The initial overheating may be caused by excessive currents, overcharging or high external ambient temperature.The breakdown of the SEI layer starts at the relatively low temperature of 80✬ and once this layer is breached the electrolyte reacts with the carbon anode just as it did during the formation process but at a higher, uncontrolled, temperature.
As with over-voltage operation, when the electrodes can not accomodate the current flow, the result is reduced power and Lithium plating of the anode with irreversible capacity loss. Details of this process are given in the section on Charging Ratesįuthermore, at low temperatures, the reduced reaction rate (and perhaps contraction of the electrode materials) slows down, and makes makes more difficult, the insertion of the Lithium ions into the intercallation spaces. In other words its power handling capacity is reduced. This translates to a reduction in the current carrying capacity of the cell both for charging and discharging. ( Arrhenius Law) The effect of reducing the operating temperature is to reduce rate at which the active chemicals in the cell are transformed. Heat is a major battery killer, either excess of it or lack of it, and Lithium secondary cells need careful temperature control.Ĭhemical reaction rates decrease in line with temperature.
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See more about State of Charge and how to measure it. Operating outside of these limits will adversely affect the life of the battery. The voltage restrictions necessary to avoid the problems outlined above can be translated into recommendations for the operating range of the State of Charge of the battery shown in the following diagram. With Lithium Iron Phosphate cells this can happen over a few cycles. Keeping the cells for prolonged periods at voltages below 2 Volts results in the gradual breakdown of the cathode over many cycles with the release of Oxygen by the Lithium Cobalt Oxide and Lithium Manganese Oxide cathodes and a consequent permanent capacity loss. This is a dangerous situation which can ultimately cause a short circuit between the electrodes. This increases the self discharge rate of the cell however, as the voltage is increased again above 2 volts, the copper ions which are dispersed throughout the electrolyte are precipitated as metallic copper wherever they happen to be, not necessarily back on the current collector foil. Allowing the cell voltage to fall below about 2 Volts by over-discharging or storage for extended periods results in progressive breakdown of the electrode materials.įirst the anode copper current collector is dissolved into the electrolyte. Rechargeable Lithium cells suffer from under-voltage as well as over-voltage. See below.Įxcessive current also causes increased Joule heating of the cell, accompanied by an increase in temperature.
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The consequence of this is a reduction in the free Lithium ions and hence an irreversible capacity loss and since the plating is not necessarily homogeneous, but dendritic in form, it can ultimately result in a short circuit between the electrodes.Īnother major cause of Lithium plating is low temperature operation and it can also be caused by non-uniformities in the cell elements due to manufacturing defects or abuse. With excessive currents the Lithium ions can not be accommodated quickly enough between the intercalation layers of the carbon anode and Lithium ions accumulate on the surface of the anode where they are deposited as metallic Lithium.
If the charging voltage is increased beyond the recommended upper cell voltage, typically 4.2 Volts, excessive current flows giving rise to two problems.
Once outside the box permanent damage to the cell will be initiated. The diagram below shows that, at all times, the cell operating voltage and temperature must be kept within the limits indicated by the green box. The performance of Lithium Ion cells is dependent on both the temperature and the operating voltage. Woodbank does not monitor or record these emails
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