Energy Materials

Question 1

Describe the components of a Li-ion battery with a graphite based anode. Describe how lithium enters into the anode on charging. What it understood with “staging”?

Answer 1

• Cathode (during discharge): Li-transition metal oxide as positive pole with an attached Al current collector
• Anode (during discharge): Graphite/carbon negative pole, connected to Cu current collector
• Electrolyte: Organic, containing a dissolved/dissociated Li-salt (water free environment)
• Separator: A permeable membrane, separating the two electrodes, which prevents a potential shortcircuit

Question 2

Why is the liquid electrolyte not always stable at the interface towards the anode and cathode materials?

Answer 2

Very reducing potential at the anode side and very oxidizing potential at the cathode side.

Question 3

Which mechanism can possibly make a Li-ion battery unsafe, and trigger thermal runaway?

Answer 3

Oxygen evolution at the cathode side - if overcharged - alternatively formation of reactive peroxide.

Overcharging/discharging, oxygen evolution, Li-dendritic growth, physical damage….

1) Heating
2) Break-down of protective layer
3) Electrolyte breaks down into flammable gas
4) Separator melts, possible cause of short circuit
5) Cathode break-down, oxygen generation

Question 4

What is meant by the “SEI layer”? Describe what mechanism might take place at the interface between the electrode materials and the electrolyte.

Answer 4

Solid Electrolyte Interface:

https://www.youtube.com/watch?v=s0nuyWN_et4

Question 5

Which chemical parameters are essential for providing a high energy Li-ion battery cell? Consider redox aspects of the cathode and anode materials.

Answer 5

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Question 6

What could be a positive role of an ultrathin coating layer with an inert material on the fine particles of the cathode material?

Answer 6

It may prevent dissolution of cations into the electrolyte. It may stabilize the particle from breaking up - at conditions where lithiation/delithiation result in major volume work owing to differences in molar volume between the involved compounds.

Question 7

Explain what is understood with an ionic liquid, and provide one example.

Answer 7

• Negligible vapor pressures
• High ionic conductivities
• Wide electrochemical stability window
• Thermally stable
• Easily dissolve lithium salts

(slide 78 LN_29-30)

Question 8

Why can a solid electrolye be beneficial as replacement of the liquid electrolyte? What is the main challenge for having a well working solid electrolyte that may facilitate reasonable charge/discharge times? What is understood wih a charge rate of 1C?

Answer 8

Benefits:

• Avoid organic flammable liquid
• Avoid formation of SEI; loss of electroactive material
• Can have higher potential for the cathode material
• Reduced risk for thermal runways/short circuiting

Challenges:
- Sufficiently high ionic conductivity

Question 9

Why is it important to add conductive carbons to the powder mix that is used to fabricate the electrodes? What is typically the electronic properties of the redox active materials of the cathode?

Answer 9

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Question 10

Are such ion-batteries limited to lithium? Suggest other possible ions as “rocking ions”, shuffling the charge between anode and cathode. What could be the challenges/open questions?

Answer 10

Na & K (I), Mg & Ca (II)
Issues:
- Potential, not much change
- Anode materials (graphite not cool with Na)
- Diffusion divalent cations - too slow
- Lack of structural stability by removing 2+ cations

Question 11

What is the performance problem with alloying anodes, e.g silicon? What can be done to improve their cycling stability?

Answer 11

Main issue:

• Huge volume work
• Breaking up of particles
• Loss of connectivity to the conducting network of carbons