Design of Bi-Tortuous, Anisotropic Graphite Anodes for Fast Ion-Transport in Li-Ion Batteries

Abstract

Thick Li-ion battery electrodes with high ion transport rates could enable batteries that cost less and that have higher gravimetric and volumetric energy density, because they require fewer inactive cell-components. Finding ways to increase ion transport rates in thick electrodes would be especially valuable for electrodes made with graphite platelets, which have been shown to have tortuosities in the thru-plane direction about 3 times higher than in the in-plane direction. Here, we predict that bi-tortuous electrode structures (containing electrolyte-filled macro-pores embedded in micro-porous graphite) can enhance ion transport and can achieve double the discharge capacity compared to an unstructured electrode at the same average porosity. We introduce a new two-dimensional version of porous-electrode theory with anisotropic ion transport to investigate these effects and to interpret the mechanisms by which performance enhancements arise. From this analysis we determine criteria for the design of bi-tortuous graphite anodes, including the particular volume fraction of macro-pores that maximizes discharge capacity (approximately 20 vol.%) and a threshold spacing interval (half the electrode's thickness) below which only marginal enhancement in discharge capacity is obtained. We also report the sensitivity of performance with respect to cycling rate, electrode thickness, and average porosity/electroactive-material loading. Present-day oil demand and supply statistics show a clear need for alternatives in energy production, management, and storage. Amongst the various energy-storage devices available, Li-ion batteries have benefits of high gravimetric and volumetric energy-density.1-3 Modern Li-ion battery electrodes are porous composites of solid-state activematerial particles bound together by a conductive carbon-binder mixture, with an ion-conducting liquid electrolyte filling the pores. When a battery operates, electrons and ions are simultaneously transported to the surfaces of active-material particles, where electrochemical reactions take place. The rates at which ions are transported depend on the microscopic structure of the composite electrodes through a parameter called tortuosity. The microstructure in an electrode results from the particular choice of material constituents and processes that are used to fabricate the electrodes. To maximize the energy density of a battery, we would like to have electrodes with low porosity (maximizing the density of active material) and high thickness, reducing the number of inactive components (separators, current collectors) that are required for a given amount of active material, saving considerable cost. Unfortunately, electrodes with low porosity generally have high tortuosity, 4 making ion transport slow, an effect whose importance is magnified when electrodes are thick. Thus, techniques that produce thick, dense electrodes with enhanced ion transport could enable the development of batteries with high energy-density and high power density at a lower cost than...

Capacitive Performance and Tortuosity of Activated Carbon Electrodes with Macroscopic Pores

“…Another result of the effective resistance of the b We note that this expression uses electrostatic potential rather than solution-phase potential (or reduced electrochemical potential of cations) that is commonly used in Li-ion batteries. 5,9,29 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address.…”

“…In contrast with the cation intercalation reactions considered in Ref. 9, the charge-sign of ions that adsorb to the EDL, and consequently the adsorbed salt, depends on interfacial polarization φ = φ s − φ E S , where φ s is the solid-phase potential and φ E S is the electrostatic potential. In porous electrode theory for cation intercalation cells electrolyte current conservation is posed in terms of "solution-phase potential" φ e , which is more precisely defined as the reduced electrochemical potential of cations in solution given by φ e = φ E S + RT /Fln(c e ), where c e is salt concentration.…”

“…Excluding electrosorption processes, we model all of the preceding mechanisms using the governing equations and boundary conditions presented in Ref. 9. In contrast with the cation intercalation reactions considered in Ref.…”

“…In porous electrode theory for cation intercalation cells electrolyte current conservation is posed in terms of "solution-phase potential" φ e , which is more precisely defined as the reduced electrochemical potential of cations in solution given by φ e = φ E S + RT /Fln(c e ), where c e is salt concentration. 5,9,29 We model the EDL using Helmholtz theory for which the stored charge due to anions is Q − = −ρ total C E DL φH ( φ) and due to cations is Q + = −ρ total C E DL φH (− φ). C E DL is the specific capacitance for which we assume a value of 50 F/g, similar to the results from low sweep rate capacitance measurements.…”

“…13 While the tortuosity of microstructures with monodisperse pores typically increases with increasing total porosity, 15 microstructures with two disparate structural length scales can be used to decrease apparent tortuosity at fixed porosity. 9 Such bi-tortuous electrodes have been studied in the context of Li-ion batteries experimentally 14 and theoretically. 9,16,17,a The orientation of solid particles comprising porous electrodes for energy storage has also been used as a strategy to control tortuosity experimentally, and thereby enhance cycling performance relative to conventionally fabricated electrodes.…”