Lithium Battery Failures
The performance of Lithium Ion cells is dependent on both the temperature and the operating voltage. 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.
Once outside the box permanent damage to the cell will be initiated.
If the charging voltage is increased beyond the recommended upper cell voltage, typically 4.2 Volts, excessive current flows giving rise to two problems.
- Lithium Plating
With excessive currents the Lithium ions can not be accommodated quickly enough between the intercalation layers of the anode and Lithium ions accumulate on the surface of the anode where they are deposited as metallic Lithium. This is known as Lithium plating. The the consequence 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. Lithium plating can also be caused by low temperature operation. See below.
Excessive current also causes increased Joule heating of the cell, accompanied by an increase in temperature. See next section below.
- Under-voltage / Over-discharge
Rechargeable Lithium cells suffer from under-voltage as well as over-voltage. 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.
First the anode copper current collector is dissolved into the electrolyte. 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. This is a dangerous situation which can ultimately cause a short circuit between the electrodes.
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. With Lithium Iron Phosphate cells this can happen over a few cycles .
Heat is a major battery killer, either excess of it or lack of it, and Lithium secondary cells need careful temperature control.
- Low temperature operation
Chemical reaction rates decrease in line with temperature. (Arrhenius Law) The effect of reducing the operating temperature is to reduce rate at which the active chemicals in the cell are transformed. This translates to a reduction in the current carrying capacity of the cell both for charging and discharging. In other words its power handling capacity is reduced. Details of this process are given in the section on Charging Rates
Futhermore, 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. 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.
- High temperature operation
Operating at high temperatures brings on a different set of problems which can result in the destruction of the cell. In this case, the Arrhenius effect helps to get higher power out of the cell by increasing the reaction rate, but higher currents give rise to higher I2R heat dissipation and thus even higher temperatures. This can be the start of positive temperature feedback and unless heat is removed faster than it is generated the result will be thermal runaway.
- Thermal runaway
Several stages are involved in the build up to thermal runaway and each one results in progressively more permanenet damage to the cell.
- The first stage is the breakdown of the thin passivating SEI layer on the anode, due to overheating or physical penetration. 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 80ºC 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. This is an exothermal reaction which drives the temperature up still further.
(Lithium Titanate anodes do not depend on an SEI layer and hence can be used at higher rates.)
- As the temperature builds up, heat from anode reaction causes the breakdown of the organic solvents used in the electrolyte releasing flammable hydrocarbon gases (Ethane, Methane and others) but no Oxygen. This typically starts at 110 ºC but with some electrolytes it can be as as low as 70ºC. The gas generation due to the breakdown of the electrolyte causes pressure to build up inside the cell. Although the temperature increases to beyond the flashpoint of the gases released by the electrolyte the gases do not burn because there is no free Oxygen in the cell to sustain a fire.
The cells are normally fitted with a safety vent which allows the controlled release of the gases to relieve the internal pressure in the cell avoiding the possibility of an uncontrolled rupture of the cell - otherwise known as an explosion or more euphemistically "rapid disassembly" of the cell. Once the hot gases are released to the atmosphere they can of course burn in the air.
- At around 135 ºC the polymer separator melts, allowing the short circuits between the electrodes.
- Eventually heat from the electrolyte breakdown causes breakdown of the metal oxide cathode material releasing Oxygen which enables burning of both the electrolyte and the gases inside the cell.
The breakdown of the cathode is also highly exothermic sending the temperature and pressure even higher. The cathode breakdown starts at around 200 ºC for Lithium Cobalt Oxide cells but at higher temperatures for other cathode chemistries.
By this time the pressure is also extremely high and it's time to run for the door.
- Law suits will follow.
See methods used to avoid these problems in the section on Cell Protection
Alternative Lithium cathode chemistries
Lithium Cobalt Oxide was the first material used for the cathodes in Lithium secondary cells but safety concerns were raised for two reasons. The onset of chemical breakdown is at a relatively low temperature and when the cathode breaks down, prodigious amounts of energy are released. For that reason alternative cathode materials have been developed. The diagram below shows the breakdown characteristics of several alternative cathode materials.
