6.2 Lithium Batteries and Lithium-Ion Batteries
121
Layered Lithium Cobalt (LiCoO 2 ) Cathode Material
LiCoO 2 is the earliest commercialized layered oxide cathode material, which has the
advantages of simple production technology, high working voltage, stable charging
and discharging performance, and so on. The research of LiCoO 2 started in 1980. J.B.
Goodenough and others put forward that LiCoO 2 can be considered as the cathode
material for lithium-ion battery and was commercialized (Ozawa 1994) by Japanese
Sony Corp in 1991.
LiCoO 2 has a layered structure (the structure of alpha-NaFeO 2 ), which belongs
to the R-3 m space group (Perkins et al. 2010), Li
+ , Co
3+ and O
2− occupy 3a, 3b,
and 6C positions in the spatial structure, respectively. Co
3+ ions and Li
+ ions are all
in the eight faces of O
2− ions. In the direction of C-axis, there is a layer of space
structure with Co
3+ ion layer and a lithium-ion layer alternately arranged. During
charging/discharging process, the lithium-ion can be reversibly removed/embedded
from its layer. The theoretical specific capacity of LiCoO 2 cathode material is about
274 mAh g
–1 , but in practical application, the specific capacity (only 140 mAh g
–1 ) is
about half of the theoretical value. This is because when the charging voltage reaches
4.3 V, there will be some side reactions, resulting in an irreversible transformation
of the structure, and the increase of the battery impedance. Only about one half of
lithium ions can be removed from the structure (Aurbach et al. 2003). Therefore, the
charging voltage of LiCoO 2 is generally less than 4.4 V. Besides, cobalt resources
are scarce, expensive, and unfriendly to environment, which makes LiCoO 2 have
many limitations in the application of Li-ion batteries.
Layered Lithium Nickel (LiNiO 2 ) Cathode Material
Compared with LiCoO 2 , LiNiO 2 has higher actual capacity, and has more advantages
in price and resources. It has been considered as extremely promising cathode material for LiCoO 2 . The LiNiO 2 structure, like LiCoO 2 , belongs to the layered structure
of the alpha-NaFeO 2 , and the R-3 m space group. The theoretical specific capacity of
LiNiO 2 is 275 mAh g
−1 , meanwhile, the actual specific capacity is between 190 mAh
g
−1 and 210 mAh g
−1 , and the self-discharge rate is lower (Liu et al. 2001). However,
LiNiO 2 also has shortcomings (Kanno et al. 1994), for example, the ionic radius of
Ni
2+ is very close to the ionic radius of Li
+ . In the process of material synthesis, the
Ni
2+ ions in the transition metal layer are very easy to migrate to the Li
+ layer and
have cation mixing with Li
+ . So far, no pure, structurally stable, strictly stoichiometric LiNiO 2 materials have been synthesized. Many LiNiO 2 are synthesized in
the form of nickel rich compounds (Li 1−y Ni 1+y O 2 ). At the same time, this is also the
reason why LiNiO 2 is still not commercialized. In addition, the thermal stability of
Li 1−x NiO 2 is poor. Under the same conditions (such as electrolyte composition and
termination voltage), the thermal decomposition temperature of Li 1−x NiO 2 is about
200 °C, and the heat release is more than that of Li 1−x CoO 2 . This is because, at the
later stage of charging, the Ni
4+ at high price is unstable. It is not only easy to oxidize
and decompose electrolytes, but also releases heat and gas from the collector, and it
is unstable and easy to heat up and generate O 2 . When the heat and gas are gathered
to a certain extent, there may be an explosion.
121
Layered Lithium Cobalt (LiCoO 2 ) Cathode Material
LiCoO 2 is the earliest commercialized layered oxide cathode material, which has the
advantages of simple production technology, high working voltage, stable charging
and discharging performance, and so on. The research of LiCoO 2 started in 1980. J.B.
Goodenough and others put forward that LiCoO 2 can be considered as the cathode
material for lithium-ion battery and was commercialized (Ozawa 1994) by Japanese
Sony Corp in 1991.
LiCoO 2 has a layered structure (the structure of alpha-NaFeO 2 ), which belongs
to the R-3 m space group (Perkins et al. 2010), Li
+ , Co
3+ and O
2− occupy 3a, 3b,
and 6C positions in the spatial structure, respectively. Co
3+ ions and Li
+ ions are all
in the eight faces of O
2− ions. In the direction of C-axis, there is a layer of space
structure with Co
3+ ion layer and a lithium-ion layer alternately arranged. During
charging/discharging process, the lithium-ion can be reversibly removed/embedded
from its layer. The theoretical specific capacity of LiCoO 2 cathode material is about
274 mAh g
–1 , but in practical application, the specific capacity (only 140 mAh g
–1 ) is
about half of the theoretical value. This is because when the charging voltage reaches
4.3 V, there will be some side reactions, resulting in an irreversible transformation
of the structure, and the increase of the battery impedance. Only about one half of
lithium ions can be removed from the structure (Aurbach et al. 2003). Therefore, the
charging voltage of LiCoO 2 is generally less than 4.4 V. Besides, cobalt resources
are scarce, expensive, and unfriendly to environment, which makes LiCoO 2 have
many limitations in the application of Li-ion batteries.
Layered Lithium Nickel (LiNiO 2 ) Cathode Material
Compared with LiCoO 2 , LiNiO 2 has higher actual capacity, and has more advantages
in price and resources. It has been considered as extremely promising cathode material for LiCoO 2 . The LiNiO 2 structure, like LiCoO 2 , belongs to the layered structure
of the alpha-NaFeO 2 , and the R-3 m space group. The theoretical specific capacity of
LiNiO 2 is 275 mAh g
−1 , meanwhile, the actual specific capacity is between 190 mAh
g
−1 and 210 mAh g
−1 , and the self-discharge rate is lower (Liu et al. 2001). However,
LiNiO 2 also has shortcomings (Kanno et al. 1994), for example, the ionic radius of
Ni
2+ is very close to the ionic radius of Li
+ . In the process of material synthesis, the
Ni
2+ ions in the transition metal layer are very easy to migrate to the Li
+ layer and
have cation mixing with Li
+ . So far, no pure, structurally stable, strictly stoichiometric LiNiO 2 materials have been synthesized. Many LiNiO 2 are synthesized in
the form of nickel rich compounds (Li 1−y Ni 1+y O 2 ). At the same time, this is also the
reason why LiNiO 2 is still not commercialized. In addition, the thermal stability of
Li 1−x NiO 2 is poor. Under the same conditions (such as electrolyte composition and
termination voltage), the thermal decomposition temperature of Li 1−x NiO 2 is about
200 °C, and the heat release is more than that of Li 1−x CoO 2 . This is because, at the
later stage of charging, the Ni
4+ at high price is unstable. It is not only easy to oxidize
and decompose electrolytes, but also releases heat and gas from the collector, and it
is unstable and easy to heat up and generate O 2 . When the heat and gas are gathered
to a certain extent, there may be an explosion.