Passivation, friend or foe?
One of the issues I am most frequently asked about by equipment designers is passivation in liquid cathode lithium primary cells and batteries. Questions regarding ‘avoiding’ or ‘preventing’ passivation are not uncommon, but the hard truth is that these cells types would not be practical without passivation. The high energy densities possible with the liquid cathode cell design are very appealing to equipment designers, with the added benefits of a wide operating temperature range and long shelf life. Depending on the precise chemistry and cell construction, reasonable (for a primary cell) power is achievable too. Now long established in the market place, with a good safety and reliability reputation, liquid cathode lithium primary cells are often the first choice power source for portable powered equipment. The drawback is the problems associated with a slow start up, sleepy cells with lower than expected voltages when power is demanded from them.
Overview of Passivation
At the surface of the lithium anode a cell, not under load, will form an insulating layer, known as the passivation layer. This is vital as without it the ‘liquid cathode’, present throughout the cell, would be in permanent contact with the lithium resulting in a continuous discharge. Obviously disastrous for cell life! A very small amount of active material in the cell is used up during the formation of this layer, which starts to form as soon as the cell is ‘filled’ during manufacture. As conditions at this stage are reasonably controlled, in terms of material age, temperature and so on, the characteristics of the passivation layer in a new, unused cell, are well understood.
What does it mean to the user?
When the cell is first required to provide energy to the equipment, this passivation layer needs to be broken down to allow the lithium to be accessed. At first application of the load this manifests itself as a drop in voltage for a few milliseconds before recovering to the expected level. Depending on the amplitude of the load, and other factors such as temperature, this voltage drop can be significant resulting in a voltage level below that at which the host equipment can operate. Heavy initial loading can also extend this low voltage period leading to users thinking the battery has failed. A cell that has gone through this voltage dip is described as de-passivated.
As soon as the external load is removed, the passivation layer starts to re-form. The nature of this re-formed passivation layer will now be dependent on other factors, such as cell age, state of charge, usage pattern, and most importantly temperature. A very small amount of capacity will, again, be used in re-forming the passivation layer, as it will each time the cell is partially discharged.
Avoidance and prevention are not options (sorry), but with correct management passivation should not be an issue. Introducing de-passivation routines into equipment start up procedures is a good start, but the passivation layer will have different characteristics each time it re-forms and so the same routine may not work every time. Another approach is to draw a continuous low current from the cell, just enough to keep the passivation layer in a state that is easily, and quickly, broken down. This is a particularly useful method where sudden higher power, short duration, burst are required. The downside being that energy is being continuously drawn from the cell, and it is likely that the higher the power burst, the higher the background current will need to be. More complex load profiles, and very long operational life (15 years+ claimed), all bring their own challenges to cell management.
Of course it may be that in managing passivation, the advantages in using liquid cathode cells and batteries is lost. Knowing what you need your battery to do at the outset helps in producing a reliable product with the most appropriate power solution. There are many different cell and battery types to choose from, but making sure you find the best match for your design is more difficult than many think. Designing equipment to make the most of the battery, rather than choosing a battery once the equipment is designed, will give the equipment a better power solution.