Formation and Stability of Interface between Garnet-Type Ta-doped Li7La3Zr2O12 Solid Electrolyte and Lithium Metal Electrode

Abstract

Garnet-type Li 7-x La 3 Zr 2-x Ta x O 12 (LLZT) is considered a good candidate for the solid electrolyte in all-solid-state lithium batteries because of its reasonably high conductivity around 10 −3 S cm −1 at room temperature and stability against lithium (Li) metal with the lowest redox potential. In this study, we synthesized LLZT with a tantalum (Ta) content of 0.45 via a conventional solid-state reaction process and constructed a Li/LLZT/Li symmetric cell by attaching Li metal foils on the polished top and bottom surfaces of an LLZT pellet. We investigated the influence of heating temperatures and times on the interfacial charge-transfer resistance between LLZT and the Li metal electrode. In addition, the effect of the interface resistance on the stability for Li deposition and dissolution was examined using a galvanostatic cycling test. The lowest interfacial resistance of 25 Ω cm 2 at room temperature was obtained by heating at 175 • C (5 • C lower than the melting point of Li) for three to five hours. We confirmed that the current density at which the short circuit occurs in the Li/LLZT/Li cell via the propagation of Li dendrite into LLZT increases with decreasing interfacial charge transfer resistance.

Properties of Lithium Trivanadate Film Electrodes Formed on Garnet-Type Oxide Solid Electrolyte by Aerosol Deposition

“…LLZT pellets were prepared using a conventional solid-state reaction process reported in our previous work [19,35]. All starting materials were obtained from the Kojundo Chemical Laboratory (Saitama, Japan): stoichiometric amounts of LiOH·H 2 O (99%, 10% excess was added to account for the loss of Li at high temperatures), La(OH) 3 (99.99%), ZrO 2 (98%) and Ta 2 O 5 (99.9%), which were then pulverized and mixed by planetary ball-milling with zirconia balls (5 mm in diameter) and ethanol for 3 h in a zirconia pot, and then calcined at 900 °C for 6 h in air using a Pt-5% Au alloy crucible.…”

“…The conductivity at 27 °C and activation energy for LLZT were confirmed to be 9 × 10 −4 S cm −1 and 0.40 eV [19]. …”

“…Before the cell construction, an Au film as a current collector was deposited onto the LVO film by sputtering. A heat treatment at 175 °C for 5 h was applied to the cell after the cell construction, to reduce the interfacial charge–transfer resistance R Li-LLZT between Li and LLZT [19]. …”

“…LLZ has two different crystal phases, one is the cubic phase [7,8] and the other is tetragonal one [9,10], but the former has two orders higher conductivity at room temperature than the latter. Partial substitution of the Zr 4+ site by other higher valence cations, such as Nb 5+ [11,12] and Ta 5+ [13,14,15,16,17,18,19] stabilizes the highly-conductive cubic phase. The conductivity at room temperature for both Ta- and Nb-doped LLZ with optimized Li contents (6.4–6.5) in crystal framework attain up to 1 × 10 −3 S cm −1 , but the former has much higher chemical stability against Li metal electrode than the latter [20,21].…”

“…Although Li-stuffed garnet-type oxide is a good candidate for SE in a solid-state battery, high-temperature sintering at 1000–1200 °C is generally needed for densification [7,11,12,13,14,15,16,17,18,19,20,21] and this temperature is too high to suppress the undesired side reaction between the majority of electrode active materials and SE and the formation of impurity phases [22]. Li + conducting Li 3 BO 3 with a low melting point (~700 °C) has been applied to form the interface between LiCoO 2 and garnet-type SE by a co-sintering process [23,24], but the conductivity for Li 3 BO 3 is low (10 −7 –10 −6 S cm −1 at room temperature) and there are currently limited electrode materials that can be used for solid-state batteries with garnet-type SEs developed by the co-sintering process.…”