Batteries: Power is nothing without control

As part of our Thought Leaders series, Mewburn Ellis Partner Sam Bailey explores some exciting developments in battery chemistry and the resulting need to control the increasing energy density of batteries.

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The phrase “Power is nothing without control” was a slogan used (to some acclaim) by a well-known Italian tyre manufacturer, but it applies just as well to the battery technology that is now being used to power the next generation of vehicles.

Ever since lithium-ion (Li-ion) batteries took over from nickel-cadmium as the rechargeable battery of choice, scientists have worked to squeeze more and more performance out of the Li-ion system. But the greater the energy density (the amount of charge per unit volume of battery), the more thought needs to be given to how to get that energy into and out of the battery in a controlled manner.

I think this will be one of the big growth areas for innovation as battery chemistry develops further in the coming years.

Going with the flow

The charging and discharging of Li-ion batteries involves two main processes. Lithium ions must flow from being embedded in the structure of one electrode through an electrolyte and into the structure of the other electrode. While that is happening in the battery, electrons flow through an external circuit: either driven by an energy source when charging or using power from the battery to drive a device such as an electric motor when discharging.

Recent developments in the chemistry of battery electrodes have the potential to dramatically increase the power of a Li-ion battery. For example, the use of silicon in anode materials allows more lithium ions to be squeezed into the same electrode space. Meanwhile, the work being done to replace traditional flammable liquid electrolytes with novel solid materials is not only increasing safety but also opening up possibilities for higher-voltage batteries because the electrolyte material can tolerate a higher voltage before it breaks down.

While these exciting and necessary developments in battery chemistry enable higher-power batteries, manufacturers are faced with a competing customer demand for faster charging rates and longer time between charges. In simple terms, this means that more energy needs to be moved into and out of the battery in a shorter space of time.

Tabless technology

Charging and discharging quickly and without causing components to overheat presents some serious challenges – and this will only get worse as battery power increases. However, a few recent innovations in battery architecture and design are going some way towards addressing the issue. Notably, the ‘tabless’ technology developed by Tesla and championed at its recent Battery Day event claims to reduce resistive heating during charging and discharging. Meanwhile, some fascinating work from Penn State University on the temperature at which cells should be charged offers the prospect of a headline-grabbing ten-minute charge time for an electric vehicle.

However, as improvements in battery chemistry give access to greater energy density, the electrical current required to charge the cell rapidly increases. And for every doubling of the charging current, the waste heat produced increases fourfold, which both reduces charging efficiency and potentially poses significant safety concerns.

What’s more, it is known that careful control of the level to which a Li-ion battery is charged – by squeezing fewer lithium ions into one electrode – can significantly extend its lifetime. This strategy is often neglected in consumer electronics, where time between charges usually trumps battery lifespan. However, due to the high cost of battery replacement in an electric vehicle, the lifetime of the battery becomes a critical consideration and the optimisation of the charge and discharge process becomes far more important.

With this in mind, the need to control an increasing reservoir of power in novel battery systems and extend the lifetime of valuable battery packs will continue to demand fresh innovation over the next few years, both in terms of battery design and charge/discharge systems.