Operando Sum-Frequency Generation Detection of Electrolyte Redox Products at Active Si Nanoparticle Li-Ion Battery Interfaces

For the first time on nanoparticle-based Si electrodes, we monitor electrochemical reduction products of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) using interface-sensitive operando spectroelectrochemical sum-frequency generation (SFG). We observe SFG signatures that suggest carbon monoxide (CO) evolution on nano-Si proceeds at distinct lithiation potentials for different electrolyte solvents. EC reduction to yield CO-associated species occurs at potentials associated with silicon’s most highly lithiated state (10 mV), whereas FEC is reduced to CO-associated species at 10 mV and 500 mV (vs Li/Li + ). These results suggest that EC reduction is more sensitive than FEC to the lithiation state, validating previous computational predictions describing the reduction of both solvents. Our results suggest that low molecular weight oligomers that readily diffuse from the interface are formed during cycling, leading to SEI instability and an absence of SFG signal. Only upon prolonged EC reduction at 10 mV do we observe SFG signatures for poly(EC), which we hypothesize are due to the formation of higher molecular weight chains that remain on the electrode surface during SFG acquisition. Potential-dependent FEC reduction to Li 2 CO 3, LiF, and CF-containing moieties evidently induces the predominant stabilizing effects to the interface, irrespective of the lithiation time scale. These results provide new, precise insight on the stability of high-capacity anodes.