The introduction of electric vehicles (EVs) has posed many technical challenges to the automotive industry; in particular, the state-of-the-art lithium-ion (Li-ion) batteries add massive weight to vehicles both in the form of battery weight and supporting systems, immensely hindering vehicle performance and efficiency. A primary source of this challenge is that the battery packs serve only one purpose – energy storage, as current EV batteries do not carry loads or absorb collision impact energy. Recently introduced by the authors, a hybrid energy storage design, called the Multifunctional Energy Storage Composites (MESC), possesses the unique feature of simultaneously carrying tremendous structural loads and providing energy-storage capabilities (1-3). It has been demonstrated that the concept potentially leads to significant weight and volume savings in electric vehicle and system designs.

In brief, the MESC involves a unique integration technique for embedding Li-ion battery materials in structural carbon-fiber-reinforced-polymers (CFRP). The crux of the multifunctional design is the incorporation of three-dimensionally interlocking “rivets” (1-3). The electrode layers, which make up the battery stack, contain a rectangular array of perforations (with diameter on the order of millimeters), instead of being solid, continuous, planar films. The perforations in each layer of the stack line up to fit the interlocking rivets, which three-dimensionally interlock the electrodes and rigidly anchor onto the stiff CFRP faceplates. The interlocking rivets exploit the intrinsic mechanical properties of the Li-ion battery electrodes, allowing them to carry mechanical loads. Despite the non-conventional cell construction, the MESC has demonstrated cycling and cycle-life performance tantamount to commercial pouch cells, with similar mechanical load carrying capabilities to automotive structural materials (2).

Even though the rivets are indispensable in the MESC design and offer immense mechanical benefits, they present a slight electrochemical challenge. The presence of the perforations influences the current flow path, current density distribution, and depth of discharge (DoD) uniformity. Therefore, in this work, special attention is given to the numerical modelling and optimization of current density distribution in the electrodes, with the presence of a perforation array. Standard non-perforated graphite/NMC pouch cells are used to determine the DoD dependency of the intrinsic open-circuit voltage, electrochemical conductance, and resistance. The governing equations for charge conservation, polarization, and current density are solved numerically in COMSOL simulation (4-6). The perforations are included as boundary conditions on the electrode domains. The model is parameterized to capture various geometrically feasible perforation patterns, array densities, and diameters. Under different constant-current profiles, the non-uniformity of current density distribution and DoD is quantified. The increase in cell impedance, due to the local constriction and spreading of electrical current, is also evaluated.

Complete functional relationships between perforation parameters (e.g. pattern, density, and hole diameter) and the electrochemical performance (current density distribution, DoD distribution, cell impedance, etc.) are established. Counter-intuitively, our results confirm minimal electrochemical impact in MESC cells with current path disruption sufficient for mechanical stabilization. Compared to unperforated cells of the same dimensions, less than a 10% increase in non-uniformity of current distribution and less than a 5% increase in cell impedance are observed. Beyond these indicative results, the model provides meaningful physical insights into the current flow path around the holes, as well as near the current collector tabs. Together, these allow the MSEC electrode design to be optimized in the electrochemical-mechanical tradeoff. Additionally, towards a broader context, it is believed this study will shed light upon electrode design and the optimization of conventional high-energy, high-power Li-ion batteries. To this end, a first step has already been taken to generalize the model and employ advanced optimization schemes e.g. genetic algorithms (GA).

References

1. P. Ladpli, R. Nardari and F.-K. Chang, Multifunctional Energy Storage Composites, WO patent application 2016127122, published 2016-08-11.
2. P. Ladpli, R. Nardari, R. Rewari, H. Liu, M. Slater, K. Kepler, Y. Wang, F. Kopsaftopoulos and F.-K. Chang, Multifunctional Energy Storage Composites: Design, Fabrication, and Experimental Characterization, in ASME 2016 Energy Storage Forum, Charlotte, NC (2016).
3. P. Ladpli, R. Nardari, Y. Wang, P. A. Hernandez-Gallegos, R. Rewari, H. T. Kuo, F. Kopsaftopoulos, K. D. Kepler, H. A. Lopez and F. Chang, Multifunctional Energy Storage Composites for SHM Distributed Sensor Networks, in International Workshop on Structural Health Monitoring 2015, Stanford, CA (2015).
4. U. S. Kim, C. B. Shin and C.-S. Kim, Journal of Power Sources, 189, 841 (2009).
5. K. H. Kwon, C. B. Shin, T. H. Kang and C.-S. Kim, Journal of Power Sources, 163, 151 (2006).
6. J. Newman and W. Tiedemann, Journal of The Electrochemical Society, 140, 1961 (1993).