Classification of lithium salts in lithium battery electrolytes

Lithium salt electrolyte, as an important component of lithium-ion batteries, 

not only provides free shuttle ions for lithium-ion batteries, but also plays a role in ion transport within the battery.

At the same time, the electrolyte can also form a protective layer on the surface of the electrode material, 

which largely determines the capacity, operating temperature, cycling performance, power density, 

energy density, and safety of lithium-ion batteries.

At present, the lithium salt electrolytes used in lithium batteries mainly include inorganic lithium salt electrolytes 

and organic lithium salt electrolytes. 

This article mainly summarizes common inorganic lithium salts and organic lithium salts, 

and reports the advantages and disadvantages of lithium salt electrolytes.

Inorganic lithium salt electrolyte

Generally speaking, inorganic lithium salts used in lithium-ion batteries have the advantages of low cost, 

low decomposition, high potential resistance, and simple synthesis. 

The common inorganic lithium salts in electrolytes mainly include lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), 

lithium hexafluoroarsenate (LiAsF6), and lithium hexafluorophosphate (LiPF6).

1. LiClO4

Lithium perchlorate (LiClO4) is a highly soluble lithium salt electrolyte, exhibiting high ionic conductivity. 

Its room temperature ionic conductivity in carbonate organic solvents can reach 9 mS/cm. 

In addition, the electrochemical stability window of LiClO4 as a lithium salt electrolyte can reach 5.1 V vs. Li+/Li, 

with relatively good oxidation stability. 

This characteristic also allows the electrolyte to match some high-voltage positive electrode materials, 

thereby exhibiting high voltage performance. 

The energy density of lithium batteries.

In addition, LiClO4 has the advantages of simple preparation, low cost, and good stability, 

and has been widely used in laboratory basic research. 

However, due to the highest valence state of Cl in LiClO4 being+7, 

it is highly susceptible to redox reactions with organic solvents in the electrolyte,

 leading to safety issues such as combustion and explosion in lithium batteries. 

Therefore, LiClO4 is rarely used in commercial lithium batteries.

2. LiBF4

The anion radius of lithium tetrafluoroborate (LiBF4) is relatively small (0.227 nm), 

so the coordination ability between lithium salt electrolyte and lithium ions is relatively weak. 

It is easy to dissociate in organic solvents, which is beneficial for improving the conductivity of lithium batteries and thus enhancing battery performance.

But precisely because its anion radius is relatively small, it is easy to coordinate with organic solvents in the electrolyte, 

which also leads to low lithium ion conductivity, so LiBF4 is rarely used in room temperature lithium batteries.

But LiBF4 has high thermal stability and is not easily decomposed at high temperatures, 

so it is commonly used in high-temperature lithium batteries. 

Meanwhile, LiBF4 also exhibits good battery performance at low temperatures, 

mainly due to the small interfacial impedance of LiBF4 based electrolytes at low temperatures.

In addition, LiBF4 has certain corrosion resistance to the current collector Al. 

Therefore, LiBF4 is often used as an electrolyte additive in lithium-ion batteries to increase 

the corrosion potential of the electrolyte on the current collector Al.

3. LiAsF6

Lithium hexafluoroarsenate (LiAsF6) has the same ionic conductivity as LiBF4, 

and the lithium salt electrolyte does not corrode the current collector Al. 

In addition, the electrochemical window of LiAsF6 lithium salt electrolyte can reach 6.3 V vs.

 Li+/Li, which is much higher than the electrochemical stability of general lithium salts. 

However, due to the highly toxic As element in LiAsF6, it is not commonly used in commercial lithium batteries.

4. LiPF6

Lithium hexafluorophosphate (LiPF6) is currently the most commonly used lithium salt electrolyte in commercial lithium batteries, 

with good ion conductivity and electrochemical stability in non proton organic solvents. 

In addition, LiPF6 electrolyte can form a protective film with the current collector Al, 

thereby reducing the corrosion of the electrolyte on the current collector Al.

