In Situ Thickness Measurements During Cycling of Li-Ion Pouch Cells with Silicon Negative Electrodes

Abstract

Cell expansion and contraction are major concerns for battery pack and module developers. This work is an effort to understand the behavior of cell expansion due to formation, lithiation and cycling, with measurements made by a system that uses a Linear Variable Differential Transformer (LVDT) sensor. For a NCM622-Silicon cell, impact of factors such as initial compression pressure and calendaring of electrodes on cell expansion were explored. Reversible expansion during charge/discharge in a cycle is a function of cell capacity and reversible expansion can be reduced with increasing initial compression. Irreversible expansion during cycling grows linearly with the number of cycles and can also be reduced with initial compression of the cell. With initial compression of 45 psi, measurements showed 3% cell expansion during formation, 4% reversible expansion in a charge/discharge cycle with C/5 rate, and 12% irreversible expansion over 220 cycles. From the data, a hypothesis was developed suggesting that SEI growth and plastic deformation of the silicon particles could be the main causes for irreversible expansion of the cells with silicon electrodes, with the assumption that cells show less than 20% capacity fade over life.