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LI-ION BATTERY

Layered cathodes (2D)
  • LCO: Lithium Cobalt Oxide (LiCoO2) (by John B. Goodenough in 1980)
  • NCM (NMC): Lithium Nickel Cobalt Manganese Oxide (LiNiCoMnO2) (for Panasonic NCR18650B)
  • NCA: Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) (for Samsung INR18650-25R)
  • Cobalt is rare, toxic, expensive and has low energy density, and cells ages rapidly
  • Ni can substitute Co, giving higher energy density (higher voltages, same capacity), but is not very thermally stable
  • NCM and NCA have properties from all three constituent metals
Spinel cathodes (3D)
  • LMO: Lithium Manganese Oxide (LiMn2O4) (by Goodenough and Thackery in 1983)
  • LMO is cheaper and safer than LCO, but can have short lifetime due to the manganese dissolving into the electrolyte under some conditions (additives can be added to help prevent this, but this “art” is presently well guarded by trade secrets)
Olivine cathodes (1D)
  • LFP: Lithium Iron Phosphate (LiFePO4) (by Goodenough in 1997)
  • LFP has low cost and low toxicity, but also has low energy density due to a low open-circuit potential and low specific energy due to heaviness of Fe
  • 1D structure tends to have high resistance, which can be overcome in part by using very small particles and including conductive additives
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Panasonic NCR18650B: Nickel-cobalt-rechargeable (NCR)
Panasonic NCR18650PF: Nickel-cobalt-rechargeable (NCR)
Samsung ICR18650-26F: Lithium-cobalt-rechargeable (ICR)
Samsung INR18650-25R: Lithium-nickel-rechargeable (INR)
LG ICR18650HE2: Lithium-cobalt-rechargeable (ICR)
LG LGDBHG21865 (INR18650HG2): Lithium-nickel-rechargeable (INR)
A123 ANR26650M1-B: Lithium Iron Phosphate (LFP)

When no current flows through a battery for a certain period of time, the battery is considered to be in equilibrium state and the corresponding electrode potential is called equilibrium electrode potential. When current flows through the battery, the electrode potential deviates from the equilibrium potential; this phenomenon is called electrode polarization or overpotential. There are three types of polarization:
  • Activation polarization: the voltage required to drive a current (see Butler-Volmer equation)
  • Concentration polarization: it is caused by the fact that the reactants consumed on the electrode surface cannot be supplemented from the electrolyte (or some reaction products cannot be evacuated in time)
  • Ohmic polarization: it represents the resistance of electrolyte, electrode material, and conductive material and their contact resistance
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Mechanical abuse:
  • crash/crush
  • nail penetration
  • drop
  • immersion
  • roll over
  • mechanical shock
  • vibration
Thermal abuse:
  • thermal stability
  • fuel fire
  • rapid charge/discharge
  • elevated temperature storage
  • thermal shock cycling
Electrical abuse:
  • overcharge/overvoltage
  • short circuit
  • overdischarge/voltage reversal
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Causes:
  • Corrosion: (undesired chemical reaction with environment) consumes some of the active chemicals in the cell leading to increased impedance and capacity loss
  • Crystal formation: Electrode particles evolve as larger crystals are formed. This reduces the effective surface area of the electrodes and hence the current carrying and energy storage capacity
  • Dendritic growth: Formation of treelike structures on electrodes, which can ultimately pierce separator and cause short circuit.
  • Chemical loss through evaporation: Gaseous products resulting from over-charging are lost to atmosphere causing capacity loss.
  • Passivation: Growth of a resistive film layer that builds up on the electrodes, impeding the chemical action of the cell
  • Shorted cells: Cells that were marginally acceptable when new may have contained latent defects that become apparent only as the aging process takes its toll: poor cell construction, contamination, burrs on metal parts leading to a short circuit
  • Electrode or electrolyte cracking: Some solid electrolyte cells such as lithium polymer can fail because of cracking of the electrolyte
Results:
  • Increased internal impedance
  • Reduced capacity
  • Increased self-discharge
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Negative electrode:
  • Aging at surface of particles
    • SEI formation and growth
      • The lithiated graphite has low potential, which is outside the electrochemical voltage stability window of the organic solvents in the electrolyte. When the solvents come into contact with the surface of the lithiated graphite particle, they are reduced and form reaction products that coat the surface of the particle with a solid-electrolyte interphase (SEI) surface film.
