Chapter 13 - Li-O and Li-S batteries

Question 1

What are the theoretical specific energy of Li-O2

Answer 1

~3500 Wh / kg

Question 2

What is the difference between an aqueous and non-aqueous Li-O2 cell?

Answer 2

Non-aqueous: Use organic electrolyte, and have 2Li+ ions react with 2 electrons and O2 to form Li2O2.

Aqueous: Use water based electrolyte. Must have a Li+-conducting membrance to keep water from the Li metal. At the cathode 2Li+ reacts with 2 electrons, 1/2 O2 and H2O to form 2LiOH. Includes a O2-evolution electrode as well.

Question 3

What are some shared challenges between Li-O and Li-S batteries?

Answer 3

1) To make use of Li-metal anode, as only improving the cathode will give only limited gains.
2) Formation of insulating species (Li2S2 and Li2O2)
3) Electrolyte must dissolve intermediate species, but also keep them in place

Question 4

What are the Li-metal problems for Li-O and Li-S?

Answer 4

The usual…

1) If using aqueous electrolyte, the reaction with water.
2) If using non-aqueous electrolyte, the formation of SEI

Question 5

What is the difference between Lithium-air and Lithium-O2?

Answer 5

Air:
Takes oxygen from the air. Requires filtering (and weight the comes with this). Need to remove CO2 as they form irreversible carbonates.

O2:
Pure O2-source. Pressurised for example? Need extra weight to carry tanks.

Question 6

What are typical overpoential in a Li-O2-battery?

Answer 6

Large overpotentials. Caused by difficult charge transfer. Redox mediators can improve kinetics.

Question 7

What are the difference in pathways between discharge and charge for Li-O batteries?

Answer 7

Discharge:
O2 + e- -> O2-
O2- + Li+ -> LiO2
2LiO2 -> Li2O2 + O2
(LiO2 + Li+ + e- -> Li2O2)
(Li2O2 + Li+ + e- -> 2 Li2O)

Charge:
Li2O2 -> 2Li+ + 2e- + O2

Question 8

What is the problem with Li2O2 formation in Li-air batteries?

Answer 8

If it forms on the surface, further reaction is blocked. Need to solvate but still keep it. Current focus on large proes to store it.

Question 9

What is the problem with carbon or carbon dioxide in Li-air batteries?

Answer 9

They trap Li in Li2CO3. Need to stabilise carbon scaffold by surface treatment.

Question 10

What is a redox mediator?

Answer 10

It is a charge carrying additive that improves charge transfer to inert Li2O2 surfaces. It reduces overpotentials and improves kinetics.

Question 11

What kind of non-aqueous electrolytes can we use?

Answer 11

Ethers is an OK choice, but not good. Parasitic reactions with oxygen.

Carbonates are no good - they react with O2^2-

Question 12

For a LiO2-battery with an aqueous electrolyte, how can we solve the issue of LiOH formation near the O2-source leading to blocking of O2-supply?

Answer 12

Membrance allowing LiOH to form in electrolyte outside cathode, combined with an extra O2-evolution cathode.

Question 13

What is the PolyPlus battery?

Answer 13

A commercial Lithium - O2 battery (primary). Uses solid electrolyte membrane. Uses the oxygen in the seawater. Seawater also functions as electrolyte.

Question 14

What are some pros and cons of LiS?

Answer 14

Pros:
Low weight of cathode

Cons:

• Low voltage
• Requires a lot of electrolyte
• Scaffolding required to handle elctron transport due to resistance of Li2S

Question 15

What kind of electrolytes can be used for Li-S?

Answer 15

Can use:

• Ethers
• Tetrahydrofuran (THF)
• 1,3 dioxolane DIOX
• Dimethoxyethane
• Tetra(ethylene glycol)-dimethyl ether (TEGDME)

Can’t use:
- Carbonates - reacts with sulfur

Question 16

Why do we need high electrolyte volume in Li-S batteries?

Answer 16

Polysulfides have low solubility

Question 17

What is important to consider when designing an aqeuous Li-S battery?

Answer 17

Keep pH of electrolyte > 7 to avoid formation of H2S.

Question 18

If using an aqeuous electrolyte for a Li-S battery, how can the cell be designed?

Answer 18

Anode separated from an aqueous polysulfide solution by a solid electrolyte (conducting Li+). Aqeous polysulfides in a porous electron transfer matrix.

Question 19

What is a major advantage of using an aqeuous Li-S battery?

Answer 19

Polysulfides have a high solubility in aqeuous electrolyte.

Question 20

What is the problem of the sulfur cathode?

Answer 20

Sulfur and discharge products are insulating in nature, causing low sulfur utilisation.

Question 21

What is done to solve the insulating nature of sulfur cathodes?

Answer 21

Making a carbon scaffolding for sulfur. Examples include microporous carbon-sulfur, highy ordered mesoporous carbon-sulfur, core-shell particles.

Graphene scaffolding can perhaps also be used.