Fluorinated cyclic siloxane additives for high energy density Li-ion batteries with high nickel cathodes and silicon–carbon anodes

It is a big challenge to enhance the interfacial stability and cycling performance of high nickel cathodes and silicon carbon anodes in Li-ion batteries with high energy density. In this work, we propose a new type of fluorinated cyclic siloxane additive, 1,3,5-tris(3,3,3-trifluoropropyl)methylcyclotrisiloxane (3FO), to improve cycling stability of high nickel cathodes (NCM811) and silicon carbon anodes (Si–C). The Li/NCM811 cells with Baseline electrolytes+1 wt% 3FO have a significant capacity retention (63.3%) after 400 cycles at 1C current rate than that with Baseline electrolytes (37.5%). Furthermore, the Li/Si–C cells in the combined electrolyte of 3FO and fluoroethylene carbonate (FEC) can be cycled well, and the capacity retention of Si–C/NCM811 full cells is as high as 93.3% after 800 cycles at 1C/1C rate. Our results show that 3FO can scavenge part of the HF in the electrolyte and stabilize the interface structure. The results of EIS, SEM and TEM show that 3FO can help to generate a stable and uniform CEI film, suppress the increase in impedance and reduce cell polarization. Theoretical calculations, NMR and XPS studies found that 3FO can result in ring-opening reactions during cycling, forming a stable interface film with SiOx and C–F, and improving the electrochemical stability of lithium-ion batteries.