Structural and Chemical Changes in Si Nanoparticle-Based Anodes in Lithium-Ion Batteries during the (De)lithiation Processes Studied by In Situ Raman Spectroelectrochemistry

Nanostructured silicon is considered one of the most attractive anode materials for high-energy-density Li-ion batteries (LIBs) because it can provide a high capacity and extended cycle life compared to bulk Si anodes. However, little is known about the electrochemical lithiation mechanism in nanosilicon due to the lack of suitable measurement techniques. In this study, nanostructured anodes based on Si nanoparticles (approximately 6 nm) integrated within a conductive carbon-based matrix are studied by an in situ Raman spectroelectrochemical (SEC) method in modified coin cells in LIBs. Additionally, cyclic voltammetry and galvanostatic charge-discharge cycling are used to determine the stability of the solid electrolyte interphase (SEI) layer and the long-term capacity degradation of the Si nanoparticle-based anodes. The in situ Raman SEC provides unique insight into the crystal lattice changes and degradation/amorphization pathways of the Si nanocrystals and the electrolyte (LiPF₆ in EC/DMC) decomposition during the electrochemical lithiation and delithiation processes. The evolution of the spectral parameters (shift, line width, intensity) of the first-order Raman peak of crystalline Si at 520 cm^-1 is found to be related to the stress buildup in the nanoparticles. This stress originates from the (i) SEI layer formation on the electrode surface within the initial charge/discharge cycle, (ii) the lithiation-induced stress in Si nanoparticles and the native oxide on their surface, and also (iii) the progressive crystalline-to-amorphous Si phase transition. The structural changes in the anodes determined using in situ Raman SEC show good agreement with the results obtained from cyclic voltammetry measurements, revealing a progressive crystalline-to-amorphous Si phase transition and a complex energy storage mechanism in nanostructured silicon anodes in LIBs.