Improvement of Electrode/ Electrolyte Interfaces in Graphite/LiNi0.5Mn1.5O4 Batteries at High Voltage with Lithium Trimethyl Alkyl Borates As Electrolyte Additives

Lithium trimethyl alkyl and aryl borates (LTMB) has been prepared and investigated as a novel cathode film forming additive to improve the performance of LiNi_0.5Mn_1.5O₄ cathodes cycled to high potential (4.8 V).  Addition of LTMB to 1.2 M LiPF₆ in EC/EMC (3/7) improves the capacity retention of graphite/LiNi_0.5Mn_1.5O₄ cells cycled at 55 ^oC.  The added LTMB is sacrificially oxidized on the surface of the cathode during the first charging cycle.  The LTMBs can be used as a functional group delivery agent via reaction of trimethyl borate with of the appropriate lithium alkoxide.  Over 40 different LTMBs have been prepared and investigated.  Ex-situ surface analysis of the LiNi_0.5Mn_1.5O₄ by Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR), Transmission Electron Microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) reveals inhibition of electrolyte oxidation on the cathode surface and the presence of a borate based passivating layer.  The novel cathode electrolyte interface (CEI) leads to the performance improvements.