Porous Structuring of Si Microparticles for Li-Ion Battery Anodes by Urea-Assisted Etching

Silicon-based anodes offer substantially higher theoretical capacities than graphite in lithium-ion batteries, but their practical deployment is hindered by severe volume changes that induce mechanical degradation and unstable interfacial chemistry. While nanoscaling strategies can mitigate these effects, they often suffer from low tapped density, complex synthesis, and limited scalability. Porous silicon microparticles provide a promising alternative by partially accommodating volume expansion while preserving processability and electrode-level integrity. Here, a HF-free urea-assisted etching strategy is employed to generate porous silicon microparticles under mild conditions, leveraging the coupled action of thermally induced structural disruption and chemically driven surface modification. Control experiments confirm that the combined action of these effects is essential to achieve BJH-resolved mesoporosity and increased surface area. The resulting porous silicon exhibits oxygen- and nitrogen-containing surface functionalities. Composite electrodes prepared with nanographite and sodium alginate binder at graphite:silicon:binder ratios of 8:1:1, 7:2:1, and 4.5:4.5:1 demonstrate improved electrochemical behavior. In half-cell testing, electrodes containing 10-20 wt % porous silicon deliver stable redox activity and retain 630-880 mAh g^-1 after 100 cycles at 0.1 C, with Coulombic efficiencies of 98.8-99.7%, whereas higher silicon loadings lead to rapid capacity decay. Cycling-resolved impedance and differential-capacity analyses reveal the formation of a thicker yet mechanically resilient interphase that stabilizes charge-transfer kinetics, while rate capability tests show 65-74% capacity retention at 2 C.