Lithium Diffusion Coefficient of Amorphous Lithium Phosphate Thin Films Measured By Secondary Ion Mass Spectroscopy
Authors: Lu Xiaoli , Kuwata Naoaki , Miyazaki Takamichi , Kawamura Junichi 
Published: February 2020

1． Introduction

Thin-film solid-state batteries have attracted attention due to their low risk of fire and explosion, reduced environmental impact, and high energy density. Amorphous lithium phosphate (a-Li3PO4) is used as a solid electrolyte for thin-film batteries because of its wide electrochemical window and negligible interface resistance. Lithium diffusion is an important factor affecting the characteristics of solid-state batteries. However, the technique for measuring lithium self-diffusion in solid-state batteries is limited because the lifetime of lithium radioactive isotope, 8Li, is too short to measure the diffusion coefficient. Stable isotopes, 6Li and 7Li, are applied to measure the trace diffusion of lithium. Secondary ion mass spectrometry (SIMS) is useful as it is able to distinguish the isotopes of 6Li and 7Li.

In this study, we intended to measure the tracer diffusion coefficient of lithium (D*
Li) directly using secondary ion mass spectrometry (SIMS) with isotope exchange methods because they have negligible interface effect on the transport of lithium. Here, we used the “ ion-exchange method ” and the “ mask method ” to prepare diffusion couples (6Li/7Li). Analyzing the diffusion equation allowed us to obtain the tracer diffusion coefficient of lithium (D*
Li). The Haven ratio (H
R) of a-Li3PO4was obtained by contrasting lithium tracer diffusion coefficient with the lithium conductivity diffusion coefficient (Dσ).

2． Experimental

The 6Li3PO4 target (6Li : 7Li = 95% : 5%) was synthesized by the solid-phase reaction method. The target was applied to prepare a-6Li3PO4 thin films (6Li : 7Li = 95% : 5%) by the pulsed laser deposition (PLD) method with an ArF excimer laser at room temperature. The structure of the target and the thin films were analyzed by XRD.

Diffusion couples (6Li/7Li) were prepared using two methods, ion-exchange method and mask method. In ion-exchange method, the thin films were immersed in a natLiClO4/PC electrolyte solution with a natural abundance ratio (6Li : 7Li = 7.5% : 92.5%) for 10 hours at room temperature. In mask method, the a-6Li3PO4 thin films were half covered with the a-natLi3PO4 thin films (6Li : 7Li = 7.5% : 92.5%) using a mask. The thin films prepared by the two methods were cut into several pieces and annealed at different temperatures (25~160 ℃) and time.

The line analysis was performed using a double-focusing magnetic sector SIMS (IMS-7f). Isotope concentration ratios of 6Li and 7Li could be calculated from the counts of 6Li and 7Li isotopes obtained from SIMS.The isotope profiles of 6Li and 7Li were analyzed to obtain the tracer diffusion coefficient of lithium (D*
Li) by fitting them using the diffusion equation.

Moreover, alternating current (AC) impedance method was used to measure the ionic conductivity of thin films (σ). The a-6Li3PO4 thin films were deposited on interdigitated array (IDA) platinum electrodes. Impedance spectra were collected using an impedance/gain-phase analyzer (SI-1260) in the temperature range 0~140 ℃. The conductivity diffusion coefficient of lithium (Dσ) was obtained by using the Nernst-Einstein equation from the ionic conductivity.

3． Results & Discussion

Fig.1. Shows the temperature dependence of D*
Li and Dσ of a-Li3PO4 thin film. The lithium tracer diffusion coefficient determined by the ion-exchange method marked as D*
Li ion-exchange, and the mask method marked as D*
Li mask. The tracer diffusion coefficient of a-Li3PO4 thin film is D*
Li ion-exchange-RT = 6.0 × 10-13 cm2/s at 25 ℃ obtained by ion-exchange method, and D*
Li mask-RT = 6.85 × 10-13 cm2/s at 25 ℃ obtained by mask method. The ionic conductivity is σRT = 2.87 × 10-7 S/cm at 25 ℃ obtained by alternating current impedance method.

The activation energy and pre-exponential factor is Ea
(D*
Li
) = 0.58 eV, D0 = 3.92 × 10-3 cm2/s, Ea
(D
σ
) = 0.56 eV, D0 = 3.86 × 10-3 cm2/s. The tracer diffusion coefficient of lithium obtained by those methods were confirmed to be almost coincident with Dσ. Haven ratio, which is defined by ratio of tracer and conductivity diffusion coefficient as H
R ≡ D*/Dσ, H
R = 0.55.

Figure 1