1.Lithium-ion battery electrolyte
Lithium-ion battery electrolysis is a mixture of organic solvents and lithium salts, plus functional additives. The electrolyte itself is electrically neutral, and conducts ions but not electrons in the battery, and it is one of the important components to ensure the normal operation of lithium-ion batteries
1.1.Role of Electrolyte
Lithium-ion battery electrolyte is an important medium responsible for the shuttle of lithium
ions in the positive and negative electrodes;
Good electrochemical stability over a wide range of voltages;
Good electrochemical and chemical stability over a wide range of temperatures;
Good ion transmission performance;
It can form a good interface at the positive and negative electrodes to ensure the normal
operation of the battery cell performance.
2.Common composition of electrolyte
2.1.Solvents
1.EC: Polar solvent, dissolves lithium salt and has film-forming effect, is an essential component.
2.DMC: weakly polar solvent, low viscosity, conducive to the increase of conductivity, mostly used for rate type and electrolyte requiring good wettability.
3.EMC: Easy to decompose into DMC and DEC in small amounts, and is mostly used in aluminum shell batteries with EC.
4.DEC: High boiling point, mixed with EC and PC, mostly used for high-temperature electrolyte.
5.PC: Polar solvent with high boiling point, used for high-temperature storage electrolyte, but poor compatibility with natural graphite.
2.2.Lithium salts
LiPF6：good conductivity, outstanding chemical and electrochemical stability, disadvantage is slightly poor thermal stability and easy hydrolysis, is the main salt in the current commercial application of formulations.
2.3.Common additives
1.VC：Vinylene carbonate, containing unsaturated double bonds, is chemically more reactive than PC and EC, and can break the double bonds at higher decomposition potentials than PC and EC during discharge, forming a network of macromolecules involved in the formation of the SEI layer. Its usage is generally no more than 2%, which can effectively reduce the first capacity loss of lithium-ion batteries, improve the stability of the SEI layer at high temperatures, and increase the cycle life.
2.FEC: Fluoroethylene carbonate, FEC has one more -F substituent group than EC, and this group has a strong ability to absorb electrons, so it can be explained that at higher potentials, FEC can undergo reductive decomposition reactions. The circulation efficiency and safety performance of the electrolyte in the electrolyte formed by fluorocarbonate are also improved.
2.4.Additives
1.PS: 1,3-propanesulfonolactone, a high-temperature additive, can form an effective protective film at the cathode, so that the reactivity between the cathode and the electrolyte is reduced, and the high-temperature performance of the battery cell can be well improved.
2.From the data, it is clear that VC and FEC are beneficial to the cell cycle, while PS alone does not bring beneficial help to the cycle.
3. Physicochemical properties and test
methods of electrolytes
These five indicators are the basic requirements of the industry for electrolyte!
3.1. HF and moisture of the electrolyte
1. When LiPF6 is dissociated directly in the aqueous solution, the change of HF is relatively constant.
2. When the LiPF6 electrolyte (LiPF6 is dissolved in an organic solvent), the following reactions occur to promote the production of more HF:
3.Therefore, in the process of ideal electrolyte configuration, the moisture of the solvent pack before adding salt should be strictly controlled, preferably below 10ppm, and then added main salt, and the temperature of the solvent pack during the addition process should also be controlled, preferably not more than 40°C (special formulas should also have special requirements). The result of the electrolyte configuration should be to control the lowest HF and moisture content.
3.2. Conductivity and density of the electrolyte
Conductivity and density are equivalent to the abscissa and ordinate of an electrolyte
formulation, and when the formula is constant, the conductivity and density should be constant.
1.The density increases with the solubility of lithium salt;
2.The conductivity begins to decrease as the solubility of the lithium salt increases to an extreme point. To put it simply: conductivity = electrolyte viscosity + lithium salt solubility
The density and conductivity of the electrolyte are important indicators to measure the stability of each batch of electrolyte!
3.3 The chromaticity of the electrolyte
There are several factors that affect the color of the electrolyte:
1.Solid particles may puncture the separator and cause a short circuit in the battery;
2.Special additives: Special additives may dissolved in the electrolyte, and the color will be high;
3. Storage environment: Temperature plays a key role in it.
Recommendations for sample-level electrolyte storage:
1.The best storage conditions for conventional electrolyte: in the glove box (≤5ppm), temperature ≤ 24°C;
2. Storage container selection: aluminum bottle, steel cylinder, fluorinated bottle, glass container is not recommended;
3. Special type of electrolyte: under closed conditions, the recommended storage temperature is below 20°C.
4.Meaning of special types of electrolytes:
(1)High EC content (e.g., formula EC content above 40% volume ratio) electrolyte;
(2) Electrolyte with high content of DMC (e.g., formula DMC content above 40% volume ratio);
(3) Electrolyte with high content of FEC (e.g., formula FEC content above 5% mass ratio);
(4) High lithium salt solubility, lithium salt >1.20M, electrolyte with DTD and other special additives.
3.4 Development Direction
1.Wide temperature range requirements:
In the field of conventional digital and power applications, it is a technical problem for electrolytes to be urgently broken through to expand the use of high-temperature electrolytes at lower temperatures, or low-temperature electrolytes to be used at higher temperatures.
