Chapter 8 - Separators

Question 1

What is the point of a separator?

Answer 1

Electrical insulation. Should prevent cathode and anode from touching each other.

Question 2

What are the functions of a separator?

Answer 2

• Electronic insulation
• Chemical stability
• Electrochemical stability
• High mobility of positive ions (large porosity for liquid electrolyte or ion conductivity)
• Thin (low weight, low electrolyte consumption)
• Mechanically strong
• No deformation during wetting with electrolyte or when heated
• Homogenous (pore size, thickness, no holes)
• Shut down function
• Good behaviour after shut down
• Environmentally friendly
• Low cost

Question 3

What happened with the Samsung Note 7?

Answer 3

Main cause: High welding burrs on the positive electrodes. Caused penetration of the insulation tape and separator, and shorted the battery.

Additional cause: Missing insulation tape on some batteries.

Question 4

What are relevant material properties of separators?

Answer 4

Chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, molecular weight

Question 5

What are relevant structure properties of separators?

Answer 5

Thickness, porosity, pore size, pore shape, pore tortuosity, pore distribution

Question 6

What are relevant physical properties of separators?

Answer 6

Electric resisitivty, air permeability, mechanical strength, thermal shutdown, electrolyte wettability, electrolyte retention

Question 7

What is the typical thickness of a separator?

Answer 7

20-25 micrometers

Question 8

What are some measurements of permeability?

Answer 8

Macmullin - ionic conductivity of electrolyte soaked separator vs value of electrolyte alone.

Gurley - permeability for air

Question 9

What kind of separators are used?

Answer 9

Polyolefins
Non-woven film
Celgard 
Polyamide-reenforced PVdF
Dreamweaver
Composites
Gel-filled membranes
Anodic Aluminimum Oxide
PVDF (Polyvinylidene fluoride)
Polyacrylonitril
PMMA (Polymethyl methacrylate)

Question 10

What are polyolefin separators?

Answer 10

Polyolefin are polymers of simple alkene monomers. The most common are polyethylene and polypropylene, often in combination.

They are characterised by molecular weight and crystallinity.

Can make microporous polyolefins with a dry and wet process.

Question 11

How does degree of crystallinity affect polyolefin separators?

Answer 11

Low crystallinity = very liquid (0-20%)
Medium crystallinity = ductile thermoplastics (20-50%)
High crystallinity = rigid/brittle (>50%)

Question 12

What is the difference between polyolefins obtained from a dry vs wet process?

Answer 12

Wet process gives more tortuous membrane. Reduces the probability of dendrite penetrating, and increases resistance.

Wet process also gives more symmetric tensile strength.

Question 13

How can a mixing of polyolefin separators be used to protect against fire?

Answer 13

Melting point of PE is below that of PP. When temperature increases, PE melts, while PP maintains the structural integrity.

Question 14

What kind of separator is the celgard?

Answer 14

It is a ceramic coated separator. The ceramic improves temperature resistance, and acts as a HF scavanger.

Question 15

What are some pros and cons of non-woven films for separators?

Answer 15

Pros:
Good performance, low weight, high porosity, no direction selectivity of pores / fibres.

Cons:
Takes up much space, need much electrolyte. Large risk of particles penetrating. Large risk of dendrites penetrating.

Question 16

What are some pros and cons of anodic aluminium oxide as a separator?

Answer 16

Pros:

• Self-organised system
• Very low tortuosity
• Very high thermal stability

Cons:
- Brittle

Question 17

What are some ways we can physically modify the separators?

Answer 17

• Fill pores with gel polymer electrolyte (increases wettability and retention of electrolyte)
• Coating with surfactant (increases wettability)
• Inorganic filler (e.g. SiO nanoparticles. Increases wettability and add mechanical separation in case of separator melting)

Question 18

What are some ways we can chemically modify the separators?

Answer 18

• Covalent grafting of polar groups into separator surface (Increases hydrophilicity, increasing wettability)

Question 19

What are some pros and cons of a PVDF separator?

Answer 19

Pros:
Better wetting, retention and thermal properties than polyolefins

Cons:
Swells when filled with electrolyte. Possible side reactions with Li-salts forming LiF.

Question 20

What are some pros and cons of PAN separators?

Answer 20

Pros:
- Good processability, high thermal stability, high electrochemical stability. Polar C=N bond acts as Li+ channel, incrasing ion conductivity.

Cons:
- Suffer from electrolyte leakage during long term storage.

Question 21

What are some pros and cons of PMMA separators?

Answer 21

Pros:
- Good in almost all aspects

Cons:
- Poor mechanical strength - brittle.