Interesting video of a lecture by Dr. Jeff Dahn, professor of Physics and Chemistry at Dalhousie University in Canada on what causes lithium ion batteries to deteriorate, and how the situation can be improved. Dahn was involved in development of lithium ion battery technology, and maintains an large, active research program devoted to studying materials for batteries and fuel cells.

The video is over an hour long, and the pace is a little slow. I’ll be updating the post with my notes as I go through it.

Highlights

• Manufacturer testing and rating of battery lifetime uses a simple cycle test that takes place under a compressed timeframe, with unfortunate results.
  • Charge/discharge cycles are done back-to-back, at charge/discharge rates that might not mirror real-world use.
  • This underrepresent the impact of the parasitic reactions during charge/discharge that lead to cell degradation.
• During charge and discharge, lithium ions intercalate (intermingle) and de-intercalate with the the electrode material. This causes a modest, “benign” structural change of 3% in the anode and 10% in the cathode.
• The materials used in lithium cell electrodes are stable in open air, and batteries can be assembled in open air.
  • As cells are charged, the material at both the cathode and anode become highly reactive with the electrolyte solution.
  • Fortunately, the reaction products are solid and form passivating surfaces that protect the electrodes from rapid degradation.
• For a perfect Li-ion battery, the coulombic efficiency, the number of electrode in during charging vs the electrons out during discharge, should be exactly 1.
• The difference between theoretical and actual coulombic efficiency, outside of the measurement area, is attributable to  undesirable parasitic reactions that lead to cell degradation.
• With sufficient measurement precision, it is possible to predict lithium cell lifetime without intensive testing.
  • Requires at least 4 digits of precision and accuracy
• Battery Electrolyte
  • Cyclic and linear organic carbonates
    • Ethylene Carbonate
    • Propylene Carbonate
    • Dimethyl Carbonate
    • Ethylmethyl Carbonate
  • Lithium Phosphorous Hexafluoride solute
  • Solvents are typically mixed
    • Some have low boiling points
    • Others form excellent passivating layers
• Approximate 3-billion 18650 cells made a year
• Parasitic reactions between electrodes and electrolytes reduce the capacity of the cell as lithium ions are consumed/trapped
  • Capacity change after first charge/discharge cycle is relatively large.
  • Loss in subsequent cycles is lower, because the reaction products form a passivating layer that protects the electrodes.
• Extent of parasitic reactions can be quatified with a precise measure of coulombic efficiency.
  • A good cell is close to 100%.
  • 10,000 cycle life would require a cell with 99.99% efficiency
  • Need 4th digit in accuracy and precision.
• Precision charging device
  • Developed by
    • Aaron Smith, PhD, graduated in 2012, now at Tesla in charge of Tesla’s Battery Lifetime Group
    • Chris Burns, PhD Student
  • Specs
    • 60 channels, each with precision current supplies with 5 digit precision
    • Cells under test are in temperature controlled boxes ( 0.05C)
    • Current is fed through precision resistors to measure current.
• Longer charge cycles have higher coulombic inefficiency, and therefore more destructive parasitic reactions.
• Coulombic Ineficiencies of common chemistries
  • Test conditions
    • Commercial 18650 cells, two per test condition
    • Charge / Discharge rates: C/24, C/50, C100
    • 30C, 40C, 50C, 60C
  • High-level
    • Coulombic inefficiency has linear relationship to charge time.
    • Strongly suggests that time under charge is the major factor in determining coulombic inefficiency for a given chemistry, at a given temperature.
  • Lithium Cobalt Oxide (LiC02)
    • Lowest parasitic reactions at 30°C
    • Moderate at 60°C, but still lower than Lithium Manganese Oxide at 30°C
    • Tesla has used chemistries with similar characteristics
  • Lithium Cobalt Manganese Oxide (Li[NiMnCo]02)
    • Moderate at 30°C
    • ~50% of Nissan Leaf and Chevy Volt packs
  • Lithium Iron Phosphate (LiFePO4)
    • Moderate parasitic reactions at 30°C, high at 60°C
    • Used by Fiskar
  • Lithium Manganese Oxide (LiMn02)
    • Highest parasitic reactions of all examples at 30°C and even worse at 60°C
    • ~50% Part of the Nissan Leaf and Chevy Volt battery pack
    • Ill-suited to use in battery packs without thermal management, like the Nissan Leaf.
• Electrolyte additives
  • Vinylene Carbonate: reduces electrolyte oxidation on positive electrode. 2% by weight makes a huge difference
