Chapter 5 - Cathodes Flashcards
What are the demands on a cathode material?
- High electrochemical potential vs. Li
- Mechanical stability (low volume expanion during cycling, structure loss / irreversible phase changes, electrochemical stability of everything expect the Li)
- Electrical conductance
- Safety in case of overcharge
- Lithium diffusivity
- Good contact to percolating network (for electron transport)
- Good contact to electrolyte (low ion transport activation energy)
- Compatability with available electrolytes
- Specific capacity
- Cost, abundant material, environmentally friendly ++
How can we look for material with a high electrochemical potential?
1) We look for high electronegativity. Main candidates are F, O, Cl and S. Oxygen is good candidate.
2) Find element which can keep the electronegative material in place while allowing a redox couple (electron not part of the binding structure). Transition metals are good candidates.
3) Find an element that will be covalently bonded to the low electronegativity element to stabilize the structure (Optional). Inductive effect, through e.g. the P-O bond in PO4.
Given that a cathode makes up 30% of the total battery weight, what do we get from a 1V increase in operating voltage, from 4 V to 5 V?
The potential is valid for the whole battery. So it will give a 100% - (5V / 4V)*100 = 25% increase.
Given that a cathode makes up 30% of the total battery weight, what do we get from a 200 to 400 mAh g^-1 increase?
With a 100% increase in capacity, we now need 15% less of the total weight of the battery for the same amount of energy.
What was the first layered attempt at a cathode?
TiS2 (Whittingham). Low voltage (1.4 - 1.9 V vs. graphite). Ok cycleability. Better conducitivity than many present oxides.
What is the classic option for a cathode?
LiCoO2 (Goodenough).
Which trend does the battery potential follow looking at the periodic table?
Higher filling of d-band, higher potential.
How does the specific capacity change with the periodic table?
Higher weight, lower specific capacity. Going towards the right in the d-block gives lower speciifc capacity. However, the increase in potential gives us a higher specific energy.
Can we use a layered LiMnO2 cathode?
No - Mn3+ is not stable in octahedral site, and will diffuse to the tetrahedral sites and form a spinel, LiMn2O4. Reduces performance.
Can we use a layered LiNiO2 cathode?
A pure LiNiO2 cathode is not used, but going towards higher and higher Ni-content. Still some Co and Mn left.
What can be said about the LiMn0.5Ni0.5O2 as a cathode?
Ni is active as long as only half the Li is extracted. Mn maintains the structure.
How is LiMn0.5Ni0.5O2 prepared?
Through ion-exchange from NaNi0.5Mn0.5O2. Direct synthesis yields disordered material with less beneficial behaviour.
What is an NMC material?
NMC material is a layered cathode consisting of Ni, Mn and Co in different proportions.
We want to have a cathode with as high Ni-content as possible, as this increases the voltage. However, Ni4+ is highly reactive.
Co gives structural stability, Mn gives chemical stability.
How is the chemical composition of NMC-materials denotes?
E.g. NMC 811 - 80% Ni, 10% Mn and 10% Co.
What happens with NMC if you cycle it at higher voltages? What are some possible ways to improve this?
Gives increased capacity and increased voltage, but degrades faster (oxidation of electrolyte and transition metal dissolution at interface).
Can try to make a single crystalline cathode. Few sites for oxygen evolution.
Can coat with Al2O3.
