550mAh g1 even after 100 cycles, as well as good
cycling performance. The results demonstrate the power
of the strategy of using elastic hollow carbon spheres as
buffer and container and could be extended to other
anode and cathode materials.
iii) Mesoporous materials: There has always been a need for
developing highly porous electrode materials with large
surface areas readily accessible to electrolyte, which, as
a result, reduce the transport lengths for both electronic
and ionic transport and improve the rate capability. In
this context, more emphasis should be laid on synthesis
of optimized pore sizes and connectivity rather than
striving for micropores ordered arrangements. Mesoporous
electrodes with pore sizes ranging from 2 nm to
50 nm are currently of interest based on the above
consideration. Even with micrometer-sized particle size
like bulk materials, due to the presence of nanometersized
sub-grains inside individual large particle, mesoporous
electrodes usually exhibit better electrochemical
performance than bulk materials.[22] Furthermore, the
relatively large particle size enables easy operation in
terms of separation or film-formation and better nanoparticle
contact compared with nanometer-sized
particles. However, the electronic conductivity of the
mesoporous electrode remains unimproved.
iv) Hierarchical 3D mixed conducting networks: Though
mesoporous electrode materials have shown enhanced
electrochemical performance compared with bulk
materials due to the ‘ionic wiring’, the rate performance
enhancement of such materials is still limited especially
if the pore-walls themselves are poor electronic conducting.
An example of this is the mesoporous anatase
TiO2 sub-micrometer spheres (particle size ca. 300 nm,
grain size ca. 7 nm, pore size ca. 3–30 nm, BET specific
surface area ca. 131m2 g1, porosity ca. 48%)[19] which
exhibit comparable performance to nanometer-sized
anatase (5 nm TiO2) at low current rates, while at high
current rates above 10 C, the performance becomes
worse. This indicates that for semiconductors like
TiO2, the electronic conductivity becomes insufficient
at very high rates, and ‘electronic wiring’ is also needed.
An optimized nanostructure design of electrode materials
for high energy and high power lithium-ion batteries is shown
to be the introduction of hierarchical 3D mixed conducting
networks on both nanoscale and microscale levels through
which the effective diffusion length is reduced to only a few
nanometers (Fig. 2e).[19] The nanoscopic network structure is
composed of a dense net of metalized mesopores that allow
both Liþ and e to migrate. This network with mesh size of
about 10nm is superimposed by a similar net on the microscale
formed by the composite of the mesoporous particles and the
conductive admixture (Fig. 2e). While the nanometer-sized
network provides negligible diffusion times, enhanced local
conductivities and possibly faster phase transfer reactions, and