  • Trimethoxyboroxine, 1-3 propane sultone: impedence reducers.
  • Triethylphosphate and others: wetting agents to aid speed in manufacutre
  • Others for lifetime, low or high temp performance, and safety
  • Typical Li-ion cell will have 5 additives
• Precision coulombic efficiency measurements allow impacts on cell lifetime to be assessed relatively quickly, traditional testing of discharge capacity vs cycle number requires testing to end-of-life.
  • Short term test results correlate with long term test results
• Predicting catastrophic failures
  • Hypothesis
    • Reaction byproducts from cathode are migrating to anode, where they are reduced and gradually foul the material until the pores in the anode material are clogged. Once the pores are clogged, lithium starts plating out, and cell capacity plummets.
  • Predictions
    • The more compacted the anode material, the sooner the cells will fail.
      • This predicted behavior has been observed, and exploited to make cells that are designed to fail early in order to test other variables.
    • Coulombic Inefficiency * Cycles = A Constant
      • Coulombic Inefficiency = amount of byproducts
      • Constant is the amount of material required to clog the pores
      • A strong correlation has been observed for many additives
  • Exceptions
    • Some unknown additives from battery manufacturers have been tested and shown that make huge (20x) contributions to cycle life while falling outside the predictive model
    • Model assumes a solid reaction byproduct on negative
    • Additives may produce a reaction byproduct with different characteristics (ie liquid, gas)
  • Other observations
    • Coulombic efficiency and cycle life tend to increase with the number of additives
      • And yet most academic research on electrolyte additives only looks at one at a time.
• New grant to put a 100 channel system on line with the capability of testing automotive scale cells
  • 10 ppm CE accuracy
• Research Directions
  • Tests require highly uniform cells. Lab made cells can be rather inconsistent
  • Have established relationships with Chinese manufacturers of pouch cells
    • Get machine-made pouch cells without electrolyte in lots of 2,000
    • Mix and add electrolyte themselves.
    • Seal cells in vacuum sealer
  • Microcalorimeter
    • Measure the heat from parasitic reactions
    • TA Instruments TAM III
      • 12 separate calorimeters
      • 10nW sensitivity
      • Baseline stability of 500nW/month
      • Isothermal, all experiments at 40C
      • Holds AA and 200mAh cells.
    • Three sources of heat during charging (~1Wh cell capacity)
      • Entropy changes from electrodes
      • Internal resistance/polarization
      • Parasitic reactions
        • 100 uW would consume all the electrolyte in a year BAD
• Questions from audience
  • Audience asked about the unknown additive outliers that didn’t conform to the simple model of reaction products clogging the pores on the anode.
    • Dahn replied that if those additives formed a liquid or gas reaction product, then one would expect the impedance to be lower. He then showed data showing that, indeed, the cells with outlier additives did have lower impedence.
  • Asked about using these techniques with silicon electrodes.
    • Silicon electrode cells at this point are so poor, that you can see the parasitic inefficiency with an ordinary charger.
    • Silicon electrodes need a lot of development before they need this precision.
    • Lithium Titanate
      • Incredibly durable already.
  • How can these metrics be applied to drive cycles?
    • GM wants to relate this to drive cycles.
      • Dahn’s initial approach is:
        • How does existing technology work for driving?
        • How does it look with precision charging?
        • What are the new technologies?
        • How do they perform under precision charging?
        • Based on result, he can say if the new stuff should be better or worse.
  • How do real-world discharge profiles relate to constant discharge rates seen in lab tests?
    • The biggest factor is time spent at highest voltage.
      • Longer is worse
      • Most of the parasitic reactions happen above 4v.
  • Lithium Ion Cell Overhang
    • Negative Electrode is slightly wider than the Positive
    • Prevents lithium ions from the positive electrode from plating out the edge of the negative and forming lithium metal dendrites
    • There is an overhang of graphite
    • At the beginning of charging there is some odd behavior as the ions equilibrate with the overhang
  • To extent the life of your phone or laptop battery
    • Keep it as cold as possible 🙂
    • Put it in the fridge.