Mechanical behavior of solid electrolyte materials for lithium-ion batteries

RWTH Publications (RWTH Aachen)

Abstract

All solid-state lithium-ion batteries (ASSLIBs) being based on solid state electrolytes are at present regarded as a promising alternative for conventional batteries on account of their higher ionic conductivity, energy density as well as higher chemical stability and safety. The solid electrolytes are expected to possess enhanced ionic conductivity and in addition a mechanical stability that warrants a safer separation of electrodes and is envisaged to permit them to withstand long-term cycling operation. However, in contrast to the widely investigated electro-chemical properties of solid electrolytes, the mechanical properties, which are important for long-term reliability, need to be studied deeper. Hence, the main aim of this work is the mechanical characterization of respective ceramic materials as candidates for solid electrolytes and the relationship to materials’ microstructures. For this purpose three types of solid electrolytes, NASICON type Li1+xAlxTi2-x(PO4)3 (LATP), garnet type Li7La3Zr2O12 (LLZO), perovskite type Li0.350La0.557TiO3 (LLTO), were chosen due to their previously verified promising electro-chemical properties. With the aim of understanding the mechanical properties of the LATP material, LATPs sintered at different temperatures (950 – 1100 °C) were characterized via indentation method in this work. The results revealed that LATP sintered at higher temperature possesses higher elastic modulus, hardness and fracture toughness. The anisotropy of the mechanical properties of the solid electrolyte material LATP sintered at 1100 °C was investigated in this work via indentation mapping test in a depth control mode at room temperature with associated EBSD characterization. The experimentally derived elastic modulus and hardness of LATP show similar trends, i.e. that the rotation angle between two prismatic type planes had no detectable influence, whereas when the rotation angle from basal plane to prismatic plane increased, elastic modulus and hardness value decrease conspicuously. Furthermore, to assess the fracture reliability of electrolytes, Li1.5Al0.5Ti1.5P3O12 mixed with SiO2 (LATP:Si) and a LLZO material were selected to investigate the macroscopic mechanical properties, concentrating on fracture strength and Weibull modulus. The Weibull moduli of LATP:Si and LLZO are in a similar range as for other ceramic materials, whereas the fracture strength of LATP:Si is higher than that of LLZO, which indicates potential advantages of the LATP:Si material. Due to materials availability the stress for a tolerable failure probability was only derived for the LLZO electrolyte; yielding for example that the stress should not exceed 21 MPa for 3 years lifetime to guarantee a failure probability of 1%. As the electrolytes in all-solid-state lithium batteries are to be operated under cyclic voltage condition that are associated with chemical expansion effects, exemplified for LLTO, a material being close to application, voltage effects on mechanical properties were studied. Elastic modulus, hardness and fracture toughness of LLTO at different voltages were tested, revealing the same trend, i.e., both conductivity and fracture toughness were proportional to the lattice parameter, elastic modulus and hardness were inversely proportional to the lattice parameter and hence behave opposite to conductivity and fracture toughness. Overall, the investigation of the effects of sintering temperature, grain orientation and charge state on the mechanical behavior of the electrolytes as well as the fracture reliability study aid an understanding of the mechanical properties of the materials and provide a data base for further improvement of design and processing of battery materials and cells.

Authors 1

1. Gang Yan corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen