A gravimetric capacity of 240 Wh/kg and a volumetric energy density of 700 Wh/l. Sounds like a great cell? Hand on heart! Who can really make sense of this data off the top of their head? This article helps to clear up any ambiguities. What performance data can we really expect from cells today? And how should future battery chemistries perform?
Anyone who wants to launch a new battery cell on the market today usually publishes a number of performance parameters. These usually include
- Cell chemistry
- Capacity
- Temperature range
- Voltage curve
- Maximum charging and discharging current
- Cycle life
- Volume and weight
However, it is often difficult to assess how a cell performs in comparison. While the permissible temperature range is still very easy to get an overview of, it becomes much more difficult with other parameters. How good is a cell with 50 Ah? That depends on the size of the battery, of course. That is why there are two other parameters:
- Gravimetric capacity (Ah/kg): This relates the capacity to the weight of the cell
- Volumetric capacity (Ah/l): Here, the capacity is set in relation to the cell volume.
With these two parameters, it is then possible to compare cells of different sizes and formats. Even more frequently, the capacity is multiplied by the nominal voltage (for LFP cells this is approx. 3.2 V and for NMC/NCA this is approx. 3.6 V), so that the energy density in Wh/kg or Wh/l can be specified .
But how good is a lithium cell with 240 Wh/kg?
It is not easy to answer this question, so a market analysis was carried out that takes freely available cell data sheets into account and indicates the energy densities that can be achieved depending on the cell type. This table can then always be used as a reference for a new cell. The result of the research is shown in Figure 1.
Figure 1: What capacity + energy density (volumetric and gravimetric) should a good lithium NMC, NCA or LFP cell achieve in 2024?, own illustration.
Here are a few notes on the significance of the data (boring, but necessary):
Some of the data sheets available on the Internet, which form the basis for this evaluation, are very outdated. A large proportion of the cell data sheets are five years old or more. Old data sheets were ignored if sufficient other data was available (which was not always possible due to the sparse data available).
Large cell manufacturers in particular, such as CATL, BYD etc., sometimes do not publish any data sheets at all, as they are only active in the B2B market. Although the smaller manufacturers do publish data sheets online, they systematically achieve poorer cell parameters than their larger competitors.
So to sum up: There is a systematic negative bias in the data that is available. The capacity of the cells is systematically understated.
For this reason, publicly available roadmaps from manufacturers were also included in the data analysis. In these roadmaps, manufacturers provide forecasts of the energy densities they would like to achieve in the coming years. There is now a strong positive bias here, as these roadmaps are usually mere declarations of intent and it has often been shown in the past that announcements have not been implemented. Announced energy densities tend to be set too high.
For the data analysis, both data sets were combined, weighted and mean values were determined. This results in values that are realistic for 2024. The result is the table in Figure 1.
Back to the initial question: How good is a lithium cell with 240 Wh/kg?
This can now be easily answered with the table. The cell would be well suited for use as a stationary energy storage device (high energy cell).
How good will Sodium-Ion and Solid-State batteries be?
Now that the performance parameters expected on the market have been defined for the common cell chemistries LFP and NMC/NCA: What about new technologies such as Lithium Solid-State batteries and Sodium-Ion batteries?
Very little data is available so far, meaning that only rough estimates can be made. The available data is also
- very incomplete,
- are forecasts of the manufacturers
- or come from comparatively small manufacturers.
Nevertheless, it is worth analyzing this data. They can serve as benchmarks for new announcements and show how the market will change over the next few years. Figure 2 shows the available data for Solid-State and Sodium-Ion cells.
Figure 2: What are the (volumetric and gravimetric) capacity + energy densities to be achieved for sodium-ion batteries and lithium solid state batteries?, own illustration.
How can the cell industry 2024 be summarized in one picture?
A Ragone diagram is often used in the battery industry to illustrate the performance parameters. This compares the gravimetric power of a cell with its gravimetric capacity, making it easy to assess whether it is an energy cell or a power cell.
This representation is modified here and instead the gravimetric energy density is compared with the volumetric energy density (see Figure 3).
Figure 3: What energy densitities are typical for common cell chemistries? For solid-state batteries and sodium-ion batteries, only a few measurement data exist, so these are shown as dots, own illustration.
For lithium LFP and NMC, the spectrum from Figure 1 was drawn as a rectangle. The new cell chemistries sodium-ion and lithium solid-state are only shown as dots due to the low availability of data.
