Investigation of the influence of mechanical pressure on the aging and the expansion of silicon-containing lithium-ion batteries

The demand for Battery Electric Vehicles (BEVs) is constantly increasing resulting in the need for optimization of today’s Lithium-ion battery cells (LiBs) in terms of volumetric energy density, lifetime, and high power ability. Due to their high specific capacity, next generation anode active materials as silicon (Si) play an important role to achieve these future goals. However, the high volume expansion of the Si of up to 300% during the lithiation leads to numerous drawbacks on system and cell level. Apart from numerous technical challenges for the application of those LiBs in module or pack constraints, the repetitive volume changes lead to particle cracking, contact loss, and delamination from the current collector foil. Besides several approaches on material level, i.e. new binders, modification of the active material, and the utilization of tailored electrolytes, developments on cell level are necessary to enable the extensive application of Si in automotive applications. One promising idea is the application of a mechanical pressure on the Si-containing cells to prevent contact loss, particle fractures, and layer detachment. In this work the influence of such a mechanical compression on different Si-containing anode materials is investigated in full cells with a Ni-rich layered oxide cathode material. Initially, the effects of the applied pressure on the microstructure of the electrodes are investigated via Electrochemical Impedance Spectroscopy (EIS) measurements of symmetrical cells. The results show a reduction of the Li-ion mobility within the pores of compressed electrodes. The data gained from a combination of EIS with compressibility measurements of the examined electrodes allows the interpretation of the current rate stability of the LiBs depending on the applied pressure. For electrodes with a high mass loading, the reduced pore volume in the compressed electrodes results in a hindered ion mobility. This is pronounced at high current densities reducing the available capacity by an increased polarization. Contrariwise, at low current densities the improved electric contact in the cells is beneficial for their lifetime and their performance compared to uncompressed cells. In the next step, the gained knowledge about the pressure-dependent sensitivity of the electrochemical processes in the electrodes is applied on Si-containing LiBs. Two different material concepts are presented. Firstly, the influence of the mechanical pressure on a C/Sicomposite with ∿8 wt% Si is investigated. The capacity of this composite is fully utilized resulting in a high vertical expansion. Secondly, a Si-rich composite with ∿50 wt% Si is examined, which was designed with embedded porosity in an encapsulated C-matrix preventing the vertical expansion. To enable the comparison with state-of-the-art applications, the cells are tested at different flexible and fixed pressure configurations at certain pressure levels of 0.08 MPa, 0.42 MPa, and 0.84 MPa. The flexible configuration enables the expansion of the cells under a…