Reasons

• Charging at low temperature leads to Lithium plating
• Charging Li-ion cells at higher voltage accelerates degradation
• Storing a cell at a high temperature and state-of-charge accelerates capacity fade
• Reasons for atypical cell degradation

Physical causes

• Loss of lithium inventory to side reactions
  • Cathode
  • Anode
• Loss of active material: cracks exclude parts of anode or cathode from cycling.
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Symptoms

• Increase of Cell internal resistance
• Cell capacity fade. This can also lead to the change in the open-circuit voltage of a cell, but not necessarily (or significantly).
• Increased Cell self-discharge: electrodes can swell, increasing pressure on separator and, as a consequence, increasing the self-discharge rate of the cell. (Full Lithium atoms move through the separator? unclear) Possible interpretation: very thin dendrites cause "soft short circuit" because of their high resistance. These dendrites is the vehicle for self-discharge.
• Changed shape of Cell open-circuit voltage (see Degradation diagnostics for lithium ion cells)

Mechanisms

• SEI layer on anode grows thicker over time (a. k. a. passivation)
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• Lithium plating
• Crystal formation, dendritic growth (preconditioned by lithium plating)
• Metals from cathode can dissolve into electrolyte, reducing the number of sites for Lithium ions to intercalate
• Corrosion (e. g. formation of Lithium oxide when overdischarging the cell, or corrosion of other metals dissolved from the cathode material)
  • Metal oxides have high resistance (not very conductive)
  • Over-discharge creates latent short-circuit risk: Corrosion of copper current collector, releasing positive Cu+ ions into the electrolyte, when a cell is severely discharged (to about 1.5V), at least locally.