2. Nanostructured Electrode Materials for
Lithium-Ion Batteries
Lithium-ion battery currently represents the state-of-the-art
technology in small rechargeable batteries because of its many
merits (e.g., higher voltage, higher energy density, and longer
cycle life) compared with traditional rechargeable batteries
such as lead acid and Ni-Cd batteries. Typically, a lithium-ion
battery consists of a negative electrode (anode, e.g., graphite),
a positive electrode (cathode, e.g., LiCoO2), and a lithium-ionconducting
electrolyte (Fig. 1a). When the cell is charged, Li
ions are extracted from the cathode and inserted into the
anode. On discharge, the Li ions are released by the anode and taken up again by the cathode (Fig. 1a). Although such lithiumion
batteries are commercially successful, especially in smallscale
devices, these cells are still objects of intense research to
enhance their properties and characteristics, which is largely
promoted by the increasingly diverse range of applications they
need to power, such as next-generation wireless communication
devices (e.g., 3G mobile phones, MP4), EVs, HEVs, power
tools, uninterrupted power sources (UPS), stationary storage
batteries (SSBs), and microchips. Since no single lithium-ion
battery type can meet all the demands of such a large variety of
applications, different types of batteries with specific properties
for certain applications should be considered, including:
i) high-energy lithium-ion batteries for modern communication
taken up again by the cathode (Fig. 1a). Although such lithiumion
batteries are commercially successful, especially in smallscale
devices, these cells are still objects of intense research to
enhance their properties and characteristics, which is largely
promoted by the increasingly diverse range of applications they
need to power, such as next-generation wireless communication
devices (e.g., 3G mobile phones, MP4), EVs, HEVs, power
tools, uninterrupted power sources (UPS), stationary storage
batteries (SSBs), and microchips. Since no single lithium-ion
battery type can meet all the demands of such a large variety of
applications, different types of batteries with specific properties
for certain applications should be considered, including:
i) high-energy lithium-ion batteries for modern communication
Yu-Guo Guo received his Ph.D. in Chemistry from ICCAS under the supervision of Prof. Chun-Li
Bai and Prof. Li-Jun Wan. From 2004 to 2007 he worked with Prof. Joachim Maier at the Max
Planck Institute for Solid State Research at Stuttgart (Germany) first as a Guest Scientist and then a
Staff Scientist. He joined ICCAS as a professor in 2007. His current research interests are centered on
the nanostructured materials for advanced energy conversion and storage devices, the sizedependent
properties of energy materials, as well as ion/electron transport in nanoscaled systems.
Jin-Song Hu received his Ph.D. in Chemistry (2005) from ICCAS with Prof. Chun-Li Bai and Prof.
Li-Jun Wan as his supervisors. He joined ICCAS as an assistant professor in 2005 and was prompted
as an associate professor two years later. His current scientific interests are focused on functional
nanomaterials for environmental remediation, energy system and electronics.
devices; ii) high-power lithium-ion batteries for HEVs, EVs
and power tools; and iii) long-cycle-life lithium-ion batteries
for UPS and SSBs. Many anode and cathode materials with
appropriate properties have been considered for different
types of lithium-ion batteries (Table 1). However, commercial
batteries are mostly based on micrometer-sized electrode
materials, which are limited by their kinetics, lithium-ion
intercalation capacities, and structural stability. The performance
of currently available lithium-ion batteries can only
meet the requirements of these different applications to some
degree; challenges still remain, naturally, in developing new
electrode materials for high energy density, high power density
(viz., higher rates), longer cycle life, and improved safety. The
development of nanostructured electrode materials is considered
to be the most promising avenue towards overcoming
the current limits and achieving these goals.[1–12] However, it is
necessary first of all to know how nanomaterials impact on the
performance of the lithium-ion batteries and what kinds of
mechanisms these nanomaterials exhibit. Here, we address the
benefits of nanometer size effects and the disadvantages of
using ‘nano’, as well as strategies for solving these issues and
fulfilling the nanomaterials’ potential.
2.1. Benefits of Nanometer Size Effects
Nanomaterials can play a large role in improving the
performance of lithium-ion batteries, because in nanoparticle systems the distances over which Liþ must diffuse are
dramatically decreased; the nanoparticles can quickly absorb
and store vast numbers of lithium ions without causing any
deterioration in the electrode; and nanoparticles have large
surface areas, short diffusion lengths, and fast diffusion rates
along their many grain boundaries. Much enhanced capacities,
high rate performance, capacity retention abilities, and many
novel Li storage systems have been found to benefit from
nanometer size effects.
2.1.1. Enhanced Lithium Storage Kinetics
Lithium-ion batteries are amongst the most promising
candidates for applications in EVs, HEVs, and power tools in
terms of energy density, while the achievement of high power
density is hindered by kinetic problems in the electrode
materials, i.e., the slow Liþ and e diffusion. For solid-state
diffusion of Li in electrode materials, the mean diffusion (or
storage) time, teq, is determined by the diffusion coefficient, D,
and the diffusion length, L, according to the following formula:
teq ¼ L2=2D (1)
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
rutile (Lab¼5 nm).
Nanometer-sized electrode materials may not only increase
the electroactivity towards Li insertion but also enhance the
high rate capability (high power), as has been observed for
many anode materials. The high rate capability results directly
from the transport advantages of the fine particle size, such as
shorter transport distances for both e and Liþ transport as
well as a larger electrode/electrolyte contact area resulting
from the larger surface area. The former makes full Li diffusion
possible within a short storage time, i.e., at high charge/
discharge rates, and the latter greatly reduces the specific
current density of the active material. For example, the abovementioned
nanometer-sized rutile TiO2 with a specific surface
area of ca. 110m2 g1 also exhibits an excellent high rate
performance (100mAh g1 at 10C and 70mAh g1 at 30 C,
where 1C¼336mAg1), which makes it a promising anode
material for high-power lithium-ion batteries.[13] These findings
have encouraged people to reinvestigate materials that were
thought to be electrochemically inactive in bulk form due to
poor electronic and Liþ conductivity, but that could present
improved electrochemical performance at the nanoscale. More
examples should present themselves in due course.
2.1.2. Enhanced Structural Stability
Since structural transition to thermodynamically undesirable
structures can only occur if the particle radius rp is larger
than the critical nucleation radius rc for that phase, it is possible
to eliminate such transitions by using nanoparticles with rp>rc.
Thus, small particles would more easily accommodate the
structural changes occurring during the cycling process where
Li is inserted and extracted. For example, layered LiMnO2
suffers from structural instability during cycling and as a result,
exhibits significant capacity fade. As a way to overcome such
difficulties, nanocrystalline structures have attracted increasing
attention, since the lattice stress caused by Jahn–Teller
distortion can be accommodated more easily, and hence they
exhibit much higher Li-intercalation capacity than their
conventional crystalline counterparts.[4]
In nanoparticles the charge accommodation occurs largely
at or very near the surface and the smaller the particles are, the
larger the portion of these constituent atoms at the surface.
This fact reduces the need for diffusion of Liþ in the solid
phase, greatly increasing the charge and discharge rate of the
cathode as well as reducing the volumetric changes and lattice
stresses caused by repeated Li insertion and expulsion.
2.1.3. New Lithium-Storage Mechanisms
Anothe