Self-Driven SiO₂/C Nanocomposites from Cultured Diatom Microalgae for Sustainable Li-Ion Battery Anodes: The Role of Impurities

Nanostructured SiO2 shells from diatom microalgae are a promising feedstock for the production of high-performance SiO2 anodes for next-generation lithium-ion batteries (LIBs), and diatom biomass has been proposed as a carbon source for producing SiO2/C nanocomposites of improved cyclability. A standard approach before implementing diatoms as an anode material involves an acid washing step for removing minor impurities from diatom shells. In this work, we perform the first comprehensive analysis on the effect of minor chemical species present on diatom shells on the electrochemical properties of diatom-SiO2/C anodes. Unwashed and acid-washed single species cultured diatoms containing their original biomass content were subjected to thermal treatments at 600, 700, 800, and 900 °C, and the resulting SiO2/C composites were fully characterized by XRD, BET, TGA, Raman, SEM/EDX, and TEM techniques. The electrochemical performance of the resulting anodes reveals the key role of impurities in improving the cycling properties. While acid-washed SiO2/C composites displayed higher surface area, their electrochemical performance was comparable to non-coated SiO2. On the other hand, unwashed SiO2/C anodes exhibited a specific capacity up to twice that of SiO2. The best-performing SiO2/C anode was the unwashed diatom-SiO2 heat-treated at 800 °C, showing a specific capacity of 661 mAh·g–1 after 100 cycles at a current density of 200 mA·g–1. Results on the beneficial effects of impurities on SiO2/C anodes are crucial for an effective implementation of diatoms in LIB technology.