1.Baking
1.1.The main purpose of baking is to remove moisture from the bare cell
H2O can cause the decomposition of LiPF6, leading to an increase in HF levels:
H2O can react with organic solvents in the electrolyte to produce alcohol and CO2, for example:
During the formation process, H2O can decompose, producing H2, consuming lithium ions,
reducing the initial efficiency and capacity of the battery, and damaging the battery interface.
1.2.For some special materials, such as lithium iron phosphate and lithium titanate with large
specific surface areas, the general control standard is overall < 400ppm due to their strong water absorption properties.
Moisture control targets for lithium battery baking (taking lithium cobalt oxide as an example):
1.3.Three key factors for controlling baking: temperature, vacuum level, and time
Increasing the temperature is a viable method, but due to the poor heat resistance of the
separator in the cell, the general baking temperature for the cell is below 85°C. In order to improve efficiency and shorten the baking time, increasing the vacuum level has become a practical solution.
1.4.Three modes of heat transfer: conduction, radiation, and convection
In a vacuum state, a conventional oven can only rely on thermal radiation for heat transfer,
while a contact-type oven can achieve rapid heat transfer through both thermal conduction and radiation
2.Water Content Testing
The Karl Fischer method is a recognized water content testing method widely used in the lithium battery industry.
Methods for Testing Water Content
3.Liquid Injection
1)Quality Requirements for Canrd Electrolyte
The moisture content in the bare cell, excluding materials such as the positive electrode, negative electrode, and separator, needs to be controlled through baking. The moisture content in the electrolyte also needs to be strictly controlled (moisture ≤ 20ppm)
2)Selection of Liquid Injection Volume
(1) Firstly, based on the true density of the material, porosity, and compaction density of the electrode sheet, the theoretical demand for electrolyte (i.e., the minimum amount of electrolyte required) can be estimated.
(2) On this basis, considering the size of the battery, application requirements (such as rate-type, energy-type, etc.), and incorporating a certain amount of excess, determined the injection volume Common injection coefficients: lithium cobaltate system ~2.0g/Ah, ternary system 3.0g/Ah.
4.Standing
Purpose of Standing
(1)he main purpose of standing is to allow the electrolyte to fully infiltrate into the interior of the electrode sheets and the pores of the separator.
(2)To accelerate infiltration, the standing process is conducted under vacuum conditions, which speeds up the infiltration of the electrolyte.
(3)Since the electrolyte can only infiltrate from the head and tail of the wound cell, a certain temperature and time are required to ensure the infiltration effect (if the cell is thick and long, the infiltration conditions need to be optimized).
5.Moisture Effects
Effects of Moisture on Battery Performance
LiF and Li2CO3 are the main components of the SEI. Therefore, an appropriate amount of water
(below 150ppm) helps to form a stable, uniform, and dense SEI film dominated by Li2CO3. Once the SEI film completely covers the negative electrode, the irreversible reactions stop immediately.
After the formation of SEI, if water is still present in the battery (water content above 150ppm),
the H2O will continue to consume active Li and cause the decomposition of LiPF6, leading to battery bloating, increased internal resistance, and dissolution of the SEI, which deteriorates the battery performance.
1)The presence of H2O in a battery during the initial charging process leads to the following reaction at the negative electrode:
This process consumes active Li, reducing the battery's cycle efficiency. Additionally, the generated H2 can increase the internal pressure of the battery, affecting the consistency of the first charging current distribution. The LiOH and Li2O produced by the aforementioned reaction eventually react with HF to form LiF, which deposits on the surface of the negative electrode.
2)During the initial charging process, the solvent can also be reduced at the negative electrode, consuming active Li:
3)The alkyl lithium carbonate produced by the single-electron reduction process can further react with trace amounts of water in the electrolyte:
4)After generation CO2, further reactions occur on the surface of the negative electrode:
6.HF Effects
Effects of HF on Battery Performance:
1)HF in the electrolyte undergoes the following reactions:
2)The LiF produced by this reaction, while thermally stable, exhibits poor lithium conductivity, leading to increased impedance at the electrode/electrolyte interface and subsequently higher internal resistance within the battery
3)HF dissolves the SEI film by reacting with Li2CO3 present in the SEI, generating LiF. This not only compromises the stability and density of the SEI but also increases the interfacial impedance of the electrode. Additionally, the H2O and alcohols produced by this reaction can, in turn, promote the hydrolysis of LiPF6 to produce more HF. This process continues in a cycle, leading to a gradual decrease in battery capacity, a decline in cycle efficiency, and ultimately severe degradation of performance.
Moreover, HF in the electrolyte can react with oxide cathode materials, causing the dissolution of metal ions from the cathode. For instance, LiMn2O4 is particularly susceptible to Mn ion dissolution in electrolytes containing LiPF6, resulting in degraded performance:
7.Canrd Brief Introduce
Canrd use high battery R&D technology(core members are from CATL) and strong Chinese supply chain to help many foreign companies with fast R&D. We provide lab materials, electrodes, custom dry cells, material evaluation, perfomance and test, coin/pouch/cylindrical cell equipment line, and other R&D services.