More importantly, the carbonate electrolyte based on LiPF6 lithium salt electrolyte can form 

a solid electrolyte interface (SEI) on the graphite negative electrode, 

which protects the adverse reactions between the electrolyte and the graphite negative electrode. 

Lithium ion batteries have good long-term cycling performance.

However, the thermal stability of LiPF6 lithium salt electrolyte is poor. 

In addition, it is prone to react with trace amounts of water to generate strongly acidic PF5. 

PF5 is prone to side reactions with organic solvents in the electrolyte, leading to battery performance degradation. 

The main LiPF6 enterprises in China include the top 10 lithium hexafluorophosphate enterprises.

Organic lithium salt electrolyte

Compared with inorganic lithium salts, the commonly used organic lithium salts in lithium-ion batteries 

can be adjusted by adding electron withdrawing groups to the anions of inorganic lithium salts. 

Common electrolyte organic lithium salts mainly include lithium oxalate borate (LiBOB), lithium difluorooxalate borate (LiDFOB), 

lithium difluorosulfonylimide (LiFSI), and lithium trifluoromethanesulfonimide (LiTFSI).

1. LiBOB and LiDFOB

Xu Kang et al. synthesized lithium oxalate borate (LiBOB). 

As a lithium salt electrolyte used in lithium batteries, LiBOB lithium salt electrolyte has the advantages of high ionic conductivity, 

wide electrochemical stability window, good thermal stability, and good cycling stability.

In addition, studies have shown that it can form a stable passivation film with the current collector Al, 

protecting Al from electrolyte corrosion. 

However, LiBOB has a significant drawback, which is its low solubility in non proton solvents, 

resulting in a lower conductivity of the electrolyte composed of it, 

limiting the rate performance of batteries based on this salt.

In order to overcome the disadvantages of poor solubility and low ionic conductivity of LiBOB, 

Zhang et al. studied the drawbacks of poor solubility and low ionic conductivity of LiBOB. 

Another novel lithium salt electrolyte, lithium difluorooxalate borate (LiDFOB), 

was synthesized using partial LiBOB and LiBF4 lithium salt electrolytes. 

Research has shown that LiDFOB has much higher ionic conductivity than LiBOB.

In addition, it also has good electrochemical stability and good compatibility with the positive and negative electrodes. 

In addition, batteries based on lithium salt electrolytes also exhibit good low-temperature performance. 

Based on the above advantages, LiDFOB has been widely used in current lithium batteries.

2. LiFSI and LiTFSI

Lithium bis (fluorosulfonyl) imide (LiFSI) has the advantages of high ionic conductivity and low sensitivity to water. 

In addition, the decomposition temperature of LiFSI is higher than that of LiPF6, indicating relatively better safety. 

However, LiFSI has strong corrosiveness towards the current collector Al, which to some extent limits its application in lithium-ion batteries.

Lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) is another organic lithium salt developed by Michel Armand. 

Its negative electrode ion consists of a nitrogen (N) atom with strong electronegativity 

and two sulfur (S) atoms connected to a strong electron withdrawing group (CF3). 

This structure disperses negative charges, making it easier for positive and negative ions to dissociate, 

thereby significantly improving their ionic conductivity.

Adding other lithium salts that do not corrode the current collector, introducing long-chain perfluorinated groups, 

and adding additives to LiTFSI can significantly increase the corrosion potential of LiTFSI on the current collector. 

Although these two lithium salt electrolytes have the characteristic of corroding the current collector Al, 

they have been widely used in lithium-ion batteries and all solid state batteries due to their high ionic conductivity, 

good thermal stability, and good electrochemical stability. 

Lithium polymer batteries and lithium sulfur batteries.

conclusion

Lithium salt electrolyte, as an important component of lithium batteries, not only provides and transports lithium ions, 

but also to some extent determines the overall performance of lithium batteries.