1.Expression of the formula
1.1.The yellow part is the main electrolyte formulation for the experiment in this article, let's translate into the table below
1.2.So how to configure the electrolyte exactly?
From the figure above, it can be observed that the industrial prefers to use mass ratio expressions, which is significantly different from the volume ratio expressions commonly used in academic circles. This is largely due to the following reasons: 1. Industry is based on quality control unit in the production of electrolyte; 2. The volume of solvents is a function of temperature, and it is greatly affected by environmental temperature in practical use, making it unsuitable for large-scale industrial applications!
1.3.The solvent mass expression method has the following differences:
Addition method: when the laboratory test group is continued, in the case of the control group, the method of adding additional additives, this method is easy to compare and simple to operate when actually doing experiments, the disadvantage is that when you add the method, the actual mass fraction of the additives is diluted because the additives you add are diluted, which has little effect when the additives are small,but when the amount of additives is large, the impact on the mass fraction is obvious.
Internal addition: the quality of additives in each group of the experiment is certain, when you set it to 2%, the actual formula is 2%, which affects the mass proportion of the solvent. This kind of method, in the actual experiment, is troublesome to configure, and each recipe has to be specially configured. The advantage is that the mass fraction of the combination ratio of the additives is constant, and it is easy to explain the mass ratio expression to the customer, so it is not easy to misunderstand.
1.4.So how to configure the electrolyte exactly?
Through the above explanation, you should understand the deviation of industry and academia in the understanding of formula expressions. It is particularly emphasized here that the industry will not use the method of "constant volume" volume to configure an electrolyte with a certain molar concentration of lithium salt.
Back to the configuration steps in this case: (Control Group)
1.Find the density of EC and DEC.
2.According to the volume ratio of 1:2, the mass ratio is calculated to be ~40.4:59.6.
3.1M lithium salt at 12.5% wt.
4.The mass-to-mass ratio formula for the Control group is shown on the right
5.The additive group is set to 1 in the control group, and the corresponding amount of additives is (additive).
2.Analysis of electrolyte discoloration
2.1.We mentioned it in the previous lecture 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. The meaning of special types of electrolytes: high content of EC (such as formula EC content above 40% volume ratio) Electrolyte: high content of DMC (such as formula DMC content above 40% volume ratio) Electrolyte:high content of FEC (such as formula FEC content above 5% mass ratio) Electrolyte:high lithium salt solubility, lithium salt > 1.20M, electrolyte containing DTD and other special additives.
2.2.From the suggestions, we can know that the main factors affecting the discoloration of
electrolyte are:
1. LiPF6
2. Type and content of additives
3. Storage environment (container, temperature, etc.)
The factors involved in the problem of discoloration are mainly the above three categories, which mainly affect the generation of pigments brought about by transesterification reaction, lithium salt catalytic decomposition, solvent catalytic decomposition, and ultimately affect the color of the electrolyte?
2.3.Here we would like to share with you some of our experimental results, so that you
can have a certain feeling about the electrolyte discoloration：
50℃,10d,blank experiment, no LiPF6 was added, and the colors of the four groups did not
change
2.4.50℃,10d, add LiPF6, Degree of discoloration：DEC>EMC>DMC>EC~PC
2.5.Here we would like to share with you some of our experimental results, so that you can have a certain feeling about the electrolyte discoloration：
Room temperature, 40d, add LiPF6, the degree of discoloration：EP>PP>DEC>EMC>DMC>PC~EC
2.6.We can see the general discoloration rules of different solvents, but it is very difficult
to make predictions! Nevertheless, we have done experiments on monopole sheets! Hope
to inspire you in future experiments!
1.Simulates a full charge of the negative electrode: solvent（EC,EMC,DMC,DEC,PC,PP,EP
+12%LiPF6+Lithium flakes
2.60℃,7d, DEC>EMC>DMC>EC~PC
3.The DEC solvent on the right without lithium salt changes color under the immersion of the
Lithium flakes, indicating that DEC will react with the Lithium flakes. Experiments have
found that no other carbonate solvent reacts with Lithium flakes!
2.7.Simulates a fully charged cathode: solvent（EC,EMC,DMC,DEC,PC,PP,EP）+12%LiPF6+electrode（4.4v4.45v LCO）
1. 60℃,8d, 4.4vfully charged cathodeDEC>EMC>DMC，EC~PC
3.Criteria for judging the quality of electrolytes
3.1.Through these two lectures, we should know more about how to judge the quality index of electrolyte, and here we will roughly sort out the quality of electrolyte industry judgment standards.
4.Selection of electrolyte
4.1.Let's take the LCO system as a starting point to look at the cell cycling performance of the same electrolyte at different operating voltages
In the same electrolyte, with the increase of LCO voltage, the voltage of the cell increases, more and more Co ions precipitate, and the catalytic electrolyte decomposition will be faster under the action of high voltage, so the performance of the cell will decrease with the increase of voltage.Therefore, the electrolyte of each specific system can be optimized to a certain extent, and the effect of optimizing the performance of the battery cell has been achieved.
4.2.Regardless of the influence of factors such as cell design, cathode and anode material matching, cell type (buckle, aluminum shell, steel shell, soft pack) and other factors, we have listed the optimization strategies of electrolyte formulations under different systems for your reference.
