Longer range with the same weight: Silicon battery passes lab tests
A major breakthrough in battery research: Sheet cells with silicon-rich anodes maintained over 80 percent of their capacity after more than 1,000 cycles in tests. Here’s what factors play a key role and what this progress means for electric vehicles.
Image: Fraunhofer ISE
Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) have, as part of the “StrOboBatt” project, managed to operate a self-made 25-Ah thin-film cell with silicon-rich anodes for over 1,000 full charge-discharge cycles. After the testing, the cell still retained more than 83 percent of its original capacity. This marks an important milestone in battery development: 1,000 full cycles with at least 80 percent capacity retention is considered the minimum requirement for commercial use.
Silicon: high energy density, but extreme expansion
The special feature of the membrane cell is its silicon-rich anode, as it enables a higher energy density compared to conventional graphite anodes. However, silicon cannot currently be used as an anode material because its volume expands by up to 300 percent (a fourfold increase) when charged due to the absorption of lithium ions, and contracts again when discharged. This significant movement causes a loss of electrical contact within the electrode and accelerates the decomposition of the electrolyte. The result is that the cell ages too quickly to be practically useful. For comparison, graphite expands by only about ten to twelve percent.
To reduce mechanical stress caused by excessive expansion, researchers at Fraunhofer ISE used a silicon-carbon composite that makes up 51 percent of the anode active material. Together with a nickel-rich cathode, they manufactured the thin-film cell on an industry-scale pilot line.
Pressure and charging dynamics are key factors
In the laboratory, the battery cell prototypes were also subjected to an axial pressure of 1,000 kilopascals (about 10 bar) during charging and discharging. The electrode contact remained stable despite the movement of the material. However, at a lower pressure of 300 kilopascals (about 3 bar), the usable capacity declined much more rapidly. Thus, external mechanical compression proved to be a critical parameter.
In addition, tests under dynamic real-world conditions showed that the usable capacity declines earlier at high performance levels compared to gentler testing cycles. This provides an important insight for future use in electric vehicles, whose battery management systems must take this into account.
The cells tested achieved a gravimetric energy density of 280 Wh/kg. However, Fraunhofer ISE explains that around 330 Wh/kg can be achieved through industrially optimized mass production. A battery with 330 Wh/kg would represent a gain of about 20 to 35 percent over current NMC and NCA cells, and nearly double the performance of LFP cells.
European raw materials for greater range
In practical terms, this means that an electric car could carry almost 100 kWh of storage capacity with the same battery weight, up from around 75 kWh today. This would increase the realistic range of a medium-sized vehicle from about 450 kilometers to well over 600 kilometers.
In addition to simply increasing range, this technology also brings strategic advantages to the industrial landscape. Since graphite currently has to be imported mostly from outside Europe, greater use of silicon composites allows for a partial shift of raw material value creation to Europe, as silicon can be sourced locally and synthetically.