Smart Cells - batteries with integrated sensors for condition monitoring

Modern battery cells, especially Li-ion cells, must be monitored with regard to their state of charge in order to avoid reaching safety-critical states. If, for example, charging or discharging limits are exceeded, there is a risk of cell damage, which may lead to cell destruction. Similarly, too high a temperature, cell operation at too low a temperature or charging or discharging with too high currents can have similar consequences. Until now, Li-ion cells have been controlled by an electronic monitoring system - the battery management system (BMS) - which uses the cell voltage in particular as an input variable, but often only the total voltage of several interconnected cells (“modules”).

Figure: Readout unit for the use of temperature sensors and reference electrodes.

For large cells in more demanding applications, especially in the automotive sector, this monitoring is no longer sufficient, which is why there is an increasing move towards single-cell monitoring, in which several complementary input variables are considered. This is referred to as “smart cells” [1].

In this context, the Fraunhofer Institute ISIT is working on the integration of various sensor systems directly on the cell body or inside the cell. The aim is to obtain real-time information on the status of the cell during operation. With the help of this information, the BMS can directly optimize the operation of the cells with regard to service life or intervene at an early stage in the event of safety-critical cell states.

Figure (temperature board): Layout of the sensor board.

The monitoring of parameters inside the cells in particular promises a major advantage here, as critical values can be detected earlier than on the outer wall of the battery. However, a number of points must be taken into account for the successful integration of sensors inside the cell:

• The sensors used must be inert to the conditions inside the cell, particularly with regard to the electrolyte, and must not have any mechanical or electrical influence on the cell.
  
• It must be possible to lead the measurement signal out of the cell.
  
• The sensor system must not interfere with the processes in the cell, as this would lead to additional cell ageing.
  
• The sensors should be as small and light as possible.
  
• The price and market availability must also correlate with the potential applicability.
  

Despite these framework conditions, Fraunhofer ISIT has succeeded in integrating various sensor systems into Li-ion cells. These can be successfully used to characterize the cells or to monitor cell operation.

Figure (temperature board): “Spacer plate” to compensate for the sensor thickness.

Temperature sensor technology

The temperature in the cell has a significant influence on cell ageing - and safety. Temperatures above around 50°C lead to a gradual decomposition of the electrolyte and thus to gas formation and an increase in the internal resistance of the cells. As heat generation is greater at higher resistance, this has a self-reinforcing effect that reduces the service life of the cell [2].

Monitoring the internal cell temperature and also the temperature distribution in the cell is therefore valuable information for finding the optimum operating conditions. Unfortunately, the internal temperature of the cell cannot be measured directly from the outside. Neither can the temperature distribution, as the aluminum foil covering the cell distorts the information.

To address this problem, a printed circuit board was developed at ISIT [3,4], on which up to 15 temperature sensors can be inserted into the cell and read out in real time. By selecting the materials, it was possible to achieve and prove the stability of the system under the conditions in the cell. Both NTC sensors and digital sensors proved to be suitable as possible sensors. Figure (temperature board) shows the board with the sensors and the “spacer plate” for mechanical protection. This allows the temperature to be mapped in one plane inside the cell. This enables a kind of “temperature tomography” during cell operation. The figure (stack with circuit boards) shows a cell stack with 2 measuring circuit boards. The figure (cell with circuit board) shows a complete cell with the connection of a cell circuit board. The figure (measured values) shows the temperature curve during a charge/discharge cycle of 2C at the different sensors of a measurement board. One limitation of this technology is that the thickness of the circuit boards + sensors is in the range of several electrode layers, i.e. it is still not possible to measure directly in the individual electrode layers.

Upper figure (stack with circuit boards): Cell stack of 10 Bicell layers with 2 sensor boards for temperature measurement.

Bottom figure (cell with circuit board): Cell with integrated sensor board for temperature measurement.

Reference electrodes

The cell voltage measured between the arresters of the cell provides information about the voltage of the entire cell, but is not a clear measure of the electrochemical potential prevailing at the individual electrodes. It is therefore possible, for example, that the measured cell voltage still has a non-critical value, but the anode is already in a safety-critical range. Figure (Potential ratios) illustrates this phenomenon. If the potential at the anode falls below 0V against Li/Li+ during charging, Li metal is deposited, which leads to the formation of Li dendrites inside the cell and ultimately to an internal short circuit. This must be prevented, otherwise cell damage will occur. Exceeding the maximum discharge potential at the cathode can also lead to decomposition of the electrode material.

Fraunhofer ISIT uses reference electrodes that have a stable potential independent of the battery cell to directly determine the electrode potentials. On this basis, an integration process suitable for Li-ion cell technology was developed: A Li-titanate reference electrode is connected to the separator next to the cell body (e.g. by means of the lamination technology used at ISIT). 

The figures (stack with ref) and (cell with ref) show an unpackaged or finished cell with reference electrode. The system of Li-ion batteries equipped with reference electrodes developed in this way was also patented here [5].

Figure (Li-plating) shows the course of the cell voltage and the electrode potentials of a cell in which Li-plating is detected by the anode potential. The deposited lithium can be recognized in the microscopic post-mortem image.

Upper figure (stack with Ref): Unpackaged cell with reference electrode.

Lower figure (cell with ref): Finished cell with integrated reference electrode.

Readout and evaluation unit

The use of the sensors requires adapted readout electronics that track and process the signal from the sensors in real time and pass it on to the BMS. A readout system (hardware and software) was developed, built and tested in operation for the combined use of the temperature sensors and a reference electrode. This provides a complete sensor + readout unit solution.

Figure (potential ratios): Representation of the voltage levels in a Li-ion cell; if the two electrode potentials are shifted to high or low values at the same time (e.g. excessively high charging/discharging currents or operation at excessively low temperatures), a limit voltage can be exceeded without the cell voltage being in the abnormal range.

First figure (measured values): Temperature curve at the sensors of a sensor board during a 2C charge/discharge.

Second figure (measured values): Corresponding cell voltage and current curves (bottom). The board was located in the center of a cell with 10 Bicell layers.

Third figure (potential ratios): Representation of the voltage layers in a Li-ion cell; If the two electrode potentials are simultaneously shifted to high or low values (e.g. excessively high charging/discharging currents or operation at excessively low temperatures), a limit voltage can be exceeded without the cell voltage being in the abnormal range.