The
spinel LiMn2O4 cathode is considered
a promising cathode material for lithium ion batteries. Unfortunately,
the poor capacity stability, especially at elevated temperature, hinders
its practical utilization. In this study, the atomic layer deposition
(ALD) technique is employed to deposit a TiO2 nanocoating
on a LiMn2O4 electrode. To maintain electrical
conductivity, this amorphous coating layer with high uniformity, conformity,
and completeness is directly coated on cathode electrodes instead
of LiMn2O4 particles. Among all the samples
studied, the TiO2-coated sample with 15 ALD cycles exhibits
the best cyclability at both room temperature of 25 °C and elevated
temperature of 55 °C and has the higher specific capacity of
136.4 mAh g–1 at 0.1 C that is nearly close to the
theoretical capacity of LiMn2O4. Meanwhile,
this sample realizes lower polarization and less self-discharge. The
improved electrochemical performance is ascribed to the high conformal
and ultrathin TiO2 coating, which enhances the kinetics
of Li+ diffusion and stabilizes the electrode/electrolyte
interface. Also, the deconvolution of Ti 2p X-ray photoelectron spectroscopy
shows a weaker peak of Ti–O–F after cycling, which indicates
that the coexistence of TiO2 and TiO
x
F
y
layers can inhibit Mn dissolution
and electrolyte decomposition.
A high-capacity Si anode is always accompanied by very large volume expansion and structural collapse during the lithium-ion insertion/extraction process. To stabilize the structure of the Si anode, magnesium vapor thermal reduction has been used to synthesize porous Si and SiO (pSS) particles, followed by in situ growth of carbon nanotubes (CNTs) in pSS pores through a chemical vapor deposition (CVD) process. Field-emission scanning electron microscopy and high-resolution transmission electron microscopy have shown that the final product (pSS/CNTs) possesses adequate void space intertwined by uniformly distributed CNTs and inactive silica in particle form. pSS/CNTs with such an elaborate structural design deliver improved electrochemical performance, with better coulombic efficiency (70% at the first cycle), cycling capability (1200 mAh g at 0.5 A g after 200 cycles), and rate capability (1984, 1654, 1385, 1072, and 800 mAh g at current densities of 0.1, 0.2, 0.5, 1, and 2 A g, respectively), compared to pSS and porous Si/CNTs. These merits of pSS/CNTs are attributed to the capability of void space to absorb the volume changes and that of the silica to confine the excessive lithiation expansion of the Si anode. In addition, CNTs have interwound the particles, leading to significant enhancement of electronic conductivity before and after Si-anode pulverization. This simple and scalable strategy makes it easy to expand the application to manufacturing other alloy anode materials.
The electrochemical performance of lithium-oxygen (Li-O 2 ) batteries depends largely on the architecture and catalytic effectiveness of the oxygen cathode. Herein, in this study, a graphene aerogel decorated with MoS x nanosheets (MoS x /HRG) with a three-dimensional porous framework synthesized using a one-step hydrothermal reaction followed by freeze-drying is reported. The MoS x /HRG aerogel possesses hierarchical mesopores and micropores, which could facilitate electrolyte impregnation and oxygen diffusion, and provide much more accommodation space for the reaction products. The lithium-oxygen batteries based on this MoS x /HRG aerogel cathode show improved electrochemical performance, with a high initial discharge capacity up to 6678.4 mA h g À1 at a current density of 0.05 mA cm À2 and better cycling capability with a cut-off capacity of 500 mA h g À1 at a current density of 0.1 mA cm À2 , compared with the lithium-oxygen batteries based on an HRG aerogel cathode. The enhanced performance is ascribed to the excellent catalytic activity of the MoS x nanosheets and the unique three-dimensional porous architecture.
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