Email： contact@canrd.com Phone/Wechat/WhatsApp： +86 19867737979
Canrd Official Web Canrd Company Vedio Canrd Company profile
Website : www.canrud.com
8.
Q & A
HF has three main effects:
1.Formation of LiF: HF generates LiF, which becomes a component of the SEI (solid electrolyte interphase) layer, leading to an increase in internal resistance.
2.Dissolution of SEI: HF dissolves the SEI layer, compromising its stability and compactness, which results in degraded cycling performance.
3.Impact on cathode materials: For certain cathode materials, HF can cause the dissolution of metal ions, leading to performance degradation.
The key takeaway from today’s lesson is that it is crucial to strictly control the moisture content in both the materials and the electrolyte to achieve high-performance batteries. Additionally, the SEI layer will be discussed further in the next session, focusing on the formation process. Stay tuned!
This concludes today's lesson. Thank you all!
During this Q&A session, Dr. Ke addressed each of the questions raised by the audience one by one.
韬光养晦: "Why is the temperature controlled by the separator? Are the electrode sheets and separator dried together?"
Dr. Ke:
"This course focuses on the entire cell manufacturing process. When the whole cell is baked, it exists in its assembled form, which already includes the separator."
Steve-NJTECH-Lithium Silicon Anode: "After baking at 80°C in a vacuum oven, can electrode sheets be taken out immediately? Could thermal stress cause cracking, and is gradual cooling necessary?"
Dr. Ke:
"This situation should not cause the phenomenon you mentioned. Cracking in electrode sheets is usually due to other factors and not directly related to temperature drops after opening the oven. However, the biggest issue with opening the oven is that air exposure can cause the dried electrode sheets to quickly absorb moisture, exceeding acceptable levels. For coin cells, electrode sheets are baked after being cut: negative electrodes are typically baked at 85°C, while positive electrodes can go above 100°C or be baked at 85°C as well."
BlackBlack: "Will sulfur volatilize during lithium-sulfur electrode baking?"
Dr. Ke:
"Sulfur sublimates at around 95°C, and the particle morphology of sulfur also influences sublimation. For lithium-sulfur electrodes, experimental optimization of baking conditions is necessary."
BlackBlack: "After cutting, positive electrode sheets sometimes crumble or lose material during baking. What causes this?"
Dr. Ke:
"If this happens after cutting, check whether the material in the electrode sheet changes at high temperatures. It seems that the bonding strength has decreased. We discussed material detachment during coating in a previous course; you can review the course notes. Key factors include slurry formulation, current collector, and coating process."
BlackBlack: "For moisture measurement, how much sample weight is typically used for electrode sheets and electrolytes?"
Dr. Ke:
"Electrode sheets are about 2g, and electrolytes are a few milliliters."
Jacosec: "How should electrode sheets be sampled for moisture testing?"
Dr. Ke:
"Typically, bare cells are disassembled, and small strips are cut in a dry room using scissors. These strips are sealed in a specialized glass container and sent for testing."
Angel Offline: "Can the SEI layer be observed with SEM?"
Dr. Ke:
"No, it’s typically only tens of nanometers thick. You’ll need TEM for that."
Nitrobenzene: "Are electrode sheets baked continuously in the oven or baked right before assembly?"
Dr. Ke:
"They need to be baked right before assembly."
Bizarre Raccoon: "Does SEI exist on both the positive and negative electrodes?"
Dr. Ke:
"Yes, SEI exists on both electrodes, but we usually focus on the negative electrode SEI because it has a greater impact on battery performance."
Nitrobenzene: "Are there specific temperature and time requirements?"
Dr. Ke:
"The best practice is to ensure that the electrode sheets do not contact air after drying and are transferred directly into the glove box. This prevents moisture absorption."
Steve-NJTECH-Lithium Silicon Anode: "How can electrode sheets be transferred directly into the glove box without air exposure in a lab setting?"
Dr. Ke:
"Glove boxes now have large compartments equipped with heating functionality."
Bizarre Raccoon: "Is SEI only present in the first few cycles? Does its thickness and stability relate to the voltage range?"
Dr. Ke:
"SEI forms during the first cycle and becomes relatively dense in the initial cycles. However, throughout the battery's life cycle, it is constantly damaged and repaired. Its thickness and stability are indeed influenced by the voltage range, along with many other factors. We can discuss this further after the next session on 'Formation Process Introduction.'"
Melancholy Wanderer: "If cell moisture is controlled at 250ppm and electrolyte moisture at 20ppm, can moisture transfer from the cell to the electrolyte?"
Dr. Ke:
"Yes, transfer is possible, but not guaranteed. One is in the solid phase, and the other in the liquid phase. Moisture measured in the solid phase includes both free and bound water, whereas liquid phase moisture is entirely free water. In actual production, these parameters are determined through experimental results. Moisture in the electrolyte must be controlled because it can generate HF, while moisture in the electrode sheets is often consumed or decomposed during the formation process. Both have moisture control requirements, but the standards differ."
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