and 10 wt % sodium alginate, cast on Cu foil and dried at 80 °C. The electrolyte was 1 M LiPF₆ in a 1:1 EC/DEC mixture. Charge–discharge tests were conducted on a Neware battery tester (0.01–1.5 V), and cyclic voltammetry (CV) was performed with a CHI650d workstation (0.2 mV s⁻¹).

Characterization

PXRD (Bruker D8), SEM (Hitachi S‑4800), TEM (JEM‑2100), BET surface analysis (Micromeritics ASAP 2020), Raman spectroscopy (Horiba LabRAM Aramis), XPS (PHI 5000 VersaProbe), and TGA (NETZSCH STA449F3) were employed to assess crystallinity, morphology, porosity, and composition.

Results and Discussion

The ionothermal reaction proceeds via the following overall process: Mg₂Si + 4 AlCl₃ + 4 [Bmim]Cl → Si + 2 MgCl₂ + 4 [Bmim]AlCl₄. PXRD confirmed the crystalline Si phase (JCPDS No. 27‑1402) with an average grain size of ~40 nm. Raman spectra displayed the characteristic Si‑Si stretch at 518 cm⁻¹ and a surface‑oxide shoulder near 303 cm⁻¹. N₂ adsorption revealed a type‑IV isotherm with a hybrid H2(b)/H3 hysteresis, yielding a BET surface area of 450 m² g⁻¹ and a narrow micropore distribution centered at 1.27 nm. SEM/TEM images showed interconnected 20–100 nm particles with a uniform porous network.

Coating with nitrogen‑doped carbon transformed the material into a composite (pSi@NC). PXRD showed an amorphous carbon peak (~23°), while Raman D/G intensity ratio (I_D/I_G ≈ 1.07) indicated low graphitization. XPS revealed N–C bonds (285.85 eV) and nitrogen species (pyridinic, pyrrolic, graphitic) at 397.85, 398.72, and 400.57 eV, confirming successful doping.

Electrochemical testing displayed a stable first discharge plateau at ~0.1 V (Li_xSi formation) and a Coulombic efficiency of 72.9 %. After 100 cycles at 1 A g⁻¹, the reversible capacity remained 1000 mAh g⁻¹, with a capacity retention of 79 % after 110 cycles at 0.1 A g⁻¹. Rate capability was excellent, delivering 2360, 1690, 1570, 1470, 1320, and 850 mAh g⁻¹ at 0.1, 0.3, 0.5, 1.0, 2.0, and 5.0 A g⁻¹, respectively, and recovering 2160 mAh g⁻¹ when the current was reset to 0.1 A g⁻¹. In contrast, commercial Si@NC electrodes suffered severe capacity fading, underscoring the advantages of the porous architecture and nitrogen‑doped carbon shell.

Conclusions

We have developed a simple, ambient‑pressure ionothermal protocol that converts Mg₂Si into nanoporous silicon with an 82 % yield. When paired with a nitrogen‑doped carbon coating, the resulting pSi@NC composites exhibit high reversible capacity, excellent cycle life, and a robust initial Coulombic efficiency. The mild reaction conditions and high throughput make this approach suitable for large‑scale production of silicon‑based anodes.