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

Abstract

Increasing the thickness of composite electrodes in lithium‐ion batteries from about 80–100 μm in state‐of‐the‐art commercial cells to several 100 μm would enhance the energy density, however at the expense of the power density. Despite this common knowledge, quantitative studies on the impedance and rate capability of composite electrode in dependence of the electrode thickness are scarce. Therefore, we have carried out a case study on LiCoO2 composite electrodes with thicknesses up to about 250 μm. We first demonstrate that conventional composite electrode preparation leads to ion transport tortuosities in the electrolyte‐filled pores, which are virtually independent of the thickness. The thickness‐dependent impedance of these electrodes decreases with increasing thickness and follows the predictions of a generalized transmission line (GTLM) model. We use the GTLM model for analyzing in what way cycling of ultrathick electrodes (250–300 μm) with a rate of 1 C should be achievable.

Mitigating the Ion Transport Tortuosity in Composite Cathodes of All-Solid-State Batteries by Wet Milling of the Solid Electrolyte Particles

“…Remarkably, our values for τ eff of electrodes with LWM-based preparation of the SE particles are in the same range as tortuosity data obtained for composite electrodes of laboratory-scale LIBs with different volume fractions of the liquid electrolyte, ε LE , see the blue area in Figure . This blue area was obtained from an interpolation of values given in refs , . Together with the recent finding that the Li + exchange current densities in composite cathodes of ASSBs can compete with or even exceed those obtained for cathodes of conventional LIBs, the present results indicate that high power densities of ASSBs should be achievable even at volume fractions of the solid electrolyte in the range of ε SE ≲ 0.3.…”

“…The solid electrolyte particles were prepared either exclusively by HDM (open symbols) or an additional LWM step. The data obtained in this study are compared to those of Minnmann et al and Park et al In addition, the blue area indicates the data range obtained for laboratory-scale LIBs containing a liquid electrolyte. , The dashed lines are guides for the eye.…”

“…The ion transport tortuosities 23 and Park et al 36 In addition, the blue area indicates the data range obtained for laboratory-scale LIBs containing a liquid electrolyte. 19,20 The dashed lines are guides for the eye.…”

“…Based on a transmission-line model analysis of the results, they reported τ eff values increasing from 3.0 to 7.0 with the electrolyte volume fraction decreasing from 0.7 to 0.3 . Cronau et al carried out the same type of measurements with an additional variation of the thickness of LiCoO 2 cathodes and reported τ eff values increasing from 2.0 to 4.5 with the electrolyte volume fraction decreasing from 0.55 to 0.30 . Ebner et al carried out numerical diffusion simulations in a reconstructed LiNi 0.33 Co 0.33 Mn 0.33 O 2 cathode, with the reconstruction being based on synchrotron X-ray tomography data .…”

“…They obtained effective tortuosity values τ eff increasing from 1.3 to 2.0, when ε LE decreased from 0.6 to 0.4. However, this result has to be taken with caution since it was demonstrated that diffusion simulations can underestimate ion transport tortuosity as compared to values obtained from impedance spectroscopy 20. Overall, studies on laboratory-scale LIBs yield effective ion transport tortuosities between 4.5 and 7 for a liquid electrolyte volume fraction close to 0.3.In the case of ASSBs, studies of effective ion transport tortuosities at different volume fractions of the solid electrolyte, ε SE , are scarce.…”