Introduction of lithium battery electrolyte additives

2022-04-27 23:52

Throughout the history of battery development, we can see three characteristics of the current development of the world battery industry

First, the rapid development of green and environmentally friendly batteries, including lithium-ion batteries, hydrogen-nickel batteries, etc.

Second is the transformation of primary batteries to accumulators, which is in line with sustainable development strategies.

Third, the battery further development to small, light, thin direction.

In the commercialization of rechargeable batteries, lithium-ion batteries have the highest specific energy, especially lithium-ion polymer batteries, which can realize the thin shape of rechargeable batteries. Because of the high volume specific energy and mass specific energy of lithium-ion batteries, rechargeable and non-polluting, with the current battery industry development of the three major features, so there is rapid growth in developed countries. The development of telecommunications, information market, especially the use of cell phones and laptops in large numbers, to lithium-ion batteries have brought market opportunities. The lithium-ion battery polymer lithium-ion battery, with its unique advantages in safety, will gradually replace the liquid electrolyte lithium-ion battery, and become the mainstream of lithium-ion battery. Polymer lithium-ion battery is known as “the battery of the 21st century”, will open up a new era of battery, the development prospects are very optimistic.

Lithium-ion battery is a secondary battery system with two different embedded lithium compounds capable of reversibly inserting and removing lithium ions, respectively, as the positive and negative electrodes of the battery, generally using lithium alloy metal oxide as the positive electrode material and graphite as the negative electrode material, using a non-aqueous electrolyte.

When charging, lithium ions are removed from the lattice of the cathode material and inserted into the lattice of the cathode material after the electrolyte, making the negative electrode lithium-rich and the positive electrode lithium-poor; when discharging, lithium ions are removed from the lattice of the negative electrode material and inserted into the lattice of the cathode material after the electrolyte, making the positive electrode lithium-rich and the negative electrode lithium-poor.

The reaction that occurs on the charging cathode is

LiCoO2 == Li(1-x)CoO2+XLi++Xe-(electron)

The reaction on the negative electrode is

6C+XLi++Xe- = LixC6

Total reaction on the rechargeable battery: LiCoO2+6C = Li(1-x)CoO2+LixC6

Lithium battery electrolyte additives are mainly divided into the following categories.

  1. Film-forming additives.

Excellent SEI film (solid electrolyte film) has organic solvent inadmissibility, allowing lithium ions to freely enter and exit the electrode without solvent molecules crossing, thus preventing solvent molecules from co-intercalating to damage the electrode and improving the performance of the battery such as cycle efficiency and reversible capacity.

It is mainly divided into inorganic film-forming additives (small molecules such as SO2, CO2, CO and lithium halide, etc.) and organic film-forming additives (fluorinated, chlorinated and brominated carbonates, etc., which improve the electric power gain capacity of the central atom by the electron-absorbing effect of halogen atoms, so that the additives can reduce and effectively passivate the electrode surface under higher potential conditions to form a stable SEI film). Another Sony patent reports that adding trace amounts of anisole or its halogen derivatives to the non-aqueous electrolyte of lithium-ion batteries can improve the cycling performance of the battery and reduce the irreversible capacity loss of the battery.

  1. Conductive additives.

The research on additives to improve the conductivity of electrolyte is mainly focused on improving the dissolution and ionization of conductive lithium salts and preventing the destruction of electrodes by solvent co-intercalation.

According to the type of action can be divided into the role of cations (mainly including some amines and molecules containing more than two nitrogen atoms of aromatic heterocyclic compounds, as well as crown ether and cavity compounds), and anionic role (anionic ligands are mainly some anion-acceptor compounds, such as boron-based compounds) and the role of electrolyte ions (neutral ligand compounds are mainly some electron-rich groups bonded to electron-deficient atoms N or B) (formed by some electron-rich groups bonded to electron-deficient atoms N or B compounds, such as aza ethers and alkyl borons).

  1. Flame retardant additives.

As a commercial application, the safety of lithium-ion batteries is still an important factor limiting the development of their applications. Lithium-ion battery itself has many safety hazards, such as high charging voltage, and the electrolyte is mostly organic and flammable, if used improperly, the battery can be dangerous or even explode. Therefore, improving the stability of the electrolyte is an important way to improve the safety of lithium-ion batteries. Adding some high boiling point, high flash point and non-flammable solvents in the battery can improve the safety of the battery.

Mainly divided into (1) organic phosphorus compounds (2) organic fluorine substitutes (3) haloalkyl phosphate

  1. Overcharge protection additives.

For the use of redox pairs for internal protection has been extensively studied, the principle of this method is by adding a suitable redox pair to the electrolyte, during normal charging this redox pair does not participate in any chemical or electrochemical reaction, and when the battery is fully charged or slightly above this value, the additive begins to oxidize on the positive electrode and then diffuses to the negative electrode for a reduction reaction, as shown in the following equation.

Positive electrode: R→O+ne-

Negative electrode: O+ne-→R

The optimal overcharge protection additive should have a cut-off voltage of 4.2 to 4.3 V, thus satisfying the requirement of a lithium-ion battery with a voltage greater than 4 V. In general, this part of the research work needs to be further investigated.

Additives to control water and HF content in electrolyte.

The presence of traces of water and HF in organic electrolytes has a role in the formation of excellent performance SEI films, which can be seen from the reactions of solvents such as EC and PC at the electrode interface. However, the high content of water and acid (HF) will not only lead to the decomposition of LiPF6, but also destroy the SEI film. When Al2O3, MgO, BaO and carbonates of lithium or calcium are added to the electrolyte as additives, they will react with the trace amount of HF in the electrolyte, reduce the content of HF, stop its damage to the electrode and its catalytic effect on the decomposition of LiPF6, improve the stability of the electrolyte, and thus improve the battery performance. However, these substances are slow in removing HF, so it is difficult to stop the damage of HF to the battery performance. Some anhydride compounds can remove HF faster, but at the same time, other acidic substances will be produced to destroy the battery performance. Alkane diimine compounds can form a weaker hydrogen bond with water molecules through the hydrogen atoms in the molecule, thus preventing water from reacting with LiPF6 and producing HF.

  1. Additives to improve low-temperature performance.

Low temperature performance is one of the important factors to broaden the use of lithium-ion batteries, and is also a must in current aerospace technology. n,N a dimethyl trifluoroacetamide has low viscosity (1.09mPa-S, 25°C), high boiling point (135°C) and flash point (72°C), good film-forming ability on the graphite surface, and also has good oxidation stability to the cathode, and the assembled battery at low temperature has excellent cycling performance. Organic borides, fluorocarbonate is also conducive to the improvement of battery performance at low temperatures.

  1. Multifunctional additives.

Multifunctional additives are ideal additives for lithium-ion batteries, they can improve the performance of the electrolyte in many ways and have a prominent role in improving the overall electrochemical performance of lithium-ion batteries. It is becoming the main direction of future additive research and development.