Project Phoenix: How Smart Sensor Technology Supports the Self-Healing of Batteries

The Fraunhofer ISC has made significant contributions to the successful achievement of a milestone in the EU project PHOENIX with its materials research and technology expertise. PHOENIX is developing sensors and triggers for self-healing batteries. As part of the milestone, a new generation of sensors and triggers with optimized properties has been produced. The prototypes have already been delivered to the partners for evaluation.

For PHOENIX, the Center for Smart Materials and Adaptive Systems (CeSMA) of Fraunhofer ISC developed HASEL actuators that apply mechanical forces to stressed battery cells. HASEL stands for "hydraulically amplified self-healing electrostatic" – a new class of self-monitoring artificial muscles in the form of high-performance actuators. These couple electrostatic and hydraulic forces to achieve various action modes, initiating pressure-dependent self-healing effects in the PHOENIX battery cell. The 300 µm thin, polymer film-based HASEL actuators were able to generate pressures of up to 70 kPa (0.7 bar).

To additionally initiate self-healing effects through temperature, flexible thermal triggers were developed. These self-regulate their maximum temperature to avoid exceeding the optimal temperature range for the battery cell. With the help of PTC materials (PTC = positive temperature coefficient), a homogeneous temperature distribution across the entire battery surface can be achieved.

Further work by CeSMA focuses on optimizing battery sensor technology. Both the thermal and mechanical behavior of the battery are utilized to obtain information about its condition. The properties of materials with NTC behavior (NTC = negative temperature coefficient) have been adapted for PHOENIX. The modified materials can be printed on thin substrates and can be cross-linked at moderate processing temperatures below 120 °C. They are used in PHOENIX as flexible and thin temperature sensors to measure temperature development in the battery cell and draw conclusions about the state of health (SoH).

Thin, pressure-sensitive elastomer sensors with adapted sensitivity can also be integrated with the battery cell in the module. They capture the expansion of the battery cell and the tension pressure in the module. These sensors provide valuable information about the state of charge (SoC) and the state of health (SoH) of the cell.

As a third sensor technology, ultrasonic sensors are used, placed on both sides of the battery cell. They measure the time-of-flight to determine the expansion of the battery and changes in mechanical behavior (e.g., modulus of elasticity). It has already been demonstrated that the state of health (SoH) of the cell can be monitored using this data.