“For the first time, the world’s fastest production car is electric,” said Tesla boss Elon Musk when he recently launched the company’s latest battery. The new 100kwh device can propel Tesla’s cars to about 97kph in just 2.5 seconds and allow them to drive 20% further before recharging, compared to previous batteries.

But Musk also admitted that the current design and chemistry of the battery means this is quite close to the theoretical limits that it can achieve. From here it will become exponentially harder to increase the amount of energy a battery of the same size can store. So where could Tesla and other electric vehicle manufacturers go next?

Tesla’s current P90D design uses a battery pack that sits underneath the floor of its “skateboard” chassis. This allows the car to carry a large volume of battery cells while maximising the interior space of the vehicle, although it also leaves the battery vulnerable to damage in an accident.

The new P100D battery pack looks the same from the outside and appears to use the same two rows of lithium ion cells. Yet, remarkably, it packs 100kwh of energy density into the same battery model that previously stored 90kwh and weighed just 4% less. That’s over 11 times the amount of electricity that a medium British house uses on an average day.

Solid-state batteries

The difference is in the way the pack is put together, the cooling system and the electronics. For example, a significant improvement in the way the batteries are cooled could have created enough room to fit 56 extra cells to provide the additional 10kwh energy. The 4% increase in weight suggests more components have been added and that perhaps the cell arrangement has been reworked to allow for this extra weight in the same volume.

For Tesla to go much further, however, it may need to consider a completely different way of storing energy. One early-stage technology being studied by companies including Toyota, Volkswagen, Bosch and Dyson is solid-state batteries. These are potentially safer, store more energy for their size and could lead to low-cost battery module developments.

image-20160905-15466-22w0it — Battery ‘skateboard’. Oleg Alexandrov/Wikimedia, CC BY-SA

Typical lithium ion batteries such as the one Tesla uses contain a flammable liquid electrolyte, while solid-state batteries use a solid electrolyte that is comparatively safer. This also opens up the possibility of using lithium metal instead of a graphite electrode, as this has a higher energy density and longer cycle life. Recent improvements in electrolyte additives and ceramic shields could solve the problem of lithium electrodes sprouting fibres or “dendrites” that eventually short-circuit the battery.

Autolib, a Paris-based electric car-sharing service has already started using these solid-state batteries in its 3,000 or so cars. Bosch’s battery company, Seeo, claims to have developed prototype batteries with an energy density of 350 Wh/kg (watt-hour per kilogram). In comparison, the Panasonic 18650 cells that Tesla uses have an energy density of just 254 Wh/kg. Simply replacing Tesla’s current cells with these solid-state batteries (once they are ready for production) would help the firm go from a 100kwh battery pack to a 118kwh model – almost twice the improvement Tesla’s new P100D has made on its previous design.

Some believe such strategies could help produce safe batteries that can carry enough charge to really compete with petrol engines. Donald Sadoway, a materials chemist at MIT, says that achieving such high energy densities is key to widespread adoption of electric vehicles. “If we had batteries with 350 Wh/kg we’d have EVs [electric vehicles] with 350 miles of range, and that’s the end of petroleum,” he said.

Sodium alternative

However, continuing to use solid-state batteries with lithium electrodes may not be possible because the metal’s rarity means it comes with high financial and environmental costs, especially compared to carbon-based electrodes. Two potential alternatives are sodium ion-based and possibly sodium metal-based batteries, which have higher energy densities than lithium ion batteries. One such prototype battery has demonstrated 650Wh/kg of energy density, which means 650 miles of range for an electric vehicle in a single charge – more than twice what current lithium ion batteries offer.

Sodium is much more abundant than lithium and its salt (sodium carbonate) is ten times cheaper than the equivalent lithium salt. As the costs of the electrodes and electrolyte take up more than 50% of the cost of a typical cell, batteries using sodium-based reactions will have a key advantage in that respect. Sodium ion batteries can also be completely discharged without damaging the active materials and without creating a hazard – unlike lithium ion batteries, which can catch fire if stored without charge in them.

With these developments in solid-state lithium and sodium technology, we can expect to see electric vehicle batteries with higher energy densities and lower costs than the ones recently revealed by Tesla. But that doesn’t mean we’ll necessarily see them in all electric cars. An alternative strategy would be to put more effort into reducing the body weight of the vehicles by using carbon composites so they can carry more batteries. This could potentially get us to the range of more than 350 miles in a single charge without the need for a new type of battery. But whatever the innovation is, Tesla and other manufacturers still need a final advance to put electric vehicles within reach of ordinary drivers.