(Keynote) Overcoming Hurdles Towards Fast-Charging Solid-State Batteries
Authors: Richter Felix H. 
Published: December 2020

Solid-state batteries with lithium metal promise to outperform lithium-ion batteries. In practice, this remains a tough target. While the focus of most solid-state battery research remains on discovering new solid electrolytes, cell chemistries and increasing energy density, many solid electrolytes provide a key advantage for rate performance: a lithium transference number close to 1. There are several examples of solid electrolytes that achieve more than 5 mS/cm ionic conductivity and are single-ion-conductors. This prevents polarization resistance from building up in the solid electrolyte at high current. Therefore, fast charging is in principle possible.

However, there remain several significant challenges to be overcome, which are not or less of an issue in batteries with liquid electrolyte. To overcome the energy density of lithium-ion batteries, lithium metal needs to be used as anode. Hence, the dominant impeding factor regarding fast-charging of solid-state batteries is lithium growth through the solid electrolyte occurring at one or more mA/cm² of current density. At present, there is no easy fix for this problem, the underlying mechanisms of which are different for different types of electrolytes. The garnet-type solid electrolytes are particularly interesting for studying the interface of the solid electrolyte with lithium metal fundamentally as they are one of the few solid electrolytes that are not reduced by lithium and form a stable interface. This interface was shown to have extremely low charge transfer resistance, which accomplishes another milestone on the way to fast charging lithium batteries.

Another key aspect of lowering internal resistance is the lithium transport resistance through the solid electrolyte. Two aspects are particularly relevant, but in different parts of the cell. In the cathode composite, high lithium-ion conductivity is crucial as the solid electrolyte volume fraction is low and tortuosity of the pathways increased. In the separator layer, lithium-ion conductivity is not as big of an issue as the ideal separator layer is very thin. This brings along a new challenge though: preparation of thin sheets of brittle inorganic materials. The processing of thin inorganic layers has been demonstrated before. Where necessary, composites of inorganic and polymer electrolytes may be used. Layer thicknesses are expected to continue to decrease.

On the cathode side, several aspects are relevant. For one, the active material needs to retain contact with the solid electrolyte. This is particularly challenging for the contact of two inorganic materials, whereas it is often easier for a polymer to wet an inorganic surface. The volume change of the cathode active material during cycling and the electrochemical degradation of the interface add complexity to maintaining contact and a low lithium transfer resistance. Finally, lithium diffusion inside the cathode active material as well as electronic conductivity in the cathode composite all need to be considered to identify and improve the weakest link in the chain to optimize the rate performance of solid-state batteries.