Electrochemical Properties of Amorphous Li3PS4 Films Synthesized by Pulsed Laser Deposition using Li2S-Excess Targets
Authors: Quan Zhen , Hirayama Masaaki , Sato Daichi , Zheng Yueming , Yano Taka-aki , Hara Kosuke , Suzuki Kota , Hara Masahiko , Kanno Ryoji 
Published: February 2020

Introduction

For conventional all-solid-state thin-film lithium batteries with oxide- and oxynitride- electrolyte, large bulk-resistance limits the battery performance due to the low ionic conductivities in the order of 10−6–10−8 S cm−1.1 Highly conductive sulfide materials, which have been proposed as solid electrolytes for bulk-type batteries, are strongly desired to be applied to the thin film batteries.2,3 Pulsed laser deposition (PLD) in inert atmosphere is a promising technique for fabricating sulfide-electrolyte films. However, a deviation in the composition of the films from target materials is an issue to be sloved for lithium-containing oxides and alkaline-earth-metal sulfides due to loss of the lithium with light weight and sulfur with high vapor pressure, which reduce the ionic conductivity.4 Therefore, it is important to control the composition of lithium-containing sulfide films for fabricating highly conductive film electrolytes. In this study, amorphous Li3PS4 films were synthesized by PLD using Li3PS4 targets with excess amounts of Li2S, and the structures and electrochemical properties of the thus-produced films were investigated.

Experiment

The crystalline Li3PS4 targets were synthesized by solid-state reaction.5 The targets had a Li2S-excess composition of Li3+2x
PS4+x
(x = 0, 0.21, 0.34, 0.71, and 1) to compensate the loss of lithium and sulfur during the PLD process. Li3PS4 films were grown on MgO(100) or Al2O3(0001) substrates at room temperature using a PLD apparatus. The structure of the thin film was examined by X-ray diffraction (XRD) method and Raman spectroscopy. AC impedance, DC polarization, cyclic voltammetry (CV) and constant current charge-discharge measurement were carried out to evaluate the electrochemical properties of the sulfide-electrolyte thin film.

Results and discussion

The XRD patterns of the Li3PS4 (x = 0) and Li3.42PS4.21 (x = 0.21) targets showed similar pattern of the γ-Li3PS4, while diffraction peaks assigned to lithium-excess phase of Li7PS6 appeared for the targets of x ≥ 0.34.Structural characterization of the sulfide thin films confirmed that all the films are amorphous phase and have extremely low surface-roughness. Raman spectroscopy revealed that Li4P2S6 phase and sulfur cluster formed in the thin film during ablation of the Li3PS4 target due to the Li loss, while the Li2-x
S cluster formed in the film using 50% Li2S-excess Li4.42PS4.71 (x = 0.71) as the target. The Li3PS4 thin film synthesized with 14% Li2S-excess Li3.42PS4.21 (x = 0.21) target included few impurities, indicating that the composition deviation in the Li3PS4 films is associated with a loss of Li and S during PLD process, and could be compensated by using of the targets with an appropriate excess amount of Li2S. As a result, the Li3PS4 thin film synthesized using Li3.42PS4.21 (x = 0.21) target exhibited the highest ion conductivity of 7.29 × 10−4 S cm−1 at 25 °C. Furthermore, the Li3PS4 thin film shows negligible electronic conductivity and high electrochemical stability against Li metal up to 5 V. It was also confirmed that a LiCoO2/Li3PO4/Li3PS4/Li all-solid-state thin-film battery delivered the reversible battery reaction under current densities in the range of 0.1–10 C, which implied that the Li3PS4 film could act as solid electrolyte in thin film batteries.

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