4.3.Electrolyte strategy：
LCO: A very mature and widely used cathode material, there is no big problem with normal cycling at room temperature, only when high temperature requirements need to think about the problem of Co dissolution, then the role of ADN and SN in the nitrile.
LMO: Inhibition of Mn leaching is the key to this type of system, and the commonly used additive is LiBOB.
NCM333、-->NCA
The surface alkalinity of ternary materials increases with the increase of Ni content, and the Ni dissolution intensifies with the increase of Ni content. The formula of the electrolyte is simple to increase the content of PS, add nitriles, and inhibit the dissolution of transition metals.
4.4.40V LCO Under the action of different additives, the cycle is very obvious!
4.5.Electrolyte strategy:
1. Lithium flakes mainly consider the influence of the cathode!
2. Lithium flakes mainly consider the influence of the cathode!
5.The difference between commercial electrolyte vs university electrolyte
6.The expression of the formula
1M LiPF6 is commonly agreed to refer to 12.4%~12.5% wt by default in the industry, and the theoretical
1M lithium salt solubility on the far right of the figure below can be compared with the difference.
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8.
Q & A
In this Q&A session, Dr. Ke also answered all the questions raised by everyone.
Tommy: "Does adding EC to the electrolyte lower its freezing point?"
Dr. Ke:
"If it's for use in low-temperature environments, you can use EA/EP solvents."
Tommy: "What is the shelf life of the electrolyte at room temperature in the glove box?"
Dr. Ke:
"Generally, the optimal shelf life of electrolyte is three months, and it's best not to exceed six months."
0.0: "Why shouldn’t electrolytes be stored in glass containers?"
Dr. Ke:
"If the electrolyte contains hydrofluoric acid, it will corrode the glass."
Black: "In the 7th slide, how do you convert 1M to 12.5%?"
Dr. Ke:
"When the solvent and the type of lithium salt are fixed, you can calculate it based on the molar fraction."
Lithium-ion battery anode: "What should the normal water content of the electrolyte be?"
Dr. Ke:
"The normal water content should be less than 20 ppm."
Xi'an Jiaotong University: "How is the fully charged positive electrode treated before storage in different solvents? Do we need to soak it in DMC, dry it, and then store it?"
Dr. Ke:
"Yes, it needs to be cleaned with DMC. Soak it for about half an hour, then vacuum dry it. This can be done in the glove box vacuum channel."
Black: "Does high water content also cause discoloration?"
Dr. Ke:
"High water content will affect HF and may also lead to discoloration."
Kaiyou Xue - Lanzhou University of Technology - Electrolyte: "I have a question. If DEC can react with lithium metal, then shouldn’t LiPF6-DEC/EC electrolyte systems be used in half-cell tests?"
Dr. Ke:
"Yes, solvents like EC can passivate the lithium metal. Why doesn’t EC decompose? Because the lithium metal has a very low potential and its surface activity is very high."
Kaiyou Xue - Lanzhou University of Technology - Electrolyte: "What is the highest voltage electrolyte system you've developed?"
Dr. Ke:
"It depends on the material. Recently, using our nickel-manganese material with high-voltage electrolyte, we can reach 4.8V."
Kaiyou Xue - Lanzhou University of Technology - Electrolyte: "I personally doubt the conclusion that ‘fully charged LiCoO2 soaked in electrolyte turns red because of Co.’ I have tested other electrodes and they also turn red, even without Co."
Dr. Ke:
"This is a great observation, and we welcome everyone to discuss and supplement. Could it be that other metals from the cathode are leaching out and playing a similar role to Co?"
UESTC - Electrolyte: "Is low-temperature testing just charging at room temperature and discharging at low temperature?"
Dr. Ke:
"Low-temperature testing depends on the specific application. It includes low-temperature charging, discharging, or even low-temperature cycling."
Little Penguin: "What is pre-cycling?"
Kaiyou Xue - Lanzhou University of Technology - Electrolyte: "Pre-cycling is when the negative electrode is pre-formed with the SEI film, consuming some of the lithium ions from the half-cell so that the lithium ions from the positive electrode won't be consumed in the full-cell, reducing irreversible capacity loss."
Luke - Battery Design: "Does biphenyl cause the electrolyte to turn pink?"
Dr. Ke:
"We haven't tested biphenyl."
Black: "When considering lithium metal, what is the main impact from the positive electrode?"
Dr. Ke:
"Lithium metal is quite different from other electrodes, so it has fewer varieties to consider."
Huashu - Silicon Carbon: "Is the high consumption of electrolyte in silicon-carbon due to volume expansion?"
Dr. Ke:
"Yes, the deformation leads to the rupture of the SEI film, and more electrolyte is needed."
Black: "Does DEC improve high-temperature performance? Is DEC commonly used in low-temperature electrolytes?"
Dr. Ke:
"DEC performs better for high-temperature performance among chain esters like DMC, EMC, and DEC. For low-temperature electrolytes, it depends on the required low-temperature performance. If the requirement is high, DMC still dominates."
Black: "Can low-temperature electrolytes work without DEC?"
Dr. Ke:
"Yes, they can."
Lithium-ion battery anode: "If my electrolyte contains too much water, how can I remove the water?"
Dr. Ke:
"If there is excess water, it's recommended to reconfigure the electrolyte. Buy high-quality electrolyte and store it in the glove box as soon as possible. If needed, you can divide it into smaller bottles in the glove box for storage."
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