Lithium-ion battery electrolyte additives, selection criteria for
high-voltage electrolytes. With the continuous improvement of the energy density
of lithium-ion batteries, people's expectations for the improvement of the
energy density of lithium-ion batteries are getting higher and higher. Lithium
battery electrolyte additives can effectively reduce the internal resistance of
the battery, improve the battery's ability to accept large currents, and extend
the service life of the battery. It is a good additive for quickly rechargeable
batteries.
Introduction to lithium-ion battery electrolyte additives
1. Boron-containing additives
Boron-containing compounds are often used as additives in lithium-ion
batteries with different cathode materials. During the battery cycle, many
boron-containing compounds will form a protective film on the surface of the
cathode to stabilize the interface between the electrode/electrolyte, thereby
improving battery performance. . Considering this unique property of
boron-containing compounds, many scholars have begun to try to apply them to
high-voltage lithium-ion batteries to enhance the stability of the cathode
interface.
2. Organophosphorus additives
According to the relationship between frontline orbital energy and
electrochemical stability: the higher the HOMO of the molecule, the more
unstable the electrons in the orbit are, and the better the oxidation property;
the lower the LUMO of the molecule, the easier it is to obtain electrons, and
the better the reduction property. Currently used organophosphorus additives
also include phosphate ester compounds. XIA et al. applied triallyl phosphate
(TAP) additive to Li[Ni0.42Mn0.42Co0.16]O2 (NMC442) graphite full battery and
found that the Coulombic efficiency will be significantly improved when TAP is
present. After long-term cycling, , still with high capacity retention. XPS
results show that during the cycling process, the allyl group may undergo
cross-linking electropolymerization reaction, and the resulting product covers
the electrode surface to form a uniform SEI film.
3. Carbonate additives
Fluorine-containing (PFA) compounds have high electrochemical stability and
possess both hydrophobic and oleophobic properties. When PFA is added to organic
solvents, the solvent-phobic PFA will aggregate together to form micelles. Due
to this characteristic of PFA, ZHU et al. tried to add ethylene carbonate
substituted by perfluorohydrocarbon groups (TEM-EC, PFB-EC, PFH-EC, PFO-EC in
the figure below) into the electrolyte of high-voltage lithium ion batteries.
For Li1.2Ni0.15Mn0.55Co0.1O2 graphite batteries, when 0.5% (mass fraction) of
PFO-EC is added, the performance of the battery is significantly improved during
long-term cycling. This is mainly because the additive forms a double layer
during the cycling process. layer of passivation film, while reducing the
degradation of the electrode surface and the oxidative decomposition of the
electrolyte.
4. Sulfur-containing additives
In recent years, there have been many reports on the application of organic
sulfonate esters as additives in lithium-ion batteries. PIRES adds
1,3-propanesultone (PS) to the high-voltage lithium-ion battery electrolyte,
which effectively inhibits the occurrence of side reactions on the electrode
surface and the dissolution of metal ions. ZHENG et al. used
dimethylsulfonylmethane (DMSM) as an electrolyte additive for high-voltage
LiNil/3Col/3Mn1/3O2 graphite batteries. XPS, SEM and TEM analysis results showed
that the presence of MMDS has a good modification effect on the positive
electrode SEI film, even if It can also significantly reduce the
electrode/electrolyte interface impedance under high pressure and improve the
cycle stability of the cathode material. In addition, HUANG et al. studied the
cycle performance of trifluoromethylphenylene sulfide (PTS) additive in
high-voltage lithium-ion batteries at room temperature and high temperature.
Analysis of theoretical calculation data and experimental results shows that PTS
is oxidized preferentially than solvent molecules during the cycle, and the SEI
film formed improves the cycle stability of the battery under high voltage. In
addition, some thiophene and its derivatives are also considered for use as
high-voltage lithium-ion battery additives. When these additives are added, a
polymer film will be formed on the surface of the positive electrode to avoid
oxidative decomposition of the electrolyte under high pressure.
5. Ionic liquid additives
Ionic liquid is a low-temperature molten salt that is widely used in
lithium-ion batteries because of its advantages such as low vapor pressure, high
conductivity, non-flammability, thermal stability and high electrochemical
stability.
Selection criteria for high voltage lithium battery electrolyte
The performance of high-voltage lithium-ion batteries is mainly determined
by the structure and properties of active materials and electrolytes. Among
them, the matching of electrolyte is also very important. Because as the energy
density increases, generally the compacted densities of the positive and
negative electrodes are relatively large, the wettability of the electrolyte
becomes worse, and the liquid retention capacity decreases. Low fluid retention
capacity will result in poor cycle and storage performance of the battery.
1. Choose some solvents with higher oxidation potential and wider
electrochemical window (such as sulfones, nitriles and fluorinated
solvents).
2. Some positive electrode protection additives can be added to the
electrolyte to improve the interface properties of the positive electrode
material.
3. Add positive electrode film-forming additives to the electrolyte to
inhibit the reaction between the electrolyte and the positive electrode material
interface.
4. Add new high-voltage resistant lithium salt as an additive to the
electrolyte. For example, adding bisoxaloboric acid (LiBOB) to the electrolyte
can also form a film on the surface of the cathode material, preventing side
reactions between the electrolyte and the electrode material.
High-voltage additives generally oxidize prior to solvent molecules during
the circulation process, forming a passivation film on the surface of the
positive electrode, stabilizing the electrode/electrolyte interface, and
ultimately achieving the stable existence of the electrolyte under high
pressure. I believe that with the advancement of technology, the use of
high-voltage lithium batteries will be the future trend.
The electrolyte is very critical. We call the electrolyte the blood of the
ion battery. On the one hand, the electrolyte is the bridge connecting the
positive and negative electrodes. On the other hand, the electrolyte is also the
medium for the migration and transmission of ions. We need to develop a
high-voltage electrolyte system. , additives are very critical. Some additives
can use competing ions to introduce ions and lithium ions for competitive
solvation, thereby changing the solvation structure of the electrolyte.
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