138
6 Nanomaterials for Batteries
facilitates the formation of a particularly stable SEI membrane. Therefore, fluoridecontaining lithium salts have been the main body of lithium salts in Li-ion battery
electrolytes, and it is also an important direction of development.
6.2.3.2 Inorganic Anion Electrolyte Lithium Salt
Many simple lithium salts such as LiF, LiCl, and LiBr have not been used in Li-ion
batteries that owe to the low solubility in lithium salts. Although LiI-based electrolytes have moderate conductivity, LiI is difficult to prepare in non-aqueous conditions, and I is easily oxidized. Li 3 AlF 6 and Li 2 SiF 6 have low solubility in organic
solvents (only about 0.1 mol/L), and their electrical conductivity is generally at 10–
5 S/cm. LiSbF 6 and LiAlCl 4 have larger anions and smaller lattice energies. They
have good conductivity as lithium salt organic electrolytes, such as the conductivity
of 1 mol/L of LiSbF 6 + THF and LiAlCl 4 + THF electrolyte is 16 × 10
–3 S/cm.
However, Sb(V) and Al(III) are easily reduced, and the SEI membrane formed
is permeable to Li
+ but also permeable to SbF6
−
, AlCl4
−
. LiTaF 6 and LiNbF 6
based organic electrolytes also have suitable electrical conductivity, but they are
very expensive and not easy to obtain the high purity, and on lithium electrodes, like LiSbF 6 and LiAlCl 4 , Ta(V) and Nb (V) is easily reduced to metal Ta and
Nb. In addition, LiSbF 6 and LiTaF 6 were also found to initiate polymerization of
cyclic ethers in the electrolyte. Therefore, among the numerous lithium salts, only
LiClO 4 , LiPF 6 , LiBF 4 , and LiAsF 6 may be used in lithium-ion batteries. LiClO 4 is
the longest researched lithium salt with appropriate electrical conductivity, thermal
stability, and oxidation stability. However, it is generally accepted in the international lithium battery industry that it is only suitable for research work systems and
cannot be used in practical applications. In batteries, this is because LiClO 4 itself is
a strong oxidant, and it is feared that under certain uncertain conditions, it may cause
safety problems; LiBF 4 not only has poor thermal stability, is easily hydrolyzed,
but also has relatively low electrical conductivity. Among the known lithium salts,
the LiAsF 6 -based electrolyte has the best cycle efficiency, relatively good thermal
stability, and almost the highest conductivity. However, the potential carcinogenic
effect of the As(V) reductant has limited its application. Therefore, LiPF 6 has been
applied in commercial Li-ion battery and Li AsF 6 is mainly used in military lithium
batteries. LiClO 4 is mostly used in experimental studies because it does not have
good hydrolysis and thermal stability.
LiPF 6 has the following advantages as a lithium-ion battery electrolyte lithium
salt: (1) It can form an appropriate SEI film on the electrode, especially on the
carbon negative electrode; (2) It can effectively passivate the positive electrode
current collector to prevent it from dissolving; (3) It has a wider power chemically
stable window; (4) It has appropriate solubility and high conductivity in various
non-aqueous solvents; (5) It has relatively good environmental friendliness. LiPF 6
has outstanding oxidation stability. In a single solvent DMC electrolyte system,
the oxidation potential of several electrolyte lithium salts changes according to the
6 Nanomaterials for Batteries
facilitates the formation of a particularly stable SEI membrane. Therefore, fluoridecontaining lithium salts have been the main body of lithium salts in Li-ion battery
electrolytes, and it is also an important direction of development.
6.2.3.2 Inorganic Anion Electrolyte Lithium Salt
Many simple lithium salts such as LiF, LiCl, and LiBr have not been used in Li-ion
batteries that owe to the low solubility in lithium salts. Although LiI-based electrolytes have moderate conductivity, LiI is difficult to prepare in non-aqueous conditions, and I is easily oxidized. Li 3 AlF 6 and Li 2 SiF 6 have low solubility in organic
solvents (only about 0.1 mol/L), and their electrical conductivity is generally at 10–
5 S/cm. LiSbF 6 and LiAlCl 4 have larger anions and smaller lattice energies. They
have good conductivity as lithium salt organic electrolytes, such as the conductivity
of 1 mol/L of LiSbF 6 + THF and LiAlCl 4 + THF electrolyte is 16 × 10
–3 S/cm.
However, Sb(V) and Al(III) are easily reduced, and the SEI membrane formed
is permeable to Li
+ but also permeable to SbF6
−
, AlCl4
−
. LiTaF 6 and LiNbF 6
based organic electrolytes also have suitable electrical conductivity, but they are
very expensive and not easy to obtain the high purity, and on lithium electrodes, like LiSbF 6 and LiAlCl 4 , Ta(V) and Nb (V) is easily reduced to metal Ta and
Nb. In addition, LiSbF 6 and LiTaF 6 were also found to initiate polymerization of
cyclic ethers in the electrolyte. Therefore, among the numerous lithium salts, only
LiClO 4 , LiPF 6 , LiBF 4 , and LiAsF 6 may be used in lithium-ion batteries. LiClO 4 is
the longest researched lithium salt with appropriate electrical conductivity, thermal
stability, and oxidation stability. However, it is generally accepted in the international lithium battery industry that it is only suitable for research work systems and
cannot be used in practical applications. In batteries, this is because LiClO 4 itself is
a strong oxidant, and it is feared that under certain uncertain conditions, it may cause
safety problems; LiBF 4 not only has poor thermal stability, is easily hydrolyzed,
but also has relatively low electrical conductivity. Among the known lithium salts,
the LiAsF 6 -based electrolyte has the best cycle efficiency, relatively good thermal
stability, and almost the highest conductivity. However, the potential carcinogenic
effect of the As(V) reductant has limited its application. Therefore, LiPF 6 has been
applied in commercial Li-ion battery and Li AsF 6 is mainly used in military lithium
batteries. LiClO 4 is mostly used in experimental studies because it does not have
good hydrolysis and thermal stability.
LiPF 6 has the following advantages as a lithium-ion battery electrolyte lithium
salt: (1) It can form an appropriate SEI film on the electrode, especially on the
carbon negative electrode; (2) It can effectively passivate the positive electrode
current collector to prevent it from dissolving; (3) It has a wider power chemically
stable window; (4) It has appropriate solubility and high conductivity in various
non-aqueous solvents; (5) It has relatively good environmental friendliness. LiPF 6
has outstanding oxidation stability. In a single solvent DMC electrolyte system,
the oxidation potential of several electrolyte lithium salts changes according to the