The graph above shows that Lithium Iron Phosphate cathodes do not break down with the release of oxygen until much higher temperatures and when they do, much less energy is released. The reason is that the Oxygen molecules in the Phosphate material have a much stronger valence bond to the Phosphorus and this is more difficult to break. The other cathode chemistries are based on Lithium metal oxides which have much weaker valence bonds binding the Oxygen to the metal and these are more easily broken to release the Oxygen.
Note that consumer concern about the safety of Lithium batteries tends to be focussed on the Lithium cathode materials, whereas in reality, thermal runaway is initiated at the anode, NOT the cathode.
The electrodes of Lithium cells expand and contract during charging and discharging due to the effect of the intercalation of the Lithium ions into and out of the crystal structure of the electrodes. The cyclic stresses on the electrodes can eventually lead to cracking of the particles making up the electrode resulting in increased internal impedance as the cell ages, or in the worst case, a breakdown of the anode SEI layer which could lead to overheating and immediate cell failure.
See also the causes of Cyclic Mechanical Stress
A similar process, possibly augmented by the accumulated release of small amounts of gas due to the slow deterioration of the electrolyte each time it is heat cycled, could result in swelling of the cell and ultimately rupture of the cell casing.
The effects of voltage and temperature on cell failures tend to be immdiately apparent, but their effect on cycle life is less obvious. We have seen above that excusions outside of the recommended operating window can cause irreversible capacity loss in the cells. The cumulative effect of these digressions is like having a progessively debilitating disease which affects the life time of the cell or in the worst case causes sudden death if you overstep the mark..
The graph above shows that starting at about 15 ºC cycle life will be progressively reduced by working at lower temperatures. Operating slightly above 50 ºC also reduces cycle life but by 70 ºC the threat is thermal runaway. The battery thermal management system must be designed keep the cell operating within its sweet spot at all times to avoid premature wear out of the cells.
Beware: the cycle life quoted in manufacturers' specification sheets normally assumes operating at room temperature. This would be totally unrealistic for automotive applications. Graphs showing how cycle life varies with tempeature like the one above are seldom provided by cell manufacturers.
Battery Management System (BMS)
One of the main functions of the BMS is to keep the cells operating within their designed operating window (the green box above). This is not too difficult to achieve using safety devices and thermal management systems. As an additional safety factor some manufacturers set their operating limits to more restricted levels indicated by the dotted lines.
There is however very little te BMS can do to protect aginst an internal short circuit. The only prevention action that can be taken is strict process control of all the cell manufacturing operations.
Lithium Charged but Not Guilty?
The cause of many fires has been attributed to Lithium batteries and there is a fear of Lithium because of its well known vigorous reaction with water. Under normal circumstances, most (but not all) batteries do not contain any free Lithium. The Lithium content is combined into other compounds which do not react with water. The amount of Lithium deposited during the Lithium plating when cells are damaged as described above is very small and not usually responsible for the fires which have occurred. Furthermore, many of the reported fires are due to burning electrolyte rather than the Lithium compounds.
The guilty party
Although investigation has shown that some Lithium fires are due to internal short circuits as described above, many, if not most fires are caused by abuse by the user. This may be "deliberate or negligent" abuse such as overcharging or operating in a high temperature environment or physical damage due to mishandling, but quite often it is unconscious abuse. Surprisingly many of the most serious fires have been initiated by inadvertent short circuits caused by careless disposal of cells in the rubbish. While strict regulations for transporting Lithium batteries by air have been implemeted, the sources of several aircraft / transport fires have been identified as spare laptop batteries being carried in passenger luggage shorting against other items packed with them.
Note: Large batteries such as those used in automotive applications usually incorporate short circuit protection, but smaller laptop batteries do not usually have this facility.
See also more general Battery Failure Modes and New Cell Designs and Chemistries.