More than many brands offering electric cars, Porsche must find the right formula in battery cells. Not inclined to wait long in front of a charging station, its customers are likely to empty them quickly while driving sportily.
To achieve significant recharging powers without making the various components impacted too heavy, the voltage of the pack fitted to an electric car must be high. This is why the Porsche Taycan has an 800 V battery.
This is about double what is typically found on common models available in the trendy market. Rather, it is electric trucks and buses that receive lithium-ion batteries with a voltage greater than 700 V. For these vehicles, it is a question of being able to regenerate the batteries during the few hours of shutdown when they are at the depot. .
The Taycan on the market already accepts charging power up to 270 kW. This makes it possible to recover in 22 minutes approximately 80% of the capacity of its 90 kWh pack.
Full in six minutes
Porsche Engineering has already developed a prototype capable of handling 400 kW from an ultra-fast charger in its research laboratory. By installing an innovative cooling system for the battery, vehicle and charging system. Enough to regain 100 kilometers of range in less than 180 seconds, and reduce energy refueling to 15 minutes between 10 and 80%.
It’s already very good, but the German manufacturer is hoping for much more. Its objective is to be able to refuel in 6 minutes, which is the average time to go to the pump with a gasoline or diesel model.
Porsche is working to achieve this, within the FastCharge research project, alongside, among others, the Siemens group and the German electronics component supplier Phoenix Contact E-Mobility.
Difficult to win on all fronts
A professor specializing in electrochemical energy conversion and storage technology at the University of Aachen, Dirk Uwe Sauer warns: “There will be no universal general-purpose battery.”
According to him, it is not possible to significantly increase the performance, charging speed and life of a lithium-ion battery. Combining ultra-fast charging and high energy density would result in a particularly short service life. When one parameter is optimized, it is to the detriment of the others.
At Porsche, we are convinced: “Energy storage systems for electric vehicles are constantly improving, but lithium-ion cells will remain the technology of choice for the foreseeable future”.
Why rely on lithium-ion? Due to the high reactivity of lithium, and the high density of cells which offers a good compromise between the space taken up by the elements and the amount of energy that can be stored.
In addition, there is good robustness in use, a substantial depth of discharge, no memory effect, and an already satisfactory service life around “2,000 discharge / recharge cycles in an all-electric vehicle”.
Significant gains in energy density are still expected. The latter has doubled in 10 years to currently stand at 250 Wh / kg (or 500 Wh / l). It could be further doubled by 2030.
Director of the battery electrochemistry research group at the Helmholtz Institute in Ulm, Stefano Passerini believes that the charging speed can still be improved significantly, but together with high demands on safety.
The future of solid-state batteries? Lithium sulfur?
On semiconductor batteries referred to as “solid”, a polymer or ceramic is used instead of the liquid electrolyte. Immediate benefit: A significant gain in compactness which translates into an increase in energy density while substantially reducing the risk of cell inflammation. In theory, these could be recharged more quickly. This “has not yet been proven” in practice, moderates Stefano Passerini.
In lithium-sulfur batteries currently under study, the cathode consists of a mesh of sulfur which completely replaces the usual structure made up of the trio of cobalt, manganese and nickel. If we gain in lightness, it is to the detriment of the production cost which seems for the moment to reserve this technology for large-scale projects, such as future air taxis. In addition, “their durability is still considered problematic”, emphasizes Porsche.
Among the other avenues followed which could lead in a few years to cells of better energy density: technologies based on a silicon-carbon composite electrode, cathode materials rich in nickel or high voltage elements (around 5 V ).
In the longer term, approaches which are still at the research stage could prove to be very interesting. Thus with “sodium ions instead of lithium ions, or metal-oxygen combinations”, highlights Porsche.
The price limits autonomy
“In the end, a vehicle’s range is not limited by the weight of a battery, but by its price,” assures Dirk Uwe Sauer. The latter went from 400 euros in 2013 to 107 in 2019.
However, this trend should not be expected to continue at this rate. In question, a growing demand for cells. “Raw materials represent up to 75% of the cost of a battery,” explains the researcher from the University of Aachen. That is why he envisions “evolutionary but not revolutionary” progress in the future.