Biomedical Engineering Reference
In-Depth Information
Based on the morphology of the coating, the surface coating can be
divided into three subgroups - rough coatings, a core-shell structure, and
ultra-thin film coating (Chen et al., 2010). For rough coatings, the majority
of coating strategies have been based on solution techniques such as the sol-
gel method (Cho et al., 2000; Sun et al., 2009). These coating technologies
can result in an incomplete coating or a thick coating, which brings a trade-
off between Li
+
diffusion (power and/or energy density) and protection
(lifetime) controlled by the thickness (nm) of the film (Kim et al., 2002). The
core-shell structure takes advantage of both the high electrochemical
performance of the core material and the excellent stability of the shell
material. However, core-shell structured materials generally have a thick
coating shell, sometimes up to 1
m, which brings problems of crystal
structure mismatch and slow transport of electrons and Li-ions. Normally,
relatively thick films deposited onto substrates exhibit mechanical stresses,
including intrinsic and thermal stresses. In battery systems, repeated charge/
discharge cycling will create additional mechanical stresses between the core
and the shell due to volumetric changes, which can lead to fatigue and the
formation of cracks between the two types of materials.
By contrast, an ultra-thin film coating of thickness down to sub-
nanometer levels has the potential to reduce the mechanical stress and to
alleviate the requirements on the electron and Li-ion conductivity for the
coating materials. ALD can prepare pinhole-free surface coatings on battery
electrode powders with precise control of the coating layer thickness down
to 0.1 nm. Since ALD is a layer-by-layer process, the slow growth rate can
reduce the intrinsic stress induced during the growth of the films. Unlike
physical coating methods, strong chemical bonds are created to maintain the
physical integrity between the substrate and coating layer prepared by ALD.
Ultra-thin ALD films are much more flexible than thick coatings, and the
thermal stress can be greatly reduced.
Jung et al. (2010a) demonstrated that conformal Al
2
O
3
ALD films greatly
increased the performance of LiCoO
2
powders, as shown in Fig. 8.8.
Alumina ALD films were deposited on commercially available micron-sized
LiCoO
2
powders at 180
μ
C using alternating reactions of TMA and water.
The coated LiCoO
2
powders with thicknesses of only 0.3 to 0.4 nm using
two ALD cycles exhibited a capacity retention of 89% after 120 charge-
discharge cycles in the 3.3-4.5 V (vs. Li/Li
+
) range, while the bare LiCoO
2
powders displayed only a 45% capacity retention. However, with thicker
alumina ALD films, the capacity started to decrease significantly and
showed a negligible value of 20mA h g
1
after ten ALD cycles (
8
2 nm thick).
The loss of capacity resulted mainly from the electronically insulating
character of the Al
2
O
3
ALD film. The electronic conductivity of LiCoO
2
powders was significantly reduced from 2
~
10
5
Scm
1
after
only two ALD cycles. The conductivity continuously decreased with
10
4
6
to 5
6
