THESIS
2015
xiv, 102 pages : illustrations ; 30 cm
Abstract
The high-voltage LiNi
0.5Mn
1.5O
4 spinel cathode is becoming more attractive for lithium-ion
batteries because it offers fast, 3-dimesional Li-ion diffusion with high power density. However,
its commercialization is hampered by capacity fade due to the aggressive reaction between the
cathode and the electrolyte with dissolution of the active material observed under high-voltage
operating conditions.
In this study, a Li-ion conductor, Li
3VO
4, was coated onto the micron-sized LiNi
0.5Mn
1.5O
4
powders via wet chemistry followed by a low temperature thermal-treatment.
The experimental results showed that Li
3VO
4-coated LiNi
0.5Mn
1.5O
4 cathode material exhibits
much better cycling stability and rate capability than pristine LiNi
0.5Mn
1.5O
4 at both room
temperature (25°C) and high temperatu...[
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The high-voltage LiNi
0.5Mn
1.5O
4 spinel cathode is becoming more attractive for lithium-ion
batteries because it offers fast, 3-dimesional Li-ion diffusion with high power density. However,
its commercialization is hampered by capacity fade due to the aggressive reaction between the
cathode and the electrolyte with dissolution of the active material observed under high-voltage
operating conditions.
In this study, a Li-ion conductor, Li
3VO
4, was coated onto the micron-sized LiNi
0.5Mn
1.5O
4
powders via wet chemistry followed by a low temperature thermal-treatment.
The experimental results showed that Li
3VO
4-coated LiNi
0.5Mn
1.5O
4 cathode material exhibits
much better cycling stability and rate capability than pristine LiNi
0.5Mn
1.5O
4 at both room
temperature (25°C) and high temperature (60°C). The sample coated with 2 wt. % Li
3VO
4
shows the optimum electrochemical performance with a capacity of 124.6 mAh/g at 20C (2940 mA/g) at room temperature, compared to that of 112.7 mAh/g for the pristine sample. The
capacity retention at 1C (147 mA/g) and 60°C is 90.5% at cut-off voltage of 4.9V, while the
pristine LiNi
0.5Mn
1.5O
4 retains only 83.9% of its initial capacity. XRD (X-ray diffraction) and
XPS (X-ray photoelectron spectroscopy) results reveal that the Li
3VO
4 coating layer reinforced
the surface of matrix material, which benefited structural stability of LiNi
0.5Mn
1.5O
4 during
long-term cycling. CV (cyclic voltammetry) and EIS (electrochemical impedance spectroscopy)
tests indicate that the improvement of the electrochemical performances could be attributed to
a higher Li
+ conductivity, the suppression of Mn and Ni dissolution from LiNi
0.5Mn
1.5O
4 and
the decreased polarization during cycling with Li
3VO
4 layer on LiNi
0.5Mn
1.5O
4 surface acting
as a relatively stable protective barrier as well as an excellent Li-ion conductor. Furthermore,
Li
3VO
4 coating remarkably reduced the amount of the heat released in DSC measurement, thus
improved the thermal stability of LiNi
0.5Mn
1.5O
4.
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