Electric vehicle owners will surely be delighted to learn that the life of the battery in their cars is exceeding expectations. But what is the chemistry behind this process that is only just beginning to be understood?
Vehicle manufacturers usually offer a standard eight year or 100,000-mile warranty on the battery – more in some parts of the US – but used car dealers and telematics companies are observing longer lifetimes before battery capacity drops below 80% – the point at which it can no longer hold charge as effectively. This figure is an indicator of battery health, but doesn’t mean the battery is dead. It can be repurposed for less demanding applications such as energy storage.
Batteries degrade for a range of reasons. They all degrade initially, explains Paul Gasper, a scientist specialising in battery testing and degradation at the National Laboratory of the Rockies, US. This is due to the formation of a protective layer between the graphite anode and the electrolyte on the first cycle during manufacturing. Lithium is trapped in this layer, known as the solid electrolyte interphase.
‘Without that layer the battery won’t work. But once it starts forming, it never stops – it’s a very similar process to rust formation,’ Gasper adds. ‘So, there’s a game in battery design … I have to have this layer [but] once I form it the first time, I want it to grow as slowly as possible after that.’ This is because as this layer grows, more lithium ions are trapped in it, meaning that the battery stores less energy.
The early data from an electric vehicle (EV) might show 4% degradation in the first year, implying it wouldn’t last 10 years, but ‘we’re just now starting to have almost a decade of data on some electric vehicle models in the real world and we see that rate of degradation slow down’, says Gasper.
Tesla says that even after 200,000 miles some of its models have lost just 20% of capacity, irrespective of battery chemistry.
Battery degradation is usually tested in the lab with accelerated ageing – constantly charging and discharging a battery to estimate life expectancy. But in real world driving conditions with stops and starts, and different length trips, batteries could last up to 38% longer than forecast, according to researchers at Stanford University.
Their study tested 92 lithium-ion batteries for more than two years across four different types of discharge profiles, including real discharge profiles from field data. The charging profile was the same across all experiments. The more like actual driving behaviour the battery was exposed to, with short, sharp acceleration, the greater the battery life expectancy – a finding that surprised them.
Why does stop–start driving conditions extend battery life?
Lithium ions move in and out of the electrode material during charge and discharge – a process called intercalation. The rate and completeness of the process affects battery performance and longevity.
Alexis Geslin, a materials chemist who took part in the Stanford study, suggests that continuous cycles create ‘a constant influx of lithium ions which means they may not all have time to intercalate, creating side-reactions that can increase resistance, cause electrolyte degradation or clog the electrode pores’. Any of these processes will affect battery performance. However, breaks during driving, for example at a red light, ‘could allow the batteries to “relax”, meaning that the gradients of concentrations of lithium ions that occurred in the battery will tend to dissipate’, he adds.
Understanding what’s happening at the electrode is an active area of research but testing new battery chemistries and materials using real world demand profiles will mean better estimates of battery life, says Geslin. ‘That might, in turn, help you to work out how to make longer-lasting batteries that might degrade more slowly.’ Manufacturers could also update battery management systems with the insight that dynamic driving conditions are a positive, he adds.
Gasper observes that almost every battery component affects degradation. Rapid charging creates electrochemical stress on the battery as materials expand and contract with each charge and discharge cycle. Lithium ions travelling more quickly through the electrolyte create a steeper concentration gradient that can cause, for example, lithium plating on the negative electrode.
Scientists at the University of Cambridge have found that applying a constant level of pressure on the batteries they tested could help balance these competing degradation pathways, doubling battery longevity.
The increased availability of fast charging capabilities may be showing up in real world data too. Telematics group Geotab’s 2025 survey of over 22,700 electric vehicles suggests that batteries charged more often with direct current fast charging degraded, on average, at 2.5% a year, compared with 1.5% a year for those rapidly charged less often.
Heat management is also critical. If a thermal management system doesn’t uniformly cool the pack, the hotter cells will degrade more quickly limiting overall battery lifespan, explains Gasper. ‘Battery manufacturers again play this trade-off of how to design the thermal management system so that it’s just big enough to keep the battery pack cool during fast charging, and the temperature across the pack is as uniform as possible, while still being cheap to manufacture.’ He adds that there’s enough research that batteries could be made to last 30 years or longer or operate better at low or high temperatures for example, but would be more expensive.
Other research suggests that enabling a battery management system to switch out degraded cells could theoretically extend battery lifetime by 20%.
This knowledge could have a range of environmental benefits. Finding ways to make EV batteries last longer and enhancing their lifespans also offers another way to reduce the volume of materials mined and provides breathing space as recycling technologies scale-up.
