Chapter 6 - Anodes Flashcards
What are the demands on an anode material?
- Low electrochemical potential vs Li
- Mechanical stability (low volume expansion, no structure loss/irreversible phase changes, electrochemical stability apart from Li)
- Electrical conductance
- Safety
- Lithium diffusivity
- Good contact to percolating network (electron transport)
- Good contact to electrolyte (low ion transport activation energy)
- Compatability with available electrolytes
- High specific capacity
- Cost, abundant materials, environmentally friendly ++
What are the steps to finding a material suitable as an anode?
1) Find element with low electronegativity
What are the anode materials that are available on the global market now?
- Natural graphite (2015: 49%)
- Artificial graphite (2015: 42%)
- Amorphous carbon (2015: 6%)
- LTO (2015: 1%)
- Si compounds (2015: 2%)
What are pros and cons for using Li metal as an anode?
Pros:
- No dead weight
- Low potential (0 V vs Li)
- Good conducitvity
- Excellent Li surface kinetics (unless there is LiO, LiN, LiF formation or other degradation)
Cons:
- Dendrite formation
- Large volume expansion (0 - 100%)
- Usually low or not controlled surface area
What are some factors that affects dendrite growth?
- Li mobility along surfaces. SEI can reduce mobility of Li along surface. LiF stabilises.
- Balance between charge transfer and ion diffusion in electrolyte. Increased salt concentration reduces dendrite growth. Ion depletion in electrolyte at high current rates.
- SEI inhomogeneities. Li diffusion from electrolyte varies due to SEI density variation. Li growth causes SEI cracks - increases local growth.
- Electron concentration at tips. Negative surface charge accumulates on surfaces/tips. Cations in electrolyte reduce local negative charges, reducing dendrites.
- Capacity loss due to stable SEI shells. Li cut off from rest of Li metal, floating around as dead waste.
Is dendritic growth a problem only for Li metal?
No - can also occur on graphite if intercalation is not fast enough. This means that at low temperatures, or at high C-rates, this can be a problem also for carbon based anodes.
What are ways to reduce dendritic growth?
- Using the right salts (forming homogenous LiF interface)
- Using the right solvents (homogenous SEI, good ion conductance)
- High salt concentration
- Cations in electrolyte
- Using solid electrolyte
What are the consequences of dendritic growth?
Internal short - dendrites grow through separator and causes electrical contact between cathode and anode. Causes fires, outgassing. Thermal runaway.
Capacity fade - some Li is lost for use in cycling.
What is the theoretical capacity of graphite?
372 mAh / g
How is graphite filled?
In steps, which also yields different voltages vs. Li. Li intercalates between graphene sheets. Other Li will fill in the same layer due to increased interlayer distance. Next layer that is filled is far away from an already filled one. Eventually next neighbours will fill (at lower potential, as this is less favourable).
Different models for this. One (Rüdorff model) assumes full layer filling. Another (Damus-Herold model) assumes islands filling, that separates filled Li in different layers first.
What is the difference between the bulk graphite layers and the open ends of the graphite layers?
The graphite layers are dense, and have few defects. There is little degradation.
The open ends of the graphite layers have many dangling bonds, and here there is aggressive degradation.
How can we prevent degradation of graphite anodes?
Coating it with amorphous carbon coating. This prevents “attacks” on graphite layer ends with many dangling bonds.
Which electrolyte can cause exfoliation of graphite?
PC.
What is the difference between graphite and hard carbon?
Hard carbon is prepared at lower temperatures. It includes hydrogen remnants and nano-cavaities. Has a theoretical high lithiation capacity (up to 1000 mAh / g) but a large 1st cycle irreversibility (possible because Li is bound in Li-H-C bonds?)
Gives a gradual potential slope, which is better as a state of charge indicator.
What is the difference between natural graphite and artificial graphite?
Natural graphite is mined. Artificial graphite is made through thermal treatment in inert circumstances.
Natural graphite has high crystallinity and comes as flakes (shaped to spherical particles for anode). Crystallinity of artifical carbon is low.
Natural graphite has a very ordered orientation of the crystal structure, wheras artifical graphite has random orientation.
What is the raw materials used for production of artificial carbon?
Coke, pitch produced from coal/petroleum.
How does the crystallinity of the graphite affect discharge capacity?
High crystallinity is beneficial for discharge capacity. The lower temperature artificial graphite is made at, the more amorphous it is, and the lower the discharge capacity.
What is the benefit of random orientations of crystals of graphite?
Since the Li-ion diffuse in from the edge of the particle, having very orienated graphite flakes perpendicular to the enterance angle, creates awkward paths for Li to enter the graphite. Can’t just go straight in, but need to find an entrance. For random oriented crystals, this is much easier.
What are some selling points for hard carbon?
