Contact between Garnet-Type Solid Electrolyte and Lithium Metal Anode: Influence on Charge Transfer Resistance and Short Circuit Prevention

Abstract

Interface structure between Li and garnet-type Li6.5La3Zr1.5Ta0.5O12 (LLZT) solid electrolyte was investigated by means of electrochemical impedance spectroscopy (EIS) on symmetric cells of Li | LLZT | Li. Charge transfer resistance (RCT) between Li and LLZT was investigated using LLZT pellets with various roughness. RCT and activation energy (Ea) obtained on the flat interface is as high as 746 Ω cm2 and 0.51 eV at 25°C, respectively, indicating that the charge transfer reaction at Li | LLZT (grit number: #8000) interface is a kinetically slow process, which may suppress rate capability of all solid-state batteries. Although the lowest RCT of 363 Ω cm2 was obtained by heating up to the melting point of Li for the LLZT pellet polished with an emery paper (#400), it is supposed that electrochemically effective contact area was saturated for rougher surfaces of LLZT. To prepare interfaces with large effective contact area, Li electrodes were deposited on a LLZT pellet by vacuum-evaporation, which exhibited further low RCT of 69 Ω cm2. The interface with large effective contact area is also a key to prevention of short circuit, and a high critical current density of 0.4 mA cm−2 was demonstrated.