Supercapacitors on batteries: More than meets the eye
May 30, 2014 Dr Peter Harrop
It is now commonplace for supercapacitors to be placed across lithium-ion batteries. Indeed, in some applications, the battery is now omitted and the supercapacitor does the job on its own as with some hybrid and pure electric bus traction systems and power tools. This is despite supercapacitors having one tenth of the energy density of lithium-ion batteries and much higher cost on most criteria other than power density. That interim stage is misunderstood however. It is more effective and future proof than first meets the eye.
Indeed, designers rejecting the option for most cars, buses and beyond, are probably taking too simplistic a view. It is popularly believed that the benefit of putting a supercapacitor in parallel with the battery permits fast charge and discharge, improving charge time and acceleration, and protects the battery so it lasts longer because all too many of them expensively expire well within the life of the vehicle.
However, look closer and it is clear that a battery so protected no longer has to be a compromise of construction necessary for high power density, with many thin electrodes and a large area of separator wasting space and cost but it can be suitable just for high energy density. The resulting simpler design is lower cost, so not only is less battery needed, what remains is cheaper battery type.
Another largely ignored factor is that, if the battery tolerates deep discharge the supercapacitor achieves this resulting in 10% more energy being made available from the battery. That directly translates into at least 10% greater range for a pure electric car, as has been demonstrated in Japan. This confers huge competitive advantage. Although cycle life typically decreases with increased depth of discharge, progress can be made in improving batteries in harmony with supercapacitors. Special cell constructions and chemical mixes maximise the potential of deep cycle batteries whereas many other cell chemistries will not tolerate deep discharge and cells may be permanently damaged if fully discharged.
Finally, as tracked by market leading manufacturer Maxwell Technologies, leading user Bombardier and analysts IDTechEx, supercapacitors are improving faster than lithium-ion batteries in most respects, so a gradual increase in the supercapacitor component and decrease of the battery component is in prospect.
The improvements go beyond direct performance. Form factor is involved with the advent of structural components of, say a vehicle being made that double as supercapacitors and smart skin and transfer printed forms also revealed, none of which are in production but clearly in the lead for eventual roll out. That may make it practicable to have high capacity supercapacitors across batteries in everything from tablets to electric aircraft, something less likely to happen with batteries though there has been some work there too.
Although supercabatteries (asymmetric electrochemical double layer capacitors AEDLC) have long been available that compromise the benefits and disbenefits in one package. That compromise is locked in at manufacture, so many designers prefer the versatility of the discrete component approach. Clearly any designer of a product incorporating lithium-ion batteries should consider the option of putting a supercapacitor across it and, in doing so, go through the many potential benefits, most of which leverage each other, before making a decision.
For further reading download the report Electrochemical Double Layer Capacitors: Supercapacitors 2014-2024.