title: Investigation of the influence of mechanical pressure on the aging and the expansion of silicon-containing lithium-ion batteries
Creators: name: Müller, Verena
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Lithium-ion battery
subject: mechanical pressure
subject: aging mechanism
subject: expansion behavior
subject: 620

description: 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 constant pressure, while the fixed configuration inhibits the expansion leading to the increase of the pressure in the fixture. Operando expansion measurements with a high accuracy are conducted within the flexible configuration. Furthermore, a new pressure mapping system introduced herein enables the spatially resolved evaluation of the pressure distribution on the surface of the pouch cells. The results demonstrate that the application of a mechanical pressure is of an utmost importance to prevent accelerated degra- V dation in the Si-containing anodes and to prolong the cycle life. Furthermore, different pressure induced aging mechanisms can be identified in the case of the C/Si-composite with a high vertical expansion. When cycled in a heavily flexible pressure configuration, the C/Sianode tends to an accelerated film formation due to the repetitive swelling. Therefore, the cycleable Li is reduced causing a linear degradation of the cells. Contrariwise, the attempt of preventing the swelling in the heavily fixed configuration leads to the development of pressure hot spots in regions with a high current density. Thereby, pore closure of the separator is induced reducing the available capacity drastically by an increased polarization. In case of an anode active material providing an embedded porosity, the volume expansion can be reduced by a combination of an adjusted density on electrode level together with a heavy compression on cell level. In addition, the lifetime of these cells is prolonged significantly. However, due to the void space provided in the composite material, this improvement comes at the expense of the volumetric energy density. Finally, the results of the pouch cells are transferred to a prismatic cell format consisting of Si/C-anodes (∿50 wt% Si) and Ni-rich cathodes. Employing operando expansion and X-ray computer tomography measurements, the mechanical deformation of the jelly roll during the compressed and uncompressed cycling is evaluated. The lifetime of the prismatic cells can be prolonged by mechanical compression. Aging processes and expansion behavior of next-generation active materials are investigated as functions of mechanical compression as well as of the cell geometry, therefore this work provides a new baseline for a well-defined implementation of these materials in future applications. The understanding of the effects of mechanical pressure on the electrodes, the pouch cells, and the prismatic cells facilitates the design and the development of modules and packs and can promote the extensive utilization of Si as active material in LiB applications.