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Super comprehensive introduction of rechargeable lithium battery electrolyte

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  • Time of issue:2022-01-05 17:43
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(Summary description)Everyone must have heard about the recent explosion of Samsung Note7, but instead of lamenting those shocking photos, we should analyze the reasons for the incident.

Super comprehensive introduction of rechargeable lithium battery electrolyte

(Summary description)Everyone must have heard about the recent explosion of Samsung Note7, but instead of lamenting those shocking photos, we should analyze the reasons for the incident.

  • Categories:Industry News
  • Author:
  • Origin:
  • Time of issue:2022-01-05 17:43
  • Views:
Information

Everyone must have heard about the recent explosion of Samsung Note7, but instead of lamenting those shocking photos, we should analyze the reasons for the incident. Afterwards, Samsung Electronics my country said that the supplier of Galaxy Note7 sold in my country was ATL, and the battery that would explode was not made by the supplier Samsung SDI. According to the comments and analysis of SDI insiders, the reason for the fire of the mobile phone battery was due to the short circuit of the positive and negative electrodes at the R angle of the battery. The battery was made by a winding process and used a conventional liquid lithium-ion battery electrolyte.

Well, the key is here! We understand that conventional lithium-ion batteries use non-aqueous organic solvents. When the battery is heated due to an internal short circuit, the electrolyte is decomposed by heat and gas is formed, which will cause the battery to expand in light, and cause the battery to explode in heavy.

So what is the "mysterious" electrolyte? The editor has sorted out the relevant knowledge about the electrolyte by searching for various information, and then listen to the editor to analyze it one by one.

In traditional batteries, the electrolyte system with water as the solvent is usually used, but since the theoretical decomposition voltage of water is 1.23V, considering the overpotential of hydrogen or oxygen, the highest battery voltage of the electrolyte system with water as the solvent is only 2V. Left and right (such as lead-acid batteries); in lithium-ion batteries, the operating voltage of the battery is usually as high as 3~4V, and the traditional aqueous solution is no longer applicable, so a non-aqueous electrolyte system must be used as the electrolyte for lithium-ion batteries. The non-aqueous organic solvent is the main component of the electrolyte.

First, the important components of the electrolyte

The electrolyte is mainly composed of three parts:

(1) Solvent: cyclic carbonate (pC, EC); chain carbonate (DEC, DMC, EMC); carboxylate (MF, MA, EA, MA, Mp, etc.); (for dissolving lithium salts) )

(2) Lithium salts: LipF6, LiClO4, LiBF4, LiAsF6, etc.;

(3) Additives: film-forming additives, conductive additives, flame retardant additives, overcharge protection additives, additives to control the content of H2O and HF in the electrolyte, additives to improve low temperature performance, multifunctional additives;

Electrolytes used in lithium-ion batteries should generally meet the following basic requirements:

a. High ionic conductivity, generally should reach 1x10-3~2x10-2S/cm;

b. High thermal stability and chemical stability, no separation occurs in a wide voltage range;

c. Wide electrochemical window, keep the electrochemical performance stable in a wide voltage range;

d. Good compatibility with other parts of the battery such as electrode materials, electrode current collectors and separators;

e. Safe, non-toxic and non-polluting.

Physical and chemical parameters of important solvent components:

Reaction of important solvent components during charging:

A simple performance comparison of several commonly used lithium salts:

LiBF4: low temperature performance is better, but the price is expensive and the solubility is relatively low;

LipF6: The overall performance is better, but the disadvantage is that it is easy to absorb water and hydrolyze;

LiAsF6: The overall performance is better, but the toxicity is too large;

LiClO4: The overall performance is better, but the strong oxidizing property leads to low safety;

LiBOB: The high temperature performance is better, especially the insertion and destruction of the solvent to the negative electrode can be simulated, but the solubility is too low.

Reaction of electrolyte lithium salt during charging:

Some physical and chemical parameters of electrolyte lithium salt:

Second, the important classification of electrolyte additives:

Film-forming additives:

Excellent SEI film (solid electrolyte film) is intolerant to organic solvents, allowing lithium ions to freely enter and leave the electrode without solvent molecules passing through, thereby preventing the co-insertion of solvent molecules from destroying the electrode and improving the battery's performance 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 (fluoro, chloro and bromocarbonates, etc., which improve the central atom by the electron-withdrawing effect of halogen atoms. The ability to obtain electricity can make the additive reduce and effectively passivate the electrode surface under higher potential conditions to form a stable SEI film.) Another patent report of Sony Corporation, adding a trace amount of anisole to the non-aqueous electrolyte of lithium ion batteries or its halogenated derivatives, can improve the cycle performance of the battery and reduce the loss of the irreversible capacity of the battery.

Conductive Additives:

The research on additives that improve the conductivity of electrolytes focuses on improving the dissolution and ionization of conductive lithium salts and preventing the destruction of electrodes by solvent co-insertion.

According to the type of use, it can be divided into cationic type (mainly including some amines and aromatic heterocyclic compounds containing more than two nitrogen atoms in the molecule, crown ethers and cryptates), and anionic type (anionic ligands are mainly Some anion acceptor compounds, such as boron-based compounds) and electrolyte ion-use types (neutral ligand compounds are mainly compounds formed by some electron-rich groups bonded to electron-deficient atoms N or B, such as aza ethers and alkyl groups) boron).

Flame Retardant Additives:

As a commercial application, the safety of lithium-ion batteries is still an important factor restricting its application development. Lithium-ion batteries have many potential safety hazards themselves, such as high charging voltage, and the electrolytes are mostly organic flammables. If used improperly, the battery will be dangerous or even explode. Therefore, improving the stability of the electrolyte is an important method to improve the safety of lithium-ion batteries. Adding some high boiling point, high flash point and non-flammable solvents to the battery can improve the safety of the battery.

Importantly divided into (1) organic phosphide (2) organic fluorine compound (3) halogenated alkyl phosphate

Overcharge Protection Additives:

There has been extensive research on the method of using redox pairs for internal protection. The principle of this method is to add a suitable redox pair to the electrolyte, which does not participate in any chemical or electrochemical reaction during normal charging. , 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: R→O+ne-

Negative pole: O+ne-→R

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

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