6.2 Lithium Batteries and Lithium-Ion Batteries
139
following rules: LiPF 6 > LiBF 4 > LiAsF 6 > LiClO 4 ; the change of conductivity in EC/DMC electrolyte system is LiAsF 6 ≈ LiPF 6 > LiCLO 4 > LiBF 4 .
Tarascon reported that when Li 1+x Mn 2 O 4 was used as a battery cathode material, the
oxidation stability of several lithium salts in the DMC + EC (1:1) solvent system
changed according to the following rules: LiPF 6 > LiClO 4 > LiBF 4 > LiAsF 6 >
LiN(SO 2 CF 3 ) 2 > LiCF 3 SO 3 , Conductivity changes according to the following
rules: LiAsF 6 ≈ LiPF 6 > LiClO 4 ≈ LiN(SO 2 CF 3 ) 2 > LiBF 4 > LiCF 3 SO 3 .
The decreasing order of thermal stability of several electrolyte lithium salts is
LiCF 3 SO 3 > LiN(SO 2 CF 3 ) 2 > LiAsF 6 > LiBF 4 > LiPF 6 . In PC or EC-based
electrolytes, the ion-association was reduced by LiCF 3 SO 3 > LiBF 4 > LiClO 4 >
LiPF 6 > LiN(SO 2 CF 3 ) 2 > LiAsF 6 .
6.2.3.3 Organic Anion Electrolyte Lithium Salt
The
organic
anion
electrolyte
lithium
salts
mainly
include
LiCF 3 SO 3 , LiN(SO 2 CF 3 ) 2 and LiC(SO 2 CF 3 ) 3 , and their derivatives. So far,
LiN(SO 2 CF 3 ) 2 and LiC(SO 2 CF 3 ) 3 have the highest conductivity in all anionic
lithium salts. LiCF 3 SO 3 shows good cycle efficiency in some secondary lithium
battery systems, but not as good as LiClO 4 , LiPF 6 , and LiAsF 6 , whose conductivity
is only about half of LiPF 6 . At the same time, when LiCF 3 SO 3 based organic
electrolyte is used in Li-ion batteries, there is also the problem of corrosion of
aluminum or copper electrode current collectors and compatibility with carbon
negative electrodes and layered transition metal oxides. In all three-fluorine alkyl and
perfluoroaryl sulfonic salts, LiCF 3 SO 3 has the highest electrical conductivity and
the lowest price. LiN(SO 2 CF 3 ) 2 (LiTFSI for short) was first proposed by Armand
as a Li salt for Li-ion battery electrolyte, having close electrical conductivity to
LiPF 6 and having inherent electrochemical stability and thermal stability. It is not
easy to hydrolyze, and its thermal decomposition temperature exceeds 360 °C.
LiTFSI is considered to be the most attractive electrolyte lithium salt for highly
graphitized electrodes such as MCMBs, and it can ensure a stable discharge energy
close to the maximum energy even in repeated cycles. In each electrolyte system,
the Coulomb efficiency of almost every charge–discharge cycle except the first
cycle is close to 100%, which is mainly owing to the impact that LiTFSI can form
a low resistance and stable SEI film on MCMBs. However, when LiTFSI-based
organic electrolytes are used in Li-ion battery, there is also corrosion of copper or
aluminum electrode current collectors. This is mainly due to the fact that TFSI-salts
such as Al
3+ , Cu
2+ , and Fe
2+ are highly soluble in many organic solvents, preventing
the deposition of salt and the passivation process. There are mainly three ways to
improve the corrosion potential of LiTFSI on positive current collectors. (1) Add
a perfluorinated inorganic anion salt such as LiPF 6 to the electrolyte to form the
passivation membrane containing fluorine to prevent TFSI-adsorption; (2) Use a
low-viscosity ether solvent to reduce the solubility of TFSI complexes of Al
3+ ,
Cu
2+ , and Fe
2+ ions; (3) Replacement of LiTFSI with an imine salt with a larger
molecular radius, such as the Li + (SO 2 CF 2 CF 3 ) 2 EC + THF electrolyte system
139
following rules: LiPF 6 > LiBF 4 > LiAsF 6 > LiClO 4 ; the change of conductivity in EC/DMC electrolyte system is LiAsF 6 ≈ LiPF 6 > LiCLO 4 > LiBF 4 .
