Charge Transfer Resistance Between Garnet-Type Solid Electrolyte and Lithium Metal Anode

Abstract

Introduction Garnet-type Li7La3Zr2O12 (LLZO) and its family has emerged as one of the most promising solid electrolytes for advanced battery technology such as all-solid-state Li-ion batteries (ASSBs). Its advantages include high Li+ conductivity in the order of ≈ 4x10-4 S cm-1 at room temperature (RT) in cubic phase and electrochemical stability against metallic Li [1]. However, interfacial charge transfer resistance (R CT) between Li and LLZO is a main issue when employ metallic Li as anode, which would be strongly affected by interfacial contact. Recently, many researchers have shown the charger transfer resistance are attributed to poor interface contact between electrode/electrolyte, surface morphology and particle size of electrolyte [2, 3]. Therefore, it is important to understand the fundamental electrochemical properties at the interface, and simultaneously it is necessary to improve the intimate interface between Li and LLZO. In the present work, we investigated the influence of surface roughness of the Li6.5La3Zr1.5Ta0.5O12 (LLZT) pellets and heat treatment on R CT. The effect of lithium and gold thin layers deposited on the LLZT pellets to form the favorable contact between Li and LLZT was also studied. Experimental The LLZT pellets with a relative density of 96% were prepared by conventional solid-state reaction and following spark plasma sintering technique with a sintering temperature of 1000°C, pressure of 0.3 MPa for 10 minutes. The surfaces of the sintered pellets were polished using several pieces of emery paper with grit numbers progressing from 400 to 8000 in an Ar-filled glovebox to avoid moisture and carbon dioxide in air. The surface morphology of the pellets were observed using scanning electron microscope (SEM) and scanning probe microscope (SPM). A Li|LLZT|Li symmetric cell was assembled by sandwiching a LLZT pellet with Li metal foils (ɸ5 mm) and Cu thin foils. Dimensions of the pellet was around 1.5-mm thick and 10 mm in diameter. Polyvinylidene fluoride (PVdF) washers were used to avoid the deformation of lithium by pressure and heat, and estimated pressure on the Li|LLZT|Li cell was 1.4 MPa. R CTat the Li|LLZT interfaces was characterized using electrochemical impedance spectroscopy (EIS) from 1 MHz to 0.1 Hz in the temperature range from 25°C to 175°C~180°C. Results and discussion R CT at the Li|LLZT interface was compared for each emery paper at RT after preconditioning at 175°C as shown in Fig. 1. EIS analysis clearly indicates decreases in R CT with increasing grit number from #400 to #8000. This is due to differences in surface roughness of the pellets, which affect contact area. It was revealed that R CT for the pellet polished with #4000 was as high as 700 Ω cm2 with rather high activation energy of 0.5 eV. To further improve R CT for practical application, two approaches were followed up: one is to increase the electronic conductivity of the LLZT surface. Thin gold layers (ca. 20 nm) were prepared as a current collector between Li and LLZT by vacuum deposition. In this case, R CT could be further reduced (triangle in Fig. 1). The decrease in interfacial resistance was attributable to facilitate more charge transfer at the interface. The other approach is to increase surface contact area between Li and LLZT with densely packed interfaces. For that, a thin lithium layer was prepared at the interface by vacuum deposition. Pellet polished with #400 showed decrease in R CT compared with a pellet polished with #4000, and further decrease to lowest value (68 Ω cm2) after heat treatment as shown in Fig. 2. Thus, we conclude that the better modifications are effective to establish the favorable interfaces with large contact area and with good physical adhesion between Li metal and solid electrolyte for the further improvement in electrochemical performance of ASSBs. Acknowledgement : This work was partially supported by Advanced Low Carbon Technology Research and Development Program of Japan Science and Technology Agency for the program “Specially Promoted Research for Innovative Next Generation Batteries (JST -ALCA-SPRING)’’.