Harmonization of Testing Procedures for All Solid State Batteries

Abstract

All Solid State Batteries (ASSBs) with lithium-ion based conducting solid state electrolytes are considered the next generation high performance batteries. They enable high power densities due to their single ion conducting solid electrolyte, eliminating salt concentration gradients and related polarization losses in the cell, and ensuring an unrivalled level of safety due to their non-combustibility. Currently, a variety of ASSBs based on different solid state electrolytes such as polymers, thiophosphates, oxides and combinations thereof are being developed.One general problem with ASSBs is establishing and maintaining contact between the solid electrolyte and the active material phase during production and cycling, respectively. In conventional lithium-ion batteries (LiBs), this contact is ensured by the liquid state of the electrolyte, but in ASSBs, chemical expansion and contraction of the active material during lithiation and delithiation can detach this contact, resulting in decreased capacity due to the loss of active material. As a consequence, ASSBs are often operated under pressurized conditions, applying pressures significantly exceeding those in conventional LiBs. The same holds for the operating temperature window. Especially for polymer electrolyte-based ASSBs, they are often operated at higher temperatures to compensate for the low ionic conductivity of polymers at room temperature. With respect to cell testing, such operating requirements must be considered, and testing protocols are designed according to the individual requirements of the tested cell.This contribution aims to provide an overview of testing protocols for various types of ASSBs applied to different cells with polymer-, thiophosphate-, oxide-, and hybrid-electrolytes. These protocols will be compared with standardized testing routines for conventional LiBs. Based on this compilation, a harmonized testing procedure that covers the special requirements of the individual cell types and enables a fair comparison of different ASSBs is suggested. Additionally, examples of ASSB testing results will be discussed, taking into consideration the harmonization of different testing parameters.