Experimental determination of Li diffusivity in LLZO using isotopic exchange and FIB-SIMS

Abstract

Current lithium ion battery technology makes use of flammable liquid electrolytes and so the development of solid ceramic electrolytes for the next generation of all-solid-state batteries can offer a safer alternative. However, the lithium diffusion behaviour in these solid electrolytes is not yet well characterised, despite the importance of this information for optimising cell performance. Similarly, the transport properties at the metal anode interface are critically important, but not well understood. We propose a methodology for obtaining lithium diffusion coefficients of bulk solid ceramic garnet-type Li7La3Zr2O12 (LLZO) electrolytes by coupling dense pellets with isotopically labelled lithium metal, followed by analysis with focused-ion-beam secondary ion mass spectrometry. We report room temperature lithium diffusivities of 2–8 × 10−13 m2 s−1 for doped LLZO using an estimate of the lithium diffusion length in good agreement with electrochemical impedance spectroscopy. Simultaneous detection of positive and negative secondary ion species by SIMS enables correlation of layered interfaces consisting of metallic lithium, corrosion/surface degradation products and bulk LLZO during depth profiling. Charging of the ceramic during ion sputtering is investigated and shown to have a minimal effect on the obtained lithium isotopic fractions in the current setup. Additionally, the effect of the presence of corrosion products at the surface of garnets as a result of air-exposure is investigated. This method could be extended to any Li-metal stable solid electrolyte, or with a reactive solid electrolyte coupled with a stable interlayer. As such, this work sets the basis of a methodology for further quantitative diffusion analyses for Li-conducting solid ceramic electrolytes and their interfaces with electrodes, as used in both solid-state lithium batteries and hybrid systems coupling solid and liquid electrolytes.

Building a Better Li‐Garnet Solid Electrolyte/Metallic Li Interface with Antimony

“…The self‐diffusion coefficient in solid Li at 25 °C is 7.65 × 10 −11 cm 2 s −1 . [ 36 ] This value is much lower than the Li diffusion in Li 3 Sb with 3.5 × 10 −9 cm 2 s −1 [ 37 ] and LLZO with ≈2 × 10 −9 –8 × 10 −9 cm 2 s −1 [ 38 ] (both at 25 °C). According to Krauskopf et al, [ 35 ] from a physical and chemical point of view, the alloying interlayer fulfils two main functions.…”

Doping strategy and mechanism for oxide and sulfide solid electrolytes with high ionic conductivity

“…However, the limited spatial resolution of neutron detectors requires long diffusion path lengths to characterize the diffusion process. This highlights the importance of multiple techniques for studying Li diffusion processes in solid-state electrolytes, and this an alternative methodology for determining the lithium diffusion coefficients was proposed by Brugge et al 94 This method involves stable tracer experiments, whereby an 6 Li: 7 Li isotope couple (consisting of Li metal and a dense LLZO pellet) is left to inter-diffuse and the resulting isotopic profiles are measured using secondary ion mass spectrometry (SIMS). By using tracer experiments, direct measurement of the diffusion coefficient is possible.…”

“…For established non-tracer methods such as impedance spectroscopy and NMR spectroscopy, probing the diffusion and conductivity on different time- and length-scales requires more tracer-based experiments to obtain reliable comparative data. However, in this work, 94 an estimation of the diffusion coefficient was obtained directly by measuring the diffusion length of lithium over a fixed time. Accordingly, this method can be extended to any Li–metal-stable SE or any reactive SE with a stable protective interlayer.…”

Magnetic Sector Secondary Ion Mass Spectrometry on FIB-SEM Instruments for Nanoscale Chemical Imaging

“…Another type of SIMS spectrometer being used on FIB-based instruments is quadrupole spectrometers. 16 Chater et al. reported a single-column FIB instrument equipped with two quadrupole spectrometers, to detect both positive and negative SIs at the same time.…”

“…15 Another type of SIMS spectrometer being used on FIBbased instruments is quadrupole spectrometers. 16 Chater et al reported a single-column FIB instrument equipped with two quadrupole spectrometers, to detect both positive and negative SIs at the same time. 17 Nevertheless, these systems work in serial acquisition (meaning that ions of different masses are detected sequentially rather than simultaneously) and are not suitable for fast chemical imaging.…”