Two approaches are, at present, gaining momentum in
resolving the kinetic problems of electrode materials. One
approach is increasingD by doping the electrode materials with
foreign atoms. Although mixed conduction is thus improved,
only limited rate-performance enhancement can be achieved,
and sometimes the introduction of heteroatoms can result in
unstable crystal structure.An alternative approach is decreasing
L, which has been realized by nanostructuring of electrode
materials.[3] For example, a reduction of L from 10 mm (the
typical size of commercial electrode materials) to 100nm
reduces, for a material with D¼1010 cm2 s1 (the typical
value of electrode materials), the teq from 5000 to 0.5 s. The
effects are so remarkable that the most extensive research
work over the years has followed this direction.
It has been found that electrode materials inactive towards
Li insertion may become active when ‘‘going nano’’. For
example, rutile TiO2 has very sluggish Li diffusion along the
ab-plane (Dab1015 cm2 s1), which is why Li insertion into
rutile is usually reported to be negligible, viz., ‘inactive’
towards Li insertion. However, nanometer-sized rutile TiO2
(10nm40 nm) is able to reversibly accommodate Li up to
Li0.5TiO2 (168mAh g1) at 1–3V versus Liþ/Li with excellent
capacity retention on cycling.[13] This is mainly related to the
drastic decrease of the diffusion time teq from ca. four years for
10mm rutile (assume the mean square displacement need to
diffusion along the ab-plane Lab¼5 mm) to ca. 2 min for 10nm
Two approaches are, at present, gaining momentum in
resolving the kinetic problems of electrode materials. One
approach is increasingD by doping the electrode materials with
foreign atoms. Although mixed conduction is thus improved,
only limited rate-performance enhancement can be achieved,
and sometimes the introduction of heteroatoms can result in
unstable crystal structure.An alternative approach is decreasing
L, which has been realized by nanostructuring of electrode
materials.[3] For example, a reduction of L from 10 mm (the
typical size of commercial electrode materials) to 100nm
reduces, for a material with D¼1010 cm2 s1 (the typical
value of electrode materials), the teq from 5000 to 0.5 s. The
effects are so remarkable that the most extensive research
work over the years has followed this direction.
It has been found that electrode materials inactive towards
Li insertion may become active when ‘‘going nano’’. For
example, rutile TiO2 has very sluggish Li diffusion along the
ab-plane (Dab1015 cm2 s1), which is why Li insertion into
rutile is usually reported to be negligible, viz., ‘inactive’
towards Li insertion. However, nanometer-sized rutile TiO2
(10nm40 nm) is able to reversibly accommodate Li up to
Li0.5TiO2 (168mAh g1) at 1–3V versus Liþ/Li with excellent
capacity retention on cycling.[13] This is mainly related to the
drastic decrease of the diffusion time teq from ca. four years for
10mm rutile (assume the mean square displacement need to
diffusion along the ab-plane Lab¼5 mm) to ca. 2 min for 10nm
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