Electrochemical Properties of Amorphous Li3PS4 Films Synthesized by Pulsed Laser Deposition using Li2S-Excess Targets

Field: Materials Science (Physical Sciences)

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 Li 3 PS 4 films were synthesized by PLD using Li 3 PS 4 targets with excess amounts of Li 2 S, and the structures and electrochemical properties of the thus-produced films were investigated. Experiment The crystalline Li 3 PS 4 targets were synthesized by solid-state reaction. 5 The targets had a Li 2 S-excess composition of Li 3+2 x PS 4+ x ( x = 0, 0.21, 0.34, 0.71, and 1) to compensate the loss of lithium and sulfur during the PLD process. Li 3 PS 4 films were grown on MgO(100) or Al 2 O 3 (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 Li 3 PS 4 ( x = 0) and Li 3.42 PS 4.21 ( x = 0.21) targets showed similar pattern of the γ-Li 3 PS 4, while diffraction peaks assigned to lithium-excess phase of Li 7 PS 6 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 Li 4 P 2 S 6 phase and sulfur cluster formed in the thin film during ablation of the Li 3 PS 4 target due to the Li loss, while the Li 2- x S cluster formed in the film using 50% Li 2 S-excess Li 4.42 PS 4.71 ( x = 0.71) as the target. The Li 3 PS 4 thin film synthesized with 14% Li 2 S-excess Li 3.42 PS 4.21 ( x = 0.21) target included few impurities, indicating that the composition deviation in the Li 3 PS 4 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 Li 2 S. As a result, the Li 3 PS 4 thin film synthesized using Li 3.42 PS 4.21 ( x = 0.21) target exhibited the highest ion conductivity of 7.29 × 10 −4 S cm −1 at 25 °C. Furthermore, the Li 3 PS 4 thin film shows negligible electronic conductivity and high electrochemical stability against Li metal up to 5 V. It was also confirmed that a LiCoO 2 /Li 3 PO 4 /Li 3 PS 4 /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 Li 3 PS 4 film could act as solid electrolyte in thin film batteries. Reference 1 N. Kuwata, N. Iwagami, Y. Tanji, Y. Matsuda, J. Kawamura, J. Electrochem. Soc. 2010 , 157 , A521. 2 Y. Seino, T. Ota, K. Takada, A. Hayashi, M. Tatsumisago, Energy Environ. Sci. 2014 , 7 , 627. 3 N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, Nat Mater 2011 , 10 , 682. 4 A. Piqué, M. Mathur, J. Moses, S. A. Mathews, Appl. Phys. Lett. 1996 , 69 , 391. 5 M. Murayama, N. Sonoyama, A. Yamada, R. Kanno, Solid State Ionics 2004 , 170 , 173.