Lithium ion batteries have been in the news quite a lot recently, and not all of it has been positive. There have been numerous incidents of different products from airplanes to hoverboards to mobile phones catching fire during charging or operation. These problems occur when there has been damage to the cells’ structure or to the protective circuit. The protective circuit helps to avoid over-voltage and under-voltage (over-discharge).
If the charging voltage is increased beyond the recommended upper cell voltage, typically 4.2 Volts (Restek Leak Detector 4.275 V), 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 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.
- Excessive current also causes increased Joule heating of the cell, accompanied by an increase in temperature and thus overheating
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 (Restek Leak Decector 2.3 V) by over-discharging or storage for extended periods results in progressive breakdown of the electrode materials.
- Anodes – 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.
- Cathodes – 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.
All of these scenarios can be bad for the lifetime of a leak detector battery, and the safety of the end user and their facilities, that is why we too have the protective circuit in our Li ion batteries.
“But what does this mean to me as an owner of a leak detector?” you may ask. Well first it means you cannot over-charge and overheat the unit, but it does mean that you could discharge the battery below the 2.3 volt limit, and thus not be able to recharge it because the protective circuit will not allow it to accept current.
“So what should I do to avoid this?” I hear you ask. Leak detectors are not to be used occasionally, they should be used every day to check critical seals and the gas delivery system in GCs, and labs.
Where to check on a daily basis:
Septum and septum nut
Gas lines connections to inlet
Gas supply lines
Making this part of your daily preventative maintenance routine will ensure that leaks are caught very early, potentially saving columns from being ruined, and also wasting valuable gases. Using the leak detector regularly also means that you will have to charge the leak detector regularly. At a minimum the leak detectors should be charged every 3-4 months, but in reality they should be charged more frequently, or when not in use be left on charge – remember they can’t be over-charged because of the protective circuit.