Designing the EV battery of the future: why one size no longer fits all

Rapid advances in battery chemistry combined with evolving mobility habits are challenging the traditional approach to optimising battery performance. Here, Dr. Doron Myersdorf, CEO of extreme fast charging pioneer StoreDot, explains why battery manufacturers must find new ways to increase the versatility of battery performance at the chemistry level to support a diverse range of driver needs, and outlines some of the implications of new driving patterns for the wider EV industry.

The electric vehicle (EV) industry is evolving at a startingly rapid rate, and nowhere is this more apparent than in the field of battery development. Radical new approaches to battery chemistry are enabling technology developers to overcome challenges that would have been considered impossible even five years ago – such as the ability to fully charge an EV in just five minutes. The goal of all this hard work? To optimise the EV driving experience and accelerate the transition to fully electric mobility by overcoming today’s main barrier to adoption – range and charging anxiety. However, as battery chemistry itself continues to evolve, it is becoming apparent that the industry’s traditional approach to optimising life cycle performance must too.

Changing the mindset

Let’s start by considering the way battery life cycle performance has traditionally been measured. This is based on three key parameters – energy density, charging speed and number of charge-discharge cycles. These variables are closely intertwined, meaning that if we optimise one, the others deteriorate. As such, battery optimisation largely relies on finding the best combination of these three parameters. In many respects, this is a fairly predictable process, and it means that if we charge a battery for six hours, we know it will likely deliver around 2000 cycles.

However, the arrival of new battery chemistries, together with a growing sophistication of battery management systems and changing driving habits are altering the game. Suddenly, battery performance is no longer quite so deterministic; instead, it can change over time depending on how the vehicle is driven and charged. This means our mindset also needs to change. The fact is that we cannot continue to optimise battery performance in the same way as we have previously, as a static function. It needs to be related to the driver’s profile and it needs to be dynamic.

Identifying the EV driver of the future

For battery technology developers, the need to identify the habits of the EV drivers of the future is particularly pertinent. While battery management systems give us the flexibility to alter certain parameters over time if the needs of a driver change – for example if they change their job and suddenly need to use their car more frequently or for longer journeys – battery chemistry is fixed. This is why it is so important to ensure that the chosen chemistry and design of the battery is as closely aligned to the projected needs of the driver as possible.

This realisation led us to consider what the EV driver of the future would look like and how this would impact their driving and charging habits. In reality, there will be hundreds of different driver profiles, but for simplicity’s sake, let’s focus on just three. First, there is the suburban mother, who primarily uses her car for short journeys and charges her vehicle overnight at home. At the other end of the spectrum there is the businessperson who regularly makes long journeys, often staying overnight at hotels; for him or her, frequent fast charging is essential. Then there is the driver that falls somewhere between the two extremes, using their car for both short and long journeys and therefore requiring both slow and fast charging.

For each of these stories there is an implication on the specification of the battery. So, for example, if a driver always fast charges, we need to look at how we optimise the chemistry based on that type of behaviour to achieve the best trade-off between energy density and cycle life for that particular driver.

New thinking in action

There are many chemistry and electro-chemistry considerations that need to be taken into account to optimise battery formulation and design based on the needs of the driver. This includes establishing the optimum combination of graphite, silicon and/or other metalloids in the anode, determining the right cathode to anode load ratio, and ensuring that the battery’s upper and lower cut-off voltage limits remain within safe boundaries. However, each decision we make has ramifications for other elements of the battery chemistry and design, and so all of this needs to be taken into account during the design phase.

When we developed our extreme fast charging (XFC) technology, one of the key challenges we had to overcome was to manage silicon’s expansion during the fast-charging process. We achieved this by using nano-particles which are combined in a 3D structure to provide room for the particles to expand without dramatically impacting the overall structure or volume of the anode. When designing batteries to suit a particular operational model, we need to determine not only the best combination of materials to optimise the parameters that are most important to the driver, but also how each combination affects the structure of the anode. So, for example, while batteries optimised for both XFC and slow charging could be silicon dominant, only the XFC battery will require the use of nano-particles to control the expansion of the silicon during the fast-charging process.

To complicate matters further, the structure of the anode is also influenced by the type of electrolyte and electrolyte additives that are used as part of the SEI layer as the ions move from cathode to anode. As the choice of additives is dependent on whether a battery is optimised for XFC, slow charging, or somewhere in between the two, this also has to be considered as part of the overall anode design.

The combination of materials used in the anode will also affect the upper and lower cut-off voltage limits of the battery, so this is another variable that needs to be considered during the design stage. Likewise, we also need to take into account how the operational model of the vehicle will impact the cathode to anode load ratio (C to A ratio) of the battery cell. Establishing the correct C to A ratio is crucial to ensuring that in every charge and discharge cycle there is a fully reversible reaction, meaning that all the lithium can be fully transferred back and forth between the cathode and anode. When designing the battery, we can increase the size of either the cathode or the anode by around 5% in order to balance the lithium transfer, as each option offers a different point of optimisation. In a battery optimised for fast charging, the cathode needs to be slightly larger, whereas in a battery optimised for slow charging, the size of the anode could be increased.

Joining the dots

These are just some of the ways that battery technology can be adapted to better meet the evolving needs of future EV drivers. However, the benefits of adopting a more customer-centric approach will extend far beyond improving battery performance. For example, capturing data on driving habits on a large scale will prove invaluable for infrastructure providers when determining the number and type of charging points required, i.e. whether to deploy slow charging or fast charging points in each specific location.

This will require a standardised approach to how data is collected and disseminated to relevant stakeholders. This process should start at the point of purchase, with the retailer asking a series of questions such as, ‘on a scale of 1-10, how important is fast charging to you?’. Another approach would be to ask the customer for permission to download their Google Driver Analytics, thereby gaining access to essential information such as average journey time, distance, driving speed and charging habits, at the touch of a button. The retailer will then be able to help their customers to choose not only the best vehicle for their lifestyle, but also the optimal battery type based on their particular driving and charging habits.    

If we look further down the road, we can see the last stop in this journey will be offering fully customised batteries as standard. While we are currently a long way from achieving this goal, it is vital that the necessary building blocks are put in place now. It is only by putting drivers firmly in the driving seat that we will successfully unlock the next stage of EV battery development.

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