Batteries for electric cars Part 1: Where we are today in cell chemistry

Inhaltsverzeichnis

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For many decades, the focus in the further development of the powertrain in cars was on the engine. With the switch to battery electric drive, this is shifting, as the battery cell is now the focus of interest. Huge sums are currently being invested globally to achieve progress in this area. Energy density, durability, charging performance and, above all, costs must be optimized if the battery-electric drive is to stand a chance anywhere in the world.

In a series of articles, we want to show where we are today and where the development is heading. The European Union has set the political framework: From January 1, 2035, only passenger cars with no direct CO₂ emissions may be newly registered. According to current estimates, this will largely be battery-electric cars. Because the industry cannot simply be switched on and off, there will be a continuous ramp-up in the almost twelve years until the deadline.

Let's clarify a few terms first: The traction battery supplies the electrical energy for the drive motor. At the same time, it stores the electricity that is recovered during deceleration and braking. This is called recuperation. In addition to the traction battery, electric cars have an on-board battery at a lower voltage level to supply the driving lights and other consumers.

The battery system as a load-bearing component of the car body

In the battery system of an electric car, the individual cells are combined. Currently, there is usually still a technically superfluous intermediate level, the module, in which several cells - for example twelve - are integrated. The module only exists because it is an easily manageable unit for the automotive industry. Some Chinese manufacturers such as CATL or BYD build the cells directly into the crash-proof housing.

This method is called Cell-To-Pack (CTP). If the housing of the battery system is also a (load-bearing) part of the car body, it is called Cell-To-Body (CTB) or Cell-To-Chassis (CTC). There is no naming system, but ultimately only the manufacturers' own designations.

Temperature management is elementary for the safety and durability of battery cells. This is done both actively - in the best case via powerful heating and cooling circuits - and passively: charging and discharging currents are controlled by software.

The control via software happens in the battery management system. In addition to the tasks mentioned, it is responsible for the charging stroke: Only a portion of the maximum energy content is released. In practice, most traction batteries only use 80 or less percent of the maximum energy content. For example, they charge from a level (common abbreviation SOC for State-Of-Charge) of ten to 90 percent or less. The driver does not notice any of this; all that is relevant to him is what net energy content is actually available - and only this amount of energy is displayed to him.

C-rate as a comparative variable for the loading speed

To compare the charging speed of electric cars with different energy contents, the term C-rate is common. 1C means that the traction battery is completely charged or discharged in one hour. If an electric car does that in half an hour, it's called 2C, and so on. Manufacturers often give minute numbers like "30 minutes for five to 80 percent" SOC. The rule of three helps here: This period corresponds to a C rate of 1.5 for this charging stroke.

Let's look at the core, the cell: In terms of basic design, today's lithium-ion cells are very similar. The charge transport happens via lithium ions in the liquid electrolyte between anode and cathode.

Almost only graphite is used at the anode, which offers great potential for optimizing the volumetric and gravimetric energy density, i.e. the measure of the kilowatt hours per liter of installation space and per kilogram. We will take a closer look at the anode in the second part. What already exists in individual cases, such as the Porsche Taycan (test), is an admixture of silicon to the graphite anode in the small single-digit percentage range.