Ensuring a long service life and the reliability of lithium-ion batteries is a key factor for the market success of electromobility and intermediate storage of renewable energies. The complete understanding of the interactions between cell materials, electrodes, cell design, and materials degradation is essential to improve ageing tests in terms of prediction quality and acceleration, and service life forecasts with high accuracy.
In the MAT4BAT joint project funded since 2013 by the European Union, ZSW opened and analysed cells by various manufacturers, which were cycled and/or stored at project partners under various conditions. Deposits of metallic lithium on the anodes were identified as one of the main causes for cell ageing of high-energy batteries in cycling operation. This lithium plating forms in particular during charging at high currents or low temperatures and increasingly reduces the capacity of the cells. This also adversely affects the safety properties of the cells.
As part of the project, the operating parameters in which metallic lithium can occur, were investigated intensively (see fig. below). Lithium deposition is caused by polarisation effects in the electrode stack, which can be determined quantitatively via reference electrode measurements. Glow Discharge Optical Emission Spectroscopy (GDOES) was established in the project for determining the depth profiles of lithium distribution in the anode. A method was also successfully implemented to reliably predict operating conditions via reconstructed fuel cells and reference electrode measurements. By selecting adapted charging strategies, lithium deposits can be largely avoided and the service life of the cells can be increased significantly.