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Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07 Page 19 of 29

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The reversible capacity of the Si nanoparticle @CNTs cell after 20 cycles is 1,475 mAh g [Figure 11B],
[143]
which is approximately four times that of graphite .
[144]
The cyclic performance of the cell prepared with Fe O @CNTs is shown in Figure 11C . When the current
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density is 1,200 mA g , approximately 40% of the capacity remains, and when the current rate is decreased
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to 60 mA g , approximately 1,000 mA g reversible capacity remains. Fe O -filled MWCNTs have also been
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[145]
studied as anode materials . The specific capacity reached 220 mAh g after 350 cycles, roughly twice that
of unfilled CNTs at a current density of 2,000 mA g . Fe O NPs trapped within CNTs enhance the
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electrochemical behavior of Li-ion batteries while also preventing structural degradation.
Li-S composite system as the electrode material of high-performance batteries has been widely concerned.
Kim et al. prepared lithium batteries using Li S @CNT conductive thin films as electrode materials and
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[146]
tested their performance [Figure 11D] . As the current density reduces from 0.1 C to 2 C, the discharge
capacity decreases gradually. At the end of the cycle, when the current density returns to the initial level, the
specific capacity is 1,081 mAh g (initially 1,090 mAh g ), suggesting that the electrode has excellent
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stability. The illustration shows the relationship between cycle performance and film quality. This may be
due to the increase in weight of CNT film, which leads to an increase in active adsorption sites in its
interior, which is very favorable for the improvement of battery performance. Furthermore, Fu et al.
investigated the chemical properties of sulfur in two types of SWCNTs with distinct diameters, produced by
an electric arc (EA-SWCNTs, average diameter 1.55 nm) or high-pressure carbon monoxide (HiPco-
SWCNTs, average diameter 1.0 nm), and demonstrated the electrochemical reaction activity of sulfur with
lithium inside SWCNTs of different diameters . Specifically, relatively larger diameter EA-SWCNTs can
[78]
accommodate dissolved Li ions, similar to Li-S reactions in solution. In contrast, Li ions are blocked from
+
+
entering the tube cavity in smaller diameter HiPco-SWCNTs. Therefore, the Li-S reaction in HiPco-
SWCNTs is significantly different from when S is not encapsulated and can be attributed to interactions
with π electrons passing through the carbon walls. This finding provides a new mechanism for improving
the performance of lithium-ion batteries by filling CNTs.
It should be noted that the function of carbon nanotubes is more like a modifier: their participation mainly
serves to modify the storage of lithium, thereby enhancing the capacity of lithium-ion batteries. However,
pure carbon nanotubes are not very effective as electrode materials . When filling carbon nanotubes, the
[147]
mass ratio with the active material is generally 1:5, in the milligram range. If pure carbon nanotubes are
used as electrode materials, they exhibit high specific capacity in the first lithium-ion insertion step but
cannot be fully released in the subsequent lithium-ion extraction process . This means that a large portion
[148]
of the lithium ions is irreversibly consumed, leading to a decrease in the Coulombic efficiency of the battery.

The cycling rate is reflected in the electrochemical stability of the material during battery charging and
discharging cycles, as well as the efficiency of ion insertion and extraction processes and the degree of
material damage. CNTs can significantly improve the cycling rate performance of lithium-ion batteries,
mainly due to their conductivity, their special hollow tubular structure, and cross-linked network structure,
which enhance the efficiency of electron conduction in electrode materials and improve ion transport in
CNTs. Firstly, CNTs provide a fast ion transport path, thereby improving the electrochemical reaction of
lithium storage. Secondly, the confinement effect of CNTs allows the encapsulated active electrode material
+
to be in close contact with the carbon tubes, shortening the distance of electron and Li ion transport, which
is superior to loose materials. Finally, the cross-linked conductive CNT network disperses the stress
concentration phenomenon of materials, enhancing the structural strength of powder materials. The carbon
nanotubes also have enough space to release induced stress expansion during the charge/discharge process,