Pentafluorophenyl Isocyanate as an Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium-Ion Full Cells

Due to its high specific and volumetric capacity and relatively low operation potential, silicon (Si) has attracted much attention to be utilized as a high-capacity anode material for lithium-ion batteries (LIBs) with increased energy density. However, the application of Si within commercial LIBs is still hindered by its poor cycling stability related to the huge volume changes of Si upon lithiation/delithiation, followed by continuous electrolyte decomposition and active lithium loss at the anode side. In this work, we present the application of pentafluorophenyl isocyanate (PFPI) as an effective electrolyte additive for lithium-ion full cells, containing a pure, magnetron-sputtered Si anode and a LiNi1/3Mn1/3Co1/3O2 (NMC-111) cathode. The performance of the Si/NMC-111 full cells is significantly improved in terms of capacity retention and Coulombic efficiency by the addition of 2 wt % PFPI to the baseline electrolyte and is compared to the well-known additives vinylene carbonate and fluoroethylene carbonate. Furthermore, it is revealed that the additive is able to reduce the active lithium losses by forming an effective solid–electrolyte interphase (SEI) on the Si anode. X-ray photoelectron spectroscopy investigations unveil that PFPI is a main part of the SEI layer, leading to less active lithium immobilized within the interphase. Overall, our results pave the path for a broad range of different isocyanate compounds, which have not been studied for Si-based anodes in lithium-ion full cells so far. These compounds can be easily adjusted by modifying the chemical structure and/or functional groups incorporated within the molecule, to specifically tailor the SEI layer for Si-based anodes in LIBs.