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If a battery is to be cycled, it is essential that the chemical reactions that take place at the electrodes are efficiently reversed. If only 1% of active material is lost in one cycle, that signifies that 50% capacity would be lost after only fifty cycles. Many, if not most, electrode reactions involve a reconstructive phase change in the crystal chemistry of the active materials. Many possible processes or side reactions leading to battery deterioration or its failure can occur. These include:
loss of porosity and swelling of the electroactive material
formation of non conducting phases, isolating active material
growth of dendrites, possibly causing internal short circuits
shedding of active material from the electrode grids
drying of separator
corrosion of current collectors, isolating active material
gassing of electrodes on overcharge
The degradation of a battery is usually progressive. However, once started the degradation processes tend to accelerate. (reference)
