Chapter 3 - Battery Design Considerations Flashcards
What is FMEA?
FMEA stand for Failure Mode Effect Analysis
It is a systematic method to identify all weaknesses / failure modes.
List the components of a standard Li-ion battery
Positive metal contact Positive active material Electron conductive additive + binder (positive electrode) Separator Electrolyte Electron conductive additive + binder (negative electrode) Negative active material Negative metal contact Casing
Battery management system
Cooling system
Fire protection
What is the dimensions of a CR2032 coin cell?
d = 20 mm, h = 3.2 mm
Where are coin cells typically used?
When we need small amounts of energy. Typically lab scale testing.
When are cylindrical cells typically used?
Typically for larger consumer electronics. Also used en masse in Teslas for example.
What is a clear disadvantage of a cylindrical cell?
There is an inhomogenous strain on different part of the battery as it is wound up round and round.
What are probable dimensions of each component in a cylindrical cell?
18 winding layers, >50micrometer thick anode, ~40 micrometer thick cathode, 15 micrometer thick separator and contacts around 10-20 micrometers.
What are the dimensions of a 18650 cylindrical cell?
18 mm diameter, 65 mm height.
How are prismatic cells packed?
Efficiently in prismatic casings. FOlded around.
What are some advantages and disadvantages with pouch cells?
Advantages:
- High packing density
- Homegenous strain
- Cheap casing
Disadvantages:
- Soft casing
- May require external pressure
How does a FMEA work?
1) Identify components
2) Identify functions of each components
3) Identify ways each function can fail
4) Identify possible causes of failure
5) Identify probability of failure (occurence)
6) Identify consequences of failure, including secondary failures (severity)
7) Identify ways of detecting failures (detection)
8) Identify risk reduction measures
What is the occurence scale in FMEA?
A scale that denotes the occurence. This depends on the product. For example, it could go from
- Avoided by prevention
to
- > 1/10
What is the severity scale in FMEA?
A scale that denotes the severity. 1 not very severe, 10 very severe.
- Annoyance noticed by a few, negligible value loss
- Annoyance noticed by some
- Annoyance noticed by almost all
- Degradation of secondary function, lifetime reduction
- Loss of secondary function
- Degradation of primary function, lifetime reduction
- Loss of primary function
- Possible loss of life, warning, large material loss
- Possible loss of life, no warning, very large material loss
What is the detection scale in FMEA?
A scale that denotes the possibility of detection.
- Detection not applicable - error prevention
- Error detection and/or problem prevention
- Automated problem detection at source with process stopping
- Automated problem detection post processing with process stopping
- Manual problem detection at source by operator
- Manual problem detection post processing by operator
- Detection by inspection at relevant work station
- Detection in final inspection
- Not likely to detect at any stage
- No detection opportunity
In what areas should a battery perform?
- Power
- Energy
- Lifetime (cycle life)
- Lifetime (shelf life)
- Low weight (specific power / energy, gravimetric)
- Small volume (specific power / energy, volumetric)
- Low ocst
- Low environmental footprint (recycling)
- Safe operations (not melt, no outgassing, not burn, not explode)
- Robust against abuse (temperature, vibration, shock, overcharge, pressure)
What is a standard for measuring battery failure severity?
EUCAR (European Council for Automotive R&D)
What are some functions we require of a positive metal contact?
- Contact to outside
- In-plane electron transport
- Mechanical stability in battery
- Electrical contact to active material / additive
- Mechanical adhesion to binder
- Stable against dissolution in electrolyte
- Not degrade electrolyte (catalytic effects)
- No metal pieces breaking off
- Low weigh, take up little space
- Low cost
- Ease / cost of processing
- High yield in production
What are possible failures of a positive metal contact?
- Yield loss - crumpling in production, edge imperfections
- Loss of contact from production
- Loss of contact over time
- Resistance from production
- Resistance increase over time (corrosion)
- Internal short from production (misalignment)
- Internal short after some time (metal dendrites)
What are some typical causes of failure modes?
- Design error (thickness, structure, surface, compatbility to electrochemical window, strength, weak parts, cost)
- Production method error (tension, handling, atmosphere, storage, cost, inhomogeneity)
- Production quality error (supplier failure, human error in handling, imprecise equipment/alignment, defective equipment)
- Systemic risk (material shortages, political risk)
What is the 5 Times Why method?
