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Lithium-ion Full Cell Manufacturing Process Training--Summary of Processes

 1.Role and Selection Criteria of Separators

1.1.Role of Separator

1) The separator is an important component of lithium-ion batteries.

2) Electronic insulation and ionic conductivity.

 


 

1.2.Requirement of Separator

Characteristics of separators

1)Electronic insulation - mechanical isolation between the positive and negative electrodes;

2)Certain pore sizes and porosity, low resistance and high ionic conductivity, allowing good permeability for lithium ions;

3)Resistant to corrosion by electrolytes, with sufficient chemical and electrochemical stability;

4)Good wettability to electrolytes and sufficient ability to absorb and retain moisture;

5)Sufficient mechanical properties - such as puncture strength and tensile strength, while maintaining as thin a thickness as possible;

6)Good spatial stability and flatness;

7)Excellent thermal stability and automatic shutdown performance;

8)Higher requirements for separators in power batteries, typically using composite membranes.

 

1.3.Main Performance Parameters of Separators

 

2.Classification of Separators

  1. 1）Based on different physical and chemical properties, lithium battery separator materials can be classified into several categories such as woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, separator papers, and rolled membranes. 

  2. 2）Currently, commercial lithium battery separator materials mainly adopt polyethylene and polypropylene microporous membranes. 

  3. 3）Solid and gel electrolytes serve as a special component, fulfilling the roles of both electrolyte and battery separator in solid-state batteries. 

In the future, solid-state electrolytes are expected to replace the functions of separators and electrolytes.

 

3.Adhesive-Coated Separator

3.1 Introduction to Base Membranes - PE and PP

1）In the dry process, the crystalline interface of the crystalline polymer film is peeled off at high temperatures to form a porous structure, which can increase the pore size of the film.

 

2）The wet process, also known as thermally induced phase separation, involves adding high-boiling-point small molecules as porogens to polyolefins. The mixture is heated and melted into a uniform system, then cooled to undergo phase separation. After stretching, the small molecules are extracted using organic solvents to produce interconnected microporous membrane materials.

 

3.2 Comparison of Production Processes

 

3.3 Base Separator Introduction -PP/PE/PP

The three-layer composite separator retains the low melting point and heat sealing function of the middle PE layer, while maintaining the high melting point and safety of the PP separator.

 
 

3.4 Ceramic Separator

1）The ceramic particle-coated separator is based on a base membrane, with a layer of Al2O3, SiO2, Mg(OH)2, or other inorganic ceramic particles with excellent heat resistance coated on its surface. This coating enhances the separator's resistance to high temperatures, thermal shrinkage, and puncture strength, thereby improving the safety performance of the battery.

2）The ceramic composite layer addresses two critical issues. Firstly, it addresses the safety concerns associated with thermal runaway caused by thermal shrinkage of PP and PE separators, which can lead to battery combustion and explosions. Secondly, the ceramic composite separator exhibits excellent wettability and absorption capacity with electrolyte and positive and negative electrode materials, significantly extending the battery's lifespan.

 

3.5 Adhesive-Coated Separator

1）High-energy-density batteries are prone to deformation due to significant expansion.

 
 

2）Adopted adhesive-coated separators. By applying an adhesive coating, the separator can be bonded to both the positive and negative electrodes. Improves the battery's stiffness and effectively reduces deformation.

 
 

4. New Separator Introduction—Cellulose

1）The cellulose separator is characterized by its numerous through-holes, resulting in a very high porosity and excellent rate performance.

2）Furthermore, recent advancements in manufacturing processes have enabled the creation of straight through-holes directly on the separator. This innovation effectively improves the separator's ion conductivity, further enhancing its rate performance.

 
 
 

4.1.New-type Separators Introduction—PET Ceramic Separator

   
 

5.Development Direction of Separators

Low Cost

Functionalization: High Energy Density, High Rate, High Safety, High Conductivity

 

6.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

 

7.Q & A

During this Q&A session, Dr. Ke carefully addressed each of the questions raised by the participants.

 

Yangyang - Xidian University - Lithium Battery: "Can you briefly explain which types of separators are used in lithium-ion batteries, lithium-sulfur batteries, etc.?"

Dr. Ke:

"For lithium batteries, we generally use PP or PE separators, and for special performance requirements, we might treat them with ceramic coatings or adhesives. Domestic separators have become very popular, with Enjie and Xingyuan being the two largest suppliers in China. For lithium-sulfur batteries, we also primarily use lithium-ion battery separators, but modifications to the separator are made to reduce polysulfide shuttle effects."

Heihei: "Dr. Ke, if the porosity is smaller, the liquid absorption is lower, so the liquid retention will also be lower, right? Does liquid retention also relate to the tortuosity coefficient and pore size? Would a higher tortuosity coefficient and smaller pore size lead to greater liquid retention?"

Dr. Ke:

"Liquid retention is directly proportional to porosity and isn't significantly affected by tortuosity. Tortuosity mainly represents the ion transport path and is used to describe the ion conductivity of the separator. Currently, commercial separators have good wettability, and the pore size is generally sufficient for good liquid infiltration, so I don't think liquid retention is heavily influenced by pore size."

Yangyang - Xidian University - Lithium Battery: "When assembling the battery, do you apply PVDF to the separator?"

Dr. Ke:

"Yes, PVDF is applied to the separator, but this is typically done during the manufacturing process, not during assembly. Inorganic ceramics also contain some adhesive to ensure the ceramic and separator bond together."

Yangyang - Xidian University - Lithium Battery: "Is it the pre-prepared PVDF/NMP mixture? Do you use it before it dries?"

Dr. Ke:

"The PVDF used is usually water-based, meaning it's in an emulsion form. Of course, there are also PVDFs that dissolve in NMP, but these require a dip-coating process."

Yangyang - Xidian University - Lithium Battery: "Would this affect the conductivity of the positive electrode?"

Dr. Ke:

"Just like the separator, the PVDF-coated separator has porosity, so the electrolyte can freely pass through."

Heihei: "Does a higher porosity cellulose separator lead to higher self-discharge?"

Dr. Ke:

"Yes, cellulose separators with higher porosity tend to have higher self-discharge."

Heihei: "Is the issue of the separator having low tensile strength and wrinkling due to deformation? Does the shear resistance being weak indicate insufficient strength in the transverse direction?"

Dr. Ke:

"This issue should be analyzed on a case-by-case basis. You are referring to PP separators, right? Typically, PE separators don't have tearing problems, but PP does. The SEM structure of PP separators can help explain this."

Heihei: "If the separator wrinkles, does it indicate deformation?"

Dr. Ke:

"You can disassemble a battery with wrinkles and inspect the separator to determine if deformation has occurred. I haven't encountered a situation where PP separators easily wrinkle. Even with PE separators, different manufacturers' products can vary. The softer separators tend to show different shrinkage behaviors compared to harder ones, so adjusting the winding tension may be necessary. Softer separators can buffer the stress more easily. PP separators, which are stretched during production, have low strength in the transverse direction, making them more prone to tearing along the machine direction."

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