Solid-state battery for electric cars: key to greater autonomy

With solid-state batteries, electric vehicles should be able to achieve an autonomy (driving range) matching—and eventually surpassing—that of cars with an internal combustion engine.

For electric cars to really take off, they ideally should match—or even exceed—the driving range of vehicles with an internal combustion engine. The key to achieving this lies in the battery itself, more specifically in the use of solid-state battery-cell technology.

Fig. 1: The energy density of the Li-ion battery (LiB) cell has more than tripled since its market introduction by Sony in 1991. Continuous improvements in LiB components with LiCoO₂-graphite chemistry resulted in an average increase of 25 Wh/l per year from 1995 to 2010. Introduction of new active cathode materials, such as the NiCoAl-based and NiMnCo-based lithium-metal oxides (NCA and NMC), and the gradual addition of silicon to the graphite anode, have maintained the energy density increase ever since. However, it’s expected that with the materials known today, we will reach a practical limit for wet LiBs around 800 Wh/l. Solid-state battery technology will be needed to break through this barrier and achieve an energy density of 1000 Wh/l—and more.

At the heart of today’s EVs are lithium-ion battery (LiB) cells that contain liquid electrolytes. The best-in-class of these “wet” LiB cells have an energy density of around 700 watt-hours per liter (Wh/l), accommodating a maximum driving range of about 500 km. Yet, their energy density is expected to stagnate at around 800 Wh/l due to the characteristics of their active materials (Fig. 1).
A higher energy density can be expected from solid-state batteries, which contain a solid electrolyte instead of a liquid one. In combination with new battery-pack and battery-module developments, the autonomy of electric cars could significantly be extended.

1000-Wh/l battery cells enable more autonomy

Today, the maximum driving range of an electric vehicle is determined by the amount of energy that’s contained in the individual LiB cells of the car’s battery pack. Cells are connected in parallel and series arrangements to provide the high currents and high voltage needed to power the electric engine.

To achieve a driving range of 700 km, cells are required with an energy density as high as 1000 Wh/l (or 500 Wh/kg). Since today’s LiB cells can “only” deliver 700 Wh/l (or 230 Wh/kg), a significant boost in energy density is needed.

Battery-cell roadmaps foresee that cells of 1000 Wh/l should become available in 2030—in the form of solid-state lithium-metal batteries. We will come back to the potential of those solid-state batteries, but let’s first discuss an alternative approach to increase the amount of energy that can be squeezed out of an electric car’s battery pack—an approach leveraging so-called “smart” battery cells.