Structural and chemical characterization of SnO₂-based nanoparticles as electrode material in Li-ion batteries

Improvement of long-term stability of electrode materials in Li-ion batteries requires a detailed understanding of influence of synthesis parameters on surface chemistry and on properties. Therefore, bare SnO₂ and core/shell nanoparticles with SnO₂ core and a hydrocarbon shell are synthesized in an Ar/20% O₂ microwave plasma, deposited as porous nanoparticle films in situ on heated Ni-substrates, and finally assembled as anodes in Swagelok cells. In a comprehensive study, we investigate structure, particle size, chemistry, morphology, and water content of the nanoparticles using X-ray diffraction, transmission electron microscopy, specific surface area analysis, and coulometric water titration. The thicknesses of the nanoparticle films and their surface chemistry are investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. SnO₂ nanoparticles are crystalline, with a tetragonal cassiterite structure. Primary particle sizes around 3 nm are reached for the bare SnO₂ particles, 5–8 nm for the cores of the core/shell nanoparticles. A minimum microwave power of 900 W is necessary to synthesize SnO₂ nanoparticles without precursor residuals as pristine SnO₂ particles for the subsequent coating step. In the coating step increasing hydrocarbon content can be correlated with increasing carbon-precursor feeding rate. Water uptake, stemming either from the process, or due to atmospheric contamination, can successfully be reduced by a thermal treatment. The still remaining water is a function of specific surface area. Finally, bare SnO₂ versus core/shell nanoparticles are compared regarding the influence of the shell on the electrochemical properties. The principal improved functionality of the developed anodes in Swagelok cells is demonstrated.