Tris(hexafluoro-iso-propyl)phosphate as an SEI-Forming Additive on Improving the Electrochemical Performance of the Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂Cathode Material

Highly fluorinated phosphate ester, tri(hexafluoro-iso-propyl)phosphate ((C 3 HF 6 O) 3 PO or HFiP), has been investigated as an additive to improve the cycling performance of lithium-rich cathode material Li[Li 0.2 Mn 0.56 Ni 0.16 Co 0.08 ]O 2 . The capacity retention for the Li[Li 0.2 Mn 0.56 Ni 0.16 Co 0.08 ]O 2 electrode in 1% HFiP-added electrolyte after 130 cycles is 73.3%, which is higher than that for the reference electrolyte (64.5%). The capacity-retention mechanism is investigated via a combination of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). CV results show that HFiP additive decomposes on the surface of cathode material at a relatively lower voltage (∼4.2 V) than that of solvents in the reference electrolyte. EIS, SEM, TEM along with XPS measurements indicate that a more stable SEI layer is formed on the cathode surface with the presence of decomposition products of HFiP additive. Thus, electrodes cycled in HFiP-added electrolyte have much more stable solid electrolyte interface impedance and relatively smaller charge transfer resistance than that cycled in additive free electrolyte. The stabilization of the electrode/electrolyte interface is beneficial to enhance the electrochemical performance of Li[Li 0.2 Mn 0.56 Ni 0.16 Co 0.08 ]O 2 .