Mechanical Effects Occurring inside Large Format 94 Ah Prismatic Lithium-Ion Cells at Different Bracing during Aging

Abstract

One of the biggest obstacles to the widespread adoption of fully electrified vehicles is the limited volumetric/gravimetric energy density of lithium-ion batteries to achieve driving ranges comparable to vehicles with internal combustion engines. Electrode thicknesses are being increased to enhance the energy density of the battery cells. Furthermore, the space within the confinements of the cell housing is utilized as much as possible. Today’s state-of-the-art (SoA) electrode materials exhibit volume and Young’s modulus change during formation and cycling, e.g. a volume change up to ~10% for graphite [1] and a threefold increase of the Young’s modulus of the graphite active particles [2]. This leads to a pressure build up within the cells due to reversible and irreversible volume changes, especially of the anode materials, during cycling. The optimized packing for SoA electrodes and the volume changes of the electrodes result in increased electrochemical/mechanical interactions that leads to stress inside the cells.In this work, the interactions between mechanical bracing and aging are studied for large format prismatic 94 Ah lithium-ion battery cells [3]. The impact of external bracing is shown for lithium-ion cells cycled up to over 7000 cycles with 100% depth of discharge (Figure 1 (a)), where the braced cells show an enhanced performance in the region of 80% state of health (SOH). The braced cells reach this threshold around 900 cycles later compared to the unbraced cells. Furthermore, the unbraced cells have a thickness change of around 17.5% after more than 7000 cycles (Figure 1 (b)). After cycling, the cells are examined with post-mortem analysis, showing significantly increased aging phenomena (e.g. lithium plating) in the unbraced cell compared to the braced cell. Inductively coupled plasma optical emission spectrometry (ICP-OES) analyses confirm these assumptions (Figure 1 (c) + (d)). In contrary to the enhanced lithium plating on the anode of the unbraced cell, X-Ray diffraction (XRD) show that bracing leads to an increased structural cathode degradation compared to the unbraced cells. Those ex-situ post-mortem experiments were also confirmed by electrochemical tests in laboratory cells, showing only small capacity fading of the aged anodes of the braced cell compared to unaged reference anode cells. Nevertheless the main factor for capacity loss inside the prismatic 94 Ah cells is the loss of lithium inventory.External pressure on cells is beneficial to guarantee sufficient contact of the components within an electrode stack and hence decreased aging phenomena mainly on the anode. The downside of the increased pressure on the cells are the significantly increased morphological and structural degradation of the cathode, which, however, plays a minor role in the overall cell degradation. The external bracing results in a more ‘homogeneous’ aging over the entire cell with only little locally different degradation characteristics.