Tarascon reported that when Li 1+x Mn 2 O 4 was used as a battery cathode material, the
oxidation stability of several lithium salts in the DMC + EC (1:1) solvent system
changed according to the following rules: LiPF 6 > LiClO 4 > LiBF 4 > LiAsF 6 >
LiN(SO 2 CF 3 ) 2 > LiCF 3 SO 3 , Conductivity changes according to the following
rules: LiAsF 6 ≈ LiPF 6 > LiClO 4 ≈ LiN(SO 2 CF 3 ) 2 > LiBF 4 > LiCF 3 SO 3 .
The decreasing order of thermal stability of several electrolyte lithium salts is
LiCF 3 SO 3 > LiN(SO 2 CF 3 ) 2 > LiAsF 6 > LiBF 4 > LiPF 6 . In PC or EC-based
electrolytes, the ion-association was reduced by LiCF 3 SO 3 > LiBF 4 > LiClO 4 >
LiPF 6 > LiN(SO 2 CF 3 ) 2 > LiAsF 6 .
6.2.3.3 Organic Anion Electrolyte Lithium Salt
The
organic
anion
electrolyte
lithium
salts
mainly
include
LiCF 3 SO 3 , LiN(SO 2 CF 3 ) 2 and LiC(SO 2 CF 3 ) 3 , and their derivatives. So far,
LiN(SO 2 CF 3 ) 2 and LiC(SO 2 CF 3 ) 3 have the highest conductivity in all anionic
lithium salts. LiCF 3 SO 3 shows good cycle efficiency in some secondary lithium
battery systems, but not as good as LiClO 4 , LiPF 6 , and LiAsF 6 , whose conductivity
is only about half of LiPF 6 . At the same time, when LiCF 3 SO 3 based organic
electrolyte is used in Li-ion batteries, there is also the problem of corrosion of
aluminum or copper electrode current collectors and compatibility with carbon
negative electrodes and layered transition metal oxides. In all three-fluorine alkyl and
perfluoroaryl sulfonic salts, LiCF 3 SO 3 has the highest electrical conductivity and
the lowest price. LiN(SO 2 CF 3 ) 2 (LiTFSI for short) was first proposed by Armand
as a Li salt for Li-ion battery electrolyte, having close electrical conductivity to
LiPF 6 and having inherent electrochemical stability and thermal stability. It is not
easy to hydrolyze, and its thermal decomposition temperature exceeds 360 °C.
LiTFSI is considered to be the most attractive electrolyte lithium salt for highly
graphitized electrodes such as MCMBs, and it can ensure a stable discharge energy
close to the maximum energy even in repeated cycles. In each electrolyte system,
the Coulomb efficiency of almost every charge–discharge cycle except the first
cycle is close to 100%, which is mainly owing to the impact that LiTFSI can form
a low resistance and stable SEI film on MCMBs. However, when LiTFSI-based
organic electrolytes are used in Li-ion battery, there is also corrosion of copper or
aluminum electrode current collectors. This is mainly due to the fact that TFSI-salts
such as Al
3+ , Cu
2+ , and Fe
2+ are highly soluble in many organic solvents, preventing
the deposition of salt and the passivation process. There are mainly three ways to
improve the corrosion potential of LiTFSI on positive current collectors. (1) Add
a perfluorinated inorganic anion salt such as LiPF 6 to the electrolyte to form the
passivation membrane containing fluorine to prevent TFSI-adsorption; (2) Use a
low-viscosity ether solvent to reduce the solubility of TFSI complexes of Al
3+ ,
Cu
2+ , and Fe
2+ ions; (3) Replacement of LiTFSI with an imine salt with a larger
molecular radius, such as the Li + (SO 2 CF 2 CF 3 ) 2 EC + THF electrolyte system