Effect of Tributyl Borate on Electrochemical Performance at an Elevated Temperature of High-Voltage LiNi_0.5Mn_1.5O₄ Cathode

Tributyl borate (TBB) is studied as a protective additive that enhances the interfacial stability of a LiNi0.5Mn1.5O4 cathode/electrolyte at 5 V and 55 °C. Upon addition of 0.5 vol % TBB to the electrolyte (1.0 M LiPF6/EC:EMC:DMC, 1:1:1 wt %), the LiNi0.5Mn1.5O4/Li cell maintains a discharge capacity of 99.4 mAh g–1 at the 50th cycle at 55 °C, compared to that of the electrolyte without an additive, i.e., 36.6 mAh g–1. Furthermore, the TBB-enhanced Li/LiNi0.5Mn1.5O4 cell exhibits a higher discharge capacity of 96 mAh g–1 at 3C, whereas the cell without TBB delivers only 84 mAh g–1. Theoretical calculations and differential capacity (dQ/dV) versus voltage (V) analysis show that TBB improves the electrochemical performance at 5 V and 55 °C by preferentially oxidizing on the LiNi0.5Mn1.5O4 surface. The results obtained from electrochemical impedance spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and inductively coupled plasma-mass spectroscopy measurements reveal that TBB generates a thin, less resistive film on the surface of LiNi0.5Mn1.5O4. This TBB-derived film inhibits the decomposition of the carbonate solvent, while suppressing the dissolution of Ni and Mn ions from LiNi0.5Mn1.5O4.