Fraunhofer ISE High-Energy Battery Cells Retain Over 83 Percent Capacity After 1,000 Full Cycles

Freiburg, Germany – Researchers at the Fraunhofer Institute for Solar Energy Systems ISE have made progress in developing long-life lithium-ion batteries with silicon-rich anodes. In long-term tests, in-house-manufactured high-energy battery cells showed high cycle stability combined with high energy density.

International battery research is pursuing several approaches to improve the energy density, cycle life and cost-effectiveness of energy storage. Alongside the further development of established lithium-ion batteries, new technologies such as sodium-ion and solid-state batteries are also a focus. Silicon-rich anodes are one approach to raising the energy density of lithium-ion cells.

Silicon-Rich Cells Reach 280 Wh/kg and Complete More Than 1,000 Full Cycles

In the StrOboBatt research project, Fraunhofer ISE produced silicon-rich lithium-ion battery cells and tested them for energy density and cycle stability. More than 1,000 full charge and discharge cycles with a capacity retention of at least 80 percent is considered an important benchmark for cycle life.

The institute exceeded this benchmark with pouch cells it manufactured in-house. These are flat battery cells with a flexible foil casing instead of a rigid metal housing. In Fraunhofer ISE’s non-optimised research cell format, the cells reached a gravimetric energy density of 280 Wh/kg. After more than 1,000 full cycles at an axial contact pressure of 1,000 kPa, they retained over 83 percent of their original capacity.

The cells have a capacity of 25 ampere-hours (Ah). Their anodes consist of a silicon-carbon/graphite composite, with the silicon-carbon composite accounting for 51 percent of the anode active material.

The cells were produced with commercially available electrode foils on a semi-automatic production line in the cleanroom at Fraunhofer ISE. According to the institute, energy density of approximately 330 Wh/kg is expected in an industrially optimised pouch cell format. This figure is so far a technical expectation and has not yet been demonstrated with the research cells tested.

Mechanical Pressure Affects Cell Lifetime

A focus of the Fraunhofer ISE investigations was the effect of mechanical pressure on the pouch cells during operation. The researchers tested the cells at two axial contact pressures, 300 and 1,000 kPa. The tests were carried out, among other conditions, at 1C operation, in which the cells are charged and discharged within about one hour.

At a contact pressure of 1,000 kPa, the cells achieved the targeted capacity retention even after more than 1,000 full cycles. At 300 kPa, a significantly greater decline in usable capacity over time was observed, particularly under higher current loads.

Mechanical pressure can stabilise the electrical contacts inside the cell and reduce the risk of electrode materials detaching due to repeated volume changes. The results underscore the significant influence of mechanical design on the cycle life of silicon-rich battery cells.

The researchers also found that the residual capacity measured at a low discharge rate was higher than the capacity available during faster discharge.

Battery Technologies: Different Development Paths Toward Higher Performance and Lower Costs

The various research directions in battery technology set different priorities. While work on established lithium-ion batteries centres on aspects such as new electrode materials and higher energy densities, alternative cell chemistries and solid-state concepts pursue other approaches to reducing costs and improving safety and raw material availability.

Lithium-ion batteries currently dominate in electric vehicles. According to the International Energy Agency (IEA), lower-cost lithium iron phosphate (LFP) batteries accounted for more than 55 percent of the battery capacity deployed in electric vehicles worldwide in 2025. Nickel-containing NMC cells offer higher energy densities but are more expensive and are used in particular in electric vehicles with high range requirements.

Sodium-ion batteries do not require lithium and can offer advantages at low temperatures. According to the IEA, however, their energy density, at up to 175 Wh/kg for the latest cell generations, is lower than that of LFP cells (up to 205 Wh/kg) and NMC cells (up to 265 Wh/kg). Solid-state batteries with solid electrolytes promise advantages in energy density and safety but have yet to demonstrate these in industrial applications.

Silicon-rich anodes are intended to raise the energy density of existing lithium-ion cells and can also reduce the need for graphite. The Fraunhofer results show that, with appropriate mechanical contact pressure, high energy density can be combined with high cycle stability.

Source: IWR Online, 22 Oct 2026