Investigating solid polymer and ceramic electrolytes for lithium-ion batteries by means of an extended Distribution of Relaxation Times analysis

Abstract

Solid state electrolytes based on polymer or ceramic materials are a safe alternative to liquid electrolytes based on organic solvents. Yet their ionic conductivity does not meet the required specification for state-of-the-art lithium-ion batteries. Furthermore, their conductivity mechanisms and interfacial behavior are not fully understood, making in-depth electrical characterization necessary. The calculation of the Distribution of Relaxation Times from the impedance spectrum of an electrochemical component is a powerful approach to gain insight into the processes and mechanisms responsible for the electrical and electrochemical behavior. Here we introduce an extended Distribution of Relaxation Times to avoid error-prone preprocessing. This method includes non-resistive-capacitive elements in its impedance function to overcome the constraints usually limiting the Distribution of Relaxation Times. In this study we investigated solid polymer and ceramic electrolytes regarding their conductivity mechanisms, charge transfer mechanisms and interphase formation. While the materials all possess one major conductivity mechanism, significant differences in charge transfer and interphase behavior were observed. In the case of solid polymer electrolytes, poly (ethylene glycol) bottlebrush polymers were prepared using two different boron-based lithium salts. Additionally, solid polymer electrolytes based on triethylene glycol grafted onto a poly (glycidyl propargyl ether) backbone blended with lithium bis(trifluoromethanesulfonyl)imide were compared to a similar single-ion conducting solid polymer electrolyte with the bis(trifluoromethanesulfonyl)imide anion spatially fixed to the backbone by sequential co-click synthesis. For the ceramic electrolyte, Li5·6Al0·3La3Zr1·5Ta0·5O12 powder was synthesized via a mixed oxide route and the dense ceramic electrolytes layers were fabricated by the Powder Aerosol Deposition Method. The produced layers were post-treated at 400 °C and the influence of the thermal annealing atmosphere on the conductivity of the solid electrolytes was investigated. In this study, we present the feasibility of the extended Distribution of Relaxation Times for the characterization of the investigated materials. The time-dependent formation of an interphase layer in the polymer electrolytes is identified, separated from the charge transfer process and quantified. For the ceramic electrolyte, the influence of the annealing is depicted and the charge transfer reaction is detected.