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TEK9320 Battery Technology > Chapter 7 - Electrolytes > Flashcards

Chapter 7 - Electrolytes Flashcards

1
Q

What are the functions of an electrolyte?

A
  • Electrical insulator
  • Electrochemical stability (after initial cycles)
  • High mobility of positive ions
  • Low overpotentials from negative ion accumulations
  • Effective charge transfer to electrodes (low impedance from SEI layers, good wettability of electrodes and separator)
  • High temperature operating window
  • No phase transitions (freezing or evaporation)
  • Minimize fire probability (low flammability, large thermal window, overcharge protection)
  • Minimize hazard in case of fire (low toxicity, avoid HF gasses)
  • Environmentally friendly (production and use)
  • Low cost
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2
Q

What causes an electrolyte to be an electrical insulator and have a large electrochemical window?

A

Large band gap - that is large difference between HOMO and LUMO.

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3
Q

What are some linear carbonate solvents used in the electrolyte?

A

Diethyl carbonate (DEC) and dimethyl-carbonate (DMC)

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4
Q

What are som cyclic carbonate solvents used in the electrolyte?

A

Ethylene carbonate (EC) and propylene carbonate (PC)

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5
Q

What is the transference number of electrolytes?

A

The fraction of ionic mobility carried by the anion or the cation.

tA = mu_A / (mu_A + mu_C)

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6
Q

What should the transference number ideally be for Li-ion batteries?

A

Close to 1 (we want the the ionic mobility to be of Li+ ions)

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7
Q

What is a typical transference number of Li+ in organic solvents?

A

Pretty low - usually around 0.2-0.3

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8
Q

Why does Li+ has a low transference number in organic solvents?

A

Because Li+ usually coordinates up to 4 solvent molecules. This gives a high effective radius, and slow mobility.

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9
Q

What is the effect of concentration in weak and strong electrolytes respectively?

A

Weak: Dissolution decreases with concentration, so conductivity per mol decreases.

Strong: Viscosity increases due to ion-ion interaction, reducing conductivity.

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10
Q

How do EC and PC compare to DMC and DEC in terms of viscocity?

A

They are more viscous.

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11
Q

What is the electrolyte solvent to use for low temperature operations?

A

DEC - melting point at -74.3 C.

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12
Q

Which organic solvent causes exfoliation of graphite?

A

PC.

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13
Q

What is the most commonly used electrolyte solvent?

A

EC - typically mixed with DMC and DEC.

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14
Q

What is the purpose of fluorination of solvents?

A

It reduces viscocity, melting point, flammability. Increases surface tension. FEC for example has a higher oxidation potential than EC and can thus be beneficial for high voltage cathodes. Also gives more stable SEI on anode, in particular Si.

Limited use related to industrial use. Temperature range?

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15
Q

What role does electrolyte solvents have in SEI formation?

A

Most carbonates are not stable at 0.05 vs. Li. Requires formation of stable SEI layer to stop degradation.

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16
Q

What properties do we require of an SEI layer?

A
  • Ion conductive (must pass Li from electrolyte to anode)
  • Electron conductive (by tunneling or other mechanism)
  • Stable - not expose new surfaces. Should thus be strongly bonded to the active material and not dissolve in electrolyte.
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17
Q

Why does an SEI form?

A

Because the electrolyte is exposed to potentials where it is unstable (LUMO of electrolyte lies below the Fermi level of anode, causing reduction of electrolyte or HOMO lying above the Fermi level of cathode, casuing oxidation of electrolyte).

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18
Q

What is the SEI composed of?

A

Many different substances. Roughly speaking, it consists of two layers. The inner layer in mainly inorganic (dense, does not dissolve easily - Li2O, LiF, Li2CO3). The outer layer is mainly organic (,ore open, defects, dissolvable, polyolephines, semicarbonates).

19
Q

Why are there two different layers in the SEI?

A

In the start, two-electron processes dominate. But as the layer grows, the tunneling probability decreases.

20
Q

What can be done to control SEI formation?

A

You could add SEI forming salts (LIBOB, LiDFOB) or additives (VC, FEC).

21
Q

What are some alternative solvents to the carbonates?

A

Sulfones - high melting and boiling point, tolerates high potentials, but does not form good SEI.

Ethers - low oxidation potentials. Could be relevant for LiS and Li-air.

NItrils - High salt solubility, wide electrochemical stability window, low viscosity. However, easily reduces at the anode.

22
Q

What are possible lithium containing salts used for the electrolyte?

A

LiPF6 (most common) - Poor thermal stability, release HF.

Challengers:
LiBOB
LiFSI
LiFAP
LiTDI

Other:
LiBF4 - Low conductivity
LiAsF6 - toxic
LiClO4 - explosive

23
Q

What are some major drawbacks with LiPF6

A

Poor thermal stability and releases HF-gas.