2.High safety electrolyte:
A safe electrolyte means sacrificing room temperature performance. The heat of the current gel electrolyte or solid electrolyte is due to the fact that the main solvent of the current electrolyte is a flammable organic solvent.
3.High pressure electrolyte:
The voltage resistance capacity of electrolytes is closely related to the development of cathodes. Electrolytes under CV conditions may be able to reach 5V, but in actual application scenarios, they may not even be able to withstand 4.3V.
4.Functional development:
To adapt to different application scenarios, the demand for electrolyte is becoming more and more diverse. As the easiest of the four main materials to adjust, the flexible and changeable requirements for the electrolyte are getting higher and higher.
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5.
Q & A
In this Q&A session, Dr. Ke provided detailed answers to all the questions posed by participants.
Qingya Pai @Chunzhen: "How is water usually removed from electrolytes?"
Dr. Ke:
"We use molecular sieves to remove water. It's best to remove water during the salt drying process when preparing the electrolyte. Post-removal of water is quite troublesome."
Xiao Penguin: "Could you clarify if the poor electrochemical stability of DMC is based on literature or experimental results?"
Dr. Ke:
"From our experiments, DMC has poor oxidation resistance, performs poorly under high voltage and high-nickel cycles, and lacks stability at high temperatures. However, it is highly suitable for low-temperature battery systems."
Shenzhen Lithium Battery: "Could you share the common electrolyte formulation types?"
Dr. Ke:
"Lithium battery formulations primarily involve EC, PC, and DEC solvents, with additives tailored for specific applications such as high temperature, cycling performance, or discharge rates. For low-temperature applications, EMC and DMC esters are used."
Hei Hei: "What systems is EMC suitable for?"
Dr. Ke:
"EMC can be applied across conventional systems."
H.D. Shisan: "Does DMC tolerate high voltage?"
Dr. Ke:
"Our tests show that DMC has poor oxidation resistance, performs poorly under high voltage and high-nickel cycles, and tends to generate gas in full cells. Two major issues with EMC and DMC are: (1) poor high-temperature performance due to low boiling points; (2) higher gas generation towards the end of battery life. However, they do offer advantages such as good low-temperature performance and discharge rates. For high-nickel systems, DEC and EMC are better chain-ester choices over DMC."
Materials Development: "Why does common SBR exhibit significant swelling in PP?"
Dr. Ke:
"The swelling of polymers is related to the polarity between the solvent and polymer, following the 'like dissolves like' principle."
Misha: "Regarding electrolyte: Can wide-temperature lithium battery electrolytes maintain a conductivity of >8 ms/cm at -60°C while remaining stable at 55°C under current technologies?"
Dr. Ke:
"-60°C is very low; typically, the limit is -40°C. From the perspective of solvent selection, achieving this seems possible. For instance, ether-based solvents have shown potential in literature. However, the performance at 55°C high temperatures would need testing."
Small Tall: "Could you introduce general SEI film formation methods?"
Dr. Ke:
"SEI formation depends on the active material and the specific organic additives used. In practice, different materials require different additives to achieve optimal performance. For instance, ordinary graphite performs adequately with EC as the film-forming organic compound, but for silicon-carbon materials, additives like VC or FEC are essential."
😆: "I'm using PC as a solvent, LiDFOB as lithium salt, and a Celgard 2325 separator. Why does my LFP to lithium metal system show no capacity?"
Dr. Ke:
"I suspect the viscosity of your electrolyte is too high, leading to low conductivity and excessive internal resistance. Check the charge-discharge curves for confirmation."
Gradual Formation: "In my lithium-sulfur battery, the capacity is stable at 0.1C but increases from 100 upwards at 1C during cycling. Could this be related to the conductivity of the electrolyte? My material is a transition metal nitride with good conductivity, and the lithium-sulfur electrolyte is DOL
= 1:1 with LiTFSI (1M) and LiNO3 (0.1M)."
Dr. Ke:
"Your battery likely exhibits large polarization and poor rate performance. At high rates, not all materials fully participate in charge-discharge reactions initially. Enhancing electrolyte conductivity may help. Also, review the charge-discharge curves after 100 cycles to check for other reactions, as wettability shouldn’t typically impact cycles up to 100."
Materials Development: "Why add EC to prevent coated electrode cracking during preparation?"
Dr. Ke:
"Adding EC adjusts the solvent evaporation rate. When electrode cracks occur during coating, it's often due to excessively high inlet temperatures causing rapid solvent evaporation and surface
tension shrinkage. Since EC is part of the electrolyte composition, it won't affect performance."
Angel Offline: "Are silicon-carbon electrolytes compatible with graphite solvents?"
Dr. Ke:
"No, silicon anodes have stricter requirements for film formation compared to graphite."
Hei Hei: "Can EMC be replaced with other solvents like DEC?"
Dr. Ke:
"EMC combines the properties of DEC and DMC, making it a versatile and widely used solvent in industry. While DEC outperforms EMC at high temperatures, and DMC excels in rate performance, EMC offers a balance of properties."
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