No abstract

The performance of bulk-type all-solid-state Li batteries (ASSBs) depends critically on the contacts between cathode active material (CAM) particles and solid electrolyte (SE) particles inside the composite cathodes. These contacts determine the Li + exchange current density at the CAM | SE interfaces. Nevertheless, there is a lack of experimental studies on Li + exchange current densities, which may be caused by the poor understanding of the impedance spectra of ASSBs. We have carried out a comparative case study using two different active materials, namely, single-crystalline LiCoO 2 particles and single-crystalline LiNi 0.83 Mn 0.06 Co 0.11 O 2 particles. Amorphous 0.67 Li 3 PS 4 + 0.33 LiI particles act as a solid electrolyte within the cathode and separator, and lithiated indium acts as the anode. The determination of the cathode exchange current density is based on (i) impedance measurements on In−Li | SE | In−Li symmetric cells in order to determine the anode impedance together with the anode | separator interfacial impedance and (ii) variation in the composite cathode thickness in order to differentiate between the ion transport resistance and the charge transfer resistance of the composite cathode. We show that under the application of stack pressures in the range of 400 MPa, the Li + exchange current densities can compete with or even exceed those obtained for CAM | liquid electrolyte interfaces.

In Li–O2 batteries, charge and mass transport across the discharge product Li2O2 plays an important role for the kinetics. In general, it is distinguished between laterally homogeneous transport across dense Li2O2 layers and heterogeneous transport across porous layers. However, in many studies, the dense or porous nature was not verified. Here, we use a combination of scanning electron microscopy, atomic force microscopy-based scratching experiments, and electrochemical measurements on thin Li2O2 layers to demonstrate a simple method for verifying the dense nature of a layer. We show that dense layers with a fraction of the free electrode surface below 10–5 exhibit virtually the same charge-transfer resistance for oxygen reduction and for the redox reaction of Co­(Cp)2 +/Co­(Cp)2 redox probe molecules, indicating that both charge-transfer resistances are determined by electron transport across the dense layers. In contrast, if this fraction exceeds 10–5, the charge-transfer resistance of the Co­(Cp)2 +/Co­(Cp)2 redox reaction is much lower than that of the oxygen reduction. Our results lead to the conclusion that measuring the charge-transfer resistance of the oxygen reduction alone is not sufficient for characterizing charge-transport limitations, but additional information about the dense/porous nature of the Li2O2 layer is indispensable.