Approaches to Better Determination of Oxidative Stability of Solid-State Electrolytes

Abstract

The undesirable reaction between electrolyte and cathode material at a high state of charge (SOC), i.e. the oxidative decomposition of electrolyte, results in electrolyte consumption, capacity fading and eventual battery failure during prolonged cycling of lithium-ion batteries (LIBs). The problem is becoming more pronounced as research turns to all-solid-state lithium-ion batteries (SSLIB). In conventional LIBs employing liquid electrolyte, the species oxidized at the cathode surface can diffuse back into the bulk liquid; thus, the effective oxidized species concentration near cathode surface is low. However, due to the chemical nature and slow solid-state diffusivity, oxidized species can accumulate around the cathode-solid electrolyte interface, forming a resistive layer. In this sense, the issue of oxidative decomposition of solid electrolyte is intrinsically worse than liquid electrolyte. The pursuit of novel cathode compositions with even higher delithiation potentials and higher nickel content further exacerbates the problem, greatly increases the decomposition rate of the solid electrolyte. A thorough understanding of the oxidative decomposition behavior of solid-state electrolytes is needed. However, the most commonly used method for determining the oxidative stability of solid-state materials, linear scanned voltammetry (LSV), is predominantly a kinetic method. Onset potential generated by this method not only lacks thermodynamic significance but also varies greatly due to test cell configurations and sample preparation techniques. For example, independent research groups have reported oxidation potentials for PEO electrolytes spanning from 4-5.5V vs. Li/Li+. In practice, PEO based electrolytes are often found decomposing on cathode surfaces with charge potentials above 4V.This work focuses on establishing a more accurate estimation of the stability threshold of solid-state materials within a reasonable time. In practical SSLIB systems, the solid-state electrolyte is in contact with a considerable amount of cathode particle and conductive additives, providing a large number of possible sites for decomposition. Thus, in a more accurate measurement, large amount of conductive carbon was blended with the tested material to increase the active working electrode area. Comparing to LSV which usually carried on a polished inert surface, the new method provides a more similar scenario to SSLIB. Furthermore, in common LSV practices, the oxidation limit is usually determined by setting an onset current density. However, the current density of a particular test is strongly affected by both the conductivity/kinetics of the tested material and cell configuration. The arbitrariness of cell preparation and difference between materials made LSV a flawed method for comparing stability between different materials, yet a comparison of decomposition threshold between materials is widely seen among references. To resolve this issue, a dimensionless number R, or irreversibility factor was introduced into the analysis. R is defined as the ratio between irreversible capacity and reversible capacity. Experimentally, the carbon-infused material was scanned between OCV and a cutoff potential using cyclic voltammetry, and R is calculated from the charge passed through the cell. Using this new method, two polymer electrolytes, poly(ethylene oxide) (PEO) and hydrogenated poly(butadiene-co-acrylonitrile) (HNBR) combined with lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) have been characterized. Their stability was found 3.8V vs. Li/Li+ for PEO and 4.3V vs. Li/Li+ for HNBR. The values determined by the new method were found in good agreement with battery tests.