24
Q

What are some major advantages with LiPF6

A

Good conductivity, passivation of Al current collector and compatability with alkylcarbonates for SEI forming

25
Q

What are some drawbacks and advantages of LiTFSI?

A

Pro:
Very thermally stable

Con:
Can corrode aluminium on cathode - however, OK in right concentrations

26
Q

What are some drawbacks and advantages of LiTDI?

A

Pros:

  • Easy, low-demanding, inexpensive, one-step, high yield synthesis
  • Salt is stable in air atmosphere
  • Thermally stable up to 250 C

Cons:
- Difficult to obtain with a high purity

27
Q

What are ionic liquids?

A

Ions that has a low melting point, so that at room temperature they are liquid.

28
Q

What are some major advantages of ionic liquids?

A
  • Chemically stable
  • Thermally stable
  • Large electrochemical window
  • No volatile
  • Flame retardant
29
Q

What are additives?

A

Additives are chemicals we add to the electrolyte for various reasons. Some reasons are:

  • Overcharge protection
  • Increase chemical stability
  • Increase thermal stability
  • SEI formation on anode and cathode
  • Increase wettability
  • Decrease flammability
  • Increase cycleability
30
Q

What are some pros and cons of using additives?

A

Pros:
- Enhances desireable properties

Cons:
- Adds cost and weight. (typically below 5% of the electrolyte)

31
Q

LiPF6 is very vulernable to traces of water and alcohol (causing HF release):

LiPF6 -> H2o -> LiF + POF3 + 2HF

What can be done to help this situation?

A

We can add additives. For example:

1) LiF to shift the equilibrium
2) We can use a Lewis base to trap PF5
3) Include H2O/HF scavangers

32
Q

How can additives be used as overcharge protectors?

A

1) Redox shuttles - redox potential 0.1-0.2 V higher than the max voltage. Reversible, but limited capacity. Cannot necessarily protect against thermal runaway.
2) Shutdown additives. Polymerisation on electrode surface. Irreversible.

33
Q

What kind of additives can be used as flame retardants?

A

1) Phosphates - scavange H-radicals in flame. Large wt% needed.
2) Cyclic phosphazenes - scavange H-radicals and coates oxide, blocking access to oxygen.
3) Ionic liquids - Reduces amount of flammable electrolyte,

34
Q

What different types of solid electrolytes do we distinguish between?

A

1) Polymer electrolytes

2) Inorganic electrolytes

35
Q

What is a bipolar architecture?

A

Series connected layers within a single casing. This will increase the voltage.

36
Q

How does the polymer chain PEO electrolyte work?

A

The negative charges on O in the PEO chains coordinates Li+ and “throws it around”. Gives good mobility. Would allow the use of a metal Li anode (and was already used as such in the Bolloré blue car)

Has to be in amorphous form though, which requires it to be at above 70 C (above the glass transition temperature)

37
Q

What are some tricks for improving polymer electrolytes?

A

1) Make use of block copolymers. One set gives high Li conductivity, the other giving mechanical stability.
2) Using other Li-host atoms than O, such as S.
3) Polyacrylonitrile - flexible N-site that the Li can use as a liane
4) Inclusion of Li salt - a large anion can remain locked in the polymer, causing no movement of it and giving a transference number close to 1.
5) Mixing polymers to reduce crystallinity - reducing glass transition temperature.
6) Comb-like polymer structures to increase Li mobility while maintaining mechanical strength.

38
Q

What are the ion transport mechanisms in crystals?

A

1) Vacancy (an ion jumping into a vacant site)
2) Direct interstitial (an interstitial jumping from interstitial to interstitial)
3) Correlated / interstitialcy (interstitial jumps to an occupied site which causes the other ion to jump into another interstitial)

39
Q

At what scales must the Li be mobile in an inorganic solid state electrolyte?

A

1) Within crystal grains
2) Over grain boundaries
3) From particle to particle
4) From electrolyte to active material

40
Q

Garnets were promising for a while - what happened?

A

Dendrite formation in grain boundaries were documented.

41
Q

What happens at the interfaces between cathode and solid state electrolyte?

A

Since the cathode pulls more strongly on Li than the electrolyte, it forms space charge regions due to Li migrating to the cathode.

Thin oxides surface layers with a high voltage can reduce this depletion.

42
Q

What are the most promising solid state electrolytes now?

A

Sulphide based electrolytes (Li10GeP2S12)

43
Q

What are some pros and cons of using sulfur in electrolytes vs. oxides?

A

Pros:

  • Sulfur allows greater Li mobility than oxygen
  • Softer material gives better mechanical contact
  • Softer material gives better stability with expanding anode materials

Cons:

  • Reduced electrochemical stability
  • Hygroscopic (forms H2S)
44
Q

What are the key selling point of solid electrolytes?

A

Better cell stacking. Solid electrolyte allows high temperature, which allows a low cooling rate. Low cooling rate allows close packing of cells which increase volumetric density on module level.

TEK9320 Battery Technology (15 decks)

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