Effect Of LiODFB-TFPC/DMC Electrolyte On The Electrochemical Properties Of Lithium Ion Batteries

High-voltage spinel LiNi0.5Mn1.5O4 is a promising candidate for a next generation of lithium ion battery cathode material due to its high operating voltage (4.7 V vs Li/Li+), high energy density, excellent cycling stability, and environmental friendliness. Graphite anode materials are widely used in commercial lithium ion batteries because of their extensive merits, including low operating potential, good cycling performance, and low cost. Notably, the electrochemical performance of graphite anode is also affected by the components of electrolytes. It is well-known that LiPF6-based carbonate electrolyte can exhibit good electrochemical performance when utilizing graphite as anode material. However, the compatibility between LiPF6-based carbonate elctrolyte and LiNi0.5Mn1.5O4 cathode is poor due to its low oxidation potential and bad thermal stability of LiPF6. Under high temperature, LiPF6 salt can easily decompose to produce hydrogen fluoride (HF) to corrode LiNi0.5Mn1.5O4 cathode materials. Therefore, it is necessary to find a novel electrolyte that shows excellent compatibility with LiNi0.5Mn1.5O4 cathode and graphite anode.A novel electrolyte consisted of trifluoropropylene carbonate (TFPC)-dimethyl carbonate (DMC) as the solvent and non-traditional lithium difluoro(oxalato)borate (LiODFB) as the lithium salt was prepared. The effects of 1.0 mol·L-1 LiODFB-TFPC/DMC on the electrochemical performance and surface morphology of LiNi0.5Mn1.5O4 cathode and graphite anode were systematically studied by cyclic voltammetry (CV), galvanostatic charge-discharge, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM).First, the electrochemical performance of LiNi0.5Mn1.5O4/Li half cell was investigated with the 1.0 mol·L-1 LiODFB-TFPC/DMC electrolyte. The results showed that the 1.0 mol·L-1 LiODFB-TFPC/DMC electrolyte exhibited good compatibility with LiNi0.5Mn1.5O4 cathode for the reason that TFPC enhanced the oxidative stability of electrolyte system and LiODFB had no erosion to LiNi0.5Mn1.5O4 cathode. The decomposition products of LiODFB could form a low resistance solid electrolyte interface (SEI) layer on the surface of electrode materials. Therefore, the cyclic stability, rate capability, and · self-discharge performance of LiNi0.5Mn1.5O4/Li cells were significantly improved by using this electrolyte.Second, the effect of the 1.0 mol·L-1 LiODFB-TFPC/DMC electrolyte system on the performance of Li/graphite half cells was studied. Results showed that LiODFB was reduced on the graphite electrode at around 1.5 V in the first cycle, which resulted in the formation of a preliminary SEI film. The existence of this preliminary SEI film reduced the active site of the graphite electrode and suppressed the decomposition of electrolyte. Therefore, a compact and low resistance SEI film formed on the graphite electrode. The cycle stability and rate performance of Li/graphite half-cell were enhanced.Finally, the impact of the 1.0 mol·L-1 LiODFB-TFPC/DMC electrolyte system on the performance of LiNi0.5Mn1.5O4/graphite full cells was investigated. The results showed that the discharge capacity, cycling performance, coulombic efficiency, and mean voltage of LiNi0.5Mn1.5O4/graphite full cells were all improved by using the 1.0 mol·L-1 LiODFB-TFPC/DMC electrolyte.