Diphenyl Diselenide as SEI-forming Additive for a High-voltage LiCoO₂/Graphite Battery

In this study, we investigate the effect of diphenyl diselenide (DPDS) as a bifunctional additive on LiCoO₂/graphite batteries charged to 4.4 V. In the 3.0–4.4 V potential window, a LiCoO₂/graphite full cell suffers from poor cycle performance, with a capacity retention of 88.7% after 200 cycles. With the addition of 0.1 wt% DPDS, the capacity retention is increased to 95.2% after 200 cycles. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments indicate that DPDS is oxidized and reduced prior to the decomposition of the electrolyte. Density functional theory (DFT) calculations theoretically show that DPDS has higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO) energy levels than the electrolyte. Field-emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses demonstrate that DPDS is decomposed at the LiCoO₂ and graphite surface and modifies the properties of the SEI layer. As a result, the improved battery performance enabled by diphenyl diselenide can be attributed to the SEI layers preventing collapse of the LiCoO₂ crystal on the cathode and decreasing the reactions of graphite with the electrolyte on the anode.