Impact of Pore Tortuosity on Electrode Kinetics in Lithium Battery Electrodes: Study in Directionally Freeze-Cast LiNi_0.8Co_0.15Al_0.05O₂(NCA)

Abstract

The prevailing electrode fabrication method for lithium-ion battery electrodes includes calendering at high pressures to densify the electrode and promote adhesion to the metal current collector. However, this process increases the tortuosity of the pore network in the primary transport direction and imposes severe tradeoffs between electrode thickness and rate capability. With the aim of understanding the impact of pore tortuosity on electrode kinetics, and enabling cell designs with thicker electrodes and improved cost and energy density, we use here freeze-casting, a shaping technique able to produce low-tortuosity structures using ice crystals as a pore-forming agent, to fabricate LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathodes with controlled, aligned porosity. Electrode tortuosity is characterized using two complementary methods, X-ray tomography combined with thermal diffusion simulations, and electrochemical transport measurements. The results allow comparison across a wide range of microstructures, and highlight the large impact of a relatively small numerical change in tortuosity on electrode kinetics. Under galvanostatic discharge, optimized microstructures show a three-to fourfold increase in area-specific capacity compared to typical Li-ion composite electrodes. Hybrid pulse power characterization (HPPC) demonstrates improved power capability, while dynamic stress tests (DST) shows that an area-specific area capacity corresponding to 91% of the NCA galvanostatic C/10 capacity could be reached. With the ever-increasing demand for mobile applications and the growing concern for the replacement of fossil fuels, alternative energy storage has become an increasingly pressing but still unsolved problem.1-3 Because of their unequalled combination of high energy and power density, lightweight design and excellent lifespan, lithiumion batteries are to date the technology of choice for portable electrochemical storage, powering applications such as electronic devices, power tools and hybrid/full electric vehicles. The prevailing electrode fabrication method for lithium-ion battery electrodes includes high pressure calendering of electrode formulations that include active lithium materials, organic binder and conductive additives, in order to densify the electrode and promote adhesion to the metal current collector. However, this process also increases the tortuosity of the pore network in the primary transport direction. In practice, to meet operational C-rates desired, and production throughput objectives, the thicknesses of commercial electrodes are restricted to less than ∼100 μm. Beyond this value, ion transport becomes a limiting factor and the accessible specific capacity starts to drop dramatically. 4 With the aim of increasing cell energy density and decreasing cost by building thicker electrodes, several strategies to reduce pore tortuosity by aligning the porosity in the direction normal to the current collector have been proposed. Bae et al.5 demonstrated through modeling and experiments that a dual-sc...

Diffusion‐Limited C‐Rate: A Fundamental Principle Quantifying the Intrinsic Limits of Li‐Ion Batteries

“…[41] Similarly, the tortuosity factor (γ) of the composite has significant impact on the effective transport properties of the electrode. First approaches of designing low tortuosity electrodes for LIBs were recently developed, [42,43] showing promising results and confirming the optimization potentials predicted by the DLC principle. First approaches of designing low tortuosity electrodes for LIBs were recently developed, [42,43] showing promising results and confirming the optimization potentials predicted by the DLC principle.…”

“…[41] Lowering the tortuosity improves the high rate performance while preserving the specific capacity (Figure 5c). First approaches of designing low tortuosity electrodes for LIBs were recently developed, [42,43] showing promising results and confirming the optimization potentials predicted by the DLC principle. Increasing of the Li + concentration and diffusion coefficient of the electrolyte increases the DLC and allows thicker and less porous electrodes, which benefits the specific capacity for high rate applications (Figure 5d,e).…”

Microstructure‐ and Theory‐Based Modeling and Simulation of Batteries and Fuel Cells

“…To mitigate the transport limitations in the electrolyte, new manufacturing concepts with reduced tortuosity were suggested. Recently, Delattre et al proposed a freeze-casting technique for the production of ultra-thick Li-ion battery electrodes [53]. A water-based suspension of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) is dipped at one end into liquid nitrogen.…”

Strategies for Building Robust Traffic Networks in Advanced Energy Storage Devices: A Focus on Composite Electrodes

“…At the same time, the 3D structure of ITN (the tortuosity) is also notably affected by the calendaring process. For example, the tortuosity in the thickness direction usually increases with the reduction of thickness after calendaring . However, the thickness of the ITN is also reduced after calendering, which shortens the travel distance for the Li–ions.…”