      • This side reaction that produces SEI film consumes lithium while creating the film products, leading to both capacity fade and power fade. Most of the SEI is formed during the initial charge process of the cell, which leads to the first charge being termed the formation process. Although SEI grows fastest during the formation cycle, it continues to build over time.
      • High SoC can accelerate the SEI growth. High temperature can lead to reactions that break down the SEI layer, which can lead to new SEI forming on the newly exposed graphite. High-rate charging can force solvent to co-intercalate into the graphite so the SEI reaction can take place inside a graphite particle. Meanwhile, gasses generated cause expansive pressures that tend to crack the particle and expose more fresh graphite to solvent, leading to more SEI formation.
      • Trace water in the electrolyte combines with ionized fluorine from the electrolyte salt LiPF6 to form hydrofluoric acid, HF, which thins SEI and allows more solvent to contact the graphite to form more SEI. The acid can also accelerate positive electrode degradation, leading to dissolved ionized metals such as manganese or cobalt in the electrolyte. The ionized metals propagate to the negative electrode and help form the SEI layer, a process known as anode poisoning. The resulting SEI layer has low electronic conductivity and plug pores that would otherwise be used for lithium, leading both power fade and capacity fade.
    • Lithium plating
        During charging, when the solid-electrolyte potential difference drops below 0 V, it causes lithium ions from the electrolyte to join with electrons from the external circuit and plate solid lithium metal on the surface of the particle. Capacity is lost irreversibly. The lithium plating can also promote the growth of SEI and metal dendrites, and the latter can penetrate the separator and eventually lead to shorted cells. This side reaction is most acute at cold temperature.
  • Aging in bulk
      Charging and discharging lithium-ion cells increases and decreases the amount of lithium present in the negative-electrode active particles, which causes stresses due to the volume change. Over time, the stress can lead to cracking of the SEI layer, exposing more surface graphite and leading to more SEI growth.
  • Aging in composite electrode
    • Within the composite negative electrode of a lithium-ion cell, inactive materials like conductive additives and binders, though not often mentioned, are critical to the proper functioning of the cell. When lithiating and delithiating the active materials in the electrode, stresses leading to deformations can cause the binder to fail, leading to mechanical and electronic contact loss between the graphite particles themselves, between the particles and the current collector, between the binder and the particles, between the binder and the current collector. This results in power fade. It can also lead to capacity fade if particles become completely disconnected electronically from the current collectors.
    • Porosity of the electrode can be reduced by volume changes and SEI growth, which impedes movement of lithium ions through the electrolyte and increases cell resistance.
    • If a cell becomes overdischarged, the high open-circuit potential of the graphite can lead to the corrosion of the copper current collector. First, this reduces current-collector/electrode contact, leading to higher cell resistance. Second, corrosion products that deposit on electrode particles have low electronic conductivity, which increases SEI film resistance. Third, the corroded current collector has uneven resistance, leading to accelerated aging in parts of the cell and a preference toward lithium plating. Finally, copper plating can accelerate lithium plating, dendrite growth, and hence short circuits.
Positive electrode:
  • Aging at surface of particles
    • A film can grow on the surface of the active-material particles due to a chemical reaction between the solvent and the active-material particles. However, this mechanism is not as pronounced as it is in the negative electrode.
    • A bigger factor is the dissolution of metals from the electrode crystal structures into the electrolyte and the resulting products can reprecipitate onto the particle surface as a high-resistance film, leading to both capacity fade and power fade. This dissolution is accelerated by hydrofluoric acid in the electrolyte, and tends to happen predominantly at low or high SoC and high temperature.
  • Aging in bulk
      When lithium intercalates into and deintercalates out of the positive-electrode active particles, stresses cause strains known as phase transitions. Some of them are normal and reversible, but others lead to cracking of active particles. The stresses can also lead to a phenomenon known as structural disordering where the crystal structure of the electrode materials break down, causing lithium to be trapped within the crystal structure and loss of lithium storage sites. Both effects decrease total capacity of the cell.
  • Aging in composite electrode
      Same with the negative electrode, the binder can decompose, the conductive additives can become oxidized, and the current collector can become corroded over time.
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