<p>This study investigates how particle size and structural phase affect the electrochemical performance of silicon nanoparticle anodes in lithium-ion batteries. Si particles ranging from 10 to 100&#xa0;nm were synthesized via a radio-frequency thermal plasma process with adjustments to control size and crystallinity. Structural analysis confirmed that smaller particles were mostly amorphous, whereas larger particles retained crystalline domains. Electrochemical tests indicate that 100-nm particles exhibit the highest initial charge capacity and Coulombic efficiency, whereas 10-nm particles show the lowest. <i>Ex-situ</i> X-ray diffraction analysis revealed a strong correlation between particle size and structural phase: Amorphous-rich particles formed Li₁₂Si₇, whereas crystalline-rich particles formed Li₁₅Si₄. The 50-nm particles formed Li₁₂Si₇ and exhibited improved cycling performance than the 100-nm particles. These results highlight the importance of particle size and phase composition optimization for a balance between the capacity, efficiency, and durability of Si-based anodes.</p> Graphical abstract <p></p>