Silicon as a negative electrode material
for lithium-ion batteries
has attracted tremendous attention due to its high theoretical capacity,
and fluoroethylene carbonate (FEC) was used as an electrolyte additive,
which significantly improved the cyclability of silicon-based electrodes
in this study. The decomposition of the FEC additive was investigated
by synchrotron-based X-ray photoelectron spectroscopy (PES) giving
a chemical composition depth-profile. The reduction products of FEC
were found to mainly consist of LiF and −CHF–OCO2-type compounds. Moreover, FEC influenced the lithium hexafluorophosphate
(LiPF6) decomposition reaction and may have suppressed
further salt degradation. The solid electrolyte interphase (SEI) formed
from the decomposition of ethylene carbonate (EC) and diethyl carbonate
(DEC), without the FEC additive present, covered surface voids and
lead to an increase in polarization. However, in the presence of FEC,
which degrades at a higher reduction potential than EC and DEC, instantaneously
a conformal SEI was formed on the silicon electrode. This stable SEI
layer sufficiently limited the emergence of large cracks and preserved
the original surface morphology as well as suppressed the additional
SEI formation from the other solvent. This study highlights the vital
importance of how the chemical composition and morphology of the SEI
influence battery performance.
The nickel-rich layered oxide LiNi0.8Mn0.1Co0.1O2 (NMC811) is a promising future cathode material for lithium-ion batteries in electric vehicles due to its high specific energy density. However, it exhibits fast voltage and capacity fading. In this article, we combine electrochemistry, operando synchrotron X-ray diffraction (XRD), and ex situ solid-state NMR spectroscopy to provide new insights into the structural changes and lithium dynamics of NMC811 during electrochemical charge and discharge, which are essential for a better understanding of its fast degradation. The evolution of the interlayer spacing is tracked by XRD, showing that it gradually increases upon delithiation before collapsing at high state-of-charge (SOC). Importantly, no two-phase O3→O1 transition is observed at high SOC, demonstrating that this cannot be a major cause of degradation. A strong increase of Li dynamics accompanies the increase of the interlayer spacing, which is shown by 7 Li NMR and electrochemical characterization. At high SOC, Li mobility drops considerably, and Li/vacancy ordering can be observed by NMR. A detailed analysis of 7 Li NMR spectra at different SOC is provided, demonstrating how Li NMR can be used to extract information on the dynamics of such challenging paramagnetic samples with several hundred different local Li environments. The insights on the evolution of structure and dynamics of NMC811 will further the understanding of its cycling behavior and contribute to the efforts of mitigating its performance fade.
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