Ask “Why” five times to get to the bottom of a problem.
E.g.:
Failure: Power loss
- Why? Resistance in battery?
- Why? Resistance in contact?
- Why? Corrosion
- Why? Metal not stable in electrochemical window?
- Why? Poor quality assurance, insufficient documentation, lack of competence.
What are secondary effects of failure?
Secondary effects of failure are effects that indirectly arise from a failure.
E.g. loss of resistance can cause heating, which increase chemical degradation, increases risk of corrosion, risk of gas evolution or thermal runaway.
Power or capacity loss from design can give financial secondary effects, such as reduced sales value of th ebattery.
Power loss from ageing could cause functionality to be lost.
What are the main functions of a conductive additive?
- To increase electron conduction through forming a electron conductive network.
- Good dispersion of particles
- Percolating network through binder, SEI, electrolyte
What are some things that could go wrong with the conductive additive?
- Insufficient dispersion.
- Too much weigh
- Poor packing density
- Different expansion from active material, giving delamination.
- Excessive area giving electrolyte degradation
What is the typical choice of a conductive additive, and what are some alternatives?
Typical choice: Carbon black
Alternatives: Nanotubes, metal sponge, metal coating.
What are the functions of the active material at the positive electrode?
- High potential vs Li electrode
- High specific capacity
- Fast trannsport of Li from surface to inner part of particles (good kinetics)
- Low resistance for the electrochemical reaction
- Should not degrade the electrolyte
- Electrochemical stability
- State of Charge indicator
- Mechanical adhesion to binder
What are some ways that the active material at the positive electrode can fail?
- Insufficient potentaial
- Too high potential (causing degradation of electrolyte)
- Insufficient surface area
- Too high surface area (causing degradation of electrolyte)
- No wetting in slurry mixing
- Difficulty of dispersion in slurry mixing
- UNstable at full delithiation
- Oxygen evolution at overcharging
- Impurities with shuttle effects (ions with more than one possible oxidation number)
- Catalytic degradation of electrolyte
- Not entirely reversible lithiation / delithiation
- High overpotential during lithiation / delithiation
What are the functions of the binder?
- Good dissolution in production line
- Should be soluble in environmentally friendly solvent (H2O, not NMP)
- Good ahesion to active material, metal contact and conductive additive
- Maintain adhesion during cycling
- Leave contact electrolyte/active material
- Crosslinking over full electrode in expanding electrodes
- Electrochemically stable
- Minimal drying energy required
- No water left after drying
What are the functions of the electrode?
- Conductive network for electrons
- Ion conductive network
- Mechanical integrity
- Suitable loading (maximal, but not so much that resistance increases)
What are the functions of the separator
- SHould allow electrolyte to penetrate (ie. should be porous and allow easy wetting)
- Not allow electron transport
- Shut down in case of thermal runaway
- Prevent against internal short from particle migration
- Low cost
- Easy handling in production
What are the functions of the electrolyte?
- Should wet the separator and active material
- Should allow ion transport
- Should not allow electron transport
- Electrochemically stable in the electrochemical window of the electrodes
- High temperature window
- High flash point
- Low evaporation
- Protect active material surfaces (at both electrodes)
- Form stable SEI
- Low cost
How can the electrolyte fail?
- Can contain water - causing degradation of Li-salts and get hydrogen evolution
- Low conductivity at low temperature
- Evaporate when heated
- Insufficient electrochemical window
- Degrading of materials (EC will defoliate graphite for example)
- HF released during fires
- Explosive due to oxygen evolution potential
- High chemical energy contributing to fires
What are the functions of the negative electrode active material?
- Low potential vs Li
- Specific capacity (gravimetric and volumetric)
- Avoid Li dendrites
- Avoid Li plating
- Handling in production (slurry mixing and deposition)
- Low cost
What are the functions of the casing?
- Mechanical stability
- Contain electrolyte
- Prevent outgassing
- Safety mechanism at outgassing
- Electrochemically stable
- Electrically isolate electrodes
- Fit to standard battery holders
- Keep water out
- Keep oxygen out
