Chemical power

The Australian Grand Prix of 2014 brought with it a watershed moment. Not, as you might think, for the introduction of the 90º, 1.6 litre V6 engines or the eight-speed sequential gearbox. I wouldn’t say the event was even pivotal to the introduction of kinetic energy recovery because, as I’m sure you are aware, that was first introduced in 2009, deleted for 2010 and then reinstated a year later. No, the race was notable in my opinion for a change in thinking towards the onboard battery pack.

In the original KERS battery pack of 2009, storing only a mere 400 kJ of electrical energy was never really a problem. The issue at the time was how to deploy the 60 kW of the push-to-pass energy for 6.67 s per lap. Thus the size of the battery in terms of kW-hours per kg was not considered much of an issue. More of an issue perhaps was the power density in terms of kW per kg and the size of the electric motor to use it. For 2014, however, the amount of energy that can be stored has increased tenfold to 4 MJ, but the maximum electrical power available to the rear wheels has been increased only twofold, to 120 kW. That would seem to put greater emphasis on electrical storage over simple outright performance compared with earlier designs, and so represents quite a pivotal moment.

But battery technology has moved on a long way in the past ten years. Whereas in the early 2000s the only practical technology might have been traditional lead-acid batteries – as fitted even these days by just about every vehicle manufacturer – for the current crop of Formula One teams the only serious option, according to many is lithium-ion technology. Since lithium is the highest placed metal in the electrochemical series, and much lighter than lead or indeed any other battery technology, it is easy to understand why.

Consisting simply of an anode, electrolyte and a cathode, in many cases the anode is principally carbon or carbon-based. Most ongoing development therefore would appear to be based around the electrolyte and the cathode, and in particular the choice of lithium-based compounds. Because of its relatively high voltage output per cell (around 3.6-3.7 V), early designs were developed around lithium-cobalt chemistry. Further developments led to the introduction of manganese and nickel, which reduced the internal resistance of the cell and offered higher current flow and faster charging but at the expense of lower energy density. These days, the favoured cathode would seem to be based around nanophosphate technology which, although generating a reduced voltage (around 3.2-3.3 V), can accept higher currents and increased capacity at the expense of shorter overall battery life. In Formula One, where chassis parts (including the battery) can be replaced frequently, that is not the problem it might be in other electric vehicle applications.  

In the end though, battery development in Formula One is not just about voltage per cell or indeed energy storage, it is in effect about designing an efficient system, from harvesting all the way through to re-powering the electric motor and making maximum use of the 100 kg of fuel available. That is why Mercedes-Benz is running away from the rest of the field at the moment.

To this end, teams might already be looking at potential new developments in lithium-sulphur technology which, although offering only 1.9 V per cell if early reports are to be believed, offer vastly greater storage capacity per kilogramme.

Fig. 1 - The Periodic Table

Written by John Coxon