- High stability towards electrolyte = long life.
- High power density
What are some negative sides of hard carbon?
- Low coulomb efficiency.
What are some selling points for graphite?
- High coulombic efficiency.
- Good potential
What are some negative sides of graphite?
- Not stable towards electrolyte.
- Poor power density
What are some positive sides of an LTO-anode?
- LTO has a high potenvial vs Li (1,55V), so it allows the use of Al current collector.
- There is negligible electrolyte degradation.
- It has great cycleability, even at high C-rates.
- Almost no risk of dendrites, even in cold conditions.
- Cannot burn as it is an oxide. Better buffer between SEI/electrolyte reaction and cathode reaction.
- Almost no expansion during lithiation
- Metallic when lithiated
What are some negative sides of an LTO-anode?
- High potential vs. Li (1.55V), so it gives a lower battery voltage.
- Low specific capacity.
- Insulating when delithiated
What is the structure of LTO?
Spinel structure
What is the issue with gas generation of LTO anodes?
Interaction of lithiated Li4Ti5O12 with salt, carbonate solvent. CO, CO2, CH4 and C2H4 reported.
Mainly an issue during formative cycles, but can occur with aging.
Initated by (111) plane of titante - electrons degrade elecctrolyte.
What can be done to reduce the outgassing problem with LTO anodes?
Carbon coating LTO nanoparticles can reduce gassing.
One can modify LTO by doping with Nb. What can be the benefits here?
Nb introduces two different oxidations steps III->IV and IV->V. One higher and one lower than Ti III->IV.
The one lower is interesting, and can yield higher energy density for the full cell.
Also has a slightly sloped discharge curve.
Could scandium be used instead of titanium in LTO?
Possibly, but scandium is hella expensive!
What are some pros and cons from using silicon as an anode?
Pros:
- Medium potential, so better safety than graphite (not as good as LTO)
- Fantastic theoretical capacity (high T: Li21Si5, normal: Li15Si4).
Cons:
- Large expansion during cycling
- Very reactive surface
- Significant hysteresis from slow diffusion
What type of anode is silicon?
An alloying anode. Li forms alloy with Si.
What causes cracking in silicon? What is a large problem with this?
Volume expansion. In a particle, first the outer layer will be lithiated (and expand). When the core expands, the outer part is already as expanded as can be, and cracks start to form.
When the particle cracks, it exposes more surface area to react with the electrolyte.
What is a way to curtail the volume expansion problem in Si?
Could make core-shell particles, where we have a Si core and SiO outer layer, with carbon shell. Then remove then SiO with HF to leave void in which the Si can expand.
How can two-way lithium diffusion explain Li-trapping in Si?
We start with a pure host material. As it gets lithiated, Li diffuses inwards in the particle, creating an alloy. As it start dealloying, there can still be a higher concentration of the host material in the core, causing Li diffusion both outwards and towards the centre. The Li diffusing into the centre is trapped.
Do we need to have an anode with extreme capacity?
No - the higher capacity it has, the less an improvement in the anode will matter to the overall performance of the battery (there are still many other parts that needs to be there and weighs the battery down).
This means that there is a lot of wiggle room from the ideal theoretical capacities of Si to stabilise and still make for a great overall improvement to the battery.
How could a silicon and graphite combination anode function?
Since graphite has a lower potential than Si, Li would “escape” from the graphite first. This means that for daily use, where DoD is not 100%, only the graphite would be used. Only when needing to use the battery 100% would the Si be used. This means that the number of cycles required of the extra Si capacity is not as high.. Could support our trips to the cabin.
What is the Maxcell Ulsion battery?
Uses a nanosilicon composite particles inside graphite to make a hybrid electrode. Nano silicon embedded in a matrix of amorphous silicon oxide used as a Li-ion conductor, and with an electron and Li-ion conductive layer around.
What happens to silicon during cycling?
Silicon loses bonding to other silicon as li is intercalated. After Li is removed, there is “no reason” for Si to find back to its original structure.
How could a stable SEI be formed on silicon?
In one example given, a mechanical clamping layer is used around a silicon nanotube to keep the silicon from expanding outwards and then subsequently cause the SEI to fracture when it shrinks again.
What are conversion anodes?
In conversion anodes we see a separation of phases during lithiation. Example: MnO will separate into LiO and Mn during cycling.
Conversion anodes show sloe kinetics.
What are the three main types of anode materials?
1) Intercalation (graphite, C6 + Li -> LiC6)
2) Alloying (silicon, Si + 3.75Li -> Li3.75Si)
3) Conversion (e.g. Fe2O3 + 6Li -> 2Fe + 3Li2O)
How does the overpotential vary between intercalation, alloying and conversion anodes?
From lowest to highest:
Intercalation < Alloying < Conversion
