Enhanced electrochemical performance of LiNi0.5Mn1.5O4 cathode by application of LiPF2O2 for lithium difluoro(oxalate)borate electrolyte

Spinel LiNi0.5Mn1.5O4(LNMO) cathode with high voltage plateau at around 4.7 V (vs. Li/Li+) and high energy density has attracted great attention. However, its application is limited because of the lack of matched electrolyte. Herein, we demonstrate that lithium difluorophosphate (LiPO2F2, LiDFP) as lithium difluoro(oxalate)borate (LiODFB) electrolyte additive significantly improves the electrochemical performance of high voltage LNMO/Li half cells and LNMO/G full cells at room temperature. Capacity retention of LNMO/Li half cell with 4% LiDFP achieves 89.69% after 200 cycles in comparison with 76.31% of that without LiDFP. Even discharging at 10C, the LNMO/Li half cell with 4% LiDFP still delivers a discharge capacity of 76.25 mAh g−1 and maintains at 75.94% capacity retention after 200 cycles. The electrochemical performance of LNMO/G full cells has a similar improvement. The enhanced electrochemical performance of LNMO can be ascribed to the steady and low-impedance cathode electrolyte interphase (CEI) film formed by priority decomposition of LiDFP. Besides, in order to further reveal the mechanism of film-forming additive, we studied on the changes of CEI film during initial cycles using transmission electron microscopies (TEM), X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectrometer (FT-IR) and electrochemical impedance spectroscopy (EIS) measurements. The results indicate that the LiDFP preferential decomposition product can gradually prevent the contact between the electrolyte and the cathode with the growth of the CEI film, thereby reducing decomposition of the electrolyte. The finally grown CEI film is sufficiently dense to effectively isolate the electrolyte and the cathode, and CEI film formed by LiDFP-containing electrolyte is more conducive to the transmission of Li+,eventually leading to excellent electrochemical performance for the high-voltage LIBs.