Biomedical Engineering Reference
In-Depth Information
working voltage and thermodynamic equilibrium voltage, and the
value is about 1 V) when undergoing conversion reactions for both
lithiation and delithiation process. Therefore, the theoretical
lithiation capacity can be achieved only when the thermodynamic
equilibrium voltage of the material for conversion reaction is higher
than 1 V [33]. Cr
and MnO, with their high lithium storage capacity
and relatively low thermodynamic equilibrium voltage, are thus
more suitable as anode materials for lithium battery with enhanced
energy densities [33, 34].
O
2
3
6.3 CATHODE MATERIALS
In a lithium-ion battery, cathode materials are usually oxides of
transition metals that can undergo oxidation to higher valences
when lithium ions are removed [6, 35], while maintaining their
structural stability over a wide range of composition (for example,
from fully charged states to completely discharged states). Among
all the cathode materials, LiCoO
is the first cathode material
that has been used in commercialized lithium-ion batteries and is
still widely used. With decades of study, the research on the cathode
materials for lithium-ion batteries has focused on the following
three types of materials: a layered, structured hexagonal oxide
(e.g., LiCoO
2
2
), a spinel structured oxide (e.g., LiMn
O
4
), and an
2
), as shown in Table 6.1. In
addition, transition metal sulfides such as titanium disulfide (TiS
olivine structured oxide LiFePO
4
)
2
have also been adopted in the cathode of lithium batteries.
6.3.1 Layered Structured Hexagonal Oxide
As one of the most successful cathode materials, LiCoO
2
forms a
distorted rock-salt structure, the same as α-NaFeO
, in which the
cations order in alternating (111) planes [36]. This crystalline
structure provides planes of lithium ions through which lithiation
and delithiation can occur. Currently, LiCoO
2
suffers from two major
problems. On the one hand, the availability of cobalt is relatively
2
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