The beneficial role
of lithium bis(trimethylsilyl) phosphate (LiTMSP),
which may act as a novel bifunctional additive for high-voltage LiNi1.5Mn0.5O4 (LNMO)/graphite cells, has
been investigated. LiTMSP is synthesized by heating tris(trimethylsilyl)
phosphate with lithium tert-butoxide. The cycle performance
of LNMO/graphite cells at 45 °C significantly improved upon incorporation
of LiTMSP (0.5 wt %). Nuclear magnetic resonance analysis suggests
that the trimethylsilyl (TMS) group in LiTMSP can react with hydrogen
fluoride (HF), which is generated through the hydrolysis of lithium
hexafluorophosphate (LiPF6) by residual water in an electrolyte
solution or water generated via oxidative electrolyte decomposition
reactions to form TMS fluoride. Inhibition of HF leads to a decrease
in the concentration of transition-metal ion-dissolution (Ni and Mn)
from the LNMO electrode, as determined by inductively coupled plasma
mass spectrometry. In addition, the generation of the superior passivating
surface film derived by LiTMSP on the graphite electrode, suppressing
further electrolyte reductive decomposition as well as deterioration/reformation
caused by migrated transition metal ions, is supported by a combination
of chronoamperometry, X-ray photoelectron spectroscopy, and field-emission
scanning electron microscopy. Furthermore, a LiTMSP-derived surface
film has better lithium ion conductivity with a decrease in resistance
of the graphite electrode, as confirmed by electrochemical impedance
spectroscopy, leading to improvement in the rate performance of LNMO/graphite
cells. The HF-scavenging and film-forming effects of LiTMPS are responsible
for the less polarization of LNMO/graphite cells enabling improved
cycle performance at 45 °C.