Thickness‐Dependent Impedance of Composite Battery Electrodes Containing Ionic Liquid‐Based Electrolytes

Abstract

Lithium‐ion battery models often neglect the salt concentration polarization inside the electrolyte‐filled pores of the composite electrodes. However, this concentration polarization causes a significant impedance, in particular in the case of electrolytes with low Li+ transference numbers. Here, we analyze in detail measured and calculated impedance spectra of composite electrodes containing a solvate ionic liquid‐based electrolyte and an ionic liquid‐based electrolyte, respectively, in comparison to a conventional carbonate‐based electrolyte. For calculating spectra, we use a recently published model by Huang and Zhang. We find that the impedance at 10−4 Hz, which is relevant for battery cycling rates around 1 C to 2 C, increases in the order carbonate‐based electrolyte<ionic liquid‐based electrolyte<solvate ionic liquid‐based electrolytes, but exhibits a remarkably weak thickness dependence, when the electrode thickness exceeds 50–100 μm. This suggests that electrodes considerably thicker than the conventional 80 μm can be used in batteries without significantly deteriorating battery power.

What Limits the Rate Capability of Ultrathick Composite Electrodes in Lithium‐Ion Batteries? A Case Study on the Thickness‐Dependent Impedance of LiCoO2 Cathodes

“…The measurements were performed at 0 % state of charge (SOC), i. e., under ion-blocking-conditions. In this case, the impedance Z SOC0 can be described in the framework of a specific transmission line model: [24][25][26] Z SOC0 ¼ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi…”

“…[30] In this range, the determined tortuosities are between 2.7 and 4.5, in good accordance with values from the literature. [24][25][26]29] Considering the weak thickness dependence of the ion transport tortuosity and the rather ideal shape of the impedance spectra of the symmetric cells in Figure 1, we have no indication for a significant change of the electrode morphology with increasing thickness. In Figure 3, we show exemplary FIB-SEM cross sections of electrodes with 101 μm and with 251 μm thickness, respectively.…”

“…In order to analyze the thickness-dependent cathode impedance in more detail, we use a generalized transmission line model (GTLM) for the cathodes with cylindrical pores and high electronic conductivity. [25] In this model, the total impedance of the cathode is given by:…”

“…The measurements were performed at 0 % state of charge (SOC), i. e., under ion‐blocking‐conditions. In this case, the impedance Z SOC0 can be described in the framework of a specific transmission line model: [24–26] …”

Determination of Lithium Diffusion Coefficients in Single Battery Active Material Particles by Using an AFM-Based Steady-State Diffusion Depolarization Technique

“…D Li is often determined by using potentiostatic and galvanostatic intermittent titration techniques (PITT and GITT) or electrochemical impedance spectroscopy (EIS). − However, while these techniques yield reliable values for one-dimensional Li diffusion in chemically homogeneous thin films, their application to composite electrodes is by no means straightforward. , In composite electrodes, a second ambipolar diffusion process takes place, namely, the diffusion of cations and anions inside the electrolyte-filled pores of the composite electrode (salt diffusion). Since the time scales of Li diffusion and salt diffusion are similar, their separation in PITT, GITT, and EIS measurements is challenging. , In addition, a lack of knowledge about the active area of the particles for Li intercalation and the Li diffusion length distribution inside particles of different size and shape complicates the analysis of the data. , Furthermore, ion or electron transport limitations in the composite electrodes may lead to a spatially varying Li content and thus to a spatially varying Li diffusion coefficient. Therefore, it would be highly desirable to measure local Li diffusion coefficients on the level of single active material particles.…”