All-solid-state batteries with solid electrolytes having ionic conductivities in the range of those of liquid electrolytes have gained much interest as safety is still a major issue for applications. Meanwhile, lithium metal seems to be the anode material of choice to face the demand for higher capacities. Still, the main challenges that come with the use of a lithium metal anode, i.e., formation and growth of lithium dendrites, are still not understood very well. This work focuses on the reasons of the lifetime behavior of lithium symmetric cells with the solid electrolyte Li 6 PS 5 Cl and lithium electrode. In particular, the voltage increases during the application of a constant current density are investigated. The interface between the lithium metal electrode and the solid electrolyte is analyzed by X-ray photoelectron spectroscopy, and the resistance changes of each electrode during stripping and plating are investigated by impedance spectroscopy on a three-electrode cell. A main factor for the lifetime influenced by lithium dendrite formation and growth is the buildup of a lithium vacancy gradient, leading to voids which decrease the interface area and therefore increase the local current density. Additionally, those lithium vacancies in lithium metal represent a limitation for conductivity rather than migration in solid electrolyte. Further experiments indicate that the seedlike plating behavior of lithium also plays a key role in increased local current density and therefore decreased lifetime. Plating of only a small amount of lithium leads to small areas of well-connected interfaces, resulting in high local current density. A medium amount of plated lithium leads to larger areas of interface between lithium and electrolyte, balancing the current density distribution. In contrast, a high amount of repeatedly deposited lithium leads to lithium seed plating on top of already plated lithium. Those seed spots grown on top represent a better interface connection, which again leads to higher local current densities at those spots and therefore results in shorter lifetimes due to short circuits caused by lithium dendrites.

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The solid electrolyte interphase (SEI) on the graphite anode of lithium ion batteries plays a crucial role for the battery performance. It is believed that the SEI blocks electrons and solvent molecules, while Li + can easily migrate across the SEI. However, quantitative measurements of transport coefficients for these species in the SEI are problematic due to the complex structure of graphite composite anodes. Here we have grown model SEIs on glassy carbon electrodes and have characterized them by a combination of scanning electron microscopy, AFM-based scratching experiments, impedance spectroscopy and redox probe experiments. SEM and AFM experiments reveal a dual-layer structure of the SEI. The redox probe experiments with ferrocene molecules provide strong indication that the diffusion of the redox molecules across pores in the inner SEI layer is faster than electron transport across the SEI. Remarkably, the effective diffusion coefficient of ferrocene in the SEI is virtually identical to the effective diffusion coefficient of Li + obtained from the SEI semicircle in the impedance spectra. Moreover, both diffusion coefficients show the same temporal evolution after SEI formation. This suggests that in our model SEIs, Li + is primarily transported in the liquid electrolyte phase inside the pores of the inner layer.

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 recent years, the interest in the development of highly concentrated electrolyte solutions for battery applications has increased enormously. Such electrolyte solutions are typically characterized by a low flammability, a high thermal and electrochemical stability and by the formation of a stable solid electrolyte interphase (SEI) in contact to electrode materials. However, the classification of concentrated electrolyte solutions in terms of the classical scheme “strong” or “weak” has been controversially discussed in the literature. In this paper, a comprehensive theoretical framework is presented for a more general classification, which is based on a comparison of charge transport and mass transport. By combining the Onsager transport formalism with linear response theory, center‐of‐mass fluctuations and collective translational dipole fluctuations of the ions in equilibrium are related to transport properties in a lithium‐ion battery cell, namely mass transport, charge transport and Li+ transport under anion‐blocking conditions. The relevance of the classification approach is substantiated by showing that i) it is straightforward to classify highly concentrated electrolytes and that ii) both fast charge transport and fast mass transport are indispensable for achieving fast Li+ transport under anion‐blocking conditions.