Dynamic Changes in Charge-Transfer Resistance at Li Metal/Li7La3Zr2O12 Interfaces during Electrochemical Li Dissolution/Deposition Cycles
Authors: Matsuda Shoichi 
Published: May 2020

Lithium (Li) metal is an attractive material for the use as the negative electrode of next-generation batteries such as Li-air and Li-sulfur batteries due to its high theoretical capacity (3860 mAh g-1) and the lowest electrochemical potential (-3.040 V vs. SHE). The use of a solid electrolyte is a potential solution to these issues inherent in Li metal. Garnet-type cubic Li7La3Zr2O12 (LLZ) is promising as a solid electrolyte due to various advantages, including high Li-ion conductivity, high chemical stability against Li metal and high stiffness. However, it is known that the resistance of the Li/LLZ interface (
R
int) is high, which interferes with the operation of a LLZ-based solid-state battery at a practical rate. To date, several attempts have been made to reduce R
int; application of high external pressure and temperature, tuning the chemical composition of LLZ, modification of the surface morphology of LLZ by optimization of the particle and grain sizes, and the insertion of a lithiophilic layer between Li and LLZ. Although these studies have provided potential strategies to reduce R
int toward the successful operation of all-solid-state batteries, there is still limited information regarding how R
int dynamically changes during repetitive Li deposition/dissolution cycles.

To address this issue, AC impedance spectroscopy with a three-electrode setup, in which the interface between a working electrode and electrolyte can be examined independently from the other interface between a counter electrode and the electrolyte, is necessary. In the present work, we attempted to individually trace the dynamic change in R
int at a Li/LLZ interface during Li deposition and dissolution reactions through the use of the three-electrode AC impedance technique. As a result, we clarified that the trace the dynamic changes in the charge transfer resistance at the Li/LLZ interface during Li dissolution and deposition. R
int increased and decreased during Li dissolution and deposition, respectively, and the increase during dissolution was not completely offset during the subsequent deposition process.

Figure 1a show the time courses of the W.E. potential and R
int, respectively. The overpotential increased during Li dissolution, whereas it continued to decrease during Li deposition. Rint increased and decreased during Li dissolution and deposition, respectively, which is in good agreement with the trends for the overpotential. However, Rint did not return to the initial value after one cycle of dissolution/deposition, indicating that the change in Rint during dissolution is larger than that during deposition. The degree of R
int change during Li dissolution and deposition grew larger with cycle number and the cell short-circuited in less than 10 cycles.

Time courses of the overpotential and R
int shown in Figure 1a can be well explained by following model (Figure 1b). First, let us consider the time courses during Li dissolution. As Li dissolution progresses, the size and/or the density of the voids formed at the Li/LLZ interface is inevitably increased. Therefore, the physical contact area at the Li and LLZ interfaces decreases, which in turn increases the effective current density and thus the overpotential during galvanostatic Li dissolution. Besides, R
int also increased because R
int is inversely proportional to the contact area between Li and LLZ. On the other hand, when the polarization was switched, the voids formed during the Li dissolution can only be partially filled via the subsequent Li deposition process. The contact area of the Li/LLZ interface is smaller in the beginning; therefore, a larger overpotential is required for Li deposition to proceed. However, with the progress of Li deposition, the contact area is gradually increased and thereby R
int can be smaller. Therefore, the overpotential is decreased in association with the decrease in R
int. Figure 1a shows that R
int did not return to the initial value after one cycle of dissolution/deposition, which suggests that voids were gradually accumulated at the interface as the cycle proceeded. This is a problem from the viewpoint of obtaining stable cycle characteristics because the voids inevitably induce non-uniformity of the flux of Li+ ions at the interface, which leads to the growth of Li dendrites.

Based on the results obtained through the present work, we strongly encourage the development of a strategy to prevent the formation of voids at the Li/LLZ interface, particularly during Li dissolution, toward the real application of a Li metal anode in all-solid-state secondary batteries.

Figure 1