Optimizing Energy Density of Li-Ion Batteries Using Thick Electrodes

Low-cost, high energy density lithium-ion batteries (LIBs) have been consistently pursued in consumer electronics, electrical vehicles (EVs) and stationary (grid) energy storage. Modern Li-ion cells can have an energy density of up to 550 Wh/L, compared to only 200 Wh/L in the late 1990s [1]. The gradual energy density improvement over the last 15-20 years was mostly due to the improved engineering of the electrode coatings and, to some extent, some improvement in active material development and manufacturing. The former has significantly increased the volume ratio of active materials (which provides the capacity of the battery) from ~20% at early stages to ~45% in the state-of-art LIBs [2,3]. Thickening the cathode and anode is one effective approach to further increase the active material content, enabled from reducing current collector and separator layers in the stack, which further improves the energy density and lowers the cost of LIBs. It has been estimated that significant reduction in pack cost can be realized when compared to conventional manufacturing if electrode thickness is doubled and aqueous slurry processing is utilized [4]. However, thicker electrodes lead to poor kinetics and electrolyte salt depletion and thus underutilization of active materials. The major concern of thick coatings is electrolyte salt depletion, which results in fewer lithium ions available in the liquid phase for reaction at the active material surface. Therefore, there is an optimum thickness for maximum energy and power density. To maximize energy and power density, the effect of various coating parameters on the reaction kinetics and thus battery performance must be considered during electrode design. The operation of LIBs follows porous electrode theory and electrochemical reaction thermodynamics, and the governing equations have been summarized by Newman et al. [5]. Thus, the porous electrode model is used in the present work to investigate the effect of manufacturing parameters such as electrode thickness and porosity. In the present study, the following will be discussed: (1) The effect of thickness on active material utilization, areal capacity, voltage and energy density. (2) The effect of porosity on active material utilization, areal capacity, voltage and energy density. (3) Possible solutions to further increase the energy density of thick-electrode battery. Acknowledgement This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell). References: [1] J. F. Rohan, M. Hasan, S. Patil, D. P. Casey and T. Clancy, Energy Storage: Battery Materials and Architectures at the Nanoscale, p113. [2] R. Moshtev, J. Power Sources, 91, 86 (2000). [3] M.N. Obrovac, New Metal-Ion Battery Chemistries, Workshop on Energy, Advanced Materials and Sustainability May 29, 2015, Halifax, NS Canada. [4] D.L. Wood, J. Li, and C. Daniel, J. Power Sources, 275, 234 (2015). [5] M. Doyle, T. F. Fuller, and J. Newman, J. Electrochem. Soc., 140 (6), 1526 (1993).

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