Improving electrolyte performance of PEO by addition of LLZTO nanofillers in solid state battery applications

Abstract

With the increasing demand for energy storage technologies, traditional lithium-ion batteries becoming inadequate and require enhancement. The rising trend of electric cars constitutes a significant portion of battery usage of today. Considering the needs of electric vehicles, higher energy density, higher power density and improved safety have become key areas for improvement in lithium-ion batteries. Extensive research has been conducted on various approaches to enhancing lithium-ion batteries, and studies on electrolyte have led to the discovery of solid state batteries. Solid-state batteries differ from traditional batteries by using a solid electrolyte instead of a liquid one. This solid material also acts as a separator to prevent electrode contact. Inorganic crystalline ceramics, glassy materials, and organic polymers can be considered as solid electrolyte materials, with high ionic conductivity being the most crucial requirement. While traditional liquid electrolytes have an ionic conductivity of 10-2 S cm-1, solid electrolytes are expected to have conductivities above 10-4 S cm-1 at room temperature to be suitable for commerc fillial battery applications. Ceramics like LLTO, LLZTO, and Li7P3S11 meet this requirement at room temperature however their application as solid electrolyte is limited due to their brittle nature. On the other hand polymers can be a good candidate considering their flexible structure. However, they typically have low ionic conductivities around 10-10 to 10-7 S cm-1 at room temperature, which considered as the drawback of polymer materials for to be utilized as solid electrolytes. Composite electrolytes emerge as a solution to this problem by combining a polymer matrix with ceramic fillers to create a conductive pathway. This structure retains the mechanical flexibility of polymers while benefiting from the high ionic conductivity of ceramics. This method also can solve, if not reduce the effects of, dendrite formation, a significant issue in lithium-ion batteries, by ensuring uniform current distribution and preventing lithium ion accumulation. In this study a composite solid electrolyte with a polymer matrix and ceramic nanoparticles is formulated and fabricated. Polyethylene oxide (PEO) served as the polymer, and Lithium Lanthanum Tantalum Zirconate (LLZTO) nanoparticles were used as the ceramic additive, with Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the lithium salt. Various LLZTO concentrations, 40%, 45%, and 50%, were tested for their effects on ionic conductivity and transfer numbers. For the production of the composite electrolyte samples, solution casting method has been employed Characterization of the produced electrolytes involved techniques like Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), Linear Sweep Voltammetry (LSV), Chronoamperometry (CA), and Electrochemical Impedance Spectroscopy (EIS). These tests are used for obtaining xxii the data that is necessary for calculation of parameters such as ionic conductivity, transfer numbers and examining the electrochemical and thermal stability of the samples. Results showed that LLZTO addition improved the ionic conductivity of the PEO with 10% (wt) up to 1.76×10-5 S cm-1 and transfer number up to 92%. Although it is observed that these values are retreat with increasing LLZTO contents. This effect is believed to be related with several factors. Surface roughness of composite electrolyte increases with LLZTO content. This is expected to be related with the declined surface contact of electrolyte with the stainless steel plates that used in the measurements. Also, with the increasing nanofiller content, fillers are tend to agglomerate and this resulted with lower surface area of polymer/ceramic interface. The electrochemical stability window for all samples exceeded 5V and nanofiller addition results with increasing ESW. FTIR and XRD analyzes indicated that LLZTO reduced crystallinity of PEO, enhancing amorphous characteristics, which likely contributed to improved ionic conductivity. Additionally, TGA results demonstrated that LLZTO increased the thermal stability of PEO from 357 °C to above 380 °C.