11 Lithium-Ion Battery—3D Micro-/Nano-Structuring, Modification …
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reference-cell is significantly decreased, i.e. at C/2, its retained capacity is only
at 60%, and furthermore, at 1C and 2C such cell shows no capacity. At 2C, the
laser-cell still retains about 60% of its initial capacity. As a final cycling step, the
C-rate was set back to C/10 to determine whether any cell degradation occurred
during these galvanostatic measurements. A small capacity loss was observed at the
end of the measurements: the cell capacity decreased by 6% and 8% for laser- and
reference-cells, respectively.
11.4 Passivation Coatings on Three-Dimensional Electrodes
After introducing the first commercial lithium-ion batteries (LIB) in 1991 by Sony,
lithium cobalt oxide (LCO) became the most used cathode material for LIB for
at least two decades due to its excellent lithium-ion diffusion kinetics and simple
type of synthesis [67]. The laser-assisted formation of conical 3D microstructures
in thick composite LCO films but also thin-film LCO cathodes was discovered by
Pfleging et al. [26]. They could achieve significant electrochemical improvements
such as enhanced capacity retention at high C-rates. By applying a voltage window
of 3.0–4.2 V, LCO cells cannot provide more than half of their theoretical capacity
of about 274 mAh/g [68]. One popular approach to increase the specific capacity of
lithium-ion batteries is to increase the upper voltage, e.g. to values of about 4.5 V.
In the case of LCO, however, a structural phase transition from the hexagonal to
the monoclinic phase occurs for electrochemical cycles at higher voltages, which
leads to structural damage to the electrode material. In addition, side reactions due
to electrolyte decomposition and co-dissolution contribute significantly to a rapid
fading of specific capacity with increasing cycle number [68–70].
In order to improve the structural stability of LCO materials in widened voltage
windows with upper voltages above 4.2 V, surface coatings with metal oxides such
as Al 2 O 3 [71], AlPO 4 [72], SnO 2 [73], ZrO 2 [74], ZnO [75], CuO [69], and MgO
[76] have been suggested. Other technical approaches have dealt with coatings based
on carbon such as sucrose [77] or carbon black [78]. As far as we know, the carbon
coating on the surface of a compact LCO thin-film electrode is quite a new research
field. In particular, the role of carbon coating in preventing co-dissolution has not
been disclosed. Lee et al. [79–81] have applied fullerene C60 thin films as functional
coating layers for silicon- and tin-based anodes and for composites for lithiumion batteries and an excellent electrochemical improvement could be proven. By
applying plasma processing condition, C60 formed a polymerization [82]. It could
be shown that the C60 films were polymerized along the anode surfaces forming an
artificial solid electrolyte interphase layer (SEI) which was mechancial stable during
volume changes of the cathode while lithiation/de-lithiation takes place; and also
chemical side reactions between electrode material and liquid electrolyte could be
suppressed [79–81]. Lee et al. [83] demonstrated a new approach by applying the
polymerized C60 coating to the laser-generated 3D LCO microstructure. Prior to C60
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reference-cell is significantly decreased, i.e. at C/2, its retained capacity is only
at 60%, and furthermore, at 1C and 2C such cell shows no capacity. At 2C, the
laser-cell still retains about 60% of its initial capacity. As a final cycling step, the
C-rate was set back to C/10 to determine whether any cell degradation occurred
during these galvanostatic measurements. A small capacity loss was observed at the
end of the measurements: the cell capacity decreased by 6% and 8% for laser- and
reference-cells, respectively.
11.4 Passivation Coatings on Three-Dimensional Electrodes
After introducing the first commercial lithium-ion batteries (LIB) in 1991 by Sony,
lithium cobalt oxide (LCO) became the most used cathode material for LIB for
at least two decades due to its excellent lithium-ion diffusion kinetics and simple
type of synthesis [67]. The laser-assisted formation of conical 3D microstructures
in thick composite LCO films but also thin-film LCO cathodes was discovered by
Pfleging et al. [26]. They could achieve significant electrochemical improvements
such as enhanced capacity retention at high C-rates. By applying a voltage window
of 3.0–4.2 V, LCO cells cannot provide more than half of their theoretical capacity
of about 274 mAh/g [68]. One popular approach to increase the specific capacity of
lithium-ion batteries is to increase the upper voltage, e.g. to values of about 4.5 V.
In the case of LCO, however, a structural phase transition from the hexagonal to
the monoclinic phase occurs for electrochemical cycles at higher voltages, which
leads to structural damage to the electrode material. In addition, side reactions due
to electrolyte decomposition and co-dissolution contribute significantly to a rapid
fading of specific capacity with increasing cycle number [68–70].
In order to improve the structural stability of LCO materials in widened voltage
windows with upper voltages above 4.2 V, surface coatings with metal oxides such
as Al 2 O 3 [71], AlPO 4 [72], SnO 2 [73], ZrO 2 [74], ZnO [75], CuO [69], and MgO
[76] have been suggested. Other technical approaches have dealt with coatings based
on carbon such as sucrose [77] or carbon black [78]. As far as we know, the carbon
coating on the surface of a compact LCO thin-film electrode is quite a new research
field. In particular, the role of carbon coating in preventing co-dissolution has not
been disclosed. Lee et al. [79–81] have applied fullerene C60 thin films as functional
coating layers for silicon- and tin-based anodes and for composites for lithiumion batteries and an excellent electrochemical improvement could be proven. By
applying plasma processing condition, C60 formed a polymerization [82]. It could
be shown that the C60 films were polymerized along the anode surfaces forming an
artificial solid electrolyte interphase layer (SEI) which was mechancial stable during
volume changes of the cathode while lithiation/de-lithiation takes place; and also
chemical side reactions between electrode material and liquid electrolyte could be
suppressed [79–81]. Lee et al. [83] demonstrated a new approach by applying the
polymerized C60 coating to the laser-generated 3D LCO microstructure. Prior to C60